On 7/11/2021 9:25 PM, Adrian wrote:
I would think the better IMD3 performance would be at the higher voltage
in the SPE, at the expense of efficiency.
I happened to have measured the bandwidth of both a KPA1500 and an SPE
1K on the air transmitting FT8 on 6M, using my K3/P3 with the added SVGA
output that yields additional frequency resolution. The KPA1500 at 500W
is substantially cleaner than the SPE amp running 300W at its highest
voltage setting. It was 6 dB worse at the low voltage setting. Both
stations were within about 13 miles of me. The measurements were
motivated because a mutual friend running that SPE amp was getting
complaints, so N5KO paid him a visit to help diagnose the problem, and I
looked at the signal as they made changes. One of the first things we
checked was to make certain that AGC was not being used to set TX power.
It was not.
Observations are based on the strength and nature of the sidebands.
Sidebands from the SPE were essentially broadband noise extending on
both sides of the signal. Sidebands of the KPA1500 were at discrete
frequencies on both sides of the carrier. I didn't make written notes,
so I'm going from memory, may spaced 150 Hz. The two stations were N5KO
and W6RN.
Neighbor K6XX and I both run a KPA1500 in one of our SO2R contesting
setups, and I run an 87A in the other. We're about 5 miles apart, both
running K3s with the latest synth boards. These stations are clean
enough that on CW, we can work 500 Hz apart, one running, the other S&P,
and hear the other as nothing more than another strong signal. Bob is 50
dB down 235 Hz from his CW signal, 60 dB down at 335 Hz, 70 dB down at
540 Hz.
Bob, K6XX, is a manufacturing engineer at Elecraft, and about eight
years ago, we gave a joint presentation of transmitter cleanliness to
our large contesting club. It's here.
http://k9yc.com/K6XXAmpTalk.pdf
Here's some work I did using one of my contesting radio's P3/SVGA to
look at the other, and to other rigs loaned for testing.
http://k9yc.com/P3_Spectrum_Measurements.pdf
It's important to understand the power amps are a small fraction of the
problem. Some rigs are MUCH dirtier than others. Here's a report I
prepared several years ago that analyzed and compared ARRL Lab test data
on a selected group of rigs then being sold. ARRL Labs sent me data for
as many of their measurements as were available in a common form. I
plugged them into a spreadsheet, and plotted data for all of the rigs on
the same axes and scale, so that they could be clearly compared. Part of
the motivation was issues raised about dirty rigs in the WRTC
competition. That report is here.
http://k9yc.com/TXNoise.pdf
73, Jim K9YC
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