Maybe some clarification of what the "calibration" really does would be helpful.
The hardware AGC in the K3 operates on the post-filter FET IF amplifier and
associated diodes. The "RF" gain control (really IF gain) under the control of
the DSP adds a control voltage to the hardware AGC loop. If the manual gain
control voltage exceeds the detected AGC voltage then it controls the IF gain,
otherwise, it's in the hands of the AGC detector.
Unfortunately, there is variability (no pun intended) in the FET Gm, from
device to device, hence for a given control voltage, each FET behaves
differently. In comparison of my K3 to another, the differences were
remarkable. In mine, even a slight reduction of manual gain caused an
immediate and obvious degradation of SNR even on a BC station that was S9+50
dB. The comparison radio was much better behaved.
Anyone who has done any design work on receivers understands "delayed" AGC
where the delay serves to keep the gain nearer the front end high enough to
maintain SNR and only reduces it after the SNR is fully developed. Clearly, in
my radio's case, the gain partitioning between the IF and DSP was being
negatively affected and the SNR took a hit.
The RF gain calibration was a response to this issue and serves to measure the
dB gain reduction vs. control voltage and develop a correction in firmware that
linearizes (in dB) the curve.
Contrary to the belief of some, this isn't an S-meter calibration, although
that is a secondary benefit. Likewise, absent the development of hardware AGC
and/or the manual reduction of IF gain, it doesn't change the overall gain of
the radio.
--- On Sun, 3/14/10, r...@aol.com <r...@aol.com> wrote:
> From: r...@aol.com <r...@aol.com>
> Subject: [Elecraft] K-3 RF Gain Calibration
> To: elecraft@mailman.qth.net
> Date: Sunday, March 14, 2010, 12:22 PM
> Mike,
>
> Are you saying that your S-Meter reads 3-4 s-units MORE
> noise level now
> that you did the RF Gain calibration with the XG-2? I
> was going to make the
> same adjustment with my K-3. But if you are getting a
> 3-4 s-unit higher
> noise level after the adjustment has been done... I won't
> be making the
> adjustment as my K-3 is already more sensitive than I need
> on the low bands.
> I'd use a 20dB ATT on the low bands often if I had
> one available. Perhaps
> one day we'll be able to change the K-3's RF Gain per
> band via software to
> help adapt to our local noise environment.
> (mine is very noisy on the low
> bands with thunderstorm QRN off the Atlantic).
> For those who may be
> concerned, yes I do use the RF Gain control.
>
> 73,
> Dick- K9OM
>
>
> Sorry if this comes across more than once, I'm having some
> e-mail issues.
>
> I just did an RF gain calibration, using the XG2 and the
> K3 utility on
> my kit K3 (built in Sept. '09).
>
> There seems to be a dramatic difference in my noise
> level, 3-4 s units,
> with a calibrated s-meter. I think I can pull more
> stuff out of the
> ditches and weeds now.
>
> Does that seem reasonable?
>
> 73, Mike NF4L
>
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