Simulator Test Tools Speed Circuit Design

Posted on Feb 6, 2014

A zero-crossing detector would convert an input sine wave (Vin) to a square wave, which when high would charge an op-amp integrator. Subsequently, a reference-input square wave would discharge the integrator. The integrator output voltage at the conclusion of this charge/discharge cycle would represent the difference between the input- and reference-signal periods. This level could be captured

Simulator Test Tools Speed Circuit Design
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by comparators and an output latch (Fig. 3). I made several enhancements to my basic design, because of problems I identified before putting cursor to screen and because of problems I encountered during circuit simulation as well as construction. First, I identified a shortcoming in the marketing spec ”it failed to provide an update rate. I envisioned the output being a string of LEDs that would connect to the Figure 3 output latch. A once-per-half-cycle update at 440 Hz would obviously not be useful to the user. Because the human ear does not detect sound-wave variations slower than about 12 to 20 Hz as pitch (they`re apprehended as beats), I reasoned that an update rate faster than about 20 Hz would be unnecessary. Consequently, I decided to drive the integrator for a period corresponding to several input and reference cycles. Eight cycles came to mind, as I had on hand a couple of 74HC164 8-bit serial-in, parallel-out shift registers. I initially reduced the cycle count to seven, reserving the shift registers` eighth bit for resetting the integrator to zero after each measurement. This approach worked fine in the simulation, but the integrator did not seem to reliably reset within the allotted time at worst-case reference-to-input phase relationships in my breadboarded version, so I allocated an additional cycle for circuit reset. I realized that reducing the number of cycles over which the integrator operated would...

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