Hi David,
The TX inputs are of no importance once the DDS is disabled, the captured data will be whatever is on the air at the RX frequency.
The spectrum analyzer computes the frequency scale based on the sampling times of the blocks in the Simulink model. To get a correct scale what needs to be done is to set the 'Sample time' of any of the source blocks to match the RX_SAMPLING_FREQ setting and leave for the rest of the blocks the sample time value to -1.
Regards,
Andrei
