An RF loopback test circuit is adapted to be coupled to an antenna port of a digital radio, to determine whether the radio is functioning properly. A frequency generator outputs an auxiliary frequency corresponding to the sum or difference between the radio's transmit and receive frequencies. The auxiliary frequency is coupled to a Schottky diode mixer, which is also coupled to the input/output port. If the radio's transceiver section is operating properly, the sum or difference frequency output of the Schottky diode mixer will be passed by only the diplexer's narrowband receive path filter to the radio's receiver section. The mixer's sum frequency is blocked by the diplexer's narrowband filters. Therefore, if the receiver section provides an indication that it is receiving a sufficient signal level from the RF loopback test circuit, it can also be inferred that each of the transmitter and receiver sections is operating properly.