AGC
Understanding AGC
AGC is essential in every receiver that must handle signals of widely varying strength. Without AGC, a strong signal would saturate the receiver (causing distortion) while a weak signal would be too small for accurate demodulation. AGC adapts the gain in real time to keep the signal level optimal.
AGC Components
- Detector: Measures the output signal level (power or envelope).
- Control loop: Compares measured level to desired level, generates error signal.
- Variable gain element: PIN diode attenuator, variable-gain amplifier (VGA), or digital step attenuator that adjusts gain.
AGC Parameters
- Dynamic range: Range of input signal levels over which AGC maintains constant output. Typically 60-100 dB.
- Attack time: How quickly AGC responds to a sudden signal increase. Fast attack prevents ADC overload.
- Release time: How quickly gain increases when signal decreases. Slower release prevents pumping on modulated signals.
Frequently Asked Questions
What is AGC?
AGC automatically adjusts receiver gain to maintain constant output level regardless of input signal strength. It enables the receiver to handle both very weak and very strong signals without saturation or loss of information.
Why is AGC needed?
Signal strength varies enormously due to distance, fading, and propagation conditions. A satellite signal can vary by 20 dB due to rain; a cellular signal varies by 80+ dB as the user moves. AGC keeps the signal level constant at the demodulator input despite these variations.
What determines AGC speed?
AGC speed is set by the control loop bandwidth. Fast AGC (microsecond response) is needed for radar and burst communications. Slow AGC (millisecond response) is used for broadcast receivers. The speed must be fast enough to track fading but slow enough to not track the modulation.