3231 Hi V pulse gen man TS (Page 28)
Contents
1. 50 pF bulk modulator in parallel with a 50 Q termination will result in an impedance mismatch This mis match will result in reflections of the output pulse as it reaches the load As a result the waveform Vv will exhibit aberrations depending on the degree of the mismatch The time position and appearance of the aberrations will vary with pulse width and cable length cable transit time is approximately 1 3 ns per foot of cable In the case of a 50 pF load with a 50 Q termination in parallel the aberrations will not exceed 15 of the original pulse amplitude Resistive loads with an impedance greater than 50 Q in addition to generating aberrations will cause the trailing edge transition time of the pulse to increase The larger the load resistance the longer the trailing edge transition time will be The leading edge should not be significantly affected Another result of using higher impedance loads is that the Model 3231 may allow larger duty factor settings before going into an over current state For very high impedance loads using a 50 Q termination in parallel with the load may provide a cleaner waveform Resistive loads with less than 50 Q impedance will generate aberrations and will cause an over cur rent state at a lower duty factor than that specified for the Model 3231 If the resistance is small enough the short circuit protection may shut off the output see Short Circuit Protection p 6 TROUBLESHOOTIN2. another pulse When it does retry the output level will be raised slowly If a short circuit condition is not present pulses are produced nor mally If a short is still present the output is again disabled the LED starts blinking and the cycle repeats The Model 3231 will continue to retry every four seconds until it detects that a short cir cuit condition no longer exists Over Voltage Protection The energy of a high voltage pulse traveling down a length of cable can be reflected back if the impedance terminating the cable does not match that of the cable itself 50 Q An unterminated cable open circuit will produce a reflected wave of double the pulse amplitude For a long cable and a wide pulse the energy carried back in such a reflection could potentially damage an unprotect ed output stage To prevent this the Model 3231 employs circuitry to clamp such reflections to a safe level This circuitry does not interrupt the operation of the instrument and the front panel LED s are unaffected Over Current Protection When the specified duty factor maximum for a given pulse level is exceeded see Duty Factor Vv below the Model 3231 will go into a current limit state and the output voltage will drop below the selected value Using a low impedance load may have this effect as well even when duty factor maximums have not been exceeded Additionally dialing in a large change in the level may momentarily cause a curren
3. the Output board PCB 202H 1 for 0 6 V using R1 Check this for amplitude settings of 300 V 150 V and 20 V If nec essary readjust R1 to obtain the smallest difference between readings Note that when the amplitude is changed the output pulse should be allowed enough time to settle completely before reading TP 4 Drive Level Adjustment Switch the Polarity switch to POSITIVE Set the repetition rate to 1 kHz set the width to 500 ns and set the level to 300 V Using the DVM measure the voltage between TP 2 and TP 6 on the output board PCB 3231 1 This is the voltage directly across the high voltage power supply PS1 Compare this reading to the level of the output pulse top shown on the scope Adjust the FET drive signal with C76 located in the center of the floating island on the output board so that the amplitude of the output pulse just stops increasing This minimizes the power dissipation in the FET There should be less than 40 V difference between the two levels representing the drop across the FET and the 1 5 Q resistor R14 If necessary recalibrate the output pulse amplitude as described above Vv Minimum Pulse Width Calibration Adjust the width to 12 ns Verify that the width for a 300 V pulse is 12 ns at the 50 amplitude points If necessary a very small correction in width can be made by adjusting C76 on the output board PCB 202H 1 Change the polarity to negative Adjust the drive sig
4. voltage sources e Do not work alone e Do not operate with the cover removed e Use extreme care when handling output connec tions Do not connect or disconnect cables when high voltage is present e Be sure the ground conductor of the AC power cord connects the instrument to a solid earth ground To prevent equipment damage please observe the following CAUTION Loads connected to the Model 3231 must be able to dissipate up to 2 kW peak power and 20 W average power for 300 V operation Loads with either lower peak or average power rat ings may be damaged or destroyed The load New Focus provides with this unit meets these criteria Theory Vv The Model 3231 supplies flat topped pulses of posi tive or negative polarity It has output protection for short circuit open circuit over current and over temperature conditions The pulses produced from this pulse generator can have a peak level that is adjustable from 20 V to 300 V The baseline level is fixed at ground From the front panel you can adjust the pulse width from 20 ns to 1 ps The leading edge transition time is lt 5 ns while the trailing edge transition time is lt 20 ns The pulses are triggered either from an internal repetition rate generator which is adjustable from 1 Hz to 1 MHz or from a valid external sync signal Sync In The unit automatically synchronizes the output pulses when a valid Sync In signal is provided In either case you can adjust t
