On 5/25/2018 1:43 PM, Wes Stewart wrote:
I have no idea what you are actually measuring but it certainly isn't
IMD in the classical sense.
I think I made it plain that I was measuring the "envelope" of keying
sidebands, usually described as "clicks." This envelope is due to a
combination of the shape of the keying waveform and IMD.
FWIW, I really don't care about IMD as a number. What I'm concerned with
is clicks, splatter, and phase noise from another station that's loud
covering a weaker station I want to work.
While it might be "well known" that IMD decreases at lower power, it
isn't necessarily true.
Right -- it depends on the transmitter design and implementation. K6XX's
work shows that to be true of the K3, and my measurements (done on 20M
as I recall) confirmed it. I also recall that you noted increased IMD
around 15W, which is likely the result of the K3 firmware causing the
power amp to be turned off and the QRP output stage to be driving the
load at it's full rated power. Under that condition it is consistent
with my (and K6XX's) observations that IMD is often lower at reduced
power output.
What is really not well known is that in the K3(S) IMD is highly
frequency sensitive.
That's news to me.
The 3rd order IMD in my K3S at 10W output varies 18 dB with respect to
frequency.
Again, the power amp is not engaged and you're running on the driver.
But that much variation also implies a problem. Have you sent the rig
back to Elecraft?
Furthermore, my K3S on 10, 17, 20, 30 and 40-meters has higher IMD at
20W than at 100W.
Interesting.
73, Jim
Wes N7WS
On 5/24/2018 8:17 PM, Jim Brown wrote:
On 5/24/2018 7:30 PM, Wes Stewart wrote:
I haven't measured IMD on my KPA500 either since I don't own two
generators that are clean enough with the necessary power to do so.
I evaluate IMD by looking at keying sidebands of a long series of
dits, as I posted hear yesterday. Details of the measurement setup
and method, and lots of results, are here.
http://k9yc.com/P3_Spectrum_Measurements.pdf
It's well known that IMD in the K3/K3S increases at rated output, and
is a lot lower at the levels needed to drive a power amp. It takes
25-30W to drive the KPA500 to full power, and about twice that to
drive my Ten Tec Titans. The K3/K3S is pretty clean at these levels.
The plots for the two amps look almost exactly the same, and have the
same shape as the K3 at those drive levels.In other words, neither
amp is increasing bandwidth. Note that these measurements are of the
envelope of peaks, so they are, in essence, capturing clicks.
IMD also increases with low DC supply voltage. I've not done any
disciplined measurements to put numbers to it, but K6XX has. His data
is in this pdf slide show. http://k9yc.com/K6XXAmpTalk.pdf
73, Jim K9YC
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