Pulse generator has low top-side aberrations

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 –5 V 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 50 W termination resistor. When IC₁ goes low, Q₂ turns off, Q₃ turns on, and the output settles to 0 V. D₂ prevents Q₃ from saturating.

Figure 1.	Need a fast, clean pulse? This simple circuit provides 500-mV, 850-nsec pulses with a high degree of purity.

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.

Figure 2.	Viewed with 1-GHz bandwidth, pulses are free of discontinuities and anomalies.

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 50 Ω termination. The trims are somewhat interactive, although not unduly so, and converge quickly to give the results described.

Figure 3.	Pulse tops exhibit less than ±4 mV, or less than ±1%, aberrations.

Reference

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