Peter,
As you said, that was not phase noise, just intermodulation distortion, but
from my understanding of radios at that time, I don't think the engineers
were correct on their story. I believe the IMD/noise was being generated
before the IF.
The problem with all rigs at that time, which was right after Xtal filters
became popular, was that engineers did not pay attention to presenting a
good match between the mixer and the crystal filter. That's maybe not the
best choice of words, so I'll describe it.
The output of the mixer produced two signals, the sum and the difference
frequency.
One of the two was the desired frequency, and the other was of course
undesired.
The signal on the desired frequency was on the same frequency as the Xtal
filter and if designed properly, their impedances were a good match. That
part was OK. So far so good.
HOWEVER, the signal on the other frequency did not pass through the filter
and was reflected back, placing standing waves on the line. These standing
waves in turn went back into the mixer and unbalanced it, causing
intermodulation distortion in the mixer (not in the IF stage), which was
then transmitted down the line, through the IF stage and out the antenna.
So I don't think an additional filter on the output of the IF would have
solved the problem.
One simple way to cure this is the use of a diplexer.
Ten-Tec (!) was one of the first companies to deploy this technology, in
their original Omni A.
Must have been 1988.
The diplex was inserted between the mixer and the Xtal filter. It split the
sum and difference frequencies apart, passing the desired frequency on to
the Xtal filter and routing the undesired frequency to a resistive
termination. With proper termination of the undesired frequency, there were
no more standing waves on the line and the mixer remained balanced and
linear.
So the noise was caused by intermodulation distortion in the mixer, and
simply passed through the IF.
Perhaps I'm wrong. It's been a long time since I studied that stuff.
Too bad today's problems cannot be solved that easily.
Today's phase noise problem is simply dirty PLL's.
The only good news is, I don't think they are as dirty now as they were in
the late 80s and early 90s.
The cheap rigs back then were really terrible. Now they are just bad. (hi)
BTW, that is a lovely set of radios on your QRZ.com page.
I remember most of those radios, recognize several but on some I just don't
recall the model numbers.
Looks like a Mohican, nearly hidden on the right side. That was one of my
favorite receivers because it could be run off of batteries!
A great blast from the past!
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Peter
Bertini
Sent: Monday, July 07, 2014 5:55 PM
To: tentec@contesting.com
Subject: Re: [TenTec] On Noisy Transmitters
Synthesizer phase noise wasn't always the cause for transmitter broadband
noise issues.
Back in the 1960s I used a Heathkit Hx-10 Marauder transmitter with a
transverter to operate on six meter SSB. I also used a homebrew 4CX250B to
generate around
250
watts into a modest beam.
Several locals noted that I had broadband noise on either side of my signal,
extending for a few hundred kHz, whenever the TX was keyed.
Subsequent mail correspondence with a engineer at Heath revealed that they
were aware of the problem: the IF crystal filter was placed before the IF
amplifiers and the design required a second filter after the IF gain stages.
The issue was broadband noise generated in the IF stages. On my end, the
system worked flawlessly. But, for other operators within a 30 mile radius
the shortcomings of the transmitter design were painfully obvious.
Today, I could easily remedy the problem, but back then, as a newbie teen,
the problem was overwhelming, and I replace the rig with one that was more
friendly to my six meter neighbors.
Pete K1ZJH
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