The ZL1BPU SUPERCLOCK USER MANUAL
Contents
1. 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 Superclock power supply is low At this point the micro starts talking to the serial port and 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 14 85 MHz 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 60 Baud rate 9600 baud with 14 85 MHz 1D 00 T1 reload for 2 kHz interrupt with 14 85 MHz TCXO 13 NZDT offset FF FF FF FF FF FF FF Rest not used Page 11 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 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 local time offset So for example the first line of the EEPROM memory when ready to program should read FF FF FF FF 60 1D 00 13 FF FF FF FF FF FF FE Then program the EEP2. Murray Greenman 2002 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 e The ATMEL ISP AVR programmer ISP EXE V2 65 or similar e A programming cable e The Superclock firmware SUPRCLKn HEX from the author s website n is the firmware version e A PC running Windows 3 1 95 or 98 to run the programming software Connect the DB25 end of the cable to the PC printer parallel port and the 10 way header to the Superclock 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 Superclock 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 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
3. Normally the number is meaningless and it won t change if an external reference is not used If a precise reference pulse is connected to the PD6 input pin 11 the negative edge will be used to sample the high speed hardware counter phase The reference frequency can be any accurate division of 2 kHz down to 1 Hz and must have better precision than the TCXO so a GPS ipps signal or Rubidium locked TV 50 Hz frame frequency can be used The sampled value from the high speed counter should change only slowly up or down and it may be possible to tweak the TCXO calibration so that there is no change over several seconds Anything better than this is outside the capability of the simple TCXO Even if the number changes by one every second the error is very small about 0 1 microseconds per second or one part in 10 This technique can be used to detect offsets as small as 1 part in 10 The Rubidium locked TV network signal is a very convenient and much under rated source of precision frequency The colour subcarrier 4 43361875 MHz line frequency 15 625 kHz and frame frequency 50 00 Hz are all locked to a Rubidium reference 1 part in 10 or better 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 Operated from the Superclock s 5V supply and fed with TV video from a Rubidium referenced TV station this
4. 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 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 Tin 10 14 See NZART Call Book page 8 1 See also www irl cri nz teams ms tiservi htm 15 The NMEA time message may be correct on average but will wander in time The second tick however is frequently held within 1us of UTC 1 Temperature Compensated Crystal Oscillator 17 Oven Controlled Crystal Oscillator P
5. The source code SCLOCK BAS written for the QBASIC 4 2 compiler is also available from the website The user can easily write much more sophisticated graphical software for monitoring reference phase see the following section on Serial Data but anything fancier than SCLOCK EXE is quite unnecessary for occasional calibration use and would probably be an over kill for TCXO references of this performance Page 8 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 Serial Data Commands are sent to the Superclock 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 Superclock 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 and S commands Commands that do not stop time keeping do not need the Superclock to wait for data these are the single character commands lt gt and R The clock transmits three separate data products During normal operation the UTC time and the reference divider phase are transmitted Whenever a command which stops time keeping is sent to the Superclock it replies with a message The data format is also 9600 N 8 1 UTC Time The UTC time message is transmitted every second starting just after the second event
6. 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 eight commands 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 gt Increment local time offset by one hour lt Decrement local time offset by one hour R Retard clock by 10 ms The first three commands 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 VNG or WWV 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 However if the clock seconds don t tick over exactly in time with the radio seconds pips next use the R command repeatedly for very fine adjustment If an oscilloscope is available trigger it from the 1 Hz pulse output of the Superclock and observe the seconds ticks from the radio signal on the screen Without an oscilloscope the 1 Hz ticks can also be line up with the time signal quite easily and with remarkable precision by listening to them both with a pair of headphones If the R command is used too much causing overshoot another 100 presses are required to get close to
