Software to Analyze and Explain CMOS Frequency Counter Circuit Operation?

A frequency counter circuit designed in 1978 using CMOS counters, NAND/NOR gates, an RF frequency divider, and a crystal oscillator was revisited after 36 years. The main challenge was understanding the circuit operation, particularly how the counting start and stop signals are generated and how the final counter states are interpreted. Suggestions included using logic simulation software such as CEDAR Logic Simulator or SmartSim to analyze the schematic by breaking it into smaller blocks. It was explained that frequency measurement involves gating the input signal with a timing signal, counting cycles during the gate interval, and calculating frequency as F=1/T. The gating signal must be a submultiple of one second to correctly start and stop counting. The original design used a series of divide-by-10 counters for frequency scaling (100M, 10M, 1M, 100K, 10K Hz), with the RF input entering the 10KHz counter gated by a 1KHz timing signal. The author confirmed understanding that the gating signal controls the counting interval, enabling accurate frequency measurement. The circuit still functions after nearly four decades, partly due to the use of military-standard components.
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