[time-nuts] Divide by 3

Nick Sayer nsayer at kfu.com
Wed Jun 8 19:19:51 EDT 2016

There’s a link to the blog posting (not mine) that has the schematic on the OSHPark shared project.

The schematic there isn’t *totally* obvious - the square boxes that are otherwise unlabeled are D flip-flops with D on the left and Q on the right. I’ve simulated the circuit at CircuitLab and gotten the correct behavior, for what that’s worth.

You’re correct that multiple instances of this circuit fed from the same source would not be in phase. That’s ok with me. I’m just going to build one.

I don’t know if I’ll actually put it into service or not. I have lots of different references at my disposal for the 53220A, and at this point my thinking is that all of them are going to be a tie given this TIA’s specs.

Someone else mentioned sine vs square for the 53220A reference input. I’ve fed square waves into this TIA with no indication that it hasn’t worked just as well as sine, but I don’t have any assurance beyond that.

> On Jun 8, 2016, at 9:47 AM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
> The URL you cited doesn't have the schematic in any obvious
> place.  However, using both edges of the clock to supposedly
> result in 50% duty cycle output depends on having 50%
> duty cycle at the input.  If you have differential logic
> like ECL, this can be realistic.  Single ended logic,
> questionable.
> The other issue is that the divider can start up in any
> one of 3 phases with respect to any other frequency
> dividers in your system, unless you do something to
> synchronize the various dividers.
> This is probably old hat to most readers of time-nuts, but
> I just wanted to mention it in case some were unaware
> of it.
> Rick
> On 6/8/2016 6:55 AM, Nick Sayer via time-nuts 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:
>> 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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