7. 3 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 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 above and the TCXO X1 below The micro is an inexpensive ATMEL AT90S2313 available in New Zealand and Australia from DSE Polykomm or Jaycar To the right are the RS232 interface TR1 and TR2 the LCD connector J3 and the programming header J1 There really is very little to it A discrete transistor RS232 interface is used to reduce supply current drain Current is drawn from the supply only when transmitting data If the REF input micro pin 11 isn t used it should be grounded This is achieved by R4 The use of this phase detector input is described later Other spare pins are outputs and can be left open There is of course considerable similarity between the circuit of the Superclock and previous projects See Fig 3 for a detailed view of the circuit board The LCD connects at the top left and uses display routines specially written for operation at up to 15 MHz The trim pot top centre 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
8. The format is HH MM SS in eight bytes and is followed by a space The time sent is the same as shown on the LCD screen It would be possible to build and use the Superclock without an LCD display Reference Phase Following the UTC time the reference phase is transmitted as four ASCII characters PPPP followed by CR LF The value can be from 0x0000 zero to Ox1CFF 1999 or thereabouts depending on the reference frequency 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 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 and S are echoed back to the PC along with the command data in the same format as the commands If a command is sent to the Superclock which is not understood or if the data accompanying the command is not understood the clock sends the error message followed by CR LF Time keeping then continues although a time keeping delay will have been incurred Commands that do not suspend time keeping lt gt and R do not invoke command responses No parity 8 data bits 1 stop bit 10 Carriage Return ASCII Ox0D and Line Feed ASCII 0x0A Page 9 of 16 ZL1BPU Superclock Copyright
9. battery to ride out power failures The battery should last its shelf life For a more portable clock application a six cell NiCd pack would also work well and could be charged by fitting a resistor R1 across the battery s isolating diode D3 see Fig 2 u1 ZL1BPU SUPERCLOCK 10V 78L05 IN OUT 45V GND 1N4002 20 ue PDO 2 9N Vcc TXD D4 eo 1N4002 C6 10uF oer RDI REF Ry 25V PD6 11 R33 D2 R35 10k R32 A 4k7 1N4148 10k ie danas R4 5V aau Ra AT90S 100k J3 14X1 HDR 231 3 1 OPC LCD Disolav R2 100k c1 100nF IRESET 1 PB7 19 6 PD2 PB6 18 BZX85C4V7 A Mo 7 PD3 PB5 17 By 8 PD4 PB4 16 10 15V DC 9 PDS PB3 15 J TR1 from AG i PB2 14 BC557B oy Alaine PRIS CK J4 DC socket backup PBO 12 O B1 can be 7 2V NiCd an a a R29 J2 DB9 F r 5V 5V 10k RS232 xa o R30 100R 1 TCXO c10 100nF Do E 14 85 MHz L ne L C5 100nF 4k7 H 2 J1 2x5 HDR C8 10uF 35V D1 aman PROGRAM 1m4148 P 0V Copyright 2002 Murray Greenman ZL1BPU Fig 2 The Superclock schematic In the schematic note the power supply arrangements on the left with the diode isolated backup supply and AC supply using a 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 a DC battery supply is used it might be useful to know when the Page
10. 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 project uses a TCXO from a surplus AMPS cellular phone junked ones can often be obtained for nothing from cellular service centres if you ask nicely Fig 1 The Superclock prototype See Fig 1 The project uses an inexpensive micro controller a TCXO rescued from an old cellular phone and an LCD display also recycled from an old office telephone for minimum cost The unit is built on a simple Dick Smith Electronics H5608 project board As well as UTC time and local time the Superclock provides two reference signals a 1 kHz square wave and a 1 Hz 1pps pulse 1 ms long These can be used as references for other devices or as part of the calibration procedure The Superclock also provides time as a message via an RS232 serial port 1 See Appendix About Frequency References Temperature Compensated or controlled Crystal Oscillator Page 2 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 Description The micro controller has a hardware counter timer which is used to divide from the TCXO reference frequency and generate an interrupt at 2 kHz Thus any TCXO frequency that is a multiple of 2kHz can be used The micro does n
11. running Windows 3 1 as well as later computers Make a project from the menu Project New Project selecting AT90S231 3 as the device Devices Supported SSS Cancel pe Help AT90S7L 2333 JAT9057L54433 Fig 6 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 and a Port available message will be observed If all is well select CANCEL If not try changing the port in case itisn t LPT1 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 A See www qsl net zl1bpu micro for a suitable cable The Jaycar KC 5340 kit will also do the trick 3 There have been problems with port timeout errors with later versions Page 10 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 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 5 xf i 6670 FF_FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF Fig 7 The Data EEPROM Memory window This window ca
