The present invention is generally related to radiotelephones, and more particularly to power control circuitry for a time-division multiple-access (TDMA) radio frequency (RF) transmitter that may be advantageously used in digital cellular telephones.
Analog cellular telephones currently are continuously transmitting during a telephone call. RF transmitters of such analog cellular telephones are frequency modulated with voice signals and continuously operated at one of eight different power levels depending the quality of the RF signal received therefrom by the cellular system base station. The output power of such RF transmitters are maintained at the desired power level by conventional automatic output power control circuitry, such as, for example, the circuitry shown and described in U.S. Pat. No. 4,523,155. However, such conventional output power control circuitry is inadequate for TDMA cellular systems where it is necessary to rapidly pulse the RF transmitter on for 6.67 milliseconds and off 13.33 milliseconds every 20 milliseconds. Furthermore, it is also necessary that the RF transmitter output follow the envelope of the modulation, which has frequency components in excess of 12.15 KHz. These problems may be solved in part by output power control circuitry employing variable attenuators which attenuate the RF input to the transmitter power amplifier. An example of such output power control circuitry employing a variable attenuator is shown and described in U.S. Pat. No. 4,803,440. However, when such output power control circuitry employing a variable attenuator is operated at cellular transmitter frequencies ranging from 824 MHz to 849 MHz, substantial degradation in performance occurs due to feedthru, temperature stability, manufacturing variations, and poor linearity of the variable attenuator. For the foregoing reasons, there is a need for improved power control circuitry for precisely and reliably maintaining the maintaining RF output signal from a TDMA RF signal transmitter at one of a plurality of power levels selected by the level control signals.