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

Lux, James P james.p.lux at jpl.nasa.gov
Tue Jun 2 22:18:59 UTC 2009

> -----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.

> 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.

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