[time-nuts] Sound Cards for locking to GPSDO 10 MHz references
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
are MUCH more appropiate.
> -----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
> 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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