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Measuring phase shift from cable length

Posted: Sun May 02, 2021 2:38 pm
by carson.teale
Hi,
I'm trying to measure the phase shift that a carrier acquires when propagating through a length of cable. I have the bladeRF x40. I simply connect a 1 m sma cable from the TX port to the RX port. I'm using a Matlab script to configure the bladeRF using cli commands. My Tx samples are just ones, so that I just transmit a tone at the carrier frequency at 900 MHz. Then I expect to be able to obtain the phase acquired from propagation through the cable just by atan(imag(rx_sig)/real(rx_sig)). But when I try this, the measured phase is not consistent. It seems to be random between different measurements. Is there some fundamental reason why this won't work? Or am I just not implementing it incorrectly? I don't get why the phase measurement is not repeatable.
I should be able to use this phase measurement along with the permittivity of the cable I'm using, to back out the length of the cable (modulo the wavelength).