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

[Lyle Johnson]

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. ]