AGC has nothing to do with the S meter, its job is too level out the volume gain in the receiver, so that irrespective of received signal strength the audio output gain is relatively steady/unaffected within the range of the AGC.
if the AGC was linked to the S meter then all signals would read roughly the same on an S meter, irrespective of the actual signal strength being received at the antenna/radio front end.
Jazz I think your gonna have to change what you said about AGC having nothing to do with S Meter. A basic S meter works by measuring the agc voltage.
Automatic gain control - Wikipedia, the free encyclopedia
Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform — the sound amplitude, which correlates with loudness, is proportional to the radio signal amplitude, because the information content of the signal is carried by the changes of amplitude of the carrier wave. If the circuit were not fairly linear, the modulation could not be recovered with reasonable fidelity. However, the strength of the signal received will vary widely, depending on the power and distance of the transmitter, and signal path attenuation. The AGC circuit keeps the receiver's output level from fluctuating too much by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain an approximately constant average output level.
For a very weak signal, the AGC has no effect, allowing the receiver to operate at its maximum gain; as the signal increases, the AGC reduces the gain.
S Meter
You know, I was kind of disappointed that the good folks at Yaesu didn't do a better job on the S-meter in such an expensive radio, and otherwise fabulous radio. Obviously the
AGC-derived S-meter signal is digitized, since it can be read via the CAT port by the program S-Meter Lite.
W7AAZ's IF has an AGC threshold of -131 dBm, when referred to the RF input. That's about 23 dB below my FT1000MP's AGC threshold. So there just isn't anything available to drive my MP's S-meter below -108 dBm.
AGC and S-Mmeter
"Why have different AGC decay speeds?
AGC attempts to preserve the dynamic range of the input signal without overloading the receiver. Consider an SSB signal--the maximum gain possible before receiver overload is determined by the strength of the voice peaks, so the AGC must adjust the overall gain based on the peak signal.
Elecraft K3 AGC and S-Meter
The various RF stages that comprise a receiver (RF amp, mixers and IF) act as a unit. Each stage acts as a signal conditioner as the desired and undesired signals are amplified and filtered prior to detection.
With a properly designed receiver, increasing the gain in one unique area through modification ultimately affects the AGC'd stages since they are part of this entire loop. One typically mistakes a higher S meter reading after modification to mean 'more sensitivity'. This quite often is the receiver attempting to compensate through the AGC.
AGC Theory
Some commercial receivers either don't have an AGC (S) meter or their "M1" output will not properly drive a meter with a meaningfull scale. Most Amateur receivers do have an AGC meter, however typically give an "early" (generous) reading with weak signals. This is a waste of indication. FM receivers quite with signals, therefore, you can easily listen for these changes when checking performance. When the signal gets almost full-quiting is when you need a visual (meter) indication to observe signal strengh changes. This circuit will do just that in this version. U3 amplifies the receiver's AGC voltage, then with a strong signal will flaten out with no increase in output. This makes a handy logrimic voltage change, per RF input changes.
AGC Setup
Start by measuring the "M1" or AGC point of the receiver you are setting up. The circiut prefers to "see" around a tenth of a volt, DC, or less with no RF signal into the receiver. Adjust VR10 for this range. Then input a high RF signal into the receiver to cause a hard-limiting condition. Typically this would be in the -60 dbm range. Then adjust VR9 for a full scale meter reading. Then you can plot an AGC curve. If it's not a usable curve try different settings of the two adjustments just discribed.
It's a busy Saturday and 14070 +/- has over a dozen PSK signals on the waterfall. I select the TS-850's 500 Hz IF filter and now there's only 3 signals in the passband. I click on a faint one and begin to copy a partial exotic DX callsign when a strong domestic station comes on just 150 Hz away. The AGC kicks in and the DX signal disappears. So I turn off the AGC and cut back the RF gain a bit, and now I'm copying the DX again. If I make contact with him, what kind of "S" report should I give? My S-meter reads zero with the AGC off. It reads S-6 with the AGC on, but that's from the domestic station.
