Automatic Gain Control in Broadcasting Systems

Cadence System Analysis

Key Takeaways

AGC is a closed-loop feedback circuit which helps to maintain a constant output, irrespective of the input variations.
The basic elements of AGC are a detector, a summing circuit for error calculation, a stable reference signal, and a gain controller element.
The requirements of AGC are a dynamic range of operation, low distortion, adjustable gain, accuracy, sensitivity, stability, and speed of operation.

Figure 1: AGC regulates the strength of audio signals of various radio stations.

Consider a radio receiver receiving signals I1 and I2 from two different radio stations AM1 and AM2, respectively. Let the strength of audio signal I1 be greater than I2. If both stations are the same distance to the receiver, the broadcast volume of station AM1 should be greater than station AM2. However, if they are at different distances, their signals will experience different attenuations and the received signal strength will be a function of both volume (amplitude) and distance (attenuation).

Automatic gain control ❲AGC❳ is a closed-loop feedback circuit present in radio receivers which helps to maintain a constant output, irrespective of the input variations. In communication systems, the inclusion of AGC regulates the system output to a constant value, even if the input voltage decreases or increases.

Working Principle of Automatic Gain Control

In any closed-loop feedback system, there will be a relationship between the input signal and the actual output signal. If the input-output signal relationship is linear, then output increases linearly with input and vice-versa. If the input variations are reflecting on the output signals in a non-linear manner, they belong to a non-linear closed-loop system. Regardless of the input-output relationship, AGC helps to maintain a desired constant output in the system.

The closed-loop AGC requires a reference signal, which is the desired output. The closed-loop feedback network continuously monitors the output, and according to its variations from the reference signal, the gain is calculated to maintain the output to the constant value.

Once again, let’s use a radio receiver as an example. An amplifier receiving the input I1 gives an output O1, and shares a linear relationship with the input. Let the output O1 be greater than the reference signal, leaving the error to be negative. When the error is negative, the gain is reduced to regulate the output to a predefined constant value. For the output O2 corresponding to input I2, the error becomes positive and the gain is increased to maintain the constant output. AGC keeps the output constant by controlling the gain according to the input variations.

AGC is mainly applied in communication systems where there is a frequent need for varying gains. The basic elements of AGC are a detector, a summing circuit for error calculation, a stable reference signal, and a gain controller element. Usually, the gain controller elements used are Variable Gain Amplifiers ❲VGA❳ which control the gain through electronic means. The VGA offers controllable gain with wide frequency operations ranging from kiloHertz to megaHertz.

Requirements of an AGC Circuit

We have been discussing AGC in the context of audio communication systems. Applying AGC in any mass communication system requires the following features:

Dynamic range of operation-The AGC circuit is expected to amplify signals of high to low amplitude within a wide frequency bandwidth. The dynamic range of operation is the attribute that supports the circuit flexibility of AGC.
Low distortion-The AGC circuit should not add harmonics or distortions into the output signal. If the gain is controlled at the expense of distortions, then the AGC is not meeting the standards. The noise reduction property of AGC can be considered to be an added value which leads to improved performance.
Adjustable gain-Even though the AGC controls the gain of the system with respect to the input signal variations, it is associated with a minimum and maximum gain limit. There should be an option to adjust gain limits upon input and output range variations.
Accuracy-The accuracy of the AGC circuit solely depends on the feedback loop. If the feedback loop has an error, then the purpose of AGC will not suffice. Incorporating an error detector can increase the accuracy of the AGC circuit.
Sensitivity-The AGC circuit parameters may get affected by internal disturbances, but this should not reflect on the desired output of the system. The AGC should be insensitive to such internal disturbances, but sensitive to any small variations in the input signal.
Stability-If the output of an AGC circuit is zero for zero input and the output is bounded for bounded input, then such AGC circuits are bounded input-bounded output (BIBO) stable.
Speed of operation-The speed of the AGC is the time required to produce a stable output. Generally, a high-speed AGC is preferred, but sometimes low-speed AGCs are also required depending on the applications.

Transfer Function of AGC

Let's discuss the input-output characteristics of an AGC circuit. For low values of the input, the AGC is non-operative, and the output varies linearly with input. When the input reaches ‘threshold 1’, otherwise called knee-point, AGC is enabled and starts functioning. The knee point of the AGC should be kept as low as possible to improve performance. An ideal AGC is supposed to maintain a constant output for any input signal between the ’threshold 1’ and ‘threshold 2’. However, a flat section is not ideal for AGC operation. The overall AGC loop gain defines the slope of the characteristics between ’threshold 1’ and ‘threshold 2’, and is called AGC slope. The ideal AGC slope should be between 6 dB and 10 dB. When the input reaches ‘threshold 2’, the AGC moves back to the disabled state to prevent stability issues.

In communication systems, the signal strength needs to be maintained at an acceptable level for ensuring the reliability, integrity, and quality of the system. Despite the variety of input variations, a steady constant output is expected at the receiving end. AGC takes the responsibility of regulating the output within the acceptable range by varying its gain. Without AGC, broadcasting systems would be incomplete.

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