5. G If the output level drops or the front panel LED s blink check that a 50 Q load is being used Also check that the duty factor and repetition rate max imums are not being exceeded see Timing Requirements p 9 and Protection Features p 6 For problems relating to output pulse aberrations 13 14 Vv refer to Performance Enhancements p 11 13 FRONT PANEL DESCRIPTION Switches and Knobs The Polarity toggle switch controls the output pulse peak level polarity If it is switched while pulses are being produced the output will slowly drop to zero before the output polarity actually changes The output will then move quickly to the selected level The High Voltage On Off push button switch enables and disables the High Voltage Output and Output Monitor signals The switch will be in the disable state when the Model 3231 is turned on regardless of its status when the instrument was last powered down The Power rocker switch turns on and off the AC power to the instrument The Repetition Rate selector switch determines the range for setting the internal pulse repetition frequency A continuously variable 10 turn knob provides for adjustment within the range selected The Width selector switch determines the range for setting the pulse width A continuously variable 10 turn knob provides for adjustment within the range selected The selected width should be nar row enough to
6. Model 3231 User s Manual The High Voltage Pulse Generator n Warranty 323102 RevA 2 Contents Vv New Focus Inc guarantees its products to be free of defects for one year from the date of shipment This is in lieu of all other guarantees expressed or implied and does not cover inci dental or consequential loss Warranty Introduction Safety Precautions Theory HN NT rd BW nN Operation Appendix 1 17 AC Operating Voltages amp Fuses Appendix 2 21 Calibration Specifications 26 v Is a registered trademark of New Focus Inc Introduction Vv The New Focus Model 3231 is a high voltage pulse generator capable of driving lt 50 pF capacitive loads i e Pockels cells and bulk modulators with amplitudes as high as 300 V Typical applica tions using the Model 3231 include pulse pickers optical choppers and electronically variable wave plates It is ideal anywhere a high voltage high speed up to 1 MHz pulse generator is required This pulse generator even includes an input for triggering off of external pulse trains as from a mode locked laser and it provides a monitor out put for convenient low voltage viewing of pulse characteristics Vv Safety Precautions IMPORTANT Please read through the entire manual before operating the Model 3231 WARNING Hazardous shock can result from improper operation To avoid personal injury use safety precautions appropriate for high
7. drop and a front panel Polarity LED will flash as described in Over Temperature Protection p 9 Duty factor constraints may additionally limit the repetition rate for a given pulse width As an exam ple the last column in Table 1 gives the maximum repetition rate when a 100 ns width is used Pulse Spacing Aminimum pulse spacing of 200 ns is required between the trailing edge of a pulse and the lead ing edge of the following pulse If this minimum spacing is not met output pulses may disappear PERFORMANCE ENHANCEMENTS Transition Times and Pulse Widths While the leading edge transition time remains typically 5 ns or less over the entire range of ampli tude levels 20 V to 300 V the trailing edge transi tion time increases as the peak amplitude setting is reduced The length of the pulse top remains con stant when the peak amplitude value is changed However as a result of the change in trailing edge transition time the pulse width measured at the 50 points increases slightly as peak levels below 300 V are selected Typical trailing edge transition Table 1 Vv Typical Duty Factor Maximums for the Model 3231 Peak Maximum Maximum Maximum Pulse Rate Level Duty Factor Pulse Rate For 100 ns Width 300 V 1 400 kHz 100 kHz 200 V 2 800 kHz 200 kHz 150 V 5 1 MHz 500 kHz 20 V 25 1MHz 1MHz Table 2 Typical Pulse Width Correction Factors Peak Trailing Edge Correctio
