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The ZL1BPU GPSCLOCK USER MANUAL

1. bit C00 Low gain low noise phase locked control mode with shift register Default mode C01 Medium gain low noise phase locked control mode gain 5 C02 High gain quick locking phase locked control mode gain 15 C03 Minimum frequency used for range checking C04 Centre frequency used for reference calibration C05 Maximum frequency used for range checking C06 Ramp up from current point used for A D checking C07 Ramp down from current point used for A D checking The secret to Mode C00 is the DSP technique using a 16 bit shift register which acts as a high tech phase accumulator It allows phase comparisons to be made 32 seconds apart rather than just one second apart meaning that much higher gain and phase resolution are possible Because a shift register is used a new 32 second result is available every second important for accurate and stable control With a 10MHz reference phase resolution is thus in effect about 3ns not 100ns as you d expect The gain of Mode C00 is user adjustable using the integration time function Modes C01 and C02 lock quickly just a few minutes but not with any great accuracy They are provided mostly so you can see the difference between a high tech phase accumulator and conventional proportional control as used by others The fastest way to achieve lock is to use C04 for a few seconds to centre the frequency close to the right point then COO to lock If you can watch the fascinating pr
2. c Command sent to the Reference controller Hnn Set UTC hours to nn BCD hours Mnn Set UTC minutes to nn Snn Set UTC seconds to nn Add one to UTC seconds Subtract one from UTC seconds Increment local time offset by one hour Decrement local time offset by one hour Advance clock by 1ms Retard clock by 10 ms Synchronize clock seconds to GPS tick lt xDPAvV Before finally setting the clock on its long journey of uninterrupted timekeeping make sure that the reference is correctly adjusted and calibrated This is covered under Calibration in the next section The first four commands in the list above cause the clock to stop while data is entered so should only be used in the first stages of setting the time They should be used in the order shown while listening to WWV or some other time service Set the local offset the hours minutes and finally set the seconds to about the right value give or take a few seconds At this stage don t worry if the seconds are out by a few The clock seconds won t tick over exactly in time with GPS or the radio broadcast seconds pips so next use the X command to synchronize the clock to the next GPS pulse A and R commands can also be used repeatedly for very fine adjustment if for example you want the clock to run in advance of GPS for some reason You can also use the clock as a tool to measure clock to GPS offset and to measure propagation delay from a radio time signal x
3. TV Sync Capture Separator Register Divide by Divide by 5 P 20 000 Local Reference Fig 3 Micro controller phase comparator The Capture Register captures the number in the timer at the TV Frame sample point and so represents the phase of the local reference a count of 0 to 19 999 with a resolution of 1 microsecond This register can be read and the data sent to a PC which plots the phase and calculates the frequency offset Using this technique offsets as small as 1 part in 10 can be detected and running the PC software for an hour or so every few weeks will keep track of the ageing rate with ease Fig 4 shows part of a PC software display which plots about 9 hours of reference phase the upward sloping line The plot is magnified because only the least significant eight bits of the 16 bit count are displayed In 9 hours the phase has slipped 64 microseconds so the frequency offset is 0 00083 parts in 10 indicated on top line or 8 3 parts in 10 An error of this order is equivalent to losing one second in 38 years Phase 14338 ps Freq offset 0 00083 in 10 6 f LF PP Fig 4 PC phase comparison display The phase plot has a couple of kinks in it which attribute to occasions where the TV network is not using its precision reference The best time to run these tests is at night and the worst during the weekend when outside broadcasts are frequently used and the TV referenc
4. WWV Fig 10 If the R command is used too much causing overshoot another 100 presses are required to get close to the correct seconds phase again and time will slip one second The A command works very slowly and due to the timing relationships in the clock will not advance time every time you send the command 1ms is the finest resolution possible because this is the smallest time inverval counted After checking at the minute event use the or command to correct the seconds display It should change to 00 exactly as the WWV minute tone sounds The gt and lt commands to adjust local time can also be used any time which is convenient so stepping forward or back for daylight saving is a breeze These four commands plus the A R and X commands are the only ones permissible while the GPSClock is keeping time They do not normally affect time keeping at all although slippage can occur if you happen to send a command just as a second event occurs The slippage will be less than 10ms Finally press X to resynchronize in case there was any slippage in setting The program source code for this project is available from the author at a modest charge so the constructor can make whatever modifications are desired These are best made in the main loop where timekeeping will be unaffected There is plenty of code space so small adaptations could for example provide Time signal pips or even
5. receiver and phase locked to GPS using thermal control pulse width modulation of the oven heater GPSClock Wee led nm ete a ee a ee ee 10 min ak i T 23a T T T Fig 16 Cold start performance GPSClock is within 1 part in 10 in 10 minutes In Fig 16 the 5MHz OCXO GPS locked reference provides a continuous comparison baseline It is much less noisy than the inexpensive VTXO but uses 3W of heater power The GPSClock had power removed for an hour prior to the start of the graph The marks along the bottom are at 10 minute intervals You can see that the GPSClock pulls the VTXO to within 1e 8 within 10 minutes and the performance is close to 1e 9 give or take some noise within 30 minutes Page 22 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 GPS restored GPS loss GPSClock ele GPSClock Previn wiv VT vyyeW OE ATETEA ish Aid Abas dA Sinead rbd Pehia Vaiala 5MHz OCXO N Par 5MHz OCXO 12 hours 20 21 22 23 oo o 02 03 04 05 06 07 08 og 10 11 12 13 14 Fig 17 12 hour holdover performance In Fig 17 both references were running for more than 24 hours locked to GPS At the start of the test the GPS reference was unplugged from both devices and after 12 hours the GPS was restored Several things are obvious in this graph which has a horizontal time scale in hours 1 First the OCXO is much more stable and has much less noise than the VTXO and stays much closer to the correct freque
6. 1959 1s 0 81 63p1s xo 2040ps 1 0 0 x 0 8 Es 0 6 0 4 0 2 0 0 0 2 0 6 0 8 6 4 0 4 2 0 0 8 2 1 0 10 5 0 5 10 15 20 25 30 35 40 ms Fig 9 GPSClock 1Hz pulse red and the GPS 1PPS pulse blue If an oscilloscope is available trigger it from the 1 Hz pulse output of the GPSClock and observe the GPS pulse from J5 on the screen You can use the A and R commands to align them see Fig 9 Without an oscilloscope the 1 Hz ticks can also be lined up with WWV time signals quite easily and with remarkable Page 9 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 precision by listening to them both at the same time with a pair of headphones The only problem here is that there is a marked propagation delay in the radio signal arrival Frequently WWV WWVH and other timer services will be heard giving multiple ticks due to different propagation times In Fig 10 both WWVH and WWV ticks can be identified having 30 and 45ms delay respectively after the GPSClock 1Hz pulse v v 1 0 0 8 0 6 1 0 0 4 0 8 0 2 0 6 0 0 0 4 0 2 0 2 0 4 0 0 0 6 0 2 0 8 30 10 0 10 20 30 40 50 60 70 80 vg Fig 10 The GPSClock pulse red compared with WWVH and WWV blue If your reference is not 1pps say it s 1kHz you can t use the X command to reliably synchronize seconds As a last resort simply adjust with the A and R command so that the seconds tick lines up visually with
7. The computer is now ready to program the device From the menu select Program Erase to erase the device then Program Program Device to program it It should take 10 20 seconds depending on the speed of the computer If the device is faulty or not blank it may take longer and will report errors It may also take ages and report errors if the GPSClock power supply is low At this point the micro starts talking to the serial port and possibly garbled time messages should be seen on the Terminal screen The final step is to set the EEPROM parameters which are contained in the first few bytes of EEPROM The example shown below is for a 10 000MHz reference 9600 bps and a 13 hour default local time reference First row of EEPROM memory FF FF FF FF First four bytes not used 40 Baud rate 9600 baud with 10 000MHz 13 87 T1 reload for 2 kHz interrupt with 10 000MHz VTXO 13 NZDT offset 13 10 Integration time example is 16 seconds maximum is 31 FF FF FF FF FF FF FE Rest not used The EEPROM Data Memory window will probably show all values as FF so leave them as they are it won t matter what they contain Simply plug the values shown into the first row of numbers If the EEPROM in the device is blank reading it will restore all values in the window to FF Next count along to the 5 byte in the first row and change it to your required data rate The next two bytes set the reference division and the 8 byte the lo
8. a suitable cable The Jaycar KC 5340 kit will also do the trick as will the ATMEL ISP serial programmer Page 18 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 This window can be seen by clicking on it or from the menu Window EEPROM Data Memory If the cable is faulty or not connected an error message like this will be displayed Dongle Check i STK200 STK300 or Value Added Pack Dongle Required Fig 14 Oh dear no faulty programming cable lf the cable is connected but there is a fault at the GPSClock perhaps the programming header is wired incorrectly is plugged in backwards or there is no power applied to the clock there will be a different message AT90S2313 Part not Detected Hardware Check A AT9052313 Part Not Detected This may be due to power off on programming module Security Bits Enabled or a pre production AVR j Bety Ignore Fig 15 No communications with device to be programmed This message also appears if the wrong device has been selected in the Project Manager If everything has worked fine so far the system is ready for programming The software implicitly loads files into the current window so in order to load the executable firmware select the Program Memory window by clicking on it or from the menu Window Program Memory Next select File Load and locate and select the executable file which will likely have the name GPSCLKnn HEx The file is in Intel HEX format
