AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

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Application Note 61Figure reproduced with permissionof Tektronix, Inc.Figure 25. Tektronix P-6042 Hall Effect Based Current Probe Servo Loop.Figure 24 Replaces M18 Amplifier and Q22, Q24 and Q29TOWIDEBANDAMPLIFIERSECTIONAN61 F25AN61-20
Application Note 61Triggered 250 Picosecond Rise Time Pulse GeneratorVerifying the rise time limit of wideband test equipmentsetups is a difficult task. In particular, the “end-to-end”rise time of oscilloscope-probe combinations is oftenrequired to assure measurement integrity. Conceptually, apulse generator with rise times substantially faster thanthe oscilloscope-probe combination can provide this information.Figure 26’s circuit does this, providing an800ps pulse with rise and fall times inside 250ps. Pulseamplitude is 10V with a 50Ω source impedance. Thiscircuit has similarities to a previously published design(see Reference 7) except that it is triggered instead of freerunning. This feature permits synchronization to a clock orother event. The output phase with respect to the triggeris variable from 200ps to 5ns.The pulse generator requires high voltage bias for operation.The LT1182 switching regulator to forms a highvoltage switched mode control loop. The LT1182 pulse+1µF500kAVALANCHE BIASBIAS ADJUSTV IN V SWTYPICALLY 70V.(SEE TEXT)FBE2R3LT118212k1ME1GND V CTRIGGER INPUTT RISE = 10NS ORLESS. 1V TO 5V(SEE TEXT)50kHz MAXIMUM+5VL1820µHMUR120L22µFC23pF TO 12pF430kFigure 26. Triggered 250ps Rise Time Pulse Generator.Trigger Pulse Amplitude Controls Output Phase+50ΩL1 = J.W. MILLER # 100267L2 = 1 TURN # 28 WIRE, 1/4" TOTAL LENGTH5pF2µF10k10k+R450Ω1µFC12pFQ12N2369 (SEE TEXT)OUTPUTAN62 F26width modulates at its 100kHz clock rate. L1’s inductiveevents are rectified and stored in the 2µF output capacitor.The adjustable resistor divider provides feedback to theLT1182. The 10k-1µF RC provides noise filtering.The high voltage is applied to Q1, a 40V breakdown device,via the R3-C1 combination. The high voltage “bias adjust”control should be set at the point where free runningpulses across R4 just disappear. This puts Q1 slightlybelow its avalanche point. When an input trigger pulse isapplied Q1 avalanches. The result is a quickly rising, veryfast pulse across R4. C1 discharges, Q1’s collector voltagefalls and breakdown ceases. C1 then recharges to justbelow the avalanche point. At the next trigger pulse thisaction repeats. 6Figure 27 shows waveforms. A 3.9GHz sampling oscilloscope(Tektronix 661 with 4S2 sampling pug-in) measuresthe pulse (trace B) at 10V high with an 800ps base.Rise time is 250ps, with fall time indicating 200ps. Thetimes are probably slightly faster, as the oscilloscope’s90ps rise time influences the measurement. 7 The inputtrigger pulse is trace A. Its amplitude provides a convenientway to vary the delay time between the trigger andoutput pulses. A 1V to 5V amplitude setting produces acontinuous 5ns to 200ps delay range.A = 0.5V/DIVB = 1V/DIV(UNCALIBRATED)100 PICOSECONDS/DIVAN61 F27Figure 27. Input Pulse Edge (Trace A) Triggers theAvalanche Pulse Output (Trace B). Display Granularity IsCharacteristic of Sampling Oscilloscope OperationNote 6: This circuit is based on the operation of the Tektronix Type 111Pulse Generator. See Reference 16.Note 7: I’m sorry, but 3.9GHz is the fastest ’scope in my house (as ofSeptember, 1993).AN61-21
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AN61-1 Application Note 61 August 1994 Practical Circuitry for ...

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