AGC Independent S-Meter?

[Darrell Bellerive-2]

As I typically leave the AF gain at maximum and use the RF gain control to set
the listening level (Better signal to noise ratio.) the S-meter is not too
useful. Also with the AGC turned off the S-meter is not operational.

Anyone have any ideas for an AGC independent S-meter?

I was thinking perhaps use some sort of high impedance RF voltage detector
between D34 and T7 on the RF board. Then amplify the voltage to the level and
range required at the S-meter voltage connection (pin RA1 of the MCU), so the
existing meter could still be used. Of course the connection from the MCU to
the AGC line would have to be disconnected.

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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[Gregg R. Lengling]

If you actually duplicate the AGC circuitry in the K2 for the S-meter would
probably be a simple move.  S-meter readings are okay but I don't trust
reports from others because very few people have calibrated S-meters and
even if they are they become dependant on RF Gain Settings, Preamps and
attenuators.  They are what they are a Relative Indicator and nothing more
when it comes to most Amateur Radio operation.  I usually leave the RF gain
at full unless I'm on the TopBand and just use Xtal filters and the DSP
module to give me a clean received signal.

Anyhow on most Contests and other things you always get a 599 as everyone is
too intent on making contacts to actually give you a real report.


-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Darrell Bellerive
Sent: Sunday, March 11, 2007 12:23 PM
To: [hidden email]
Subject: [Elecraft] AGC Independent S-Meter?

As I typically leave the AF gain at maximum and use the RF gain control to
set
the listening level (Better signal to noise ratio.) the S-meter is not too
useful. Also with the AGC turned off the S-meter is not operational.

Anyone have any ideas for an AGC independent S-meter?

I was thinking perhaps use some sort of high impedance RF voltage detector
between D34 and T7 on the RF board. Then amplify the voltage to the level
and
range required at the S-meter voltage connection (pin RA1 of the MCU), so
the
existing meter could still be used. Of course the connection from the MCU to

the AGC line would have to be disconnected.

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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[Don Wilhelm-3]

Darrell,

Certainly a meter could be devised to indicate the strength of the signals
making it through the IF filter - you have the correct points in mind - all
you have to do is tap in at the input of the IF amplifier and then amplify
the signal(s) and drive whatever circuits you want (even a duplicate of the
stock IF amp and  AGC/S-meter circuit).

The biggest problem to solve is how to tap into that signal path without
degrading the signal present in the receiver - that can be accomplished as
long as you pay attention to the impedances.

You may want to look into the Analog Devices AD8307 log amplifier rather
than duplicating the IF amp, especially if you want to drive an external
meter - the '8307 has a log function built in so you can have an S-meter log
response with a linear meter.

73,
Don W3FPR

> -----Original Message-----
>
> As I typically leave the AF gain at maximum and use the RF gain
> control to set
> the listening level (Better signal to noise ratio.) the S-meter
> is not too
> useful. Also with the AGC turned off the S-meter is not operational.
>
> Anyone have any ideas for an AGC independent S-meter?
>
> I was thinking perhaps use some sort of high impedance RF voltage
> detector
> between D34 and T7 on the RF board. Then amplify the voltage to
> the level and
> range required at the S-meter voltage connection (pin RA1 of the
> MCU), so the
> existing meter could still be used. Of course the connection from
> the MCU to
> the AGC line would have to be disconnected.
>
> --
> Darrell Bellerive
> Amateur Radio Stations VA7TO and VE7CLA
> Grand Forks, British Columbia, Canada
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[N2EY]

In a message dated 3/11/07 12:24:04 PM Eastern Standard Time, [hidden email]
writes:


> Anyone have any ideas for an AGC independent S-meter?

Here's what I've used for 40+ years:


Ears

--

I have never really understood the fascination with AGC and S-meters in ham
radio for CW. For AM voice, sure, and maybe SSB when the frequency is clear,
but when it comes to typical CW operation I don't use either.

I could see where a certain amount of delayed AGC would be useful as an
eardrum-saver, but the same effect can be had by a simple limiter.

IMHO, anyway.

73 de Jim, N2EY


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[Sandy W5TVW]

Circuitrywise, an "AGC/AVC free" S-meter is an oxymoron!  Can't have one
without the other.

I pay little attention to the "S" meter on my K1.  The old "ear" still one
of the best indicators anyway.  Such things as a 199 signal is as possible
as having a 519 signal!

