[Elecraft] RF-Gain & S-Meter (again)

Sat May 28 23:08:27 EDT 2011

Well Bill, you are right. RF Gain should reduce the RF gain and it does! 

When AGC is used, when do you want the RF gain reduced? Obviously, when you
receive a strong signal. So when your receiver REDUCES the RF Gain, its
S-meter moves HIGHER, indicating a stronger signal is being received. 

When you reduce the RF gain manually the S-meter moves higher in the same
way. 

Also you are right that the S-meter is worthless if you don't leave the RF
gain "full up". But that's always been the case in almost every
communications receiver. It sounds like the SX101 was an exception, but even
if it behaved differently, was it at all useful with the gain turned down? 

My HRO-5 from the early 1940's pegged the S-meter if the RF gain was turned
down. Most receivers of that era (including the HRO-5) had a switch to
completely disable the AGC and it also disabled the S-meter so it stayed at
zero even as I reduced the RF gain. 

In older receivers the most important stage to protect from overload was the
first mixer, so RF gain controls - both manual and automatic - controlled
the gain of the RF amplifier stages between the antenna input and the first
mixer (hence, the name RF gain control). But they typically controlled the
gain of the I.F. stages after the mixer as well because simply controlling
the RF stages didn't provide the desired amount of control range. 

Automatic Gain Control (AGC) also controlled the gain of the RF and I.F.
stages, just like the manual "RF Gain". 

It was easiest to implement both AGC and manual RF gain control by simply
having a potentiometer insert a d-c bias into the AGC system like that
produced by signals. By adjusting the bias level, the gain was controlled
manually by the RF gain control to whatever level was wanted. Since large
signals cause the AGC to turn the gain down more than weak signals,
adjusting the bias to reduce the gain makes the S-meter read as if a larger
signal is being received. That is, the more you turn the RF gain down, the
higher the S-meter indicates. 

Perhaps the SX-101 (I never owned one) had a special circuit that caused the
S-meter to behave differently but, if so, it was the exception, not the
rule, in receivers from at least the 1930's onward. 

The only difference between the RF Gain/AGC system then and the system today
is that most modern rigs like the Elecraft rigs normally don't use an RF
Amplifier ahead of the first mixer, so all the control is applied to the
I.F. stages. But, since it works exactly like older RF gain controls, rigs
like the Elecraft line keep the name "RF Gain" control. 

What has seen a lot of evolution over the years is the manner in which the
AGC bias voltage is developed to provide the best attack and decay responses
for each kind of signal. But the way the RF gain bias voltage is used to
control the overall receiver system gain hasn't changed since the 1930's. 

In the Elecraft rigs there is an RF amplifier that can be switched in
manually called the "Preamplifier". Again this is an old idea going back to
at least the 1930's, using a separate amplifier stage to improve the noise
figure and to provide extra gain when needed. On our 1940/50's vintage
receivers there were a number of companies selling outboard "preamplifiers".
I had the famous R-9er hooked to my HRO-5, mostly to improve its noise
figure above 10 MHz. 

Traditionally, these "preamplifiers", being "bolt on" additions to the basic
receiver, were not included in the AGC gain control system (and so not in
the manual RF gain control system either). One switched them in or out
manually as needed.  

That is still true today in the Elecraft (and most other) rigs, even though
now the preamplifier is built into the receiver. 

I hope this clears up the confusion a bit Bill. It sounds like you have an
unusual receiver in that SX-101. Bill Halligan was famous for innovative
ideas in his receivers, but the way he handled the S-meter response to the
RF gain control was one specific to that receiver, not the general rule. 

73,

Ron AC7AC 

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Bill Swindell - K1LED
Sent: Saturday, May 28, 2011 6:41 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] RF-Gain & S-Meter (again)

I think it's weird. I still think the way is should work is to actually
reduce the rf-gain. I think the purpose of the rf-gain would be to limit the
input to the rf input of the receiver to prevent overloading. Oh well,
that;s the way it is.

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