[Elecraft] High Tx IMD with new K3s/10

[Bill Leonard N0CU]

I recently got a new K3s/10 built up and running. This week I ran some tests
to get familiar with it and one test that surprised me was the high Tx IMD
levels. I first noticed that the two tone waveform on a Tek o'scope showed
flat topping starting at about 4.5 W. As the power level was increased the
flat topping got worse. I checked the spectrum with my HP spectrum analyzer
(on 14 MHz) and measured the following 3rd order IMD levels feeding a 1.1
SWR:

1 W: -36 dBc
4 W: -31 dBc
4.5 W: -33 dBc
6 W: -34 dBc
8 W: -25 dBc
10 W: -21 dBc

It was interesting that as the power was increased from 4.5 to 6 watts, the
increased distortion didn't show up in the 3 rd order IMD, but rather the 5
th and higher order products.
Since I am new to Elecraft radios, I don't know if this behavior is expected
or if there is something in my transmitter that needs to be checked out.
I have built up a number of amps over the years and I am used to seeing flat
topping start at, or a little below the maximum CW output power. My K3s puts
out about 12 W max, so I was expecting the flat topping to start at about 11
W.

Bill  N0CU



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