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/>
