[Elecraft] K3 Transmit IMD (Mic adj for alc)

[Bob Cunnings]

Hmmm. Your advice to:

 "...*please* set it to 4 bars in soundcard "digital" modes..."

contradicts the direction given here (in step 7):

http://www.elecraft.com/K3/k3_operating_tips.htm#psk%20audio

viz. "You should not see any ALC bars on the K3 display (when set to CMP/ALC)"

This always confused me since it didn't agree with the manual, but
your explanation helps a lot, and I'll do it that way now.

Bob NW8L

On Sun, Apr 6, 2008 at 1:06 PM, Lyle Johnson <kk7p at wavecable.com> wrote:
> There is apparently some confusion about how the K3 ALC works.  Just to help
> muddy the waters a bit, I'll toss in what little I know about it.
>
>  We recommend setting the Mic (or LINE IN) Gain so you get 5 bars of ALC,
> with possibly the 6th bar occasionally flickering, with CMP set to 0.
>
>  No, this is not to ensure you have Tx distortion!  Just the opposite, in
> fact.
>
>  In this case, the ALC meter display is acting as a "VU Meter" and is being
> used to help you set the Tx audio path gain correctly.  It is not showing
> you how much you are over-driving the PA.  This is why you can set it up in
> Tx test mode, when you are not transmitting any RF at all.
>
>  If you have a little too much drive, the DSP will scale things back at the
> output of the mic amp.  Thus at the 5th ALC bar, the DSP is at the threshold
> of gain compression.  At the 6th bar, it is cutting back gain by 6 dB or so.
>
>  Assuming SSB, after the Tx signal is created from the audio, it optionally
> goes through an IF clipper. This is engaged by setting CMP to a value other
> than 0.
>
>  After the optional clipper, the signal goes through a 15 kHz IF filter
> followed by a gain section in the DSP, and is then output as a 15 kHz signal
> to the rest of the Tx strip.
>
>  From  this point, the MCU watches things and if it sees power levels going
> awry, severe mismatch, etc., it cuts back the drive to the Tx strip in the
> DSP.  Thus, the Tx strip is running at nominal gain regardless of the power
> level.  OK, if you go below 12 watts you are using the LPA and not the HPA,
> so the Tx strip gain is less, but you get the idea.  What we are *not* doing
> is varying the bias on an IF amplifier somewhere that might change its
> operating point and possibly introduce additional distortion.  The point in
> the DSP path where the change in drive is commanded is linear to the limits
> of 32-bit floating point math and the resolution of the Digital to Analog
> converter.
>
>  Enjoy!
>
>  73,
>
>  Lyle KK7P
>
>  [ Note: For PSK and other operation, you want to set things to the level
> where the 5th bar just comes on -- or just doesn't.  If you have only 4 bars
> in digital modes, that's OK. With less than 4, you are under- driving things
> and the ALC will kick in to bring up the power level to the amount you
> requested.  If you try and trick the system and run with *zero*  ALC bars in
> PSK31, the ALC has to work hard to give you your requested power - or maybe
> it will think there is insufficient signal to work with - and your Tx S/N
> may suffer.  So, *please* set it to 4 bars in soundcard "digital" modes. ]
>
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