[Elecraft] K3 Transmit IMD

[G4ILO]

Wes Stewart wrote:
> 
> 
> --- "Eric Swartz - WA6HHQ, Elecraft"
> <eric at elecraft.com> wrote:
> 
>> The K3 firmware used for the original QST review did
>> not turn off the 
>> K3's RF speech compressor clipper even when the
>> compressor was set to 
>> zero for their TX IMD two tone test. This resulted
>> in a significant 
>> increase in measured TX IMD when using an external 2
>> tone source. We 
>> corrected this in a firmware update last month.  All
>> new K3's and anyone 
>> who has updated to the latest firmware have this
>> fix.
>> 
>> 73,  Eric  WA6HHQ
> 
> [snip]
> 
> Well, that sort of explains it, but raises another
> question: Does the firmware change actually correct
> the IMD issue or does it simply return when the
> operator turns the processor on?
> 
> Regards,
> 
> Wes  N7WS
> 
Nobody seems to have answered this, so I'll try. IMD is a measure of the
linearity of the transmiitter. It is measured by feeding two pure sine waves
into the input and looking at what unwanted mixing products come out the
end.

The firmware has no effect on IMD. What it did (apparently) do is invalidate
the test results by introducing distortion on the input signal so the TX did
not have pure tones to start with.

It's impossible to talk about IMD when compression is used. Compression, by
its very nature, distorts the signal, creating IMD. If you want to transmit
the cleanest signal, leave the compressor off and make sure the ALC doesn't
show either.

Most of us, though, would rather transmit a "punchier" signal in the
interest of being heard at the other end, so we deliberately distort our
signals by using ALC and compression. Which (it seems to me) makes the whole
issue of IMD figures just another example of the pointless "numbers game"
that people obsess over. What the heck does it matter whether one TX is a
few dB better or worse than another when it is the component holding the
microphone that really determines how clean the transmitted signal is?


-----
Julian, G4ILO  K3 s/n: 222 K2 s/n: 392
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