Doesn't it derive the USB clock from the 28.8 MHz crystal? In this case you are out, USB needs the correct frequency to work.
Ralph. > -----Original Message----- > From: [email protected] [mailto:osmocom-sdr- > [email protected]] On Behalf Of Malcolm Robb > Sent: Thursday, September 11, 2014 10:33 AM > To: Kevin Reid > Cc: [email protected] > Subject: RE: Maximum sampling frequency questions. > > It's ADSB/ModeS. I'm partly responsible for dump1090. The 'bit rate' is 2Mhz, > and the RF center is 1090Mhz. > > There are several issues with the current sampling at 2Mhz, the main ones > being.... > 1) The Bandwidth is +/- 1Mhz, but some aircraft are a long way off the center > 1090 Mhz frequency so are missed because they are out of band. A higher > sampling frequency results in a wider bandwidth so would pull in more > signals. > 2) Nyquist means that you get nasty beating effects as signals drift in and out > of phase with the sampling clock. > > I had considered sampling at 3Mhz which would help both the above, but the > loss of samples would be the deal breaker. At 2.4Mhz, the maths is a bit > nasty for not a lot of improvement. The cheap and easy solution would be to > change the crystal to a faster one, if the RTL chip can stand the over clocking. > If the data loss is at the PC end, then this is unlikely to help. However, if the > data loss is at the dongle end, then overclocking should linearly increase all > the 2.4Mhz parameters without data loss, again assuming the RTL chip can > take the overclocking. > > Cheers > Malcolm > ________________________________________ > From: Kevin Reid [[email protected]] on behalf of Kevin Reid > [[email protected]] > Sent: 11 September 2014 00:26 > To: Malcolm Robb > Cc: [email protected] > Subject: Re: Maximum sampling frequency questions. > > On Sep 10, 2014, at 15:50, Malcolm Robb <[email protected]> > wrote: > > > I'm currently sampling a 2Mhz digital data stream at 2MHz, so obviously I'm > suffering quite badly from Nyquist issues. Ideally I'd like to sample at either > 4Mhz, or better still 8Mhz. I understand that theoretically I can set the > [RTL2832] dongle to sample at 3.2Mhz, but that data loss is virtually > inevitable. > > The RTL2832 gives you *complex* (I/Q) samples. This does not exactly > change the Nyquist limit of 1 MHz (for a 2 MHz sample rate), but it makes it > be *plus or minus* 1 MHz, or 2 MHz in total. > > If you have a digital radio signal which occupies 2 MHz of bandwidth, you > *should* be able to receive it with a RTL dongle, given that: > - the signal is strong enough > - you are using the 2.4 MHz sample rate, not 2.0 (filters aren't perfect, > especially not these ones, so you need some margin above Nyquist) > - there is no DC offset (i.e. not E4000 tuner), or you have corrected it, or it > doesn't matter for the modulation in question > > Perhaps you could tell us more about what signal you're trying to receive, > and what software you're currently using? > > (I don't have any answers for the other questions in your message.) > > -- > Kevin Reid <http://switchb.org/kpreid/> >
