Pulse generator has low top-side aberrations

-May 25, 2000

Impulse-response and rise-time testing often require a fast-rise-time source with a high degree of pulse purity. These parameters are difficult to achieve simultaneously, particularly at subnanosecond speeds. The circuit in Figure 1, derived from oscilloscope calibrators (Reference 1), meets the speed and purity criteria. It delivers an 850-psec output with less than 1% pulse-top aberrations. Comparator IC1 delivers a 1-MHz square wave to current-mode switch Q2-Q3. Note that IC1 obtains power between ground and –5V to meet the transistors' biasing requirements. Q1 provides drive to Q2 and Q3. When IC1 biases Q2, Q3 turns off. Q3's collector rises rapidly to a potential determined by Q1's collector current, D1, and the output resistors combined with the 50W termination resistor. When IC1 goes low, Q2 turns off, Q3 turns on, and the output settles to 0V. D2 prevents Q3 from saturating.

The circuit's output transition is extremely fast and singularly clean. Figure 2, viewed on a 1-GHz real-time-bandwidth oscilloscope, shows 850-psec rise time with exceptionally pure pretransition and post-transition characteristics. Figure 3 details the pulse-top settling. The photo shows the pulse-top region immediately following the positive 500-mV transition. Settling occurs within 400 psec of the edge's completion with all activity within ±4 mV. The 1-mV, 1-GHz ringing undoubtedly stems from breadboard-construction limitations; you can probably eliminate it by using stripline-layout techniques. The level of performance of this circuit requires some trimming. The oscilloscope you use should have at least 1-GHz bandwidth. You adjust trimmers TR2 and TR3 for the best pulse presentation. TR1 sets the output amplitude at 500 mV across the 50W termination. The trims are somewhat interactive, although not unduly so, and converge quickly to give the results described. (DI #2530)

REFERENCE

1. 485 Oscilloscope Service and Instruction Manual, "Calibrator," pg 3 to 15, Tektronix Inc, 1973.

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