Interferometry has been done at an amateur level before but only by a few people, and im pretty sure they all knew alot more about it than me when they started but id still like to have a try. This will be certainly one of the most ambitious projects I have undertaken.
This guy has a good site describing his simple digital interferometer(SIDI). He uses GPS recievers to demonstrate how the signals can be correlated using a single local oscillator. In his first incarnation SIDI(1.0)
_http://lea.hamradio.si/~s57uuu/astro/sidi1/sidi10.htm
he uses a low noise amplifier as a pre amp and then downconverts the signal using the local oscillator as a refference. I imagine this is effectively what an LNB does.
The tricky bit with using an LNB instead of the LNA preamp+downconverter is that the dishes will be placed 20m apart. This means that I will have to transmit a single oscillator signal to two seperate LNBs which accept external refferences. If I am to use 9Ghz external oscillator( Which I am not sure is even possible on standard PLL LNB) will a coax cable be sufficient to transmit this signal the required distance??
As far as I can tell the way a normal sat signal is processsed is that it is detected at the LNB and downconverted to an Intermediate frequency to be passed to the reciever(10ghz-~1ghz) then this is again downconverted within the reciever itself to a IF amplifier working at a lower frequency(correct me if im wrong on this point??) This means both the recievers will require their own oscillators as well as the LNBs.
As long as the overall effect is that both signals are amplified identically(well at least their phase relationship is maintained) then it should not really matter how accurate each of the oscillators are independantly.
It seems the LNB matching to the dish will also be essential, so what decent external refference LNBs are out there and how powerful does the external signal to these have to be?? Also is it possible to use a lower oscillator frequency to get greater bottom end frequency range. I want to scan from ~9.5ghz - 10.5ghz??
Alex |