[time-nuts] Sound Cards for locking to GPSDO 10 MHz references

[Rex Moncur]

Hi all

Thank you all for you advice and suggestions re my request.  At this stage
it does not look like there is a simple solution of a readily available USB
sound card that can be locked to a 10 MHz GPSDO reference.

The constraints of portable operation with a Laptop rule out a number of
solutions.  I have tried the software solution using Spectrum Lab but ran
into problems and perhaps this just needs more work.  One of the main
problems is that in working at milli-Hz binwidths the FTT word length needs
to be very long to cover even a few tens of Hz range and we run into memory
problems. So there is little room to have a reference frequency spaced well
away from the frequency range being used.

The SP DIF solution seems promising if I can generate the required input.
This could perhaps be done with a product that already provides the SP DIF
word output and locking that.  But that could be just as hard as locking the
sound-card in the first place.

So at this time I think I will put some more effort into locking the sound
card and let you know how I go, hi. Injection locking as suggested by some
of you may be the answer.

Some people asked for more details of the cloud scatter experiments which
area at http://reast.asn.au/optical.php  There is also a series of articles
on our work in the last 3 and next issue of DUBUS.

On the question of Doppler shift from clouds - this is much less than a mHz
and not an issue due to the fact that we are using base band and the Doppler
only applies to the audio frequency.  In addition the very narrow beamwidths
(around 2 degrees) mean that the possible paths are all very similar in
length.


Thanks again to everyone for their input.

73 Rex VK7MO