A transmitter for an RF communications system, that includes an auxiliary receiver for capturing transmit signal data for use in compensating/correcting transmit signal impairments (such as for DPD, QMC, LOL). The transmitter (such as Zero IF) includes analog chain elements that introduce transmit signal impairments (such as PA nonlinearities). The auxiliary receiver is configured to receive loopback transmit RF signals, and includes an RF direct sampling ADC to convert the loopback transmit RF signals to digital transmit RF signals. Digital down conversion circuitry is configured to downconvert the digital transmit RF signals to captured digital transmit baseband signals, and data capture circuitry is configured to generate the transmit signal data based on the captured digital transmit baseband signals.