Another interesting tale about parasitics oscillations.
-
I have found them in various audio gear. One particular
model Music Man Tube Guitar amp I service loves to
sing in the low part of the HF range. It will take out a
local hams 80 meter phone operation a half block
away.
-
cheers
skipp
-
--------- Forwarded message ----------
From: Bill Fuqua <wlfuqu00@uky.edu>
To: peter.chadwick@zarlink.com, amps@contesting.com
Date: Thu, 21 Aug 2003 09:44:42 -0400
Subject: Re: [Amps] Parasitics
There are a number of types of parasitic oscillations. In
grounded
grid amplifiers the most common is VHF. But there is a fundamental mode
as
well but more often occurs in common cathode amplifiers ( tuned
grid-tuned
plate oscillation) but does show up in poorly constructed grounded grid
amplifiers. A very rare type is a low frequency parasitic oscillation. I
have encountered this twice in the past 10 years. Once when tracking down
a
paging system that was getting into repeaters in the area. Using a
spectrum analyzer I could see the offending paging system with 2
sidebands
drifting 4 to 6 MHz on either side of the stable signal. It was
sometimes
getting into the local police and fire repeaters as well. This was a 150
MHz paging system with a 5 MHz low frequency ( drifting )parasitic
oscillation. These are usually caused by resonance in the RF chokes with
other components.
The strangest low fequency oscillation was in a 15 MHz 300 watt
amplifier used in our linear accelerator. This amplifier had large 30
Hz
amplitude oscillation. Being solid state ( copied from Motorola
Application
Notes) it used feedback to establish a constant gain. The advantage of
this
is that it did not require a regulated power supply or even a very well
filtered one. After making some measurements I discovered that the output
of the unregulated power supply was oscillating at 30Hz which is strange
because it runs off a 400Hz power source. The problem was that under the
right conditions the LC filter in the power supply ( resonant at 30 Hz)
and
the amplifier together formed a low frequency oscillator while trying to
amplify a 15 MHz signal. After a bit I realized that the amplifier
appeared to be a negative resistance to the power supply and its LC
filter. Think about it, the gain of the amplifier is constant (
determined
by negative feedback) . And if the efficiency remains constant or
increases
with an increase of the supply voltage the supply current must drop to
maintain a constant input power, and drop even more if the efficiency
increases with supply voltage. That is negative resistance. The solution
was to simply reduce the Q of the LC filter ( added a smaller bleeder
resister).
73
Bill wa4lav
At 07:58 AM 8/21/2003 +0100, peter.chadwick@Zarlink.Com wrote:
>There's a header that should wake people up!
>
>In the latest QST, the article on maintenance of linears suggests that
>poor neutralization can lead to parasitics. Not something I've come
across
>before, although obviously, it can depend on exactly what you mean by
>parasitics - personally, I tend to think of 'parasitics' being 'VHF
>parasitics' but of course, that's not necessarily the case, although
>arguably the most common.
>
>I haven't the article with me here at work, but I don't remember seeing
any
>advice about oiling fan/blower motor bearings, which I would have
thought
>was likely to be desirable, depending on the fan or blower.
>
>73
>
>Peter G3RZP
>
>
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