Hi list, hi Michel,

I'm new here, but have been working on the same solution as you are, but for an Amsat all sky tracking receiver.

First, let me point you to the signal processing textbook I've been working my way through: dspguide.com. It can be a bit verbose for a textbook, but that's been to my advantage several times.

I've run into two clear obstacles trying to build an interferometric array. Both have to do with clock synchronization. First is clock drift. No matter how expensive the crystal, or precise the temperature control, two clocks are going to drift with respect to each other. I'm planning on solving this one by pulling the crystal off the dongles and replacing it with a clock input that I'll pipe from one single clock/crystal out to each dongle. I'm assuming that I'll have to use equal length cables to keep my timing correct.

The second problem is synchronizing the separate data streams so I know which samples were taken at the exact same time. This one is a bit trickier, and I don't think there's a hardware solution for it. Most multichannel data acquisition systems have a trigger system to solve this, where all the channels get initialized but don't start to capture until the trigger is received. Unfortunately I don't think there's a trigger input pin anywhere in our dongles.

There is another way, and that's find a known RF signal that we can use for synchronization. GPS would be ideal, because there's explicit solutions for time offsets/drift that's referenced to atomic clocks. We could record 5 minutes or so on the L1 band before an observation, then mid-capture tune to our frequency of interest...assuming we can shift the e4k chip without interupting the RTL2832 in it's capture sequence.

Does anyone here know if this is possible? Shift the tuner without interupting the capture?

This does require a wide band antenna, best options seem to be spiral and conical spiral. There's probably other options, like an active gps patch antenna that's duplexed in with a frequency specific duplexer, but I'm not much of an rf guy and I like the simplicity of the single wide band antenna.

I've also been looking at options for building out an array, I've seen reports that a good LNA can get 20dB in SNR, but a wideband preamp runs $70 on minicircuits website. Plus the cost of a bias T and a clean power supply. And I'm not sure that I understand all the built-in options for amplification and filtering yet. That design trade is still pretty fuzzy. It might be more cost effective to just buy more dongles and get better SNR through summing their outputs. More research required!

Jared