8. enabled via the High Voltage On Off switch PROTECTION FEATURES The Model 3231 pulse generator incorporates cir cuitry to prevent accidental damage to the instru ment due to shorts applied to the output high voltage reflections excessive current demand and over temperature conditions Short Circuit Protection If a short or near short appears at the output while the Model 3231 is producing a pulse the instru ment will respond by rapidly turning off the pulse being generated This protects the Model 3231 out put stage from damage It takes effect if the short is Fig 1 Front panel view of the Model 3231 Vv Ve p High Voltage Pulse Generator 3231 Made in U S A Volts 0 1 1 kHz 1 10 kHz 20 100 ns 0 1 1 us Polarity Le 100 10 100 kHz C 1 10 Hz 0 1 1 MHz O Repetition Rate Delay Width Amplitude High Voltage C C C C O Power Sync Output High Voltage Tigo Input Monitor y Ci SO O O PA v NEW FOCUS Inc Mountain View CA 94041 present when the pulse begins even if the short is present before the instrument is turned on or if the short is applied after the pulse has started When the protection circuit turns the pulse off the active front panel Polarity LED see LED Indicators p 15 flashes to alert the user to a fault condition The output is disabled and held at zero for about four seconds before the instrument tries to put out
9. he delay from the trigger source from 0 to 100 ns For convenience a moni tor out signal is provided It is a scaled down ver sion of the high voltage output pulse waveform Use this monitor output for timing alignment and fine adjustment of the output width Note The Model 3231 is designed to operate with a load impedance of 50 Q For operation at 300 V the load must be rated for 2 kW peak power and 20 W average power dissipation The failure modes for loads with insufficient power ratings may vary Some loads will arc and behave as a short either temporarily or permanently Others instead will simply open up 5 Operation v POWER UP Before plugging the instrument in be sure the selection switch is set for the correct AC operating voltage and that a fuse of proper value is in place Refer to Appendix 1 for voltage selection instruc tions and fuse requirements Switching the Power switch to the on or 1 position on the front panel of the Model 3231 applies AC to the unit All functions displays inputs and outputs are functional at this point except for the High Voltage Output and the Output Monitor To turn on the High Voltage Output and the Output Monitor press the High Voltage On Off pushbutton the green LED adjacent to the High Voltage On Off switch will illuminate If AC is turned off and then re applied the High Voltage Output will remain off until
10. meet duty factor requirements for the repetition rate being used see Timing Requirements p 9 The Delay knob provides for control of the rela tive time position of the output pulses with respect to the internal trigger When synchronized to an Vv external Sync Input signal this allows for time positioning the output pulses with respect to the sync signal This 10 turn knob provides for contin uous delay adjustment from 0 100 ns note that this is in addition to a fixed delay of approximately 90 ns The Amplitude knob determines the magnitude of the output pulse peak level This 10 turn control allows for continuous adjustment of pulse height from 0 to 300 V The selected setting is shown on the Amplitude display Note that specifications do not apply below 20 V After a level change is entered the instrument may require a few seconds to settle to the new value LED Indicators The Model 3231 has five LED indicators on the front panel The LED will light when the instrument gen erates pulses of positive polarity The LED will light when the instrument gener ates pulses of negative polarity The LED that is active will flash when a short cir cuit over current or over temperature condition occurs see the section on Protection Features The Trig LED will light when valid triggers are present at the Sync Input connector This LED will light as long as the level
11. n Factor Level Transition Time 50 points 300 V 10 ns 200 V 11 ns 100 V 13 ns 1ns 20 V 18 ns 4ns 11 12 Vv times and width correction factors are given in Table 2 Where a high degree of accuracy is required pulse width should be measured at the levels being used Note that the output monitor waveform can be used to observe these slight changes in pulse shape To obtain pulses lower in amplitude than 300 V without increased trailing edge transition time set the level to 300 V and use an external attenuator Be sure that the attenuator presents 50 Q to the Model 3231 output and that the atten uator and load combination can dissipate 20 W average power and 2 kW peak power Non 50 Q Loads The Model 3231 is designed to meet all of its speci fications when driving capacitive loads as large as 50 pE A 50 Q termination however must be located at the end of the RF path i e beyond the capacitive load In most cases best results are obtained when the capacitance is located at a T with minimal stub length close to the 50 Q ter mination which should be placed at the most dis tant end of the cable system Capacitance will act as a momentary short and will generate aberra tions that depend on the size of the capacitance A capacitive load larger than 50 pF could increase rise and fall times beyond those specified and could trip the short circuit protection The use of a non 50 Q load such as a
