[Elecraft] AGC Independent S-Meter?

[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" <w3fpr at earthlink.net>
To: "Sandy W5TVW" <ebjr at i-55.com>; <N2EY at aol.com>; 
<elecraft at 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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