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Small home-built Frequency Counter

This 6-digit counter has German text on the front. It looks like home built. A TTL based frequency counter, using the rare SN74142 counter-latch-decoder IC. It has 6 NEC nixies.
Late 2014 I bought this counter on the NVHR radio swapmeet. It can measure frequency up to 500 MHz and has two ranges and two gate times. Gate times are 0,1 μs or 1 s. The counter has a single input.

There are three toggle switches on the front:

The digital circuits are on a single circuit board. This is a dual layer tin-plated PCB. The display counter consists of SN74142 IC's, each of them containing a decimal counter, latch and decimal decoder/nixie driver. The counter has a 1 MHz time base counter. Its signal is divided to a gate signal by a 6-stage divider chain made of SN7490 decimal counters. Another few TTL circuits are used to build up the control unit.

The input circuit is built up on a small circuit board. It contains a comparator for the 50 MHz input and a xxx fast counter for the 500 MHz input.

Inside view

A look inside from above, showing the ciruit board and it contents.

After a quick visual check I switched on the counter. It seemed to work but the display did not show all zeroes but 40-60 counts. There seemed to be interference caused by the fact that the input was open. When I connected it to the output of a signal generator, and put my FC7008 counter in parallel, I could see that they almost had the same readout.

I checked the frequency of the time base oscillator and got 1.000006 MHz. I decided to trust the FC7008 and set the time base to exactly 1 MHz.

This counter is working ok now. Only, its input sensitivity is not very good. And it tends to pick up interference when not connected to a low impedance source.

Copyright © 2015 by Onno's E-page         published 2015-05-03