This article continues on from Riding the RF Gain control – part 1 and explores the operating advice when applied to the next generation of receivers.
Conventional superheterodyne communications receiver designed for SSB telephony.
Move on to the generation of superheterodyne communications receivers that incorporated a demodulator designed for SSB, and an AGC system that protected the receiver intermediate and later stages (including the demodulator) from overload for even very strong signals, and you have a receiver that broadly, worked automatically from the weakest to very strong signals with RF Gain control set to maximum and little need to adjust AF gain.
The AGC system's most important function is to protect the receiver intermediate and later stages (including the demodulator) from overload. Levelling the AF output is a lesser priority.
These receivers were not perfect, the AGC was derived from the signal, and SSB suppressed carrier does not contain a consistent component from which AGC can be derived, so it is derived from the signal using a pseudo peak detector. The dynamics means that the AGC leaks away between syllables, and there is small overload during the attack time of the next syllable as it charges again, but with appropriate time constants, the distortion is small. (Some commercial HF links solved this problem by transmitting a reduced carrier typically at -26dBc from which AGC was derived.)
These receivers were subject to IMD in the front end, and operating them with no more preamplification than necessary improved handling of strong out of band signals, and in extreme interference cases inserting a front end attenuator improved IMD performance. The working configuration on low HF where external noise dominates the receiver is to improve IMD response even at the expense of Noise Figure.
These receivers were not perfect, but by and large, good implementations worked very well hands off most of the time.
AGC reduces gain ahead of the demodulator, typically in both RF and IF stages, and has the effect of increasing the receiver Noise Figure. Increasing Noise Figure degrades S/N ratio at the receiver output.
The diagram above from (Terman 1955) illustrates the behaviour for AVC, the term that was used with AM receivers at the time and superseded by AGC for SSB receivers.
It is common practice in these type of receivers to delay the onset of AGC action until the S/N is quite good, typically 20dB or more. So, there is no AGC action until the input is 20dB or so above the receiver internal noise, and since the S-meter often displays the AGC control voltage, there will be no S-meter deflection until that threshold is reached.
In the absence of significant front end IMD, using the RF Gain control to reduce gain below that which the AGC would cause simply increases receiver Noise Figure and decreases S/N ratio.
Switching the AGC off on receivers with that capability simply disables the mechanism provided to prevent back end overload with no warning of overload (other than S/N degradation). (More on that in part 3.)
In the next part, we will explore a superheterodyne receiver with DSP demodulation.
- Terman, Frederick. 1955. Electronic and Radio Engineering – 4th ed. New York: McGraw-Hill.