Abstract: In a base station of a mobile cellular telephone system of the type having time division multiple access (TDMA) channels, a radio frequency test loop is established between a predetermined downlink channel of a base station transmitter and a predetermined uplink channel of a base station receiver. The test loop is in real time, thereby eliminating the time offset of the radio channels used for communications. The radio frequency test loop includes a circuit for transposing the frequency of the carrier signal from the transmitter to a frequency that can be received by the receiver. The signal having the transposed frequency is used in tests for determining values such as bit error rate (BER), transmission signal level, or reception sensitivity.