I guess it strokes the ego of a 'phone operator to to get a 40 over 9
report.  What difference does it make if the signal report is 5 by 9 + 40 db
or 5 by 2?

Most CW ops I know don't depend or rely much on "S" meters anyway.  (Back to
"ears" again!)
73,
Sandy W5TVW
----- Original Message -----
From: <[hidden email]>
To: <[hidden email]>
Sent: Sunday, March 11, 2007 4:37 PM
Subject: Re: [Elecraft] AGC Independent S-Meter?

> In a message dated 3/11/07 12:24:04 PM Eastern Standard Time,
> [hidden email]
> writes:
>
>
>> Anyone have any ideas for an AGC independent S-meter?
>
> Here's what I've used for 40+ years:
>
>
> Ears
>
> --
>
> I have never really understood the fascination with AGC and S-meters in
> ham
> radio for CW. For AM voice, sure, and maybe SSB when the frequency is
> clear,
> but when it comes to typical CW operation I don't use either.
>
> I could see where a certain amount of delayed AGC would be useful as an
> eardrum-saver, but the same effect can be had by a simple limiter.
>
> IMHO, anyway.
>
> 73 de Jim, N2EY
>
>
> **************************************
> AOL now offers free
> email to everyone.  Find out more about what's free from AOL at
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>
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> Checked by AVG Free Edition.
> Version: 7.5.446 / Virus Database: 268.18.8/718 - Release Date: 3/11/2007
> 9:27 AM
>
>

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[Darrell Bellerive-2]

On March 11, 2007 04:16 pm, Sandy W5TVW wrote:
> Circuitrywise, an "AGC/AVC free" S-meter is an oxymoron!  Can't have one
> without the other.

Why not?

--
Darrell Bellerive
Amateur Radio Stations VA7TO and VE7CLA
Grand Forks, British Columbia, Canada
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[Don Wilhelm-3]

Sandy,

As far as I know, there is no direct relationship between the strength of a
signal and receiver AGC.

My AD8307 power meter can measure signal strengths directly off an antenna
if called upon to do that, it goes down to -80 dBm (about S-8) with no other
amplification.  This is strictly a power meter, no AGC implied or
implemented - the output is the base 10 logarithm of the input signal.

All one must do is measure the strength of the received signal to drive an
S-meter - but it must be calibrated if it is used as an absolute measurement
device.

Yes, the implementation in most receivers derive the AGC voltage from the
same signal strength measurement circuits, so things get a bit mixed up, and
often appear as a 'chicken and egg' situation.

73,
Don W3FPR

> -----Original Message-----
>
> Circuitrywise, an "AGC/AVC free" S-meter is an oxymoron!  Can't have one
> without the other.
> (snip)
> 73,
> Sandy W5TVW
>
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[Sandy W5TVW]

What I was referring to is the common circuits used in most receivers of
olden days.  (Tube era)  Most receivers depended on the AGC level for signal
strength indication.  At best a large approximation and some wishful
thinking.  Not only do 50 microvolt (if that's your "standard") S-9 levels
vary from band to band, but also at the high and low ends of the bands as
well.
I have dealt with Field strength meters where the "S meter" actually
indicates microvolts per meter, and they go thru some rather careful
procedures to make sure the receiver's gain is the same wherever the
measurements are made.  I've never used the H-P instruments that are popular
on the 500 Khz experimental band at present.
Anyway, the short of it is one can't depend on an "S meter" on a
'communications receiver' to really be accurate.  No matter who makes the
receiver generally.  Unless it is some special purpose device especially
made for measuring the input levels.
One of the silliest ideas I have ever seen was when Bill Halligan's folks
put that HUGE 4-1/2" "S" meter on the S-76 when they came out with it.  I
personally thought that they'd have done better if they had just jeft the
loudspeaker there and offered an external "S" meter.  The S-76 was a big
improvement over the old S-40, but why that big meter?  Also calibrated in
microvolts by the way!

73,
Sandy W5TVW
----- Original Message -----
From: "Don Wilhelm" <[hidden email]>
To: "Sandy W5TVW" <[hidden email]>; <[hidden email]>;
<[hidden email]>
Sent: Sunday, March 11, 2007 11:41 PM
Subject: RE: [Elecraft] AGC Independent S-Meter?

