On Wed,12/14/2016 8:12 PM, Gary Schafer wrote:
Inter modulation is just what the term implies. Two or more signals modulate
each other in the signal path. For this to happen there needs to be a non
linear device in the path to cause modulation to occur.
A single signal by itself passing thru a non linear amp will generate
harmonics but not IMD. If a second signal is present it will modulate the
first signal and the first signal will modulate the second.
Any time you want to transmit more than one signal thru an amplifier the
amplifier must be linear. Of course SSB contains more than one signal
(frequency) at any given time as does a two tone signal.
Exactly right. And a keyed CW signal DOES contain many frequencies
because it is the 100% amplitude modulation of a continuous wave by a
rectangular wave. Indeed, our use of the words "continuous wave" to
describe Morse transmitted by keying a radio transmitter is another one
of those wrong words used to describe things that cause us to
misunderstand how things work.
I posted this link a few days ago, and I will do so again. Please study
it. It illustrates many important points.
http://k9yc.com/P3_Spectrum_Measurements.pdf
Please look at slides 8-11, which are a REAL continuous carrier (that
is, keydown). The sidebands are hum from the rig's power supply. Now
look at slides 14-16, which show the spectrum of a series of dits at
about 30 WPM.
Slides 31-34 show distortion produced in the K3's output stage as a
function of power supply voltage. Some rigs have this characteristic and
some do not.
The series of slides beginning with #38 show keying bandwidth of some
other rigs that were made available for me to test. Note that all have
keyed bandwidth much greater than the K3 used in the first series of
slides, but SSB bandwidth is as good or better than the K3. The
difference in the CW bandwidth is primarily due to the rise and fall
time and the shaping of the keying waveform, which in a K3 has the shape
of a raised cosine.
Slides 51-58 show CW bandwidth of an FTDX5000, which was notorious for
being very broad on CW, before and after application of a firmware
modification that Yaesu issued to improve this issue.
Slides 69 and 70 compare an ACOM 1010 at its rated power to a Ten Tec
425 Titan at rated power. Slide 70 can be compared with an earlier slide
of the K3 barefoot -- bandwidth with the Titan is the same as from the
K3 barefoot. The same was true with the KPA500 at rated output (500W).
But the ACOM 1010 is considerably broader at 50 dB down.
What do these slides show?
1) ALL modulation produces sidebands. I learned this in a Communications
Theory class in 1963 when I studied EE.
2) NO power amplifier is perfectly linear, but some are better than
others. The KPA500 and Titan are sufficiently linear that they do not
increasing the keying bandwidth of the rig with the narrowest keying
bandwidth, while the ACOM 1010 increased the keying bandwidth.
Ah, you say, the difference is 50-60 dB down. Right. But if someone
using that amp (or one like it) is putting a lot of signal into your RX
by virtue of his QTH, band conditions, or antenna farm, he can make it
difficult for you to work someone on an adjacent channel.
3) When we say that an amp is AB or B and that it has perfect linearity,
we are assuming that it is an ideal amplifier. Almost nothing in our
world is ideal. Ideal is a simplifying mathematical concept to aid in
understanding the difference between one method and another.
4) The spectrum produced by a keying waveform is determined by the rise
and fall times, and by keying speed. I haven't published the results,
but a year or so ago I measured the keying bandwidth of a K3 with dits
varied from 20 WPM to 60 WPM. Below 35 WPM, keying speed did not affect
bandwidth. The increased bandwidth is clearly predicted by
Communications Theory.
Does this matter on the air? You betcha it does! My QTH is 3 miles
from K6XX, who runs a K3 to vintage tube amps which he carefully tunes
to minimize the IMD they produce. Another active contester about 15
miles away uses an IC 7600 and a power amp that is unknown to me. K6XX
and I can work 500 Hz apart on CW and barely know the other is there.
But that other contester chews up almost 10 kHz on CW, even though
inverse square low causes his signal to be 14 dB weaker than K6XX. In a
contest, within that 10 kHz there are typically 20-30 stations I'd like
to work but can't.
The same thing happens on SSB. Roughly half of all signals I see
ragchewing or working nets on 40M are splattering loud enough 5 kHz
either side of their intentional bandwidth to prevent a QSO on the
adjacent channel! This is NOT receiver overload, it is dirty transmitters!
5) All rigs contain an RF output stage, and some are dirty. I've let
several guys on 6M know that they're splattering, and they told me they
were running a cheap Yaesu barefoot. My measurements of the PSK31 signal
varying the power out of a K3 showed that it had far less IMD at 25W
than at 100W.
These measurements require a spectrum analyzer with rather narrow
frequency resolution. The resolution of the P3 can be set as low as a
few Hz. Neither my HP 8590D nor my Rigol analyzer have resolution narrow
enough to show it, but modern SDR receivers do. The spectrum analyzer
used for these measurements is an Elecraft P3, looking at the 8 MHz IF
of a K3 as a front end. I recently bought several low cost SDRs for use
as spectrum analyzers! Indeed, all that is required is to drive them so
that they are within their own linear range.
73, Jim K9YC
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