On Tue, 6 Apr 1999 13:30:49 +0100 "Ian White, G3SEK"
<G3SEK@ifwtech.demon.co.uk> writes:
>
>Peter Chadwick wrote:
>>
>>Something I don't understand - maybe Carl or Rich or Jon can
>explain.
>>
>>If I take an inductor (start with a 'perfect' inductor) and put a
>resistor
>>in series with it, at LF it looks like the inductor isn't there. If I
>make
>>the inductor out of resistance wire, it still looks like a resistor.
>
>>
>>Now increase the frequency. Inductive reactance rises, and the Q,
>given by
>>2*pi*F*L /R increases, eventually becoming infinite at infinite
>frequency
>>(OK,
>>3-500s don't have any gain there!) So I have a series inductive
>reactance
>>with Q increasing with frequency.
>>
>>Now take the coil, wind it out of silver tape if you like, and put a
>33 ohm
>>resistor across it. At this stage, it's a perfect resistor. At LF,
>the low
>>reactance of the coil shorts out the resistor, but as the frequency
>rises,
>>so does the coil reactance. Because it's a parallel circuit, the
>impedance
>>tends asymptotically to 33 + j0. Even a practical resistor will tend
>that
>>way. As Q is Rp/1*pi*F*L, the Q is falling with frequency - in fact,
>it will
>>have a maximum value at VLF.
>>
>>If I understand Rich's argument, the Q of a paraistic suppressor
>should fall
>>with increasing frequency. So there seems to me to be an anomaly -
>from the
>>above argument, an L-R shyunt circuit is indicated.
>>
>>can someone explain?
>
>
>Peter, what have you done? You've awoken the undead!
>Prepare for at least six weeks of mayhem and misquotation...
>
>
>I used to think that the Q of a parasitic suppressor had some meaning.
>
>having looked at the problem in detail, I don't believe that any
>more.
>
>The basic question is: will my HF amplifier oscillate at VHF?
>
>That depends on three things:
>1. Existence of a parasitic VHF resonance - we know that does exist,
> due to unavoidable stray inductance between the tube and C1 of the
>
> Pi(L) tank.
>2. Existence of a feedback path from output to input of the tube.
>3. Sufficient gain at the VHF resonance to permit oscillation.
>
>Note: ALL THREE of those requirements must be met, or else the amp
>will
>not oscillate.
>
>The parasitic suppressor operates only on #3 and #1. But in fact the
>big
>unknown is #2, because it depends on the individual design and
>construction. That's why you always have to develop the suppressor by
>cut-and-try methods.
>
>The suppressor is wired in series with the tube, but the way it works
>is
>that its resistive part creates a permanent PARALLEL load impedance
>for
>the tube at VHF, low enough to make its gain drop below the
>oscillation
>threshold. If you analyse the circuit in detail, you find that the
>series-to-parallel impedance transformation depends on the stray
>inductance that is causing the VHF resonance. There's the problem:
>the
>load impedance seen by the tube depends partly on the suppressor
>characteristics BUT ALSO on the stray inductance in the amplifier.
>
>So, however much you know about a parasitic suppressor, it's of no
>real
>use unless you also know about the strays in the individual amplifier
>in
>which it will be used.
>
>Finally, the characteristics of the suppressor consist of TWO numbers,
>R
>and X (both of which will vary with frequency... yes, even R). To
>work
>out what the suppressor will do for an amplifier, you need BOTH of
>those
>numbers separately. If you roll the two numbers together and talk
>only
>about a single value of Q, you have just thrown away any chance of
>truly
>understanding what is going on!
>
>That's why any debate about "the Q of a parasitic suppressor" is
>doomed
>to failure - it's only taking about one small corner of the whole
>problem.
>
>This has already happened at least three times. The bodies are buried
>in
>DejaNews and the web archive... buried, but not dead.
Well put Ian. I wish I had your eloquence.
Now, lets see how long battle four will be waged.
73 Carl KM1H
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