Without too much elaboration let's just say that I discovered this problem long before ARRL did. (S/N 10020)

I can tell you that my measurements show that IMD varies with Vcc (Vdd), drive level, power output and frequency. This latter parameter is, as best as I can determine, inexplicable by anyone, including the manufacturers of the transistors. Initially, I compared my shiny new K3S to my tired old, unmodified K3. At the original test frequency, 3.8 MHz the K3 had much lower IMD that the K3S under otherwise the same test conditions. Moving up to 14 MHz the situation reversed. Each of these radios had a different "sweet spot," neither of which was representative of the general case. If a testing entity happens to test at only one frequency and reports the results as typical then you get a false sense of the general performance.

Wes, N7WS

On 10/14/2016 8:03 PM, Dave Hachadorian wrote:
Yaesu FT-990, 12V finals
3rd order TX IMD  -38dB
ARRL Review 11/91

ARRL heaped glowing praise upon this rig way back in 1991 for its
Tx IMD performance. So, what was Yaesu's secret, and why haven't manufacturers figured it out and capitalized on it ever since?

Dave Hachadorian, K6LL
Yuma, Arizona

-----Original Message----- From: Kevin
Sent: Friday, October 14, 2016 5:14 PM
To: elecraft@mailman.qth.net
Subject: Re: [Elecraft] K3S QST Article

The cause for this is the use of 12V finals. It would be much
by moving to 24 or 50V finals.

Move the /K3s II/ to 50V finals, say a pair of MRF 150's, limited
to 150
or 200W max output. Design/Sell an outboard 50V switching supply
of 15-20A continuous current, with a 10A 12V switcher built in
for the
rest of the radio.

On 10/14/2016 1:33 PM, Bill Leonard N0CU wrote:
I would not expect the synthesizer (old or new) to be a
contributor the IMD
issue. Hopefully someone will correct me if that is a bad
assumption. To me,
the primary issue appears to be the linearity of the two RF

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