12. nal with C78 on the output board near Q21 so that the width for a 300 V pulse is 12 ns at the 50 amplitude points Note that if C78 is adjusted to the end of its range the output pulse will disappear 25 Specifications 26 Vv TIMING CHARACTERISTICS Internal Repetition Rate 0 Hz 400 kHz at 300 V 0 Hz 1 MHz at 150 V See Table 1 for other amplitudes External Sync Rate 1 kHz 250 MHz Adjustable Delay 0 ns 100 ns plus insertion delay of approx imately 90 ns Jitter lt 1 Sms 100 ps typical between leading or trailing edge of high voltage output pulse and the nearest sync input pulse Insertion Delay 90 ns typical between sync input pulse and the corresponding high voltage output pulse Width 15 ns 1ps at 50 points At lower amplitudes width increases see Table 2 Duty Factor 1 maximum at 300 V higher duty factor at lower amplitudes see Table 1 Vv OUTPUT CHARACTERISTICS HIGH VOLTAGE OUTPUT Amplitude 20 V to 300 V peak into 50 Q Baseline fixed at ground Polarity Positive or negative switch selectable Max Power 18 W average 1 8 kW peak Max Current 6 Apeak Leading edge 5 ns 3 ns typ into 50 Q load 10 90 8 ns 5 ns typ into 50 pF in parallel with 50 Q Trailing edge 10 ns at 300 V into 50 Q load 10 90 longer at lower amplitudes see Table 2 15 ns 10 ns typical at 300 V into 50 pF in parallel with 50 Q Aberrations 5 of amplitude 2 ty
13. of the triggers is suffi cient to be detected even if the trigger rate is not within range to allow for the output pulse to be synchronized to the trigger pulses The Sync LED will light when the high voltage 15 16 Vv output pulse train is synchronized to the trigger signal present at the Sync Input connector The High Voltage On Off LED will light when the High Voltage On Off switch has been set to enable the output to produce pulses Display The Amplitude display shows the peak level in volts of the high voltage output pulses The front panel display only shows the magnitude of the pulse level Note that this display will show the set ting even when the High Voltage On Off switch has been set to disable the output and no pulses are being generated When overload protection cir cuitry activates the display will show the resultant drop in output amplitude Connectors High Voltage Output is a BNC connector that provides the high voltage pulsed output from the instrument WARNING Shock hazard Use extreme care when handling output connections Output Monitor is a BNC connector that provides a representation of the high voltage out attenuated by a factor of 100 This output should be terminat ed into a high impedance e g 1 MQ Appendix 1 AC Operating Voltages and Fuses Vv The Model 3231 can operate from 100 120 220 and 240 V AC with AC frequencie
14. onnect cables when high voltage is present e Be sure the ground conductor of the AC power cord connects the instrument to a solid earth ground ZI 22 v PROCEDURE Calibration should be carried out in the order presented Power Supply Voltages Remove the cover and turn the power on Verify that the power supply voltages are at the appropri ate levels 12 V 12 V 5 0 V and 5 2 V Set 5 Vand 5 2 V to within 0 05 V and 12 V to within 0 1 V There are test points and adjustment potentiometers on the power supply board PCB 6040 1 and each is labeled with the appro priate voltage Repetition Rate Leave the High Voltage On Off switch in the off state Set the Repetition Rate selector switch to 0 1 1 MHz and turn the adjustment knob to maximum fully clockwise Remove any cable connected to the Sync Input connector Observe the pulse frequency at TP 1 on the Timing board PCB 3231 3 Adjust C5 on the Timing board to make this frequency 1 MHz HIGH VOLTAGE OUTPUT Amplitude With the High Voltage Output disabled connect a length of coax cable between the High Voltage Out jack of the Model 3231 and the input of the 20 W 50 Q 10x attenuator Connect this attenua tor output to the 2 W 50 Q 10x attenuator which in turn should be connected to the oscilloscope input Set the scope terminator to 50 Q Set the amplitude to 300 V set the repetition rate to 1 kHz and set the wid
15. pical into 50 Q load 15 of amplitude 8 typical into 50 pF in parallel with 50 Q Connector BNC Load Specifications apply for 50 Q terminating resistance with up to 50 pF capacitance in parallel with cable length from termi nator to capacitance no more than 6 27 28 Vv OUTPUT MONITOR Function Represents the high voltage output waveform attenuated by a factor of 100 Amplitude 0 to 3 V into 50 Q 1 of high voltage output amplitude Connector BNC INPUT CHARACTERISTICS SYNC INPUT Range 1 kHz 250 MHz Input Impedance 50 Q Min Signal Level 300 mV Max Signal Level 7 V DC or rms Threshold 0 2 V typical Connector BNC GENERAL Dimensions 8 25 Wx 5 Hx16 D Weight 14 lbs Temperature Specifications apply 10 40 C 50 104 F
