Automatic gain control
An automatic gain control (abbreviated AGC , from English Automatic Gain Control ) is used in electronic devices to keep the output level of an amplifier constant, even if the amplitude of the incoming signal changes greatly. This avoids overdriving subsequent stages, facilitates further processing and improves the signal-to-noise ratio .
The automatic gain control is part of many radio receivers, especially those for receiving amplitude-modulated signals. With AM radios, it ensures that stations with different strengths are still reproduced at about the same volume.
function
Automatic gain control requires an actuator whose voltage transfer factor can be controlled with a direct voltage (the control voltage). With analog circuit technology, the control voltage is obtained by rectifying the signal voltage and then smoothing it with a low pass . The response time depends on the dimensioning of the low-pass filter. In digital receivers such as software defined radio the control signal for the AGC is usually digitally as the square of the peak factor , referred to as English peak-to-average power ratio (PAPR), won.
The actuator can be an amplifier , the gain of which can be controlled with a voltage. It is also known as a VCA ( Voltage Controlled Amplifier ). In the days of electron tubes , special regulating tubes were used for this purpose, the steepness of which depends on the operating point.
The actuator can also be an attenuator whose attenuation can be controlled with a voltage. In undemanding cases, semiconductor diodes can be used for this, the internal resistance of which is changed by means of direct current. Today, analog or digitally controllable circuits made of field effect transistors are used, which form a controllable voltage divider.
Application examples
- With sound recordings (especially with simple recorders and dictation machines) a level control ensures that level errors are avoided. In the case of tape recorders , this was particularly important in order to achieve the best possible signal-to-noise ratio while avoiding overloading of the magnetic tape.
- Amateur video recorders automatically level out the incoming signals by means of an automatic gain control (Note: the Macrovision copy protection makes use of this by inserting short bursts of very high levels into the signal to be protected. The automatic gain control regulates the level accordingly "up" and thus causes massive picture disturbances).
- AM receivers ( radios for long, medium and short wave as well as televisions) adjust the amplification of their intermediate frequency - in some cases also the preamplifier - so that a stable signal level results even with different reception field strengths after demodulation.
- Most video cameras use automatic gain controls in order to be able to adapt to different brightness levels instead of using a mechanical shutter .
- In optical transmission links using fiber optic cables, the optical levels are often adjusted with the aid of automatic gain control.
- Automatic gain controls are used in hearing aids in order to generate a gain behavior that improves speech intelligibility
- In radio, an AGC is often used to compensate for small modulation errors on the mixer, but also often in a more severe way in order to achieve the greatest possible loudness (see Loudness War ) of the modulation (see FM sound processing )
The AGC is also used, for example, in the low-frequency section of radio telephones and high-quality landline telephones and, thanks to the automatic leveling of the voice signal, ensures good intelligibility even with loud speech or ambient noise. One then speaks specifically of automatic volume control .
Cellular phones adapt their transmission power to local conditions and can thus increase the battery life.
Individual evidence
- ↑ Tony J. Rouphael: RF and Digital Signal Processing for Software-Defined Radio . 1st edition. Newnes, 2008, ISBN 978-0-7506-8210-7 .