[time-nuts] Beginners GPS locked frequency counter question

Magnus Danielson magnus at rubidium.dyndns.org
Sat Oct 31 09:39:34 EDT 2015

Hi Chris,

On 10/31/2015 11:50 AM, Chris Wilson 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?

Your assumption is wrong.

There is a +/- 1 error in counting cycles.
Also, if the start and stop does not derive from the same trigger, 
differences in the triggers can result in a time-shift which bias the 
value. Difference in delay for start and stop events in the counter also 
produces error. Also, the white phase noise contribute.

In essence, a number of practical design limitations makes measurements 
somewhat noisier and somewhat biased compared to what theory says. It's 
important to understand these so that their effect can be estimated and 
sometimes mitigated.

>     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?

There is sound-cards you can lock to a 48 kHz clock.


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