K3 S Meter behavior

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K3 S Meter behavior

Edward R Cole
I'll take a stab at trying to explain how the s-meter works.

Now the K3 may do it differently since it may be done by the DSP, but
in normal analog radios, the IF is tapped at some point and goes to a
noise amplifier and detector.  This produces a varying dc signal that
is used by the AGC circuit to control either RF gain or IF gain or
both depending on the design.  Usually the s-meter reads this varying
dc level.  RF gain allow the operator to shift AGC by adding a dc
bias up or down, thus it should affect receiver gain.  lowering RF
gain lowers both noise and signal which may permit the receiver to
operate at a better level for  extracting the signal (i.e it sounds
quieter).  But by varying the AGC level the s-meter is fed a constant
dc bias as well (typically the s-meter rises).  In some designs the
noise amplifier also drives the NB.

So saying the s-meter is showing the level that would be needed to be
heard is a little wrong.  S-meter readings with RF gain reduced are
just not calibrated and thus of little use.

To divorce the s-meter from AGC takes more circuitry and it sounds
like the K3 has that ability by use of the ABS mode (I leave it to
the K3 experts to explain this if they desire).

Loudness of audio and strength of signal are not the same,
usually.  In SSB there is a more direct connection as there is no
carrier transmitted.  In FM often audio can be set too low (low
deviation) and still see a S9+++++ signal.  Data mode don't usually
speak in terms of loudness (unless your brain can decode FSK or PSK
signals - little joke!).  In general loudness is a function of
modulation and not signal strength.  But obviously this is a gross
generalization and not very exact.

73, Ed - KL7UW
------------------------------

Message: 30
Date: Thu, 15 Jul 2010 09:49:27 -0400
From: drewko <[hidden email]>
Subject: Re: [Elecraft] K3 S Meter behavior
To: The Smiths <[hidden email]>
Cc: [hidden email]
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=us-ascii

The way I understand it, turning donw the RF Gain does not "improve"
the signal's s-meter strength. The increasing level of the s-meter as
you reduce the RF Gain indicates the level that a signal would need to
attain in order to be heard. For example, if you turn the RF Gain to
12 o'clock you will only be able to detect signals that are approx
S9+30db or greater.

Personally, I would prefer that the s-meter not operate in this way in
ABS mode. RF Gain should have no effect on the s-meter action when in
ABS mode, just as ATT and PRE do not.

73,
Drew
AF2Z



73, Ed - KL7UW, WD2XSH/45
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