> Sandy,
>
> As far as I know, there is no direct relationship between the strength of
> a
> signal and receiver AGC.
>
> My AD8307 power meter can measure signal strengths directly off an antenna
> if called upon to do that, it goes down to -80 dBm (about S-8) with no
> other
> amplification.  This is strictly a power meter, no AGC implied or
> implemented - the output is the base 10 logarithm of the input signal.
>
> All one must do is measure the strength of the received signal to drive an
> S-meter - but it must be calibrated if it is used as an absolute
> measurement
> device.
>
> Yes, the implementation in most receivers derive the AGC voltage from the
> same signal strength measurement circuits, so things get a bit mixed up,
> and
> often appear as a 'chicken and egg' situation.
>
> 73,
> Don W3FPR
>
>> -----Original Message-----
>>
>> Circuitrywise, an "AGC/AVC free" S-meter is an oxymoron!  Can't have one
>> without the other.
>> (snip)
>> 73,
>> Sandy W5TVW
>>
> --
> No virus found in this outgoing message.
> Checked by AVG Free Edition.
> Version: 7.5.446 / Virus Database: 268.18.8/718 - Release Date: 3/11/2007
> 9:27 AM
>
>
>
> --
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.5.446 / Virus Database: 268.18.8/718 - Release Date: 3/11/2007
> 9:27 AM
>
>

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[James Duffey]

Sandy - You wrote:

"Circuitry wise, an "AGC/AVC free" S-meter is an oxymoron!  Can't  
have one
without the other."

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 QST review at the time  
commented on this feature.

So it is possible to build a receiver with an S-meter that functions  
with the AGC on or off. It was done nearly 50 years ago. I am not  
sure why receiver designers have not incorporated this useful feature  
into more modern rigs.

Now I am not a fan of AGC, in fact I think the most useful feature of  
a receiver with AGC is the ability to turn it off. :^)= - Dr.  
Megacycle KK6MC/5
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[Ron D'Eau Claire-2]

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


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[Rick Dettinger-2]

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.

Ron AC7AC
============================================

I wonder if it is possible to adjust the AGC on the K2 to only work on
strong signals, to act as a hearing protection limiter only, with little
action on all other signals.  I understand this would work for CW only rigs.
I like to use my K2 with AGC off, but value whats left of my hearing.  Was
that why old time commercial operators wore headphones forward of their
ears?  Be hard to do with modern "cushion" phones.
Rick Dettinger
K7MW



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[k6dgw]

Ron D'Eau Claire wrote:

>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.
>
Commercial CW ops wore their phones somewhat forward on their ears to
protect them against very strong signals [like being called by a ship
right off the coast 15 miles from your rx site]. In the mid-50's as a
16-yr old brand new relief op, I have a very distinct memory of being
whacked on the side of my head by my "mentor" standing behind me when I
put the cans on directly over my ears.  I still remember the raspy note
on strong signals.  Wasn't all that bad a sound, actually.  Despite the
limiter however, the beat "note" still got louder a signal got
stronger.  My guess is that as the clipping became deeper, the beat note
gained more and more spectral garbage and it just sounded louder.  I
might be wrong, however.  That's happened one or two times in my life.

The "rule" always was, "RF gain full for AM, AF gain controls volume.  
AF gain full for CW, RF gain controls volume."  I did this with my SX-28
and you really had to or strong signals in the IF would greatly exceed
the BFO level and the beat note would get weak and noisy. There was no
BFO for AM of course, but as SSB arrived on the scene, you had to use
the CW technique or it sounded like donald duck.   The "rule" went away
when product detectors arrived on the scene.  I rarely ... if ever ...
touch the RF gain on any of my rigs including the KX1.  I've never had
my K2 on SSB ... I need to try that and see how it sounds.

Somewhere in all this technical history, there appeared "hang AGC,"
billed at the time as the "greatest AGC since canned beer."  It attacked
rather quickly and then remembered it's last value for short time after
the signal went away, like when the station paused in talking.  When it
released, it did so rather quickly.  I think my Drake 2B might have had
something like it, or maybe not.  I was never fond of hang AGC and I
really enjoyed the 2B.  Did hang AGC just fade from the ham RX scene?

Put me down in the group that likes the K2 AGC action a lot.  It's
pretty much invisible, just doing what I expect it to do.  I'll have to
try it out on SSB and see if it sounds better on FAST or SLOW, but on
CW, it's just great for me.

