# [time-nuts] GPSDO Alternatives

Bob Camp lists at rtty.us
Thu Dec 6 23:13:40 UTC 2012

```Hi

The math is pretty straightforward.

Let's say the clock is 10 MHz, that's 100 ns.

Say a handful is 5 +/- 3 (2 to 8)

Your measure will bounce up and down by 6x100 ns = 600 ns.

Over a 100 second period that's going to be 6.0 x10^-9 bounce in the data.

If you run a 100 second loop as well, that's the noise in the loop (just from one source).

Six times faster clock, you get 1.0x10^-9. Step up the "handful" for the faster clock's pipeline and you may be back at the same place.

Take a bunch of readings (you only get one a second) and average - things get better by square root of the samples. That's IF you have enough jitter / dither in the system to smooth things out. If you have "lumpy" noise (as is often the case with interrupts) you may get very little gain from averaging.

With a 100 second loop, a TBolt is doing sub 1x10^-11, so you are at least 100X worse.

Bob

On Dec 6, 2012, at 5:57 PM, Chris Albertson <albertson.chris at gmail.com> wrote:

> You'd have to seriously divide down the output from the 10MHz OCXO if you
> were going to use it as an interrupt.  Maybe to divide by 10,000? and even
> at the higher clock rate you'd still have poor resolution.
>
> I image each interrupt handler would sample some internal counter and the
> background task would look at the delta between the two and adjust the DAC
> to drive the OCXO to close the difference.   The resolution would be
> (maybe?) a "handful" of clock cycles.   Given enough time, say a 1000
> second period it might wrk well enough.  I can't know without doing a more
> detailed design
>
>
> On Thu, Dec 6, 2012 at 1:59 AM, Don Latham <djl at montana.com> wrote:
>
>> Chris:
>>
>>> The question I have again is about a simple phase detector.
>>
>> I did ask if the arduino interrupt ports could be used as a phase
>> detector; one on the GPS and one on the OXCO. Too much jitter? If the 12
>> MHz clock is too slow, would an 80 MHz clock ARM arduino style processor
>> work? I'm simply too new at this to decide.
>>
>> Don
>>
>> --
>> "Neither the voice of authority nor the weight of reason and argument
>> are as significant as experiment, for thence comes quiet to the mind."
>> De Erroribus Medicorum, R. Bacon, 13th century.
>> "If you don't know what it is, don't poke it."
>> Ghost in the Shell
>>
>>
>> Dr. Don Latham AJ7LL
>> Six Mile Systems LLP
>> 17850 Six Mile Road
>> POB 134
>> Huson, MT, 59846
>> VOX 406-626-4304
>> www.lightningforensics.com
>> www.sixmilesystems.com
>>
>>
>>
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>
>
>
> --
>
> Chris Albertson
> Redondo Beach, California
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```

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