[time-nuts] Divide by 3

Nick Sayer nsayer at kfu.com
Wed Jun 8 09:55:32 EDT 2016

I’m contemplating trying my GPS board with an FE-405B. That’s a different kettle of fish, but at the end of that, if I’m successful, one of the goals would be to be able to use it for the external reference of my 53220A. Unfortunately, 15 MHz isn’t one of the options - only 1, 5 and 10.

So I did some googling and found a divide-by-3 circuit using flip-flops, and then designed a board for it:


The circuit uses 3 D flip-flops and 3 NOR gates and has a 50% duty cycle output that’s 1/3 the frequency of the input. The OSHPark project has a pointer to the original blog post that has a schematic. The only difference between their schematic and mine is that in theirs, the third flip-flop has an inverted clock input. The third NOR gate inverts the clock to achieve that in mine (also one flip-flop and one NOR gate are unused and have the inputs tied high).

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