Lux, James P skrev:
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Rex Moncur
Sent: Tuesday, June 02, 2009 3:06 PM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Sound Cards for locking to GPSDO 10
MHz references
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 reference frequency can be right on top of your signal, as long as it's
within the dynamic range. You can subtract it out before doing the FFT, after
having determined where it is and how big it is.
Considering the length of these traces, the local oscillator vs. the
reference will shift around.
What I would do is to ensure that the reference signal and input signal
is either on very different frequencies or different channels.
Then, I would in the sampling phase frequency convert the receive signal
and reference signal using digital fixed NCO/quadrature oscillators
(cos, sin) and do integrate and dump (synchronous dump for both receive
and reference signals) processing for low pass filtering and reducing
sample rate. Since the signal was fairly narrow banded, this digital
receiver approach would significantly reduce the amounts of data while
requiring a very reasonable amount of real-time processing.
The remaining sample stream still contains the crutial information if
sufficient bandwidth is maintained after the integrate and dump processing.
Cheers,
Magnus
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