On Sep 11, 2014, at 1:33, Malcolm Robb <[email protected]> wrote:
> 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. I'm pretty sure that the proper solution to both of these problems you list (beating and conversion from 2.4MHz) is to use an algorithm designed to to recover the original bit-clock. Unfortunately, I haven't myself got the knowledge of digital (de)modulation techniques to be more concrete, but I know standard such algorithms exist and don't require the input sample rate to exactly match the bit rate. > The cheap and easy solution would be to change the crystal to a faster one, > if the RTL chip can stand the over clocking. Easy as a one-off, but then every potential user would have to modify their device. -- Kevin Reid <http://switchb.org/kpreid/>
