[time-nuts] Beginners GPS locked frequency counter question

[Florian Teply]

Am Sat, 31 Oct 2015 10:50:36 +0000
schrieb Chris Wilson <chris at chriswilson.tv>:

> 
> 
>   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?
> 
In principle yes. In practice this is a bit more involved, but still
possible. The easy way would be professional audio equipment, where
A/D-Converters quite often can be fed externally with a Master clock.
This is often used in studio arrangements, where one doesn't want
sampling clocks of different modules to drift relative to each other.
But the obvious downside is that professional audio equipment that
supports this kind of operation out of the box is "slightly" more
expensive than standard consumer stuff and usually isn't made to fit
into a computer but rather a 19 inch rack.

With Consumer sound cards, this is also possible, but requires some
hardware hacking in order to replace the onboard oscillator with
something that allows for locking to a frequency standard.

In both cases, the frequencies used there are somewhat odd compared to
what the average time nut has available: common master clock frequencies
are on the order of 16, 24, 32, 48 or 64 times the sampling frequency
for said professional equipment, consumer stuff often is clocked with
something like 6.144MHz (=128 times 48 kHz) or something similar.

That's nowhere near a clean 5 or 10 MHz a time-nut is likely to have
around... Which doesn't mean this would be difficult, in the end such a
setup is no different from usual time-nuts or lab setup, just the PLL
division ratio is different to lock the oscillator to a 1PPS signal.
But still, mod'ing consumer equipment takes some time to research the
actual circuit used and come up with a way to couple this to a known
good frequency source...

Florian