9. accumulates with successive pulses the jitter is averaged out and time is accumulated with great accuracy The technique of locking a high frequency oscillator to 1pps is known as a GPS Disciplined Oscillator It is not a simple technique to implement for a range of technical reasons and equipment of this type is typically very complex and expensive Now for the first time you can have a SINGLE CHIP GPS Disciplined Oscillator with all the complexity inside the chip When you build this unit you will also need a suitable GPS unit and antenna Most of the suitable GPS units with 1pps are of the OEM type little modules that are designed to be built into other equipment The author has used the Rockwell Jupiter and the CMC Superstar but other modules such as the Trimble Lassen SQ and the Ashtech A12 pictured below would be equally suitable The GPS unit and its antenna will probably cost more than the rest of this project Fortunately there are many other uses for these modules You will be more than impressed by the performance of the GPSClock it always tells the correct time down to fractions of a second and is unaffected by temperature or power glitches and even keeps good time when the GPS reference is lost Plus it uses only one inexpensive chip 4 112mm Fig 2 A typical GPS module the Ashtech A12 Universal Time Coordinated most people will know it as GMT One power of 10 8 It therefore works wit
10. current consumption can often be improved further by lagging the oven externally to prevent heat loss Oven heat will need to be on all the time and it s a good idea to provide some automatic cut off in case the temperature controller fails You d rather replace a fuse than have to throw away a cooked reference You should be able to squeeze a thermistor inside the oven somewhere If not tape it to the outside of the oven and then lag the oven Set the trip point just above the normal operating temperature Look for oven controlled oscillators from old telecom gear test equipment and old satellite navigation Transit receivers Don t be put off by the age very often older oscillators are better because they have much lower ageing rate however they may be out of calibration and need some tweaking The more readily available and cheaper oscillators are not electrically adjustable so you may need to steam the oscillator open with a propane torch and work out a strategy for frequency control Very often the oscillator has its own internal voltage regulator and pushing this regulator up and down by about 1V will be enough to move the oscillator by 1e 7 Another method is to add a bias resistor to the oscillator transistor to alter its operating point and thus its Miller capacitance In general these methods are more stable and easier to set up than adding varicap diodes since you won t need to change any carefully selected temperature compensa
11. line and colour burst frequencies of the TV1 and TV2 networks were for many years controlled by a Rubidium Standard and now by high accuracy GPS standards The carrier frequencies of JJY on 40 and 60 kHz are very accurate and readily received in New Zealand at night The seconds ticks available from some GPS units are also very accurate and are the preferred source since these pulses are directly traceable to international standards There are several ways to use these references ranging from simple to complex which will describe The technique chosen largely depends on the quality of results required Defining Frequency Accuracy Frequency reference performance is not just a matter of quoting how far off frequency it is the offset The offset also changes over time ageing rate and has short term variations perturbations These variations are caused by temperature varying load and supply voltage and sideband noise effects Offset is usually adjustable if you have something better to calibrate against and the calibration reference needs to be at least one decade better The better the reference gets the harder it is to measure and calibrate Here are some reference examples Reference Type Ageing Rate Perturbations Inexpensive TCXO Not quoted 0 5 in 10 Good OCXO 1 in 10 per week 1in 10 Rubidium Standard 1 in 10 per month 1in 10 GPS locked VTXO Compensated 1 in 10 3 in 10 GP
12. need a superb reference standard that is much better than the oscillator under test and some technique to make comparisons A high quality ratiometric counter can be used and the results plotted but this approach is labour intensive The most painless method is to use an SSB receiver locked to the reference standard which then provides audio to a Spectrogram program on a computer An ordinary AM receiver will also do the job if you are able to generate either from your reference standard or from the oscillator an audio tone that can beat with the other source in the receiver For the graphs shown below the author used a Harris RF 505A communications receiver in USB mode It has a single 5MHz reference for all its synthesized oscillators and this reference was provided externally by an HP5065A Rubidium Standard itself calibrated to 1e 12 against GPS Measurements were made on harmonics at 30MHz receiver tuned to 29999 0kHz to improve resolution The Spectrogram was the precision sound card program SBSPECTRUM by Peter Martinez G3PLX and has a minimum span vertical direction of 2 5Hz giving a resolution of 10MHz yes milliHz or 3e 10 at 30MHz Two references using different technology were received at the same time to allow comparison to be made One was the GPSClock operating with a Rakon 10MHz VTXO200N VTXO the other an older design by the author which uses an ERC 5MHz oven cortrolled oscillator pulled from an old satellite navigation
13. 11 59 pm Local offset adjustable 13 hours Independent setting of offset hours minutes seconds via serial link Incremental time trimming in 10ms steps via serial link Automatic synchronization to GPS to within 1ms typically within 500us No front panel controls two internal adjustments reference centre frequency LCD contrast Reference Low power VTXO at 10 000MHz e g Rakon VTXO4080N 10 000MHz or higher performance V OCXO device Design will operate at any frequency from 5 15MHz that is an integer multiple of 2kHz Typical offset 1 part in 10 or better at centre of control range without GPS Typical offset 3 part in 10 or better with GPS Typical error over temperature range1 part in 10 without GPS 1 part in 10 or better with GPS Typical ageing rate 1 part in 10 per year Calibration using continuous GPS reference lt 3 parts in 10 short term 0 03Hz 10MHz 1 part in 10 long term 1 sec in 1000 years Holdover error when GPS lost Approx 1 part in 10 over 24 hours clock error lt 1ms Recovery when GPS restored reaches 1 part in 10 in under 10 minutes Cold start performance with GPS 1 part in 10 in under 10 minutes Outputs UTC time reference phase via RS232 9600 N 8 1 10 000MHz 2V p p semi sinewave phase locked to GPS see Reference above frequency depends on reference chosen 1000 0 Hz 50 square wave CMOS 5V level 1 000 Hz 1ms wide pulse CMOS 5V level positive true 12 bit PW
14. 3 15 I MOTS PB2 14 ai E72 aH NiCd 320k J4 DC socket or NiMH iy PBO 12 neo I gt face ae x xi 5V R822 i Mos R30 i 7 SHEE ro de of e H 10 00 MHz s lo ribs e_ f 3 pcos te jt L__ cs 100nF 4k7 A ns J1 2x5 HDR C8 10uF 35V INAS l5 100k ha PROGRAM 5k0 0V C3 100uF aa toou T10 00n Copyright 2002 2004 Murray Greenman Fig 4 The Superclock schematic In the schematic note the power supply arrangements on the left with the diode isolated backup supply and AC supply which consists of a simple AC adaptor or plug pack When AC power fails the input micro input PD2 pin 6 goes to zero and the micro displays BATT on the display If an external DC battery supply is used it might be useful to know when the supply drops below the back up battery rather than when it drops to zero so to achieve this replace R2 D3 and R3 with a comparator Next in the schematic are the 5V regulator U1 and the micro U2 with the VTXO X1 below The micro is an inexpensive ATMEL AT90S2313 available in New Zealand and Australia from DSE Polykom or Jaycar and in the USA from Digi key and Mouser To the right are the RS232 interface TR1 and TR2 the LCD connector J3 and the programming header J1 A discrete transistor RS232 interface is used to reduce supply current drain Current is drawn from the supply only when transmitting data The REF input J5 is where the 5V level 1Hz 1pps reference pulses from the GPS receiver enter If the G
15. 55504 10 44 39 0120455204 10 44 40 0120454904 10 44 41 011F454704 10 44 42 011F454604 10 44 43 011E454504 10 44 44 011E454404 10 44 45 011D454304 10 44 46 012C454104 The data above is an example of telemetry received on the PC during frequency adjustment in mode C04 Remember that there is a new line of telemetry each second See how the PHASE artificially coloured RED is slowly decreasing in value about every other second i e with an error of about 0 5Hz nearly close enough for this adjustment Note also that the last digit the Mode is 4 artificially coloured GREEN Operating Gain To check the operating gain of the VTXO watch the phase on the LCD display or telemetry Send the command C03 and check the frequency In other words how big the steps in PHASE are each second the above example is about 0 5Hz Then repeat with C05 and check again The range should be about 2Hz to 4Hz at 10MHz and reasonably balanced about the correct frequency By watching the phase on the computer or LCD display you can estimate by how many the numbers change per second 4Hz means the Page 12 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 PHASE changes by four each second If you have the equipment you can easily check the gain using a very stable communications receiver and Spectrogram PC program see next section You can change this frequency range by changing the GAIN resistor R23 Higher gain lower resist
16. GPSClock Murray Greenman 2002 2004
