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Comb Generator

A comb generator is a circuit which generates a signal that is very rich in harmonics. A year or so ago, I had a quick go at making one1, which is very easy and requires just one logic IC.

combgen

The active ingredient is a quad NAND-gate in a 14-pins IC, the 74AHC00. It’s mounted upside down in the picture shown here. This particular “AHC” version works on a 2V to 6V supply and has fast switching characteristics, with rise and fall times in the order of 5 ns. This is important, because the steeper the generated pulse edges are, the more harmonics this circuit will produce.

7490

A pure square wave only has odd harmonics, but with a sharp pulse you get both even and odd harmonics.

The four NAND gates in a 7400 chip can be used to accomplish this: #1 one oscillates (13/12/11), #2 buffers (1/2/3), #3 delays (4/5/6), and #4 ANDs the buffered and the delayed pulse (10/9/8), thus producing a sharp pulse.

This is the circuit, as constructed in dead-bug fashion, held down using double-sided tape:

circuit

pulse

It’s unfortunate that these pulses are negative-going, because the circuit draws a lot more current this way (being a “1” most of the time). The current consumption at 5V is 40 mA.

And sure enough the circuit generates an entire “comb” of harmonics, spread 10 MHz apart:

spectrum

The usefulness of this circuit is limited with today’s PLL technology, but it was still interesting to see the venerable “7400” chip generate a very wide and distinct frequency spectrum.


  1. With many thanks to Martyn Judd for helping a digital geek like me understand a bit of the analog world.