MATLAB: Changing the LO frequency of ad9361_RX block in ad9361_QPSK testbench model causes unexpected behavior

ad9361plotqpskRF Blocksetspectrumtestbench

I am using the 'ad9361_QPSK' testbench model in the AD9361 Testbench suite. When I modify the LO frequency and the center frequency in the ad9361 block from 2 GHz to 1 GHz, I see no change in the Spectrum RX plot. Please note that the LO frequency of the ad9361_TX block has not been changed from 2 GHz.
I am now wondering why there is no change to the Spectrum RX plot. Does no change imply that the ad9361 hardware is receiving the signal transmitted at a different frequency? Is this behavior expected?

Best Answer

If you change both the LO and the RF center frequency in the ad9361_RX block, you will notice very minor differences. The RF center frequency of the RX should the same as the TX LO center frequency. The input signal to the RX is a complex equivalent baseband signal, and the RF center frequency of the RX defines the RF center frequency of the incoming signal.
The LO RX center frequency should be close (within the simulation bandwidth) to the RF center frequency. This is described in the support package documentation. To access this, please type the following command at the Command Window -
open(ad93xx_getdoc)
If you set the TX LO frequency at 2 GHz, the RX RF frequency at 2GHz, and the LO center frequency at 1GHz, results will be incorrect.
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