For the last couple of months I've been trying to put together a 2 channel coherent receiver. For this I'm using a 9.6MHz TXCO oscillator followed by a simple tripler (from the bottom of
this page). This gives me the 28.8MHz clock signal that's required by the RTLSDR dongle. I've then taken 2 RTLSDR dongles, desoldered their crystals and replace with a coax feed from the tripler. I should then have 2 receive channels which should be always in phase with each other, and due to the use of a TXCO should not drift (too much!) with temperature.
It all seems to work well - by which I mean if I cross-correlate the signals from both receivers I get something which looks like:
which shows that the 2 channels are very correlated (the peak isn't at zero as the dongles aren't started at exactly the same time).
However, when I look in detail I see that I'm actually dropping samples - every so often the peak will shift by around 100-1000 samples. To figure out what is happening I now have each dongle in a different machine which has certainly reduced the occurrence of the drop-outs, but they still occur (the drop-out rate went from around once every minute or two, to once an hour). I need to see what I can do about this (better disk drive, update to bios, tweak USB settings somewhere) - but I'm not sure at the moment where to start (with spending lots of money!).
I'm doing this to make a simple passive bistatic radar using DAB radio as the illuminator - mainly to look at meteors and perhaps satellites (but it should also pick up planes). This is currently the sort of output I get - range is up and down and Doppler shift is left to right.
A lot of the things which are visible in this are just artefacts from structure that the format of the DAB signal. I'm in the process of verify some of the signals I think I can see!
At the moment the engineering is a bit poor, so the whole system works as a bit of a local jammer! Fortunately this isn't a problem at the DAB frequencies, but is at ~144MHz which makes it tricky to receive either GRAVES or the GB3VHF beacon. So the next step is to properly engineer the whole tripler/oscilator/dongle system into a well shielded box. I also want to increase the number of receive channels to 4 to allow me some amount of interferometric or beam forming capability. I've also started to look into using a SI5351C as the frequency driver which could make up to 8 frequency outputs from a TXCO or a rubidium clock.
