A 2GHz, 2-port VNA adapter for SDRs
I thought it might prove an interesting challenge to attempt to use an off-the-shelf SDR (LimeSDR mini in my case), and build a 2Ghz-capable RF frontend for it to allow it to neatly do full 2-port characterizations of amplifiers and filters, like a true VNA, without having to wire up a bunch of external couplers.
Given that the LimeSDR mini only provides a single RX and TX port, we're actually doing some shenanigans here to get full 2-port measurements. Ordinarily, VNAs contain more than one receiver (at least 2x RX channels), where one is used as some sort of amplitude + phase reference going into the DUT, and the other is used for receiving the test signal back from the DUT. Here, we can't do that, as we only have one RX channel! What do I do instead?
Even if we were to switch the excitation to each port as appropriate, we have nothing to compare the measurement to. The trick here to making do with only 1 RX channel (with the downside of introducing considerable error that needs to be calibrated out in software), is the PIN diode clamps close to the DUT ports.
