Automatic Gain Control (AGC)

The finite interval required for the AGC loop to reduce gain in response to a sudden increase in input signal power. It is typically defined as the time between the application of a step-function RF burst and the moment the output amplitude settles within a specified tolerance (often 90% of its final steady-state value).

• Fast Attack: Prevents ADC saturation from impulsive interference but may distort the amplitude envelope of the desired signal.
• Slow Attack: Preserves amplitude modulation (AM) but risks clipping the ADC during the transient.
• Typical Values: Ranges from < 1 microsecond for frequency-hopping systems to several milliseconds for broadcast receivers.

The interval required for the AGC loop to increase gain after a drop in input signal power. This parameter prevents the receiver from becoming deaf during short fades while avoiding noise amplification during deep fades.

• Hang AGC: A common variant where the gain reduction is held constant for a brief period before decaying, preventing gain pumping on voice or pulsed signals.
• Trade-off: A decay that is too fast causes audible 'breathing' artifacts; a decay that is too slow misses the start of a new transmission.
• Implementation: Often set significantly longer than the attack time to maintain stability.

The target amplitude the AGC loop strives to maintain at the detector output. This level is set to optimally load the ADC, balancing quantization noise against the risk of clipping.

• Threshold (Knee): The input power level below which the AGC stops applying gain reduction. This prevents the amplifier from applying maximum gain to background noise.
• Back-off: The headroom reserved between the reference level and the ADC full-scale input to accommodate the Peak-to-Average Power Ratio (PAPR) of modern modulation schemes like OFDM.
• Precision: A poorly set reference level directly degrades the Effective Number of Bits (ENOB) of the digitized signal.

The frequency response of the closed-loop feedback system, determining how quickly the AGC can track amplitude variations without oscillating.

• Narrow Bandwidth: Filters out rapid amplitude fluctuations, ideal for constant-envelope modulations like GMSK, but fails to track fast fades.
• Wide Bandwidth: Tracks rapid fades but can introduce gain ripple and distort the in-band signal envelope.
• Stability Margin: The phase margin of the loop filter must be sufficient to prevent the AGC from acting as an oscillator, which would catastrophically modulate the signal with a spurious tone.

The total range of input signal powers over which the AGC can maintain a usable output. It is the ratio between the minimum detectable signal and the maximum input power before saturation.

• Gain Control Range: The total attenuation (in dB) the variable gain amplifier (VGA) can provide.
• Instantaneous Dynamic Range: The ability to handle a strong interferer adjacent to a weak desired signal without desensitization.
• Noise Figure Impact: Inserting a VGA or digital step attenuator (DSA) before the low-noise amplifier (LNA) directly degrades the receiver's sensitivity.

The mathematical operation used to measure the signal envelope, which dictates the AGC's response to different waveforms.

• Peak Detector: Responds to the instantaneous peak voltage, essential for preventing ADC hard clipping but sensitive to impulsive noise.
• RMS Detector: Responds to the average power of the waveform. This is the preferred method for modern communication signals as it accurately reflects the true signal power regardless of the PAPR.
• Log Detector: Provides a linear-in-dB output, simplifying the loop filter design for wide dynamic range applications.
• Post-Detection Filtering: Averaging the detector output prevents the AGC from reacting to individual symbol transitions.