A test method in an RF communication system. In the communication system, at least one RF communication channel lies about a known carrier frequency in an RF communication band. According to the method, a diagnostic signal is introduced into a circuit to be tested, such as a linear power amplifier, the diagnostic signal having an RF diagnostic frequency lying within the communication band. The diagnostic signal is down converted to an intermediate frequency for diagnostic purposes so that DSP is simplified. The diagnostic frequency is selected based on the known carrier frequency of the communication channel so that the communication channel is not down converted as an image band with the diagnostic signal.