Receiver Design - AGC


Automatic Gain Control (AGC) is a mechanism to make the audio output of a receiver remain relatively constant when the incoming signal level changes by a large amount. Historically AGC was called Automatic Volume Control (AVC).

In specification terms this can mean a maximum change of output of typically 3dB (or doubling the audio output power) for an input signal change of 80-100dB or 100,000 times and this can take a significant design effort to implement.

In principle, an AGC system takes the rectified output from the signal detector and uses it to control the gain of the preceding RF and IF stages to keep the output from the detector as constant as is possible. The higher the signal level the higher is the amount of AGC. AGC can be applied to a TRF receiver but they are not in common use in amateur or professional signal system as they are/were primarily used for the reception of AM broadcast signals so this section will only consider the application of AGC to higher performance analog superhet receivers.

The designer must take into account the different modes of operation in modern communications systems which typically would include Amplitude Modulation (AM), Single Sideband (SSB), Frequency Modulation (FM), Morse or Continuous Wave (CW) and various forms of data transmissions. An AGC system is effectively a closed servo loop so the timing delays within the loop must be analysed and taken into account for correct operation - in the worst case AGC oscillation can occur.

AM presents a signal to the detector which when detected contains two parts - a DC component proportional to the carrier level and an AC component proportional to the modulation depth. To prevent the modulation affecting the AGC operation the rise and fall time constant of the AGC circuit must be significantly longer than the period of the lowest modulation frequency - 30Hz for broadcast applications and 300Hz for communications applications.

An FM only receiver does not require AGC as it uses limiters in the IF strip but where it is an addition to a multimode receiver the AGC would be that which was used on AM.

CW and SSB signals both change in amplitude as part of their normal operation so the AGC circuit time constant must have a very fast rise time and a slow fall time. More elegant designs (hang systems) have a period of constant gain before the fall time commences which produces a better overall listening experience.

Early broadcast receivers and HF communication receivers like the RCA AR88 and National HRO were primarily designed for AM reception and used a very simple AGC system with a single control line applied to all amplifier stages. This is easy to implement but had two major disadvantages - it significantly restricts the maximum signal to noise ratio and is not optimised for CW (SSB was not in common HF use until well after the end of WW II).

Broadcast receivers were generally low cost designs and used a single IF amplifier and no RF stage so to get an adequate range of control AGC had to also be applied to the mixer stage which does nothing for dynamic range and frequency stability.

The initial solution to the signal to noise problem in communication receivers was to delay the application of AGC to the RF stage until the signal level was considerably above the level at which AGC was applied to the IF stages. This improved matters somewhat but the ideal solution is to apply AGC progressively starting with the last IF amplifier and moving back stage by stage as the signal level increases until the RF stage is reached. This dramatically improves the overall signal to noise ratio but does impose significant additional dynamic range requirements on the IF amplifier stages as a result of higher signal levels.

Applying AGC to high signal level IF amplifiers can result in significant distortion so in valve based communication receivers a better technique was to reduce the amount of AGC applied to the last IF amplifier. Even better was to use a separate IF amplifier stage to drive the AGC detector with no AGC applied to that stage.

The designer must also take into account the time that it takes for the received signal to work its way through the receiver stages otherwise low frequency oscillation (motor boating) can take place in the AGC system at various signal levels. The narrower the overall IF bandwidth the longer is the signal transit time and narrow band filters for SSB or CW make the delays even longer so the design problem is not simple.

One solution that has been adopted in current high performance communication receivers is to use a wide band IF system after the mode filters where most of the AGC is progressively carried out. The IF signal is split into two components, one going to the AGC detector so the AGC response can be much faster, the other going to the mode detectors via roofing filters for each mode which optimise the signal to noise ratio by removing the wideband noise.

These additional roofing filters have no effect on the AGC signal. If additional gain control is required it can be applied to any stages prior to the mode filters but this will require additional timing analysis for each mode of operation. However, modern receiver design tell us that the gain prior to the mode filters should be the absolute minimum needed to achieve the required sensitivity.

Due to the charge time of long time constant circuits, it is possible to use two time constant networks in series to drive the AGC line - the normal long time constant and a much faster but shorter time constant that responds to signal changes while the longer time constant circuit is catching up. This results in a much faster initial reaction to increases in signal level on CW and SSB.