Measuring and controlling signal feedback between the transmit and

Abstract

A cellular communication system booster has plural narrowband and wideband channels each having down mixers, local oscillators, IF filters, and amplifiers for amplifying communication signals. On a narrowband channel preferably not in use, and preferably in a time-multiplexed manner, a reference signal of known signal strength is substituted at the output amplifier in lieu of a normal communications signal. When this amplified reference signal is broadcast at the output antenna a portion of the signal is feed back to the input antenna due to imperfect isolation between the antennas. This feedback signal is measured by a Radio Signal Strength Indicator (RSSI) circuit at the point of its receipt into the IF filter of the narrowband channel. The measured signal strength serves as a measurement of signal isolation/feedback between the antennas. A microprocessor receiving the measurement and knowledgeable of the reference signal strength controls the gain of the channel amplifier so as to maintain the feedback signal 10 dB below the reference signal, thereby preventing RF signal feedback oscillation. In different embodiments the reference signal may be derived from an injection oscillator or, alternatively, from an actual communication signal upon another communications channel. Isolation/feedback measurements are typically stepped across multiple channels spanning the entire communications frequency band, thereby providing measurement and appropriate control at all frequencies and operational conditions.