On 6/9/2021 5:51 PM, Dave wrote:
There are dirty transmitters all over, and they meet FCC specs... How
can that be you might ask yourself?
FCC Rules include numeric limits for harmonics. The Rules include a
provision that a transmitted signal shall occupy no more bandwidth then
needed for the method/mode of transmission. As a member of the Standards
Committee of the Audio Engineering Society, we used clauses like this so
that we didn't have to cover every possibility. The Rule essentially
says, you've got to be as clean as a well designed, well operated
product/station transmitting the same mode.
The summary I prepared of FCC Lab measurements of the occupied bandwidth
of then-current CW transmitters clearly showed that some occupied FAR
more bandwidth than others. At that time, the K3 was the cleanest, the
FTDX5000 was the dirtiest. My process was simple -- ARRL sent me the
data in electronic form, I plugged into a spreadsheet (actually, several
pages of a spreadsheet, one for clicks, another for phase noise), and
plotted noise amplitude vs frequency for all of them on the same graph.
That report is here. http://k9yc.com/TXNoise.pdf
What that Rule says is that if one manufacturer's rigs occupies 500 Hz
CW bandwidth 50 dB down from the key-down signal, others should meet
that performance, or at least approach it within a reasonable time for
engineering departments to catch up. The K3 was introduced in 2007,
first sold, I think, in 2008. Someone will correct me if needed. Chief
Engineer Wayne Burdick has made no secret of what he did to achieve
that, and after Flex's 6700 measured badly at the lab for bandwidth,
their engineers did something to correct it (maybe Wayne's waveshaping?)
and reports were that it was much cleaner. AFAIK, ARRL has not
re-measured it, and I've not been able to get an owner to bring one over
here to measure.
As to Yaesu's SSB bandwidth -- I first learned about this hearing a
couple of locals on 6M, several months apart. The first was splattering
badly, calling CQ, no one answering, so I called to let him know, and
suggested that maybe he was overdriving his amp. No, he responded, no
amp. What rig? A current model Yaesu. A few months later, same story,
except but the second guy and I knew each other. We went through every
thing I could think of, first, of course, turning off his amp.
My method of studying this is simply to look at the signal on the P3
waterfall. The waterfall trace of a clean signal is a vertical bar about
2-7-2.8 kHz wide, with sides that are straight lines, while one that is
splattering will see horizontal breakouts from that bar on voice peaks.
For general operation, I adjust my spectrum display (the top graph) for
a 32 dB difference between top and bottom of the screen (42 dB for high
power contests), and I set averaging for the maximum value. The result
in the spectrum will show how much bandwidth is being occupied, and I
can freeze the screen, move the cursor along it, read frequency and note
how many dB down the sidebands are away from the carrier. The signals
I've measured have sidebands that are typically only 20 dB down from the
strength of the transmitted audio for about 2.5 kHz on both sides of the
signal. In other words, for LSB, 2.5 kHz above the suppressed carrier,
and 8 kHz below the suppressed carrier. Sadly, because there are no
numbers associated with it (in the Rules), it gets ignored both by
manufacturers and ARRL, who gives passing (sometimes glowing) reviews to
dirty products.
The same spectrum measurement can be made by switching to peak mode,
accumulating peaks for a while, then killing the signal by switching off
the antenna input. In peak mode, of course, the reference point for the
vertical scale must be shifted.
With all of this, one must learn how to set up their P3 (or other
spectrum/waterfall display) to separate the forest from the trees. With
averaging turned on, random noise averages out, leaving signals and
electronic noise. Set the bottom of this noise to the bottom the
display. This will also make signals jump out of both the spectrum
display and the waterfall.
The K3/P3, especially with the SVGA module, is a very nice test
instrument! Most SDRs I've seen are significantly better, with greater
dynamic range on screen, wider possible bandwidths, an frequency
resolution that is as good or better.
73, Jim K9YC
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