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" <[EMAIL PROTECTED]>
To: "Sandy W5TVW" <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>;
<elecraft@mailman.qth.net>
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
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