Simple nanosecond-width pulse generator provides high performance

Linear Technology LT1721

Jim Williams, Linear Technology

EDN

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 ns output width with 520 ps, 5 V transitions. Pulse width varies less than 100 ps with 5 V supply variations of 65%. The minimum input-trigger width is 30 ns, and input-output delay is 18 ns.

Simple nanosecond-width pulse generator provides high performance
Figure 1. This pulse generator has 0- to 10-nsec width and 520-psec transitions. IC1 unloads
termination and drives the differential delay network. The IC2-IC3 complementary
outputs represent delay difference as edge timing skew. G1, which is high during
IC2-IC3’s positive overlap, presents circuit output.

Comparator IC1 inverts the input pulse (Figure 2, Trace A) and isolates the 50 Ω 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 80 Ω/ns. 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.

Simple nanosecond-width pulse generator provides high performance
Figure 2. Pulse-generator waveforms, viewed in 400-MHz real-time bandwidth, include
input (Trace A), IC3 (Trace B) fixed and IC2 (Trace C) variable outputs and output
pulse (Trace D). RC networks differential delay manifests as IC2-IC3 positive overlap.
G1 extracts this interval and presents circuit output.
 
Simple nanosecond-width pulse generator provides high performance
Figure 3. The 5-nsec-wide output with R = 390 W is clean with well-defined transitions.
Post-transition aberrations are within 8% and derive from G1 bond-wire
inductance and an imperfect coaxial probe path.

Figure 3 shows a 5 V, 5 ns width, measured at 50% amplitude, output pulse with R = 390 Ω. 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 4 shows the narrowest full amplitude, 5 V pulse available. Width measures 1 ns at the 50% amplitude point and 1.7 ns at the base in a 3.9 GHz bandwidth. Shorter widths are available if partial amplitude pulses are acceptable.

Simple nanosecond-width pulse generator provides high performance
Figure 4. The narrowest amplitude pulse width is 1 nsec, and the base
width measures 1.7 nsec. Measurement bandwidth is 3.9 GHz.
 
Simple nanosecond-width pulse generator provides high performance
Figure 5. The partial-amplitude pulse, 3.3 V high, measures 700 psec wide
with a 1.25-nsec base. The trace granularity is an artifact of the
3.9-GHz sampling- oscilloscope operation.

G1's rise time limits minimum achievable pulse width. The partial-amplitude pulse, 3.3 V high, measures 700 ps with a 1.25 ns base (Figure 5). Figure 6, taken in a 3.9 GHz sampled bandpass, measures 520 ps rise time. Fall time is similar. The transition of the probe edge is well-defined and free of artifacts.

Simple nanosecond-width pulse generator provides high performance
Figure 6. A transition detail in the 3.9-GHz bandpass with rise time of 90 psec
shows 520-psec rise time; fall time is similar. The granularity derives
from sampling-oscilloscope operation.

Materials on the topic

  1. Datasheet Linear Technology LT1721
  2. Datasheet Texas Instruments 74AHC08

EDN

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