Re S-meters and RST:  Does it matter if I'm S2 or S9 if the "R" is "5?"  
In fact, does "S" matter if the "R" is a "2" or "3"?  Besides, all my
contacts tell me I'm 5NN ... every time .. anyway.  The "T" long ago
outlived it's usefulness, and if we did away with S and T, think how
many more Q's DXpeditions could make in their limited time.  In fact,
since R will always equal 5 in such Q's, skip it too.  If I hear my
call, I'm in their log.  Do I really need to hear "5NN" when I and the
entire rest of the planet patiently [ ;-)  ] waiting to call knows
that's what it will be?.  Probably something wrong with that idea too.

73,

Fred K6DGW
-- Northern California Contest Club
-- CU in the Cal QSO Party 6-7 Oct
-- <www.cqp.org>


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[David Cutter]

There was an attempt to fit voltage dependent resistors (vdr) across the "receiver" (earphone) of telephone headsets to limit audio shock.  These were American headsets of an old but very common type (STC perhaps?).  When I did a brief test, I increased the voltage applied and found that the waveform across the earpiece gradually became more square.  It limited the amplitude of the waveform but the power in the earpiece still increased as it went square.  It was probably less of a volume than when the earpiece was unprotected, but it was still very loud.  Probably a similar effect to back to back diodes (this is best done with a resistor in series to smooth the response.

David
G3UNA

>
> From: Fred Jensen <[hidden email]>
> Date: 2007/03/13 Tue AM 06:52:30 GMT
> To: Elecraft Reflector <[hidden email]>
> Subject: Re: [Elecraft] AGC Independent S-Meter?
>
> Ron D'Eau Claire wrote:
>
> >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.
> >
> Commercial CW ops wore their phones somewhat forward on their ears to
> protect them against very strong signals [like being called by a ship
> right off the coast 15 miles from your rx site]. In the mid-50's as a
> 16-yr old brand new relief op, I have a very distinct memory of being
> whacked on the side of my head by my "mentor" standing behind me when I
> put the cans on directly over my ears.  I still remember the raspy note
> on strong signals.  Wasn't all that bad a sound, actually.  Despite the
> limiter however, the beat "note" still got louder a signal got
> stronger.  My guess is that as the clipping became deeper, the beat note
> gained more and more spectral garbage and it just sounded louder.  I
> might be wrong, however.  That's happened one or two times in my life.
> >

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[Darwin, Keith]

Lots of good info about AGC, but I'm thinking a lot of it is not that
useful today.

In the old days, it sounds like you HAD to turn the AGC off for things
to work right.  That's no longer the case and our operating habits can,
and have, changed.

In my case, I leave AGC on 100% of the time.  I'll turn the RF gain down
to increase the signal to noise ratio.  Turn the RF gain down to the
point where the AGC is not doing much and you have the rig running as if
AGC is off, but it is still there, able to respond to a big signal on
freq to protect your ears.

Can those of you who turn AGC off tell me why?  What does it do for you
that the AGC doesn't?

- Keith N1AS -
- K2 5411.ssb.100 -
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[Dave Van Wallaghen]

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: [hidden email] [mailto:elecraft-
> [hidden email]] On Behalf Of Ron D'Eau Claire
> Sent: Monday, March 12, 2007 11:56 PM
> To: [hidden email]
> 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
>
>
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[Ron D'Eau Claire-2]

Yep, as Fred pointed out, putting the phones ahead of the ears introduced a
sort of acoustic "hard limiter", especially if one wasn't digging for really
weak signals. In the radio room it also allowed the operator to hear what
was going on around him. Here's a picture of just such a radio operator at
his job at KPH in the California Coast in the 1970's
(http://www.radiomarine.org/historic-5.html). That's Les, K6ETY, a grand old
friend and mentor of mine, now an SK. But that wasn't always practical when
digging for a weak signal as shown by the operator in the foreground here
(http://www.radiomarine.org/historic-2.html).

If you made AGC so aggressive it attacked a signal like a hard limiter, it'd
produce the same distortion as the limiter. A limiter is easier to implement
<G>.

A handy compromise that's already a part of your K2 is to leave AGC on and
simply control the volume with the RF gain instead of the AF gain. That way
the AGC will "kick in" only if you encounter a really strong signal. Truth
to tell, I do that more that I operate with the AGC turned off completely!

