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

Hal Murray hmurray at megapathdsl.net
Wed Jun 3 13:59:57 EDT 2009


> 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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