17. M feedback at 31 25Hz HOUR CHIME output logic low for 1sec at second 00 of minute 00 ALARM CHIME every second 00 of every minute when AC power has failed Reference GPS 1PPS from any GPS receiver supporting this option 5V logic level 1Mohm input impedance GPS is the preferred option Any pulse frequency which is a common factor of 2kHz from 0 5 to 1000Hz and has the long term accuracy of GPS Any other reference with long term accuracy such as Rubidium or TV network frame pulse meeting the pulse frequency requirement Glitch free no break operation current control value is held when reference is lost and new value reacquired when reference returns LCD Display 16 column x 2 line standard HD44780 display in 4 bit mode 24x2 optional Display can be back lit but not supported by backup battery or DTR signal supply Display can be PLED but higher backup battery capacity will be required UTC time 00 00 00 UTC to 23 59 59 UTC Local time 00 00 00 am to 11 59 59 pm Local offset 13 to 13 hours Reference phase three character only on 24x2 display Control voltage three character only on 24x2 display Status and Mode three character only on 24x2 display Power failure message BATT GPS OK message GPS or blank if battery OK but no GPS only on 24x2 display Controls All via serial command Eleven commands No front panel controls Four initial setting commands which affect time keeping Seven trimming comma
18. PS is disconnected it is important that the input be grounded to prevent noise generated interrupts and thus false phase measurements This is achieved by R4 while R36 provides ESD protection The use of this GPS phase detector is described later Other spare pins are outputs and can be left open There is of course considerable similarity between the circuit of the GPSClock and previous projects especially the Superclock See Fig 5 for a detailed view of the circuit board part way through assembly The LCD connects at the top left Display routines have been specially written for operation at up to 15 MHz 13 Electrostatic Damage the series resistor limits the energy dissipated in the chip protection diodes when static electricity appears on the input All external inputs are protected in this way Page 5 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 The trim pot top centre Fig 5 sets the LCD contrast To the right of the pot are the programming header and some unused resistors left over from a previous project The micro is centre right and the power and serial connections lower left The two pin connector bottom left is for the backup battery The wiring underneath is a combination of solder bridged pad connections and short lengths of fine insulated wire The two centre tracks under the micro are OV lower and 5V power upper The same low cost RS232 interface used in other projects is in the lower centre of the boar
19. S locked OCXO Compensated 1 in 10 Tin 10 2 See NZART Call Book page 8 1 See also www irl cri nz teams ms tiservi htm In Australia use the ABC TVNZ now use Trimble Thunderbolt GPS units 5 The NMEA time message may be correct on average but will wander in time The second tick provided by some units however is frequently held within 1us or better of UTC 31 Temperature Compensated Crystal Oscillator Oven Controlled Crystal Oscillator Page 25 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Suitable References Good quality TCXOs can be purchased inexpensively or recovered from defunct cellular phones TCXOs are little silver boxes the size of a postage stamp or smaller typically on useful frequencies such as 10 000 MHz or 12 800 MHz Some cellular phone units are on 9 60 or 14 85 MHz which may not seem very useful but using a micro controller you can easily generate an accurate and useful reference The micro controller project described in this manual uses an electrically adjustable 10 000MHz version of on one of these devices For higher performance an oven controlled reference is necessary These can be home built but suitable crystals are very difficult to find and expensive to buy the oscillator has to be very good and the oven control very accurate all requiring considerable experimentation and years of experience The best solution is to hunt for surplus equipment that may contain a suitable referen
20. The ZL1BPU GPSCLOCK USER MANUAL A Single Chip GPS Disciplined Clock and Reference This manual refers to firmware Version 0 3 GPSCLK3g xxx Murray Greenman ZL1BPU February 2005 Introduction This project is a practical application of precision timing Time and frequency measurement are closely related indeed one is the reciprocal of the other and so an accurate frequency reference can make a very accurate clock This unit contains both a frequency reference and a clock with performance performance Accuracy of frequency standards is usually quoted as parts in 10 ppm or 10 ppb The time accuracy of clocks is rarely quoted but it works out that one second per 11 5 days or three seconds per month is the same as one part in 10 Three seconds per year is about the same as one part in 10 One second per 100 years is about three parts in 10 The clock in this project can depending on the reference oscillator used provide this level of performance or better Of course this order of performance is unnecessary in a clock since it will outlive its owner and clocks displaying civil time with or without Daylight Saving require attention twice a year anyway Such accuracy is however very useful in a frequency standard Electronic clocks and watches typically use a quartz crystal reference oscillating at either 32 768kHz or 4194 304kHz While the performance of these isn t too bad typically within a few seconds per w
21. and wait From the time it takes the line to move off the screen to the left or right round the back so to speak and return to where it started from the other side you can work out the offset of your reference One minute is about 1 in 10 10 minutes is about 1 in 10 and so on If the line moves to the right your oscillator is low in frequency In under an hour you can easily adjust the oscillator as close as you need This technique is called phase comparison and in doing this you are comparing the phase of your local reference with the TV line frequency at 15 625 kHz This manual technique becomes tedious if your local reference is very good or you wish to calibrate frequently In that case you need a more automated approach A simple phase comparator can be made using a flip flop and a meter as in Fig 2 Using video from a TV receiver a video recorder tuner is ideal the sync separator for example an LM1881 is used to recover the Line Sync The leading edge of the 15 625 kHz Line Sync is used to set an RS flip flop The local reference is also divided down to 15 625 kHz and the leading edge of the local reference used to reset the flip flop Thus the flip flop is set for the period between one leading edge and the other so is a measure of the phase difference Since the inertia of a meter is too high to follow the 15 625 kHz signal a simple 5V DC meter indicates the phase As in the TV set approach you can watch the meter and time t
22. cal time offset Finally set a default integration time So for example the first line of the EEPROM memory when ready to program should read Page 19 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 FF FF FF FF 40 13 87 13 04 FF FF FF FF FF FE Then program the EEPROM From the menu select Program Program EEPROM to load the data It should take just a couple of seconds If all is well you should see time messages on the Terminal screen If the message from the GPSClock is still garbled check your maths again and check the PC Terminal data rate Hints and Adaptations In its VTXO form GPSClock is very portable It will run for about 10 days from a 1 2AH SLA battery small enough to hold in your hand You might consider making the case big enough to hold such a battery rather than the suggested 450mAH NiCd pack but you ll need to arrange some other means of charging it Perhaps a solar panel GPSClock can be operated with or without an LCD display Several versions have been built for embedded frequency control applications with no display The time is still available from the serial telemetry if you need it and you could write a little PC application to display the time or simulate the LCD display if you wish all the data on the display is available from the telemetry As an embedded device you don t need any outside world connections except the GPS ipps Just build it into the host equipment that needs to be controlled For exa
23. ce Look for old Omega receivers pre GPS satellite navigation receivers telephone transmission equipment and communications test equipment The older the better to some extent as the oscillator ageing rate will be lower The two very high performance units have came from Magnavox MX4102 Satellite Navigation receivers made in the mid 1980s Unfortunately few of these devices are electrically adjustable without modification If you need high accuracy but cannot locate a suitable high performance reference consider phase locking a lower performance voltage controlled oscillator VCXO to an external reference The TV line frequency is a popular choice as the circuitry is simple but from experience lock tends to be unreliable An excellent micro controller design from the Philips Research Labs which uses an LF reference would adapt well to TV line frequency GPS is quite complex to use due the the low reference frequency 1 Hz but has proved effective using a micro controller approach The project described in this manual is a further up to date example of this technology which is much simpler and easier to use than pervious designs It is my experience that if you have a really good reference such as a top quality OCXO there is no point in attempting to phase lock it to another modest reference such as the TV The phase locked reference is likely to have much degraded short term stability and of course is quite dependent on having the ex
24. d At the far left centre is the little 5V regulator The silver box lower right is the VTXO and its adjustment trimmer can be clearly seen This reference has through hole pins making it easier to mount on the board than a surface mount device Fig 5 Circuit board detail partly assembled One interesting feature of the GPSClock is the way in which time is counted Unlike most micro controller projects which operate in binary e g a frequency counter or digital voltmeter this clock operates in BCD in other words it counts seconds 0 9 and from 0 tens to 5 tens In fact the data is stored as packed BCD where the information about seconds minutes and hours are stored in one register each Every time a mathematical calculation is made the result must be checked or adjusted to ensure that it is true packed BCD Seconds and minutes are also checked to see that they do not exceed 59 and a carry and correction made if they do This may seem a clumsy way to count but it has the advantage that displaying the result is very straightforward displaying both nibbles of the register as two ASCII characters simply displays 00 to 59 Time is displayed as HH MM SS UTC see Fig 6 14 Binary Coded Decimal Packed BCD keeps two decades in one byte In this case units right 4 bits and tens left 4 bits Page 6 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 a 37 54 UTC ts PM tIS Fig 6 The LCD display An extra check
