This chapter provides the reader with an introduction to the theory of feed forward (FF) and feedback automatic gain control (AGC) systems, followed by design examples, noise analysis, and design approaches for practical FTTx an analog receivers. This is a key chapter and has a major importance when designing any amplitude control system for hybrid fiber coax (HFC) receivers.
The purpose of AGC is to increase the receiver's dynamic range when large signal levels are applied at the receiver input. Additionally, AGC should preserve and track input signal-to-noise ratio (SNR) as the signal gets into higher levels. That means that if the input signal at the receiver front end gets higher, the carrier-to-noise ratio (CNR) (same definition as SNR) is getting better. Hence, the requirement is to preserve, with minimum degradation, the input CNR. Since any signal processing system adds noise, the output CNR is reduced and the noise figure increases. The AGC role is to preserve a constant output when the input signal level is above the AGC threshold. Hence, it can be referred to as a kind of limiting circuit. AGC attenuator for that purpose can be an analog voltage-variable attenuator (VVA), a digital-controlled attenuator (DCA), or a variable-gain amplifier (VGA).
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