12. 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 4 Fig 4 The LCD display An extra check 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 AA or pm where AA is the local time offset For negative offsets the 2 s complement of the number is entered FF for 1 FE for 2 etc Binary Coded Decimal t Packed BCD keeps two decades in one byte In this case units right 4 bits and tens left 4 bits Page 5 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 Controls and Time Setting There are no front panel controls on the Superclock After all it is accurate and battery backed so what would need adjusting 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
13. simple little 8 pin chip will generate very stable and precise pulses with the correct timing and levels to drive the REF input of the micro directly See Fig 5 Described in the Appendix Harrison spent many years regulating his first chronometers and their performance about one second week is eclipsed by modern quartz clocks See http www theorderoftime com science sciences articles awalktroughtime html 8 See hitp www national com ds LM LM1881 pdf Page 7 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 5 Composite 2 ees IN VCC VER 100nF LM1881 100nF a COMP RC GND Fig 5 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 frequency equalizing pulses and frame pulses and second there is usually no easy relationship between the line frequency and the reference oscillator 4752 5 if using a 14 85 MHz reference There is however always an easy relationship between the frame pulse and the reference 297000 1 with a 14 85 MHz reference The only disadvantage is that phase drift detection is slower This is really no problem since the phase resolution is high Of course the TV receiver and sync chip only need to be connected for initial calibration The Superclock will keep time adequately for years without further reference oscillator correction It
14. the correct seconds phase again so instead use the S command and try again After checking at the minute event use the or command to correct the seconds display It should change to 00 exactly as the 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 and the R command are the only ones permissible while the Superclock is keeping time The program source code for this project is provided free of charge so the constructor can make whatever modifications are preferred There is plenty of code space so small adaptations could for example provide Time signal pips or even time codes similar to those used by VNG Additional or alternative reference outputs such as 440 Hz 50 Hz or 1 hour Chimes 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 or timer controlled display back lighting Front panel controls could be added provided they were scanned in the main program and did not stop the clock during adjustment Using front panel controls for time setting is not recommended for obvious reasons The executable code is designed for any reference frequency from 8 to 15 MHz that is a multiple of 2 kHz achieved by setting the division rat
15. 0 00 00 UTC to 23 59 59 UTC Local time 00 00 00 am to 11 59 59 pm Local offset Reference phase four character only on 24x2 or bigger display Power failure message BATT four character only on 24x2 or bigger display Power Supply AC supply via 10 15V DC adaptor 200mW max Backup from 9V alkaline battery 15mA about 8 hrs or 500mAH NiCd cells charged in circuit several days External 12V DC supply operation 10 15V DC True glitch free no break operation power failure detection Firmware Written in machine code for ATMEL WAVRASM compiler Open source code free for private use Code is copyright no publishing or distributing patched code without permission Code size about 1k bytes Plenty of room for adaptations and improvements Page 12 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 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 Amateurs who enjoy constructing maintaining or adapting equipment often invest in or buil
16. 0 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 2 See Break In September October 2002 page 4 3 See www gsl net zl1 bpu micro EXCITER Page 16 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002
17. ROM 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 Superclock is still garbled check your maths again and check the Terminal data rate Table 1 gives the value to load for a range of common TCXOs and communication speeds of 9600 bps Crystal MHz Baud Rate T1 Reload 9 6 0x3D 0x12BF 10 000 0x40 0x1387 12 000 0x4D 0x1770 12 800 0x52 0x18FF 14 850 0x60 0x1D00 Table 1 Typical EEPROM values Specifications Clock UTC time 00 00 00 UTC to 23 59 59 UTC Local time 00 00 00 am to 11 59 59 pm Local offsets 13 hours Independent setting of offset hours minutes seconds via serial link Incremental time trimming in 10ms steps via serial link No front panel controls one internal adjustment reference frequency Reference TCXO at any frequency 8 00 to 15 00 MHz that is an exact multiple of 2 kHz Typical offset 1 part in 10 or better Typical error over temperature range1 part in 10 Typical ageing rate 1 part in 10 per year Calibration using phase detector to 1 part in 10 3 sec per year Outputs UTC time reference phase via RS232 9600 N 8 1 1000 0 Hz 50 square wave CMOS 5V level 1 000 Hz 1ms pulse CMOS 5V level positive true LCD Display 16 x 2 standard HD44780 display in 4 bit mode larger displays optional UTC time 0
