Digital control of transmitter power in a cellular mobile telephone

To satisfy OST Bulletin No. 53 a cellular mobile telephone transmitter must provide eight distinct output power levels in response to digital commands from the Cell Site. During the transmitter-off condition RF carrier leakage must not exceed -60 dBm. Frequency pulling of the transmit synthesizer during power switching must be minimized to avoid clicks in the received audio. Prior power control art has used digitally controlled RF attenuators which are expensive and wasteful of control current. A minimum of three RF switches and three RF pads (4, 8 and 16 dB) are necessary to reduce the transmitter power 28 dB in 4 dB steps. This paper describes a new type of feedback power control circuit based upon an RF detector biased with digitally controlled DC threshholds. This scheme eliminates expensive RF hardware and results in a control circuit compatible with hybrid integrated circuit technology. Two forms of the threshold detector are described and their performance is compared over a temperature range of-20°C to +50°C. Detection results are compared to theoretical models and the application of this novel threshold control to a 1 watt transmitter is described.