25. d correctly accurately over its whole range don t just rely on the software integration setting The integration time also affects all the other modes so for example if you set C46 rather than C06 the VTXO control voltage will ramp up every nine seconds rather than every second Remember the integration time is the first character in the command and the mode itself is the second character Routine Calibration Every six months or so more often at first less often as you learn how much the oscillator ages you will need to check the natural operating frequency of the reference It is important to do this without stopping the clock or losing any time The process is therefore quite different to initial calibration since it must work without changing mode and thus upsetting the timekeeping With the GPS receiver running and giving a reliable fix and the 1PPS output connected to the GPSClock connect the clock to the PC and run the terminal program If you use a 24 column display just read the value off the display the PC won t be required Note the Feedback Control Voltage If the value is significantly different to 0x800 say below 0x400 or above OxC00 adjust the 10k trimpot R21 slightly one way or the other wait 10 minutes and then note the value again If it is closer to 0x800 you re going in the right direction If not make a similar change in the other direction Mark the trimpot direction so that you will know whic
26. d via its own switch mode regulator for best efficiency The author pulse width modulates the back light using the 1kHz output from the micro to save current 7 See http www wenzel com pdffiles ncmos pdf for an excellent and very simple CMOS multiplier 8 Any amateur astronomers out there Contact the author if you are interested Page 20 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 GPSClock can be operated with a non adjustable reference i e mechanically adjusted Simply set GPSClock in mode C04 and use the telemetry or phase display to adjust the reference against GPS occasionally You can use the telemetry to measure the reference performance Once calibrated the GPS can be disconnected Of course there s no compensation for ageing or thermal variations The GPSClock can be operated for months on end without GPS if necessary without losing time or having any great effect on the reference frequency However before using the reference frequency for any important calibration connect a good GPS ipps signal to it for 10 minutes or so so that it can self calibrate If the reference frequency is to be used regularly add a buffer amplifier to the VTXO output for example a 74HC14 For bullet proof results use a high speed opto coupler after the buffer and follow it with another buffer You can make the GPSClock unit do double duty as a clock and general purpose frequency reference with multiple frequency outputs Because the di
27. e can be degraded The other line on the display in Fig 4 is the frequency offset It is essentially a horizontal line at about 1 part in 10 and it is to this line that the calibration marks refer Apart from the interesting effect at the start when the data has insufficient resolution to resolve the offset the offset shows as a dead straight line indicating that the short term perturbations are well within 1 part in 10 Over a period of weeks this line moves up slowly as the offset changes with crystal ageing My best reference illustrated here moves about 1 part in 10 per week Other Improvements A number of additional features can be easily added to a micro controlled device such as that just described An obvious thing to do is to add an LCD display to indicate phase and frequency offset digitally and since there is plenty of room for software and plenty of available processing time to add a super stable clock as well remember one second in 38 years The clock could also provide time to the PC via the serial output along with the phase and frequency offset This is the micro project described in this manual A system such as has been described need not have TV video or GPS supplied all the time and of course the PC software need only be operated when a reading is needed Thus the system is reasonably economical of power use Page 27 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 In order to compare phase over long period
28. e control voltage is also modulated the voltage is generated by Pulse Width Modulation The software processes just described happen every half millisecond and between these interrupts the micro controller is asleep LCD Digital Outputs Serial 10MHz Display Hour Chime Comms Output Laimatminigmecnmmee 9 PT Sa H A ees VTXO Programmable UTC Clock 12 bit D A Reference Divider Local Clock Converter dase aeeeeesaa GPS 1PPS Phase Reference Sampler Phase Diff LP Filter Error Algorithm PWM Command Processor Power Fault Telemetry Supply Detection Generation Inside Micro Fig 3 Block Diagram of the GPSClock The GPS reference pulses cause the programmable counter to be sampled and this number is used by the frequency control system as the phase reference acting like a digital phase detector However the phase sampler has better resolution than a conventional digital phase detector since it has 16 bit resolution it acts more like an analog phase detector Just about everything else happens in software Once every second a flag is set by the interrupt divider and this allows the main program to operate briefly before going to sleep again The main program adds one second to the UTC time carries seconds to minutes and minutes to hours calculates the local time offset and then checks the current reference phase and calculates the required control voltage for the ensuing second Finally it checks that the pow
29. e frequency 50 00Hz are 2 Unless the oscillator has an excessive ageing or thermal drift rate which is why a VTXO or better is used Page 14 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 typically all locked to a common reference which can be 1 part in 10 accuracy or better There remain two problems with using TV as a reference source a knowing which network to use when the network is using its reference and when is it not and b receiving an accurate local version of the pulses given the various propagation problems For the purposes of the GPSClock the most appropriate and generally the most stable signal is the 50Hz frame frequency The well respected LM1881 sync separator chip is an ideal source of the TV frame frequency 50 Hz reference Use the video output of a TV receiver or video recorder to drive the chip Operate the LM1881 from the GPSClock s 5V supply and feed it with TV composite video from a Rubidium or GPS referenced TV station and this simple little 8 pin chip will generate very stable and precise pulses with the correct timing and levels to drive the J5 input of the GPSClock directly See Figs 11 and 4 5V Composite P 2 e IN VCC VER 100nF LM1881 100nF L COMP RC GND Fig 11 50 Hz Reference from TV Sync The line sync output of the LM1881 is not useful for phase detection purposes in this design for two reasons First it is a composite sync output which contains twice freque
30. e positive going 5V logic level on the UTC second DC coupled and preferably 10us to 100ms wide to the GPSClock connect the clock to the PC comm port via a serial cable and run a terminal program on your PC Windows Terminal Hyperterminal even DOS Term at 9600 N 8 1 Check that telemetry is being received from the unit Type the command C04 on the terminal and note that the last digit of the telemetry on the PC screen display shows a 4 It also shows at the top right of the LCD screen on the 2x24 display VTXO Centre Frequency The next action is to adjust the 10k trimpot R21 so the frequency is exactly 10 000MHz Rather than use a receiver or frequency counter which will likely be inaccurate we use the GPS and the built in phase detector Watch the first four hex numbers after the time in the telemetry or the phase on the 2x24 LCD display If they increase in value with each consecutive reading lower the frequency If they decrease increase it You will need to adjust in tiny steps very slowly once you get close as the changes may not happen for many seconds A change of one every 10 seconds is good enough You can also adjust by watching the phase on the smaller graph in the RECORD program This adjustment does not affect the final accuracy only the calibration operating values and how long the unit will survive between calibrations 10 44 34 0123456504 10 44 35 0122456304 10 44 36 0122456104 10 44 37 0121455804 10 44 38 01214
31. e to be exceeded and lock will be lost If this occurs repeat the initial calibration and then reset the time Remember that the calibration ONLY affects the Feedback Control Voltage and the available control range By manually adjusting the median voltage applied to the VTXO you are maximizing the electronic control range from the micro You are not adjusting the accuracy of the clock or its reference which are assured by the GPS With GPS connected there is NO ERROR and even when the GPS is disconnected the error will be very small since the GPSClock remembers the Feedback Control Voltage it used when the GPS was last connected Phase Comparison Much the best source of phase reference is the GPS receiver 1PPS signal as assumed throughout this manual There are other suitable references and the GPSClock is able to lock to a remarkably wide range of signals Provided the signal is a 5V pulse it will lock to 0 1 0 2 0 5 1 2 4 5 8 10 16 20 25 50 100 200 250 400 500 or 1000Hz Thus the output of some other stable reference source such as a Rubidium Standard can be used Few other sources will have the long term accuracy that the GPS can provide Some television networks use an accurate Rubidium locked or GPS locked timing generator for some or all their reference timing PAL TV is a convenient and much under rated as a source of precision frequency The colour subcarrier 4433 61875kHz line frequency 15 625kHz and fram
32. eek it is easy and relatively i inexpensive to do much better In the Superclock a TCXO was used to provide good temperature stability resulting in an error of a few seconds per year TCXOs are widely used in UHF radio equipment such as cellular phones which need to stay on frequency over extended periods and a wide temperature range They are also used in GPS equipment frequency counters high performance signal generators and other precision equipment This GPSClock project uses a type of TCXO called the VTXO or Voltage controlled TCXO While there are better high performance devices to use for precision references especially the OCXO or Oven Controlled Crystal Oscillator devices which have less noise and lower ageing the VTXO has the advantage of reasonable availability modest cost and very low power The performance is good enough for all but the most critical applications Fig 1 The GPSClock prototype during development 1 Parts per million and parts per billion There is a very good tutorial on Timekeeping at www allanstime com Publications DWA Science_Timekeeping TheScienceOfTimekeeping paf 3 See also Appendix About Frequency References See www qsl net zl1bpu micro CLOCK Temperature Compensated or controlled Crystal Oscillator Page 2 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 The picture of the first prototype Fig 1 shows just how little there is to the unit The project uses an inexpensive
