Digital signal processing at receiver intermediate frequencies (IFs) is now commonplace, and is essential for effective demodulation of QPSK, QAM and OFDM signals. Even with simpler modulation schemes, there is a justification for replacing analogue demodulators with digital ones. The design of such systems involves consideration of sample rates, input ADC precision, and numerical conditioning of subsequent digital operations. In systems where narrowband filtering combined with nonlinear operations for demodulation is to be performed, the input dynamic range of the system can easily be reduced to well below the range implied by the precision of the input conversion. To counteract this reduction in dynamic range, automatic gain control can be implemented digitally, immediately following the input signal conversion. In the wideband digital transmission schemes used by broadcasters, sampling rates are high, precision is modest, and digital AGC gives an effective input dynamic range of 10 - 12 dB in a 6-bit system (typical of QPSK satellite broadcast ASIC demodulators). This paper discusses the design of digital AGC for narrowband receivers, implemented at low IFs, with low-cost programmable DSPs.
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