Posted by
Ron D'Eau Claire-2 on
Jul 25, 2006; 5:00pm
URL: http://elecraft.85.s1.nabble.com/Clean-CW-signal-tp392446p392449.html
Listening to the on-air signal on a nearby receiver is a really *tough* test
of the transmitter. Things such as hum or noise on the signal that someone a
short distance away would never hear will blast out loud and clear.
That can be good. If the signal is clean on a nearby receiver, it's
c-l-e-a-n!!
On the other hand, it can be difficult to determine just what is of real
concern and what isn't and, often, what is or isn't being created in the
monitor receiver itself.
With many receivers, just grounding the antenna input is not enough when
dealing with anything above QRPp levels. The usual approach for serious
monitoring is to put the monitor receiver in a completely shielded
container, so it only gets a normal level of signal: a few microvolts at
most. Of course any control lines and power lines have to be very well
shielded. A battery-operated receiver is easiest to use. Tune in the signal,
then button up the receiver is a tightly shielded box with a fine screen
mesh opening to let the sound out. Many years ago, when Hams were first
trying to accomplish a clean "d-c note" there was a popular little monitor
receiver published in QST called "The Growler". It got its name because it
was built into a popular all-metal lunch pail of the 1920's and 30's called
a "Growler".
Another approach that many of us are equipped to use is to look at the
signal on an oscilloscope. Amplitude modulation can be observed directly.
Amplitude modulation is what we're most often concerned with. Our receivers
are rather insensitive to frequency or phase modulation.
One can make a rough check for frequency or phase modulation by using a
sharp filter and tuning in the signal right on the edge of the filter
bandpass to produce "slope detection" of any FM present. If the frequency or
phase is changing significantly, noise will appear when the signal is right
on the steep slope of the bandpass. What's happening is that any change in
the frequency of the signal moves it up and down the steep filter slope is
producing amplitude modulation that the receiver can detect. (It's called,
appropriately enough, slope detection.) Once again, that is also a rigorous
test of the monitor receiver. Any frequency or phase modulation of the local
oscillator or BFO in the receiver will have the effect of moving the signal
along the slope of the filter bandpass too, producing the same noise. You
can't tell which is the result of your transmitter and which is the result
of your receiver. Most modern receivers use phase-locked-loop oscillators
and many of those have a significant amount of phase jitter in those
oscillators as they are constantly being corrected by the loop. That jitter
is indistinguishable from FM on the signal being received. If the signal
sounds bad, compare it with a known clean source such as the Elecraft XG1 or
XG2 test oscillators set on the filter slope in just the same way. Don't be
too surprised if it also sounds rough in many receivers!
Ron AC7AC
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