33. equency with a 10 second gate time resolution 1 part in 10 and adjust the counter to read the known reference frequency including any known offset To do the best possible job run the counter continuously and check it every week or so Write down the calibration date and offset on a label attached to the counter so you ll know when to calibrate next time and learn what the drift performance is like A good frequency reference makes an ideal source for a frequency synthesizer such as a phase locked loop PLL type or a direct digital synthesis VFO Unfortunately DDS type synthesizers generally require very high reference frequencies although these can of course be phase locked to a lower frequency use a 12 8 MHz TCXO as the reference in my LF Exciter which can generate any frequency from zero to 400 kHz with 85 mHz milliHertz resolution Bob ZL2CA and have experimented with precision frequency transmissions on 80m used an old 20W AM transmitter crystal controlled on 3750 kHz An output from the reference was devised at 1250 kHz 5 MHz divided by four and since this is also 3750 kHz divided by three by injecting a small amount of this signal into the transmitter crystal oscillator it could be locked to the reference It would pull in from several Hz away and provided a transmission with extremely high precision 38 See Break In September October 2002 page 4 3 See www qsl net zl1 bpu micro EXCITER Page 28 of 28 ZL1BPU
34. er supply and GPS reference are OK then sends and displays all the information before going to sleep again Because these processes happen so quickly the micro controller 12 It can count to 2 1 or 65535 Page 4 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 spends most of its time asleep and the average current drawn from the power supply is low just 10 milliamps including the LCD display and the VTXO Power for the GPSClock is provided by a small AC adaptor and is backed up by a small NiCd or NiMH battery which can be a recycled cellular phone battery to ride out power failures The unit should operate for three days from typical batteries Since the current drawn is so low the clock can also operate from the low current provided by the serial port of a PC connect a diode from J2 pin 4 to J4 The battery pack is charged slowly by a resistor across the battery s isolating diode D3 see Fig 4 ut ZL1BPU GPSCLOCK 10V 78L05 e IN ang OUT 5V 4 u2 TR2 20 PDO 2 Vcc D4 iue C9 10uF PD1 3 1N4002 te 16V PD6 11 D2 0k A aA matas P98 maa R1 b3 ART 1N4148 AT90S J3 14X1 HDR 2313 10PC POD Display R2 100k C1 1001F RESET 1 m E PB7 19 J5 Phono F 6 PD2 PB6 18 D5 R3 1 Hz BZX85C4V7 1M0 7 PD3 PBS 17 ME 8 PD4 PB4 16 i 7 pos 10 15V DC 9 PD5 PB
35. h 1000Hz 500Hz 250 100 50 25 10Hz etc See article by Dale Hughes VK2DSH in Amateur Radio June 2004 WIA 10 See products thalesnavigation com assets datasheets A12receiver pdf If you have a choice use a unit which turns off the 1pps pulse when the GPS fix is lost the Rockwell Jupiter does not and results in the clock following the unlocked GPS unit rather than going into hold Page 3 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Description The little micro controller has a 16 bit hardware counter timer which is used as a programmable divider to divide from the 10 000MHz VTXO reference frequency and generate an interrupt at 2 kHz Any VTXO frequency between 5 and 15MHz that is a multiple of 2kHz could be used but 10MHz is the most convenient as a frequency reference The micro is not involved in the counting of this high frequency or the capture of reference phase since this happens in hardware inside the chip even while the micro is asleep The 1 kHz output pin is toggled in the 2 kHz timer interrupt triggered by the programmable divider assuring that the duty cycle of the 1 kHz output signal is accurate at 50 Every alternate interrupt a further software counter is decremented This counter divides by 1000 and is used to generate the 1 Hz pulse and trigger the timekeeping process The 1Hz pulse is precisely one millisecond wide and occurs exactly at the second event At every interrupt the frequency referenc
36. h way to go next time Make one or two further adjustments each time leaving at least 10 minutes before reading the Feedback Control Voltage again this allows the adjustment you made to be corrected This process may take half an hour or so and can be most conveniently done as you change the daylight saving offset i e every six months No clock time is lost or gained provided you stay away from the H M S and C commands The idea is to move the Feedback Control Voltage value to the opposite end of the control range For example if the value when you started was Ox06FE then aim to set it to around Ox0A00 or thereabouts The actual value is not important as accucy is unaffected but offsetting it the other way maximizes the time between calibrations In between routine calibrations you can assess the reference calibration status by monitoring the Feedback Control Voltage It is a good idea to write this value and the date on a sticker on the back of the clock every month or so The nominal value when calibrated is 0x800 800 x and the range is 0x000 to OxFFF 0 to 4095 decimal So long as the value stays near the middle of the range all is well If at calibration time the Feedback Control Voltage is near 0x800 then you do not need to calibrate at all If the ageing rate of the VTXO is very high or if you used an ordinary crystal oscillator and you forgot to check the change in Feedback Control Voltage it is possible for the control rang
37. he phase rotation but a much better way is to use a chart recorder or logging DVM to record the phase drift over several hours This technique easily allows measurement to one part in 10 TV Sync Separator Divider Local Reference Fig 2 Simple Phase Comparator 38 See www rakon co nz Electronics Australia Nov 1986 page 34 3 Martin Ossmann EDN August 3 1998 page 115 Also see http www e insite net ednmag archives 1998 080398 1 6di htm 3 Brooks Shera W5OJM QST July 1998 Also see http www rt66 com shera index_fs htm 37 As in this project the results using a good OCXO will be very good Page 26 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Micro Phase Comparator Although it is possible to compare the phase of a local reference to the TV Line or Frame frequency digitally for example using digital counters and gates using a micro controller is really much simpler See Fig 3 The Divider is a timer inside the micro where high resolution is available Use an AT90S2313 Atmel device and drive Timer 1 with 1 MHz derived from the local 5 MHz precision oscillator The timer is set to divide by 20 000 and thus resets automatically every 20 milliseconds Use the TV Frame Sync to sample the value in the timer every 20 milliseconds Using the Input Capture ICP feature of the micro this is achieved without using any interrupts so the precision stuff is not at risk of timing errors See Fig 3
38. ience of Tmekeeping by David Allan and published by Hewlett Packard is well worth reading See http www allanstime com Publications DWA Science_Timekeeping TheScienceOf Timekeeping pdf See Fig 10 Page 11 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Calibration Setting Up The purpose of the calibration process is to set the natural operating frequency of the VTXO or V OCXO which allows the correction circuitry to have maximum range and therefore the longest interval between calibrations For a first time adjustment it is also important to check the operating range of the VTXO since not all devices have the same frequency control sensitivity Both adjustments are made when the unit is first assembled and the natural operating frequency is also best checked every six months or so The time between rechecks can then be modified according to the observed ageing rate of the VTXO Setting up must be done before the time is set as it upsets time keeping The GPSClock is self calibrating no special equipment is required for setup The process is easy and requires only the GPS receiver and a PC with nothing more than a Terminal program or preferably the special monitoring software RECORD supplied with the GPSClock firmware If you use a 24 column display you don t even need the PC although watching the phase plot is half of the fun With the GPS receiver running and giving a reliable fix connect the 1PPS output which must b
39. ime keeping then continues although a time keeping delay will have been incurred Commands that do not suspend time keeping lt gt A R and X do not invoke command responses Page 17 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Programming and Setup This procedure assumes the constructor uses the same programming tool as the author Since this is free it s a reasonable assumption If other tools are used familiarity with them will be required as the following procedure is specific to ISP V2 65 The following are required The ATMEL ISP AVR programmer ISP EXE V2 65 or similar A programming cable The GPSClock firmware available from the author A PC running Windows 3 1 95 or later to run the programming software Connect the DB25 end of the programming cable to the PC printer parallel port and the 10 way header to the GPSClock programming header Make sure the cable is the right way round on the header no damage will be done if it is wrong but nothing will work If two computers are available or the computer can run two applications at once connect up the GPSClock serial port to a PC serial port You will need a modem serial cable pin to pin with no crossover NOT a null modem cable Run a Terminal Emulation program Windows 3 1 Terminal is ideal setting it to the correct port and to 9600 bps This will allow progress to be monitored during programming and later allows the reference
