Greetings to all,
A couple of comments to my last post have prompted another reply.
The question must be asked, "What causes a parasitic oscillation, in
the first place?" The very basic answer is "strays"; stray inductances,
and stray capacitances that combine to produce stray resonances. The very
best design is going to have numerous stray resonances (some say infinite) .
Even with the best design techniques, it is impossible to design an
amplifier that is totally without stray, thus self resonances. Thank
goodness, most of the resonances are completely harmless and unless you have
similar resonances in the input and output circuit, at the same instant in
time, (remember, they change due to time constants in circuit components),
they are all harmless.
The self resonance that causes most of the problems in the SB-220 is
around 90 to 105 MHz It is easily measured, both in frequency and "Q". In
the TL-922A there is also a high UHF self resonance that will vaporize 1/2
(only one side) of the double-sided twenty meter fixed contact on the band
switch. Relate the size of one side of the contact to frequency and you
will get the frequency of the self resonance. It is interesting to note
that the original TL-922 (not the 922A) dose not have this problem. This is
because of a very slight, physical, re-design of the final compartment of
the later TL-922A that was sold on the American market. Good parasitic
suppressors will de-tune/attenuate these self resonances, hopefully to a
point where they will be harmless.
The next question that has to be answered is, "Why do these self
resonances cause damage to the amplifier?" The answer is very simple, LOAD,
or the LACK of a LOAD for the resonance. Remember we are not talking about
fundamental parasitic, but VHF/UHF parasitic. The antenna supplies the load
for the fundamental, but there is no load for the VHF/UHF parasitic. This
energy must be dissipated somehow, thus the "big bang", burned up band
switch contacts, and what ever else, that we are so familiar with.
Now, if we know the frequency of this unloaded parasitic, why can't
we build a trap, or load, inside the final compartment to absorb it before
it does any damage? This idea has been around for years. I spent a lot of
time trying to doing just this about thirty years ago and finally gave up.
The idea is not practical for several reasons, but mainly because the self
resonances change, as mentioned above. This brings us back to good quality,
low "Q", traditional suppressors in both the anode and cathode of the
amplifier.
73 de Lon, K5JV
1110 Golden Bear Ln.
Kingwood, TX 77339
281-358-4207
281-358-4234 FAX
281-795-1335 CELL
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