[Elecraft] S-Meter Sensitivity

Wed Jun 5 18:31:01 2002

Hi Earl!

I think if you want to make correct measurements of the gain throughout your
receive system you need to turn the AGC off.  See my post from today on AGC
if you want.  I found the AGC hitting the system too hard so I backed it off
with a minor mod to R1.

Have you tried turning the AGC off and on with no signal present?  On mine I
was getting a very noticeable decrease in background hiss with the AGC
turned on and with no signals anywhere in the passband.  Changing R1 seemed
to remendy this.  Along with this I seem to be getting more reasonable S
Meter readings.

It should be noted that this "too heavy" AGC was not accompanied by too-high
S Meter readings.  In fact, the opposite appeared to be the case.  I'd be
interested in other, more theoretically oriented folks, to comment on this
....

Peace and 73,
Stephen
W3SMK
K2 #2644

----- Original Message -----
From: <[email protected]>
To: <[email protected]>; <[email protected]>
Sent: Wednesday, June 05, 2002 11:01 AM
Subject: Re: [Elecraft] S-Meter Sensitivity

> K6SE wrote:
>
> "I estimate that my K2 requires about 100 uV to read S9 with the preamp
> ON, so the logical conclusion is that there's a loss of about 14 or 15 dB
> of gain in the receive chain somewhere before the IF amplifier U12
> (where the AGC voltage which drives the S-meter is developed)."
> ==========
> My error here -- The AGC is deleloped on the control board and is fed
> back to the IF amp at pin 5 of U12 on the RF board.
> ==========
> Don, KD5NDB wrote:
>
> "Maybe a better piece of information is the actual AGC voltage at the
> output of U2A with 50 and 100 uV in with the preamp on and off. I could
> make this measurement if you think it would help."
> ----------
> Please do.  It may be that my receiver gain is okay and the problem is in
> the AGC circuitry instead.  Measurements at pins 1, 3, 6 and 7 of U2 on
> the control board with 0 and 100 uV would be appreciated.
> ==========
>
> "If the readings are about the same as your K2 then you may just need to
> adjust the S-meter gain to get S9 at 100 uV or 50 uv your choice."
> ----------
> I'm not at all interested in setting my S-meter to one or the other of
> the "standrards".  It's just that my K2's S-meter does not even closely
> agree with the run-of-the-mill K2, and this tells me there's a problem
> somewhere that I'd like to find and cure.
>
> Tnx, 73, de Earl, K6SE
> 68 countries with K2 #2622 and counting.....
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