Fast AGC for TDMA radio systems

A circuit for providing automatic gain control in a radiotelephone system which receives signals in a succession of time slots which system includes at least one random access time slot in a broadcast frame also including traffic time slots, the circuit comprising:

• means for receiving an intermediate frequency input signal in the access time slot;
• means for detecting the amplitude of the input signal;
• means for converting the amplitude of the detected signal to a digital representative;
• means coupled to the converting means for providing an automatic gain control signal in said access time slot which is proportional to the amplitude of the detected signal only after the said amplitude exceeds a first threshold level and which is kept at a first constant value until the end of the access time slot unless the said amplitude exceeds a second threshold level which is greater than the first threshold level and, in that event, the gain control signal is held at a second constant value greater than the first constant value until the end of the access time slot;
• means for coupling the automatic gain control signal to at least one attenuator in the path of the immediate input frequency signal so as to adjust the gain of that signal in a feed forward loop;
• means for comparing a second automatic gain control signal derived from an input signal from a subsequent traffic time slot with the automatic gain control signal value derived from the access time slot, and substituting the second automatic gain control signal in the event that the difference between the two automatic gain control signal values falls outside a predetermined range.