Performance limits of feedforward automatic gain control in mobile radio receivers

In the paper, two forms of feedforward automatic gain control circuit, i.e. lowpass and bandpass, are described and their performance limits discussed in respect of nonideal circuit parameters, receiver prefiltering decorrelation and propagation-medium time delay spread. The bandpass filter limited dynamics configuration is shown to have twice the fade suppression bandwidth of the lowpass filter limited dynamics form and is particularly suited for use in VHF and UHF full carrier AM and SSB mobile radio receivers. A series of field trials in which a bandpass limited dynamics feedforward automatic gain control circuit was fitted to the audio output of a 457 MHz SSB mobile radio system is described.