Ron AC7AC

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Rick Dettinger
Sent: Monday, March 12, 2007 10:56 PM
To: [hidden email]
Subject: Re: [Elecraft] AGC Independent S-Meter?






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.

Ron AC7AC
============================================

I wonder if it is possible to adjust the AGC on the K2 to only work on
strong signals, to act as a hearing protection limiter only, with little
action on all other signals.  I understand this would work for CW only rigs.
I like to use my K2 with AGC off, but value whats left of my hearing.  Was
that why old time commercial operators wore headphones forward of their
ears?  Be hard to do with modern "cushion" phones. Rick Dettinger K7MW



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[Ron D'Eau Claire-2]

Describing the background was intended to put the AGC discussion in context
and show why AGC and S-meters are so closely integrated. A receiver with an
S-meter is just a sensitive, selective RF voltmeter. Without AGC that
"voltmeter" has a very limited useful range. That's why S-meters didn't
appear until superhetrodyne receivers with AGC came into use.

I turn AGC off a lot because AGC, in the absence of a signal, brings up the
background QRN to almost to normal listening volume. So the bands sound
noisier than they are. Our ears can follow a huge range of sound levels, but
receiver AGC fails to make use of that. It tries to deliver everything to
our ears at the same volume. Turn AGC off and set the RF gain so the band
noise and any weak signals are very faint, and you'll hear the bands as they
really are. If there's a signal down in the "mud" that you want to copy, you
can turn up the RF/AF gain to bring it up on volume for easier copy,
otherwise you need not have your ears assaulted by artificially-high QRN.

Still, I use it a lot too. It's a convenience if I don't mind its effects.
It comes in especially handy when contesting, working a net, etc. so one
doesn't have to "ride" the gain control by hand.  

Ron AC7AC

-----Original Message-----
Lots of good info about AGC, but I'm thinking a lot of it is not that useful
today.

In the old days, it sounds like you HAD to turn the AGC off for things to
work right.  That's no longer the case and our operating habits can, and
have, changed.

In my case, I leave AGC on 100% of the time.  I'll turn the RF gain down to
increase the signal to noise ratio.  Turn the RF gain down to the point
where the AGC is not doing much and you have the rig running as if AGC is
off, but it is still there, able to respond to a big signal on freq to
protect your ears.

Can those of you who turn AGC off tell me why?  What does it do for you that
the AGC doesn't?

- Keith N1AS -
- K2 5411.ssb.100 - _______________________________________________

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[N2EY]

In a message dated 3/13/07 7:29:41 AM Eastern Standard Time,
[hidden email] writes:


> Can those of you who turn AGC off tell me why?

The band sounds better that way, IMHO.

 What does it do for you
>
> that the AGC doesn't?
>

All of this is IMHO, from a 99%-CW operator with amateur-only experience.

For one thing, it puts me in total control of the receiver. A K2 really has
three
manual gain controls: PRE/ATT, RF and AF, and I use all of them.

For another, it helps in QRM-fighting. With AGC on, the receiver gain
wanders up and down according to the total signal strength at the
AGC detector. So if I am copying a weak CW signal with AGC on,
and a strong signal comes into the rx passband, the gain of the rx
will be controlled by the sum of both signals. So I will be challenged
to both ignore the unwanted signal *and* to ignore its effect on rx gain.
With AGC off, I just have to ignore the unwanted signal.

For a third, if the AGC is fast, the rx gain pumps up and down with each
CW element.  Copying code requires knowing the difference between
keydown and keyup, tone and no tone, mark and space - and fast AGC
reduces that difference!

And if the AGC is slow, the RX gain will respond to every pop and blip
that comes in. It's easier for me to copy through those things if the rx
gain doesn't wander.

---

IMHO, AGC was developed for 'phone operation. "Hang" AGC was developed for
SSB phone, to reduce pumping due to lack of a carrier. When SSB transceivers
became all the go 45-odd years ago, they all had AGC. And in many cases it
could not be turned off! So (again IMHO) a lot of hams thought it was the
only
way to go.

----

One of the great things about the K2 is that you can turn the AGC off or on,
slow or fast, or delay it by not using RF gain at full blast. Many other rigs
don't offer that
until you get into the big bucks models.

Try it and see!

73 de Jim, N2EY




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