40. is required after adding the hours to return the count to zero when 24 is reached Further checks are involved in the addition of the local time offset and allow the offset to be either negative west of GMT or positive east with accurate results For example for New Zealand Daylight Time the offset is 13 hours It might seem no big issue but performing calculations like this in BCD using machine code is not always straightforward The local time is displayed as HH MM am or pm followed by tnn the local time offset For negative offsets the 2 s complement of the number is stored FF for 1 FE for 2 etc but the user need not worry about that as adjustment is made using only UP and DOWN commands Fig 7 The 2x24 LCD display with engineering data Depending on the use intended for the GPSClock the user can choose to have a 16 column display Fig 6 and simply display time or the larger 24 column display Fig 7 and show the frequency reference management data as well In Fig 7 which shows the GPSClock operating with a 24 column display you can see extra data on the screen to the right The first 16 columns are exactly the same as appear on a 16 column display as in Fig 6 but more information about the status of the GPSClock frequency reference is visible to the right There are four blocks of information Reference Phase Status amp Mode Feedback Control Voltage and various Status Announcement
41. ltage Following the phase the Feedback Control Voltage is transmitted as four ASCII characters FFFF characters 15 18 The value can be from 0x0000 zeroa to OxOFFF 40954 and directly represents the feedback pulse width which is 1 FFFF 5000 x 100 Hence the actual DC control voltage is 5 x 1 FFFF 5000 V The value is the same as seen on the 24 column LCD screen where only the last three characters are displayed Status and Mode Character 19 is the Status byte in HEX ASCII containing the three status bits outlined on page 6 Character 20 is the Mode byte and has an ASCII value 0 F representing the current mode The table of modes is shown on page 8 Both values are the same as seen on the 24 column LCD screen Other undocumented characters may follow Command Responses When a command that suspends time keeping is received the UTC time and reference phase message sent every second is suspended along with time keeping These commands H M S and C are echoed back to the PC along with the command data in the same format as the commands 3 No parity 8 data bits 1 stop bit Carriage Return ASCII 0x0D and Line Feed ASCII 0x0A Page 16 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 If a command is sent to the GPSClock which is not understood or if the data accompanying the command is not understood the message is either ignored or the clock sends the error message followed by CR LF T
42. micro controller centre left a Rakon VTXO top left and an LCD display out of the picture recycled from an old office jelephone The unit is built on a small Dick Smith Electronics H5608 project board As well as UTC time and local time the GPSClock provides three reference signals a 1 kHz 50 square wave and a 1 Hz 1pps pulse 1 ms long from the micro and a 10MHz signal from the VTXO These can be used as references for other devices The GPSClock also provides time and other information as a message via an RS232 serial port This information can be used in the calibration procedure Long term accuracy of references such as this depends on either routine or automatic calibration to some external reference which must be at least one order better in performance The GPSClock uses continuous compensation to remove long term reference errors and also to remove small daily variations This unit automatically calibrates itself to a simple external reference 1Hz pulse commonly known as the 1pps pulse from a GPS receiver Some but by no means all GPS receivers have this option The pulse is 5V logic level and rises from OV at the exact moment of the UTC second within typically 100ns It drops back to OV from 5V some time later typically after 25ms or so It will also operate equally well from other precision sources provided the source is an exact integer fraction of 1kHz While each individual pulse may have a little jitter as time
43. mple build it into your frequency counter or communications receiver Use a small tin plate box plenty of chokes and feed through capacitors and you won t be bothered by signal leakage The author has a GPS locked HF transceiver with embedded controller This version has a LOCK indicator LED in place of the 1Hz output signal The author has built several embedded versions There is a special version for use with high performance OCXO references and even a version which will lock a crystal at fractional frequencies in the author s example a HF transceiver reference at 7 133 333 333Hz hint it samples the phase every three seconds The design concept slowly loses performance below 5MHz as the phase resolution drops off With high quality but low frequency references multiply the reference frequency and then drive the micro from the result For example the author operates one from 2MHz x 5 10MHz and another from 5MHz x 3 15MHz The 2MHz version just mentioned is an embedded device for a precision transmitter as well as generating carrier frequencies of 10 5 and 2 5MHz it also uses a direct digital synthesizer in the same micro to generate a complete time code sequence for modulating the transmitter using the VNG format It has 1kHz time coded seconds ticks and a 125Hz sinusoidal sub carrier and all products are seconds synchronous A portable version of this device which essentially operates at 10MHz would be perfect for anyone
44. ncy equalizing pulses and frame pulses and second the GPSClock is designed for low frequency references 1kHz or below This option is generally not available in countries using the NTSC TV standard where even if locked to a suitable reference the TV frame frequuency is close to but not exactly 60Hz it s 59 940059940059940 Hz In the PAL system the frequency is exactly 50 00Hz 2 See http Avww national com ds LM LM1881 pdf Page 15 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Serial Data Commands are sent to the GPSClock in ASCII format RS232 9600 bps N 8 1 Lower case is interpreted as upper case The commands are of two types those that stop time keeping and those that do not Those that stop time keeping cause the GPSClock to stop until sufficient data is received for the command to be processed after which time keeping is resumed In all cases two extra characters are expected These are the H M S and C commands Commands that do not stop time keeping do not need the GPSClock to wait for data these are the single character commands lt gt A R and X The clock transmits four separate data products During normal operation the UTC time the reference divider phase the feedback control voltage and mode are transmitted The message format is as follows HH MM SS SPPPPFFFFMM lt CR gt lt LF gt Where HH MM SS is the UTC Time is a space character is the GPSClock model iden
45. ncy when the GPS reference is removed It is after all operating at a constant temperature 2 The VTXO has a thermal variation when not GPS locked While it holds the last known feedback value for the duration of the holdover period the frequency wanders off Longer tests show a periodic variation over 24 hours of a thermal nature The maximum excursion was well under 1e 8 and more importantly not shown here the average excursion over many days is much less i e the clock will be very accurate if no GPS is connected for weeks on end 3 Note the recovery performance when GPS was restored After an initial transient the VTXO recovers to better than 1e 8 in 10 minutes The OCXO however with older software lacking the holdover function and a much slower control system has a very large transient on restoration of GPS and takes nearly two hours to recover its 1e 10 performance Unless you have a 1e 10 or better reference and locked receiver you will not be able to replicate these graphs Using this technique the author can resolve 3e 10 For higher resolution such as calibration of the Rubidium Standard a long term computer plotting technique is used It takes three days to measure to 1e 12 in this way The RECORD program and some simple maths can do the job Page 23 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 GPSClock Specifications Clock UTC time 00 00 00 UTC to 23 59 59 UTC within 1ms Local time 00 00 am to
46. nds which do not affect time keeping inc decrement seconds inc decrement UTC offset time delay increment Eight operating modes three phase locked five for setup and adjustment Power Supply AC supply via 10 15V DC adaptor 200mW max Backup from 7 2V 450mAH NiCd NiMH battery 10 15mA about 3 days charged in circuit External 12V DC supply operation 10 15V DC Can be powered by DTR signal from serial port of PC True glitch free no break operation power failure detection and warning Firmware Written in machine code for ATMEL WAVRASM compiler Source and executable code are copyright and separately available at a nominal charge no publishing or distributing patched code without permission Code size about 1k bytes Plenty of room for adaptations and improvements Phase monitoring chart recorder software supplied with code Page 24 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Appendix About Frequency References Not everyone is interested in precision frequency references but those of us that are seem to have a passion for the subject Here are some simple techniques for making and calibrating references that offer good performance without breaking the bank Using a Frequency Reference Most Amateurs are content with the calibration or lack of it of their HF transceiver perhaps use a simple crystal calibrator with older or homebrew gear and never even think about the accuracy of their VHF equipment Amate
47. ocess on the PC using the RECORD program and or an HF receiver and Spectrogram If lock is lost and the unit left to recover lock in mode C00 depending on the integration time it takes about 5 15 minutes Improved gain can be achieved by decreasing the integration time If the reference performs better in Mode C01 than COO or with low integration times i e with higher gain it might be better to adjust the GAIN setting resistor R23 lower in value Some noise is added to the reference by the feedback system so lower gain and longer integration times inject less noise Thus the gain and integration setting are always a compromise between the ability to correct the oscillator s own noise and inducing more system noise 18 Details of the integration value in the C00 command are found on page 13 Page 8 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Controls and Time Setting There are no front panel controls on the GPSClock After all it is accurate and battery backed so what would need adjusting in its or your lifetime Having no controls also prevents the clock from being upset by children with fiddly fingers The local time offset will need to be changed every six months thanks to daylight saving and at this time the clock accuracy can be checked Fortunately the local time offset can be stepped back and forth without affecting the time keeping All adjustment is achieved using the serial link to a PC There are eleven commands
