Delay line aids in one-shot simulations
Many designers use small pulse generators to delay signals, open timing windows, drive sample/hold circuits, and other functions. Though the hardware implementation of these generators does not pose any problems, the lack of dedicated circuitry sometimes puzzles the Spice simulation of the system. A common approach to this problem is to implement a time constant involving a resistor, a capacitor, and a comparator. Unfortunately, each time you need a time constant, you must recalculate the resistor value, the capacitor value, or both. Despite the fact that inline equations can do this job for you, delay lines can often offer a smarter solution. Figure 1 shows the implementation of a small pulse generator. The operating principle of the circuit lies in applying two '1' levels to the AND-gate input before the delay line switches high. Figure 2 shows the signals associated with the circuit in Figure 1.
Figure 3 shows a typical application circuit for the one-shot multivibrators. You can use IsSpice4 or PSpice to simulate this sample/hold circuit. Figure 4 shows the waveforms associated with the circuit in Figure 3. A PWM signal (top waveform) generates a kind of arbitrary staircase signal. The multiplier, X4, sinusoidally modulates the PWM signal. The circuit cascades two small pulse generators (SMALLPULSE). One creates a delay signal to sample at a given time (X1, 70 nsec); the other calibrates the width (X3, 20 nsec) of the sampling signal (second waveform). The third waveform in Figure 4 shows the sinusoidally modulated signal; the fourth waveform is the sampled signal.