16. replacing the fuse For protection against fire use only the specified fuse value Do not attempt to bypass or repair the fuses v Fig 2 Model 3231 Voltage Selection Housing San N Voltage selector card N Indicator pin Fio wer NECT ROS FUSES Ni pisco peFORE REPLA 19 v Fig 3 Voltage Selector Card Orientation Fig 4 Model 3231 Fuse Changing 20 Appendix 2 Calibration Vv New Focus recommends performing the following Model 3231 calibration procedure every 12 months EQUIPMENT REQUIRED e Peak reading 300 MHz bandwidth digital oscil loscope Tektronix TDS 520 or equivalent e 3 1 2 digit Digital Volt Meter e 25 W 50 Q 20 dB attenuator e 2 W 50 Q 20 dB attenuator e 50 Q coaxial cables RG58 or equivalent wide bandwidth cable WARNING Hazardous shock can result from improper operation To avoid personal injury use safety precautions appropriate for high voltage sources e Do not work alone e Use extreme care when operating the instrument with the cover off Use only tools with insulated handles Do not touch exposed circuitry inside the module since the storage capacitor may hold a high voltage charge even for a few seconds after powering down e Never put more than one hand near the module when power is on Make sure your body is isolat ed from ground e Use extreme care when handling output connec tions Do not connect or disc
17. s of 47 63 Hz The unit is configured at the factory for the standard AC voltage in the user s country To select a differ ent operating voltage please refer to Figures 2 and 3 and follow the directions below 1 Remove the power cord from the power entry module on the rear of the unit 2 Open the power entry module cover with a small blade screwdriver or similar tool 3 Set aside cover fuse block assembly 4 Using the indicator pin pull the voltage selector card straight out of the housing 5 Orient the selector card so the desired voltage is readable at the bottom 6 Orient the indicator pin to point up when the desired voltage is readable at the bottom note that when the indicator pin is fixed suc cessive voltages are selected by rotating the card 90 clockwise 7 Insert the voltage selector card into the housing with the printed side of the card facing toward the IEC connector and the edge containing the desired voltage first 8 Replace the cover 9 Verify the indicator pin shows the desired voltage 17 18 Vv Refer to Figure 4 when changing fuses The power entry module requires two slow blow such as Littlefuse Slo Blo series 250 V 5x20 mm fuses as follows AC Voltage Fuse Rating _Littlefuse P N 100 V AC 10A 239 500 120 V AC LOA 239 500 220 V AC 0 5A 239 001 240 V AC 0 5 A 239 001 WARNING To avoid accidental shock unplug the line cord and turn the power off before check ing or
18. t limit condition While the instrument is current limiting the active front panel Polarity LED will flash When the cause of the current limit condition is removed the Model 3231 output will slowly rise until it again produces pulses of accurate output level Over Temperature Protection When the specified maximum pulse repetition rate for a given pulse level is exceeded see Timing Requirements below overheating of the output stage can result To prevent this the Model 3231 incorporates thermal protection circuitry that will slowly lower the output level when the maximum repetition rate is exceeded The active front panel Polarity LED will flash When the repetition rate or output level is reduced the Model 3231 will return to normal operation TIMING REQUIREMENTS Duty Factor The Model 3231 instrument can operate at a maxi mum duty factor of 1 at 300 V For operation at lower peak voltages the duty factor may be increased Table 1 shows typical maximum duty factors for various voltage levels If the duty factor is too high the Model 3231 over current circuitry will respond by decreasing the pulse amplitude 9 10 Vv and flashing the active front panel Polarity LED as described in Over Current Protection p 8 Repetition Rate Independent of duty factor the maximum pulse rate depends on pulse amplitude as shown in Table 1 When the maximum pulse rate is exceed ed the output level will
19. th to 500 ns Vv Adjust the scope so that it reads the pulse top amplitude 400 ns after the leading edge of the 300 V pulse using a gated measurement mode to read the high level position both scope cursors very close to each other 100 ns before the trailing edge then read the amplitude within this narrow gate Switch the Polarity switch to positive Set the High Voltage On Off switch to the on state and turn the front panel Amplitude control to maxi mum fully clockwise Adjust R1 on the Timing board until the scope indicates a pulse amplitude of 300 V Switch the Polarity switch to negative The neg ative 300 V pulse should vary no more than 1 from the 300 V magnitude of the positive pulse If necessary adjust R2 on the Output board PCB 202H 1 so that the level for both polarities are the same to within 1 Adjust R2 on the front panel board PCB 3231 2 to make the front panel Amplitude display agree with the scope reading Width Set the repetition rate to 400 kHz and the high voltage output level to 300 V With the Width selector switch set to 20 100 ns turn the adjust ment knob to minimum fully counterclockwise Adjust R3 to narrow the high voltage output pulse width until the amplitude begins to drop This should be below 20 ns but above 12 ns 23 24 Vv Stepcal Adjustment Set level to 0 V the repetition rate to 5 kHz and the width to 1 ms Set TP 4 on
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