48. ontrol voltage Adding these phase errors every second would result in extremely high gain and probably to instability since the phase correction would take place faster than it can be measured The gain of this feedback process can be adjusted by the simple means of reducing the frequency at which the VTXO control voltage is updated by the phase comparison result This is called the integration time and is set as the first value in the CnO command up to now referred to as C00 The time in seconds is related to the value n which is a hexadecimal number 0 F Integration time 2n 1 seconds Very high performance reference oscillators double ovened high performance OCXOs can benefit from values as high as n C 25 seconds or more since they are very stable Simpler less stable oscillators such as VTXOs will normally work best with n 4 9 seconds or less The setting is a compromise between correction of short term jitter small n and accuracy of control larger n Check your oscillator performance with a Spectrogram and adjust n for the narrowest trace width Fig 17 clearly shows the difference in stability and noise performance of two different references at 10MHz 19 Obtained from http www weaksignals com Page 13 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Although system gain is affected by the integration time it is important to still have the hardware gain set so that the oscillator can be controlle
49. opposite direction and check again High performance depends on this highly linear response Without a stable receiver and spectrogram you could manually plot the phase response Write down the phase value at a constant rate say exactly every minute for the duration of the above test convert the values to decimal and plot the results There should be a straight line up and down again A spread sheet program with HEX maths capability will make this process moderately painless Setting the GPSCLOCK into Operation If all is well after the above checks send the command C04 to centre the oscillator then a few seconds later command C00 gain of three and you ll be in business The oscillator will lock in just a few minutes and the phase will no longer change once it is locked If you are impatient and want a faster lock try C01 gain of four or C02 gain of 16 and then after a few minutes revert to C00 The higher gain settings cause more noise because they use larger steps but have faster lock The default mode C00 is optimized for minimum noise but will not lock quickly and will not track excessive reference drift Once the reference is operating correctly set the clock see previous section and you re done Just set and forget Integration Time Because in mode C00 the phase comparison can be made over a long period the gain is very high Each phase comparison results in a signed or error value which is added to the VTXO c
50. or gives wider control range but poorer noise performance 4Hz range represents only about 0 59pm range and you may need to reduce the gain resistor if the clock experiences wider than normal temperature range If you do change R23 it will be worthwhile repeating the VTXO Centre Frequency adjustment It is important to have enough gain to correct the VTXO at its operating extremes otherwise lock will be lost Accurate Ramp Next you should if possible check that your feedback components are accurate and that the 4096 steps provided are linear with no unexpected jumps Use a very stable communications receiver and a spectrogram program such as SPECTRAN to monitor the signal at 10MHz Set the width of the spectrogram to 20Hz and the speed of the spectrogram as slow as possible Enter the command C04 to centre the oscillator Find the signal on the receiver and centre it on the spectrogram screen using the spectrogram adjustments Reduce the spectrogram span to 5Hz ensuring that the signal stays centred Monitor for 30 minutes or so to ensure that the receiver is stable Then send the command C06 ramp up This will make the oscillator frequency ramp up each second one step at a time Continue for 10 minutes about 600 steps and check on the spectrogram that the ramp is smooth and straight with no steps of jumps Make sure that the jumps are not from the receiver Send the command C07 and watch it come back down Continue past in the
51. s In Fig 8 these are identified Reference Status amp UTC Time Phase Mode Local Time am pm Local Time Feedback Status offset from Control Voltage Messages UTC Fig 8 The display data fields are identified Page 7 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 The Reference Phase is a 12 bit HEX value giving the current 10MHz Reference phase relative to GPS It is in fact a measure of the count of 10MHz RF cycles at the last GPS pulse with the most significant bits omitted these are irrelevant for control purposes The Feedback Control Voltage is the 12 bit HEX value sent to the PWM D A converter and represents the Control Voltage in volts The actual Feedback Control Voltage output will be close to 5 x 1 Feedback 4096 Volts The Status and Mode data has two parts hey Status and Mode well done Ignoring the M which is simply M for Mode the first character is the Status which contains three bits of information POWER GOOD Bit 7 Zero if power has been lost set with first Mode command BATT OPERATION Bit 6 Set when AC power has failed and unit runs from battery GPS OK Bit 5 Set when GPS pulses arrive regularly The second character displays the operating mode in the range 0 7 The Modes are set by the user using the C Command There are eight operating commands Those listed in GREEN are phase locked while the others are used only for setup These commands also set the POWER GOOD
52. s or include a clock you must have a backup power source Good crystal ovens use about 1W of heater power and typically operate from 12V so a 1 AH sealed lead acid battery makes an appropriate backup supply For high precision the oven supply needs to be closely regulated and when AC power is lost the reference phase will move slightly for the duration of the failure Use a 13 8 V regulated charger for the battery capable of supplying the complete load The battery drives a low dropout 12V regulator for the oven supply and a conventional 5V regulator for the logic supply Provided the power outages are short the oven supply stays in regulation as the battery voltages drops down from 13 8 to about 12 5V Outages more than an two or three hours long run the risk of phase error although the clock would not noticeably lose time Using a Frequency Reference Probably the best use of a local precision reference is as a direct reference for a frequency counter The frequencies generally used are 1 MHz or 10 MHz The well known Dick Smith Frequency Period Counter and others based on the ICL7216 chip can use either frequency My frequency counter design uses 4 MHz but adapting it to 1 MHz 5 Mhz or 10 MHz would be relatively straightforward A frequency reference can also be used to calibrate the oscillator in a frequency counter Simply read the reference with the highest precision possible for example reading the 4 43361875 MHz TV colour burst fr
53. s which i e which UTC second the 1pps pulse refers to is not so easily determined In other words there s a grey area in the time setting business between one minute and fractions of a second General public knowledge 22 GPS information Days Hours Minutes milliseconds microseconds For a start time disseminated by GPS is NOT UTC it is GPS time as defined by the US Navy which is an accumulation of seconds since 1980 without the UTC leap seconds Fortunately the length of GPS seconds is closely controlled to be the same as UTC seconds and the time offset from UTC is transmitted with the GPS data Even though the GPS UTC time difference is broadcast there s still the problem of latency through the GPS receiver to the NMEA data stream through the serial communications link and finally through the PC to the display Even if you use a stand alone GPS receiver with time display the time display is usually unreliable to better than one minute The actual GPS seconds pulse sent by the GPS unit is very good even with propagation variations it is likely to be within 1us no matter what GPS receiver you use and is frequently much better In reality the only way to know for sure which UTC second a ipps tick refers to is to use a UTC time broadcast such as WWV a telephone voice time service speaking clock or a local radio station which broadcasts time pips on the hour WWV is reasonably painless to use beca
54. ted capacitors and the last thing you want to do is compromise the performance You will need to add a buffer device if you wish to control the oscillator power supply although a bias resistor can be controlled directly If you are considering buying an oscillator be it an OCXO or VTXO TCXO look on Ebay They are for sale in small quantities all the time and the prices are good You ll pay big money for the more famous HP oscillators and for the GPSClock the difference in performance isn t worth it It would be a different matter if you were building a complex higher performance unit such as the Shera controller VTXOs are available to order for US 30 or so but few frequencies are available off the shelf You can t realistically add frequency control to a TCXO and besides the smaller ones are too small to work on Page 21 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 Performance The level of performance your oscillator achieves depends on several factors The native performance of the oscillator phase noise and thermal performance in particular These depend on the technology used double oven or single oven OCXO TCXO and on the cut and quality of crystal In general you get what you pay for Power supply noise and stability plus quality of other components such as feedback resistors and capacitors Appropriate choice of gain and integration time Actually measuring the performance can be very difficult You
