Frequency Counter

Beginning in (he May 1972 issue of 75, K20AW presented a very complete series of articles on the construction of a frequency counter with a low frequency (0—20 MHz) input and a divide-by-10 pre-scaler capable of extending the upper frequency limit to 300 MHz, The counter appeared to be just what I needed. In fact I wrote to K20AW and he generously forwarded a copy of the circuit board layout in advance of publication of the next article in 73. The counter was built using mostly surplus ICs and junk box parts, and worked the first time it was plugged in. By sorting the ICs according to speed, I was able to obtain a stable upper frequency limit of 32 MHz in the low frequency position. I have not yet added the prescaler ICs since ! had no immediate need for VHF capability, but extension of the low frequency limit did intrigue me — especially since I had heard of Heathkit counters that would go to 40 MHz, Much experimentation was accomplished on the front end of the counter, including the 40673, the 7413 and the 7400 input selector, Maximum attainable speed remained in the 32 to 34 MHz range.

The main problem appeared to stem from the limited speed of the 7473 flip-flop which comprises the divide-by-two function of the first decade. Simultaneously with my discovery of the apparent problem with the 7473, low cost Schotty TTL ICs became available from some of the surplus companies, (They are available from Solid State Systems, P.O. Box 773, Columbia MO

65201.) A 74S73 flip-flop was obtained and plugged into the ICS position on the circuit board and things started to happen. Using a four turn wire loop on the counter input and a grid dip oscillator as a signal source, the counter's upper limit was found to be 62 MHz! To say the least, 1 was flabbergasted. Subsequent checks proved that the initial test was not a fluke and the counter was now stable from 20 Hz (the lower limit of my test equipment) to better than 60 MHz.

The next limiting factor on the upper frequency limit appears either to be the 7413 Schmitt trigger or the divide-by-five function of the 7490 in the first decade. Further experimentation is under way on replacement of the 7490 with a 74196 or some other high speed device. Fm also hopeful that a Schottky 7413 will be introduced. However, a friend of mine (ILt Dan Wright) has operated a 74S00 Schottky 1C in a Schmitt trigger configuration with operating limits in excess of 100 MHz and maybe this setup can be used to replace the 7413. Anyway, there are tremendous possibilities and, who knows, maybe Fll never install the 95H90 prescaler.

My sincere thanks to friends who have added ideas and provided encouragement to my experimentation. They include ILt Dan Wright, ILt Cecil Lockett and ILt Bruce Mclntire, Most of all, thanks to Pete Stark K20AW, for his design and article on the counter.

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