BladeRF A4 - IQ Values flip when changing distance between two RX antennas

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jason_ca
Posts: 1
Joined: Mon Jul 12, 2021 10:47 am

BladeRF A4 - IQ Values flip when changing distance between two RX antennas

Post by jason_ca »

Hey all,

I'm trying to receive and process signals received from a signal generator with both RX antennas using the bladeRF-cli. I've noticed that when I move the antennas apart or closer together, at a certain point the I/Q values of RX2 swap places relative to RX1's I/Q values (which has been stationary the entire time) and messes with the processing I'm doing to the signal afterwards. As shown in the image below, at around 3.6 million samples in, the pattern changes from

RX2_Q (purple), RX1_Q (orange), RX2_I (yellow), RX1_I (blue)
to
RX2_I (yellow), RX1_Q (orange), RX1_I (blue), RX2_Q (purple)

where the expected behavior was for the same pattern to show, only with RX2's pair of I/Q's slowly slowly shifting left relative to RX1's pair of I/Q's values as can be seen in the earlier samples of the same picture (where you can see the yellow and purple curves starting slightly to the right of the orange and blue, but slowly shifting to the orange and blue's left each period).

Image
Link to clearer image

My question is, is there some sort of setting or switch inside the bladeRF that triggers it to change the way it receives samples? Is there a way to disable this behavior?

The settings I am using for the bladeRF-cli are:
frequency rx: 915MHz
bandwidth rx: 1.5MHz
samplerate rx: 3MHz
AGC: on
rx config channels: 1,2
rx config n: 10M
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