On 13/06/2023 15:17, Rob Kossler wrote:
Hi Michael,
LZ631368043US
One calibration procedure could be that you simply add a digital phase
offset to your 2nd Tx channel until your oscilloscope traces line up
to your satisfaction. It would be nice if the default FPGA image
included a simple Rx & Tx complex scalar for this exact purpose
(inside the DDC & DUC), but that does not exist (yet). If you are
using gnuradio, it should be easy to insert a complex scalar
multiplication that would allow you to adjust the samples streaming to
just one channel. In any case, if the signal on the oscilloscope
lines up well over your desired bandwidth, then you can declare
"mission accomplished". On the other hand, if your signal lines up at
one end of your frequency bandwidth but then diverges at the other
end, it likely means a delay mismatch that you could potentially
"calibrate" by adding an appropriate length of cable to one path (in
cable, 1 ns delay is about 8 inches).
Rob
I'll note that in any actual, deployed, multi-antenna phase-coherent
system, calibrating the *radio* phase is only part of the
battle. If there's any significant (where "significant" is in terms
of wavelength) cable-length between the radio and the antennas,
there will be that part to compensate, and the longer the cable, the
more it is subject to differential heating, and thus
differential phase delays.
[Old man anecdote time]
Way back in the 1970s, a radio astronomy observatory in British Columbia
was setting up their synthesis array for observations
at 327MHz and 1420MHz. They had to come up with an exotic system
for measuring the phase delays of the cables in
"near real time" to apply compensating phase offsets where the
downconversions happened at the antennas (7 of them).
One cable in full Sun, the other in a shadow (at least for part of
its length), and you get differential phase offsets. Since the
array spanned 1km, cable issues HAD to be dealt with. Even though
the IF going down those cables was at 30MHz or so...
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