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

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jun 3 03:44:49 EDT 2009


Lux, James P skrev:
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com 
>> [mailto:time-nuts-bounces at febo.com] 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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