Dr Megacycle wrote: Well, I have late 1950s Hammarlund HQ170 whose S-meter works just fine with the AGC/AVC turned on or off. As far as I have been able to ascertain, the S-meter functions identically with or without the AGC/ AVC turned on. It was designed to do so.
--------------------------------- The HQ170 S-meter rectifies the AVC signal separately in one diode section of a 6BV8 (that's a tube, folks! <G>), but it will not provide the same readings with the AVC turned off! In fact, in the HQ170 manual says, "...the 'S' meter circuit is connected to the separate AVC diode section of V8 (6BV8) and gives an indication of all types of signals in all positions of the AVC, however the 'S' meter calibration is valid ONLY WITH AVC POSITIONS SLOW, MEDIUM OR FAST and not in OFF position, although it will indicate and may be usable in MANUAL position." (Page 9 of the HQ-170 Communications Receiver Instructions and Service Information manual. The emphasis is in the manual text.) Let's take a look at how an S-meter works, starting with some background. It all started long ago with the superheterodyne invented by Col. Armstrong. That allowed a lot of amplification in a receiver that was easy to tune and without the howls and squeals that, even today, are used to signify one is tuning in an "old time" receiver! That basic superhetrodyne format is still the standard today in almost all receivers, including those in the Elecraft rigs. With the amplification the superhetrodyne provided came a nuisance: when listening to a weak station, if a strong station came on frequency it'd blast your ears! For CW signals the solution was easy. One added a "hard limiter" to the audio channel. If a really strong signal was encountered without warning, it was clipped of at some preset maximum volume (below the threshold of pain, hopefully). Most operators used the limiter at all times in case a huge signal came on frequency. The problem with a hard limiter is that is clips off the signal - turns a sine wave into an almost-square wave. The fact that the end result sounded like a buzz-saw was of no consequence. The current interest in soft, pure, sine wave tones when listening to CW is a recent pursuit that has many old timers chuckling. Everyone listened to CW signals that sounded a lot harsher than the sidetone from the stock K2 and it was (and still is) "music to their ears". But phone transmissions were a whole different story. Distorting voice (or music) with a hard limiter was a serious problem. A means to prevent blasting listeners out of their chairs without distorting the audio was needed. Automatic Gain Control (AGC) a.k.a Automatic Volume Control (AVC) was the answer. AVC (or AGC) automatically controls the gain of some of the stages of RF amplification in the receiver to turn down the signal and avoid overload and "blasting". It works like this. Phone signals were Amplitude Modulated (AM). That is, they consist of a steady carrier with sidebands containing the audio modulation. Two rectifiers are used in the receiver. One recovers the audio from the sidebands and the other, the AGC detector, produces a d-c voltage proportional to the strength of the carrier. The d-c voltage it produces is used to control the amplification of the stages in the receiver ahead of the detector. The stronger the signal, the more voltage produced, the more voltage produced, the more the amplification of the stages ahead of the detector are turned down. That produced a much-reduced change in loudness in the speaker or phones when tuning from a weak to a loud signal. The AGC circuit quickly became standard in virtually every superhet used to receive AM phone signals, from the console radio in the living room of the 1920's and 30's to the car radio of the 1940's to today's radios. A huge range of signals could be received with only nominal changes in volume - something easily controlled with the audio gain control. At some point some smart guy (or gal) realized that the AVC voltage changes in proportion to the strength of the incoming signal. That's the whole idea of the AVC! The stronger the signal, the greater the voltage! If we measure that voltage, we can show on a meter the relative strengths of various signals. The "S-Meter" was born! But AGC or AVC was only useful for AM phone reception. For CW reception we needed a beat-frequency-oscillator (BFO). The BFO is almost on the same frequency as the signal at the detector in order to produce the audio beat frequency we hear. The BFO is a huge, locally-generated signal, compared to the CW signal. It was impossible to keep the BFO out of the AVC detector. The relatively huge BFO signal made the AVC system react as if it was tuned into a very strong signal at all times, and so the AVC turned the receiver gain to minimum and kept it there. So, for decades, superhetrodyne communications receivers had a switch to turn the AVC off for CW reception and we continued to use the manual RF gain control and a hard limiter to protect our ears. CW operators never looked at an "S-meter". But some tinkerers wanted to have AVC for CW too. That interest grew as AM was replaced by SSB. SSB, like CW, requires a strong local BFO signal, so even though it was "phone" the AVC in the receivers couldn't be used. The trick was to rectify a sample of the signal to see how strong it was without letting the BFO get into the AVC. Two ways were developed. One was to rectify a sample of the audio signal *after* the detector. That produced a d-c AVC voltage proportional to the signal strength. It was called, for obvious reasons, "Audio AVC" (or AGC). It did well for SSB but had a bothersome drawback for CW. Remember, the d-c AVC voltage is produced by simply rectifying a sample of the signal. Rectifying an audio tone of, say, 600 Hz, it takes much longer for the AVC voltage to develop than when rectifying, say, and I.F. of 4 MHz. That caused a slight delay in the "attack" or time to turn down the volume when a strong signal appeared on frequency, resulting in an annoying "pop" in the speaker or phones. One approach was to let the AVC voltage return to high gain only slowly after once detecting a strong signal, so that if it was a CW signal or an SSB signal with a pause in the speech, the gain would not return to full volume before the next code element or SSB word came through. That helped, but it meant the receiver was "deaf" to weak signals for a while after the strong signal was silent. Still, audio AVC is simple and effective and often used today. The Elecraft KX1, for example, uses audio AVC. The other approach was to sample the signal well before the detector and BFO, where the signal could be isolated and avoid the BFO sneaking in. One common way to do that was to have two mixers feeding two, separate I.F. amplifiers operating on two different frequencies: one was for the signal and the other for the AVC. That way, the AVC detector was tuned to a frequency far removed from the BFO so the BFO won't interfere with it. That allows the desired fast attack time since the AVC voltage is produced by rectifying a signal at radio frequencies instead of audio frequencies. The Elecraft K2 uses this approach. No matter how AVC voltage is developed, it's the AVC (or AGC) voltage that drives the S-meter. It's possible to use a receiver to measure signal strength that doesn't have AVC. We might rectify the audio output and look at it on a meter to see changes in the signal strength, or we might sample the I.F. and rectify it. But the usefulness of such readings without the extended dynamic range provided by an active AVC is very limited for on-air communications purposes, unless one operates the RF gain control manually. If we change the RF gain control, we lose all sense of calibration of the S-meter. That's why you don't see S-meters offered in communications receivers that continue to work when the AVC is turned off. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
