[time-nuts] Divide by 3

[Nick Sayer]

> On Jun 8, 2016, at 9:59 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Nick wrote:
> 
>> 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
> 
> You can achieve substantially lower jitter (phase noise) with a regenerative divider, which also allows you to divide by 3/2 for a 10MHz output.  I've built several like that, and they work extremely well.
> 
> There are simpler divide-by-three logic circuits (generally, the simpler the circuit the closer to an exact 50% duty cycle and the lower the jitter).  See the attached image for one approach.
> 

I got the boards back, and Charles’ version with just two D FFs and a single XOR works perfectly. It works even without the delay line he indicated (sample size 1, FWIW). The version with the three D FFs and 3 NORs works just fine too, but of course it’s not as small as Charles’.