> 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.

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@febo.com > [mailto:time-nuts-bounces@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. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there.