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Design Idea
Simple nanosecond-width pulse generator provides high performance
Edited by Bill Schweber
By Jim Williams, Linear Technology Corp -- EDN, 11/11/2004
If you need to produce extremely fast pulses in response to an input and trigger, such as for sampling applications, the predictably programmable short-time-interval generator has broad uses. The circuit of Figure 1, built around a quad high-speed comparator and a high-speed gate, has settable 0- to 10-nsec output width with 520-psec, 5V transitions. Pulse width varies less than 100 psec with 5V supply variations of 65%. The minimum input-trigger width is 30 nsec, and input-output delay is 18 nsec.
Comparator IC1 inverts the input pulse (Figure 2, Trace A) and isolates the 50Ù termination. IC1's output drives fixed and variable RC networks. Programming resistor RG primarily determines the networks' charge-time difference and, hence, delay at a scale factor of approx 80Ù/nsec. Comparators IC2 and IC3, arranged as complementary-output-level detectors, represent the networks' delay difference as edge-timing skew. Trace B is IC3's fixed-path output, and Trace C is IC2's variable output. Gate G1's output (Trace D), which is high during IC2-IC3 positive overlap, presents the circuit output pulse. Figure 2 shows a 5V, 5-nsec width, measured at 50% amplitude, output pulse with R=390Ù. The pulse is clean and has well-defined transitions. Post-transition aberrations, within 8%, derive from G1's bond-wire inductance and an imperfect coaxial probing path. Figure 3 shows the narrowest full amplitude, 5V pulse available. Width measures 1 nsec at the 50% amplitude point and 1.7 nsec at the base in a 3.9-GHz bandwidth. Shorter widths are available if partial amplitude pulses are acceptable. Figure 4 shows a 3.3V, 700-psec width (50%) with a 1.25-nsec base. G1's rise time limits minimum achievable pulse width. The partial-amplitude pulse, 3.3V high, measures 700 psec with a 1.25-nsec base (Figure 5). Figure 6, taken in a 3.9-GHz sampled bandpass, measures 520-psec rise time. Fall time is similar. The transition of the probe edge is well-defined and free of artifacts.
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