18. The ZL1BPU SUPERCLOCK USER MANUAL 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 Build this little clock and you will be more than impressed by its performance it always tells the correct time down to fractions of a second and is unaffected by temperature or power glitches Adapt the software yourself to add a wide range of functions This manual refers to firmware Version 0 2 SUPRCLK2 xxx Murray Greenman ZL1BPU December 2002 Introduction Accuracy of frequency standards is usually quoted as parts in 10 ppm 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 This project should provide this level of performance Any better is unnecessary as clocks displaying civil time with or without Daylight Saving require attention twice a year anyway Electronic clocks and watches use a quartz reference either 32 768 kHz or 4194 304 kHz While the performance of these isn t too bad typically within a few seconds per week it is easy and relatively inexpensive to do much better In this project a TCXO is used to provide good temperature stability and performance resulting in an error of a
19. 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 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
20. ably 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 frequency 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 Il 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 40
21. age 13 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 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 This month s micro controller project is a clock based on one of these devices For higher performance an oven controlled reference is necessary These can be 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 The best solution is to hunt for surplus equipment that may contain a suitable reference 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 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 l
22. 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 board At the far left centre is the little 5V regulator The silver box lower right is the 14 85 MHz TCXO and its adjustment trimmer can be clearly seen This reference has through hole pins but a surface mount TCXO will work fine if you solder fine fuse wire leads to it before soldering to the board An alternative crystal reference circuit is offered on the schematic but performance without a TCXO will be severely compromised 13 10 C A C h E a EE EES S OG tes 4 Fig 3 Circuit board detail Page 4 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 One interesting feature of the Superclock is the way that 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 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
23. d 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 VNG 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 on 16 MHz The reasonably straight line is my own local frequency reference GPS locked VNG moves 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 or VNG may suffice but for anything more accurate this technique is quite inadequate as can be surmised from the picture There are other frequency references freely available in New Zealand with much better performance The frame line and colour burst frequencies of the TV1 and TV2 networks are all controlled by a Rubidium Standard
24. e relationship between the 1 kHz pulses and the line frequency is complex 125 8 but despite this the effect is easy to observe and adjust Unfortunately there is no simpler relationship between the reference and the TV line frequency so the main divider cannot be set temporarily to provide a more convenient frequency and this precludes using the simple method of observing pulses on the TV screen There are a number of other ways to achieve calibration including counting the reference at pin 4 of the micro controller a method not recommended with a conventional crystal oscillator or counting the 1 kHz output with a VERY good counter The best way of all is to use the built in phase detector Adjustment by observing changes in clock time in the traditional manner would take upwards of six months to achieve similar results Phase Comparison In Fig 4 it can be seen that there is a cryptic number 1BE9 in the top right corner of the display The software is intended to operate with a 16 x 2 character display nicely centred without this extra information The picture shows a 24 x 2 display and so time is offset to the left revealing some engineering information to the right The four characters are the hexadecimal phase count of the main high speed counter with 0 1us resolution and 500us range and can be used for calibration The same information is added to the serial time message so is still available when you use a 16 x 2 display
25. f 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 LJ Aeb aeaaaee ipani A Fig 4 PC phase comparison display The phase plot has a couple of kinks in it which I 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 reference 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
26. 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 the 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 18 See www rakon co nz 19 Electronics Australia Nov 1986 page 34 20 Martin Ossmann EDN August 3 1998 page 115 Also see http www e insite net ednmag archives 1998 080398 16di htm 21 Brooks Shera W5OUM QST July 1998 Also see http www rt66 com shera index_fs htm Page 14 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 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 digita
