Thanks for putting the historical perspective on this Ron. Makes total sense when you see the evolution.
>From a new guy perspective, I've never used the S-meter to give my signal reports (I only operate CW for now) and I do use the AGC (RF gain all the way up and use the AF gain for volume). I issue my RST report based on what I hear. I agree with Fred, that by today's standard, an R is really the most important indication of how the QSO is received. But I do use the Tone aspect of something less than 9 when I hear bad chirping or several harmonics on the signal. I then usually back that up later in the QSO so that the op knows what I'm hearing (or let me know if something wrong on my end). I did go through the exercise of calibrating my S-meter with the XGen2, but then I went with the OP1 setting as my FL1 and the meter reads 2 bars while I'm in CW mode. (I read why that may be in the reflector archives). I simply use it as somewhat of a visual indication that the strength I perceive with my ears is really kicking my butt ;-) 73, Dave W8FGU > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:elecraft- > [EMAIL PROTECTED] On Behalf Of Ron D'Eau Claire > Sent: Monday, March 12, 2007 11:56 PM > To: elecraft@mailman.qth.net > Subject: RE: [Elecraft] AGC Independent S-Meter? > > 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: Elecraft@mailman.qth.net > 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 _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
