Verifying the rise time limit of wideband test equipment setups is a difficult task. In particular, the “end-to-end” rise time of oscilloscope-probe combinations is often required to assure measurement integrity. Conceptually, a pulse generator with rise times substantially faster than the oscilloscope-probe combination can provide this information. Figure 1’s circuit does this, providing a 1 ns pulse with rise and falltimes inside 350 ps. Pulse amplitude is 10 V with a 50 Ω source impedance. This circuit, built into a small box and powered by a 1.5 V battery, provides a simple, convenient way to verify the rise time capability of almost any oscilloscope-probe combination.
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| Figure 1. | 350 ps Rise Time Pulse Generator. |
The LT1073 switching regulator and associated components supply the necessary high voltage. The LT1073 forms a flyback voltage boost regulator. Further voltage step-up is obtained from a diode-capacitor voltage doubler network. L1 periodically receives charge, and its flyback discharge delivers high voltage events to the doubler network. A portion of the doubler network’s DC output is fed back to the LT1073 via the R1, R2 divider, closing a control loop.
The regulator’s 90 V output is applied to Q1 via the R3-C1 combination. Q1, a 40 V breakdown device, non-destructively avalanches when C1 charges high enough. The result is a quickly rising, very fast pulse across R4, C1 discharges, Q1’s collector voltage falls and breakdown ceases. C1 then recharges until breakdown again occurs. This action causes free-running oscillation at about 200 kHz. Figure 2 shows the output pulse. A 1 GHz sampling oscilloscope (Tektronix 556 with 1S1 sampling plug-in) measures the pulse at 10 V high with about a 1 ns base. Rise time is 350 ps, with fall time also indicating 350 ps. The figures may actually be faster, as the 1S1 is specified with a 350 ps rise time limit.
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| Figure 2. | Avalanche pulse generator output pulse. waveform has 350 ps rise and fall times. Slightly under damped turn-off is probably due to test fixture limitations. |
Q1 may require selection to get avalanche behavior. Such behavior, while characteristic of the device specified, is not guaranteed by the manufacturer. A sample of 50 Motorola 2N2369s, spread over a 12 year date code span, yielded 82%. All “good” devices switched in less than 600 ps. C1 is selected for a 10 V amplitude output. Value spread is typically 2 pF to 4 pF. Ground plane type construction with high speed layout techniques are essential for good results from this circuit. Current drain from the 1.5 V battery version is about 5 mA.
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| Figure 3. | Alternate 90 V DC-DC converter. |
For those applications which must run from higher voltage inputs, Figure 3 is included. This circuit, which operates from inputs of 4 V to 20 V will also power the avalanche stage. Cascoded high voltage transistor Q1 combines with the LT1072 switching regulator to form a high voltage switched mode control loop. The LT1072 pulse width modulates Q1 at its 40 kHz clock rate. L1’s inductive events are rectified and stored in the 2 µF output capacitor. The 1 MΩ to 12 kΩ divider provides feedback to the LT1072. The diode and RC at Q1 ’s base damp inductor related parasitic behavior. The circuit’s output drives the avalanche stage in similar fashion to the LT1073 based circuit.