27. ine 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 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 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 external 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 prefer to operate a manually adjusted local reference continuously I never turn it off 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
28. io for example 7424 0x1D00 for 14 85 MHz Common cell phone TCXO frequencies are 9 6 10 12 8 and 14 85 MHz The only changes necessary for a different TCXO frequency are made in the EEPROM during setup Four parameters are stored the division ratio in two bytes divide the TCXO frequency in Hz by 2000 then subtract one and convert to HEX the serial port baud rate also depends on reference frequency and the default local time offset 5 See www gsl net zl1bpu micro CLOCK Page 6 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 Calibration Direct An essential part of the setup process is to set the calibration of the TCXO This can be done at any time If an oscilloscope and TV receiver are available the process is easy With the oscilloscope time base set to 10us per division trigger the oscilloscope from the 1 kHz output of the Superclock PB4 pin 16 and observe the horizontal sweep pulse of the TV receiver by hanging an oscilloscope probe near the TV set Alternatively observe the video output of a video recorder or TV receiver Tune to a TV station known to use a Rubidium standard in New Zealand TV1 or TV2 Although the line rate pulses will appear in several places on the oscilloscope screen they should be stable not drifting horizontally or drifting very slowly Watch one particular pulse and follow it for some minutes looking for drift If there is drift adjust the TCXO slightly until the drift is eliminated Th
29. l 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 TV Sync Capture Separator Register Divide by Divide by 5 P 20 000 Micro Controller a aN E E ADA EA EERE 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 o
30. n 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 STK200 STK300 or Value Added Pack Dongle Required Fig 8 Oh dear no faulty programming cable If the cable is connected but there is a fault at the Superclock 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 Abot Bety Ignore Fig 9 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 SUPRCLKn HEX The current version is SUPRCLK2 HEX The file is in Intel HEX format 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
31. 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 See this month s micro project A system such as has been described need not have TV video 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 In order to compare phase over long periods 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 Page 15 of 16 ZL1BPU Superclock Copyright Murray Greenman 2002 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 notice
32. ot need to be involved in the counting of this high frequency or the capture of reference phase which happen in hardware inside the chip even while the micro is asleep The 1 kHz output pin is toggled in the 2 kHz timer interrupt 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 This pulse is precisely one millisecond wide and occurs exactly at the second event With a little added code there could be additional outputs such as 50 Hz or pulses to drive a clock mechanism The software processes just described happen every half millisecond and between these interrupts the micro controller is asleep 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 checks that the power supply is OK then sends and displays all the information before going to sleep again Because these processes happen so quickly the micro controller spends most of its time asleep and the average current drawn from the power supply is low just a few milliamps Power for the Superclock is provided by a small AC adaptor and is backed up by a small 9V alkaline
33. would be interesting to note the rate at which the phase changes after calibration and to check it months later to see if the reference oscillator has aged noticeably The ageing rate of TCXOs is typically about 1 part in 10 per year A very simple computer program is offered which plots the reference phase over time and allows the TCXO to be trimmed This program SCLOCKk displays the reference phase on a line of text every second The UTC time is displayed followed by a field of 64 spaces containing two special characters and then the hexadecimal phase is displayed The special character indicates the coarse relative phase resolution of 32 samples while the character indicates the fine phase Using this software to monitor the RS232 output of the Superclock the user will see the offset of the reference as a slope in the column of characters Hopefully the TCXO trimmer can be adjusted so the the screen in a straight line and and even the Characters march down 23 01 23 02 23 03 23 04 23 05 23 06 23 07 23 08 23 09 23 10 w gz a F F E r OF I Ww YHOO O In the above example of the screen display from SCLOCK EXE the phase is moving very slightly as can be seen by the drift to the right of the characters Two versions of SCLOCK are offered SCLOCK1 EXE for use on serial port COM1 at 9600 baud and SCLOCK2 EXE for similar use on COM2
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