55. ternal reference connected for it to be any use Voltage controlled OCXOs or TCXOs depend on very low noise control voltages something not too easy to achieve Unless a lot of trouble is taken it is preferable to operate a manually adjusted local reference continuously and simply monitor its drift against a TV reference every few weeks When the offset becomes bothersome you can tweak the calibration so it is off in the other direction and drifts through over the next few months It is not usually possible to set an oscillator to an offset much under 1 in 10 so there will always be an offset Since the offset can be known precisely using phase comparison frequency measurements and calibrations can be made with high reliability TV Phase Comparator Calibrating a local reference which is a multiple of 15 625 kHz 250 kHz 500 kHz 1 MHz etc is really simple and costs virtually nothing Simply include a divider from the reference which generates a 15 625 kHz signal 1 MHz divided by 64 A CMOS HC4020 device is ideal for the purpose Calibration can be achieved by injecting this signal into the antenna of a TV set tuned to TV1 or TV2 and receiving a good signal Look closely at the screen and you should see one or two very narrow vertical lines superimposed on the picture One should be white the other black Decide according to picture content which is easiest to see put a sticky label on the screen to mark where the line is start your stopwatch
56. tifier PPPP is the Reference Phase FFFF is the Feedback Control Voltage and MM is the Status and Mode data Other debug or engineering characters may follow between MM and lt CR gt lt LF gt Whenever a command one which stops time keeping is sent to the Superclock it replies with a message The data format is also 9600 N 8 1 The message is also terminated by lt CR gt lt LF gt UTC Time The UTC time message is transmitted every second starting just after the second event The format is HH MM SS in eight bytes characters 1 8 and is followed by a space The time sent is the same as shown on the LCD screen It would be quite practical to build and use the GPSClock without an LCD display The full time sequence is available within 10ms of UTC time The clock can be advanced in 1ms steps so that this message can be received on time if an application requires it Reference Phase Following the model identifier the reference phase is transmitted as four ASCII characters PPPP characters 11 14 The value can be from 0x0000 zeroa to 0x1387 49994 assuming a 10MHz reference frequency is used The maximum value is the same value programmed into the EEPROM for the reference frequency divider during setup This phase value can be manually tracked or using a simple BASIC computer program graphed over time The value is the same as seen on the 24 column LCD screen where only the last three characters are displayed Feedback Control Vo
57. time codes as used by time stations WWV DCF77 JJY etc Additional or alternative reference outputs such as 440 Hz 50 Hz or 1 hour Time announcements in Morse on the hour or even voice announcements An alarm or time controlled output function A timer or stopwatch A clock that keeps astronomical time or a tide clock Push button controlled display back lighting with adjustable brightness Front panel controls could be added provided they were scanned in the main program and did not stop the clock during their action Using front panel controls for time setting is not recommended for obvious reasons The executable code is designed for any reference frequency from 5 to 15 MHz that is a multiple of 2 kHz The only changes necessary for a different VTXO frequency are made in the EEPROM during setup Five parameters are stored the frequency division ratio in two bytes divide the VTXO frequency in Hz by Page 10 of 28 ZL1BPU GPSClock Murray Greenman 2002 2004 2000 then subtract one and convert to HEX the serial port baud rate also depends on the reference frequency the default local time offset and the default error feedback integration time Notes about Setting Time It is an interesting fact of life that while time is widely known to within a minute as every broadcast station will tell you and is easy to discover in a traceable manner to within minute fractions of a second using the GPS 1pps actually knowing which second i
58. to be calibrated and the clock to be set Run the ISP software ISP EXE There is also a DOS version the most recent version supports 32 bit operating systems The author recommends V2 65 which will work on a 486 PC running Windows 3 1 as well as later computers and is required for the parallel port programming cable Make a project from the menu Project New Project selecting AT90S2313 as the device Devices Supported Devices Supported ATS0S LS 4433 Fig 12 Select the device Press OK and then on the Manager tab of the next screen give the project a title and then from the main program menu select Project Save Project to save the project file The next step is to check for communications with the micro Select Options Change Printer Port anda Port available message will be observed If all is well select CANCEL If not try changing the port in case it isn t LPT1 Next select Program Read EEPROM which should be something safe to do it won t matter since the micro is already blank If all is well the data from the micro will fill the Data EEPROM Memory window EEPROM Data Memory laaa7a FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Fig 13 The Data EEPROM Memory window 35 ISP EXE V2 65 available from user cs tu berlin de sirhenri sides up_avr avrisp zip The latest version is available from www atmel com atmel products prod203 htm 8 See www qsl net zl1 bpu micro for
59. urs who enjoy constructing maintaining or adapting equipment often invest in or build a frequency counter but how well is it calibrated What do they check it against A frequency reference is what is required The first thing most folk think about when considering frequency references is to use a Standard Frequency HF transmission such as from WWV or WWVH Those keen on the Frequency Measuring Contest will also know about HD2IOA Ecuador which transmits on 3810 kHz However while these standards may be very precise the signals received from them in New Zealand can be inconvenient no propagation when you need it and quite inaccurate due to propagation effects Fig 15 which is a spectrogram a graph of signal frequency versus time 20 Hz wide and about an hour long shows how bad reception can be The wobbly line that splits in two in places records the received carrier frequency of VNG now off the air on 16 MHz The reasonably straight line is my own local frequency reference GPS locked VNG would move in frequency up to 5 Hz as a result of Doppler caused by movement of the ionosphere s reflecting layers Fig 1 VNG reception on 15 MHz For low accuracy calibrators beating the oscillator against WWV may suffice but for anything more accurate this technique is quite inadequate as can be surmised from the picture above There are other frequency references freely available in New Zealand with much better performance The frame
60. use it transmits continuously and provided propagation is good enough you get a clear beep on the minute which identifies second 00 To use hourly pips from a broadcast station you need to be more patient they only happen once hour WWV and other services also transmit time codes which are another useful way to identify the exact minute Using software such as RADIOCLOCK you can receive the time codes and set the PC clock to UTC The PC time will then be close enough for perhaps an hour or so There are also equivalent internet services which if used with care remembering that propagation delays can well exceed one second can also be used to set PC time and identify seconds Some cellular phone services corporate computer networks and other data services send time to the user Once again the only way to be sure that the time they send is traceable to UTC is to check it yourself against a known traceable source If possible stick with WWV for a sure fire method and check other identification methods against WWV The propagation delay from WWV will rarely exceed 100ms Remember that GPSClock is a portable device just disconnect the GPS input replace the AC supply with a 12V battery and away you go It will keep time accurately for months without the GPS connection so you could transport the unit to another traceable clock set it there and then return home 17 See http gpsinformation net main gpstime htm Also The Sc
61. viders and buffers draw a lot more current than the clock it s a good idea to operate them from an independent supply that won t drain the backup battery Here we have a problem if the AC supply is off the backup battery could be drained through the buffer output and attempt to power the dividers via the clamp diodes in the CMOS devices The answer is to once again use an opto coupler Then you can keep the GPSClock and frequency divider buffer power supplies completely independent Use two CMOS gates in parallel as output buffers or better yet use a bust driver device such as the 74HC244 and connect each the output to a BNC connector via a series 100 Ohm resistor and a short length of coax cable Any simple attempt to switch the outputs will likely lead to serious cross talk If you wish to switch outputs use a CMOS digital multiplexer device not an analog multiplexer Using oven controlled oscillators is well worth while and GPSClock is fully capable of making good use of the improved performance which especially noise and thermal stability is better by a considerable margin see Fig 17 You will need to provide a very good regulated heater supply at the correct voltage typically 5V or 12V and at sufficient power typically 2 5W depending on the size and style of oscillator The micro power supply will also need to be very clean noise free and stable Use an LM723 regulator in preference to a 3 terminal one Performance and
62. wishing to tape record events along with a time code Yes you can use a PLED polymer LED display with GPSClock The software is completely compatable A few things to consider a the current drawn by a PLED display is quite a bit higher Overall the current will be about 40mA instead of 10mA so size the power supply accordingly If you operate the brightness pot connects the same place as the LCD contrast pot from the unregulated AC supply rather than the 5V regulated suppy the display will go dim and save you current when the AC power fails The brightness voltage required will be about 3V rather than the typical 0 6V ccontrast voltage for an LCD It would be a good idea to put a 4 7V zener across the pot with a series resistor to the unregulated supply PLED displays tend to fade somewhat when the same data is displayed long term so a strategy such as scrolling or blanking the display is generally used The GPSClock was not intended for a PLED display and so the leading zero digits may fade after some time Maybe a push button switch or a photo resistor to adjust the brightness on demand Yes you can use a back lit LCD display The trouble is the HUGE current drawn by the back light One the author built draws about 400mA at even modest brightness It is best to operate the backlight from the unregulated supply otherwise the back up battery will go flat in no time when there is a power failure The back light could perhaps be powere

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