# [time-nuts] Divide by 3

Mike Cook michael.cook at sfr.fr
Wed Jun 8 12:35:43 EDT 2016

```> Le 8 juin 2016 à 15:55, Nick Sayer via time-nuts <time-nuts at febo.com> a écrit :
>
> 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.

I saw the same, which put me off trying to do the same. However I am not sure that your approach will work as the specs for the external clock indicate:

- EXTernal  selects an external reference signal applied to the rear panel
Ext Ref In  connector. The signal must be:
•  1 MHz , 5 MHz, or 10 MHz
•  100 mVrms to 2.5 Vrms
•  sine wave

Your output is digital, no? It may function but I wouldn’t trust it.

It is a no brainer to get a sine from a square wave, BUT , I seriously doubt that the excellent ADEV can be maintained with all that flipping and flopping going on. I even doubt that it could be kept in a pure sine implementation.

Mike

>
> So I did some googling and found a divide-by-3 circuit using flip-flops, and then designed a board for it:
>
> https://oshpark.com/shared_projects/jxXp7wYM
>
> 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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"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw

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