[time-nuts] Beginners GPS locked frequency counter question

[Graham / KE9H]

Chris:

If by "purely mathematical" you mean with infinite precision and infinite
accuracy in the calculations, then the answer is "No."

The counter is using integer mathematics, in a world with propagation
delays and clock jitter, and uncontrolled phase relationships.
So resulting rounding errors and lost remainders in division processes.
If you read the specs on any counter, you will find that there is a
absolute accuracy PLUS an error window of (typically) plus or minus one
count.  Both error sources are very real.

I assume you mean 136 kHz.

But the answer to your sound card question is that you can't just GPS lock
the data converter on the sound card.  If it is running in a different
"clock domain" than the synchronous signal processing that follows it, then
you usually start having buffer underflows and buffer overflows which
introduce signal integrity problems.  The sound card sampling clock is
usually derived from some master clock in the computer, such that the sound
card is running in the same clock domain as the rest of the computer, so
you more likely need to GPS lock the whole computer.  This capability is
normally not provided for consumer grade computers, so you will be on your
own for the circuit modifications.

If you are doing signal processing in a desktop or laptop computer, they
are not "real time" operating systems, so understand the implications of
that, too.

--- Graham

==



On Sat, Oct 31, 2015 at 5:50 AM, Chris Wilson <chris at chriswilson.tv> wrote:

>
>
>   31/10/2015 10:46
>
>    I have a Racal counter locked to 1 MHz on its rear panel external
>    input socket from my Trimble Thunderbolt GPS. I derive the 1 Mhz
>    from a David Partridge divider board. If I also feed the counter
>    with the 10 Mhz direct output from the same GPS it reads one or two
>    Hz out. As I assume the counter is working purely mathematically
>    why would that be please?
>
>
>    As an aside, I work low frequency RF transmissions on 136 Mhz, and
>    very narrow bandwidth. Can a soundcard be locked to GPS instead of
>    its own internal crystal for precise frequency output?
>
>    Thanks.
>
> --
>        Best Regards,
>                    Chris Wilson.
> mailto: chris at chriswilson.tv
>
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