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

Ulrich Bangert df6jb at ulrich-bangert.de
Thu Jun 4 02:56:09 EDT 2009


For a lot of people the FFT seems to be the "one size fits all" solution to
any frequency and phase related problem in DSP. It is NOT! For
frequency/phase detection & comparisons from sets of sampled data the
methods explained in

http://ipnpr.jpl.nasa.gov/progress_report/42-121/121G.pdf

are MUCH more appropiate.

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Hal Murray
> Gesendet: Mittwoch, 3. Juni 2009 20:00
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] Sound Cards for locking to GPSDO 10 
> MHz references
> 
> 
> 
> > 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.
> 
> I'm missing something.  How much memory do you have on your laptop?
> 
> I'm not a DSP wizard.  If you have 10 Hz bandwidth and you 
> want milli-Hz 
> bins, that takes 2x10x1000 samples.  Right?  I'd expect that 
> to fit easily.
> 
> That's 20K samples, at 8 bytes each, round up to 10, call it 
> 200K bytes.
> 
> Jumping to micro-Hz might get interesting.  That would be 200 
> megabytes.  
> Lots of laptops have room for that.  Maybe not an old one.
> 
> Even with an old laptop without much memory, I'd expect you 
> could do several 
> factors of 2 better than milli-Hz bins.
> 
> On the other hand, how much bandwidth do you really need?  
> Junk crystals are 
> 50-100 ppm.  100 ppm at 1 KHz is 1/10 Hz.  So why do you need 
> more than 1 Hz 
> input bandwidth?  You can probably get closer than that by 
> calibrating the 
> crystals in your particular gear.
> 
> Connie's numbers were 250 micro-Hz drift with a 500 micro-Hz 
> offset.  (That 
> was with reasonably stable temperature.)  So a few milli-Hz 
> bandwidth looks 
> like enough.
> 
> 
> 
> 
> 
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
> 
> 
> 
> 
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