*Sigh!"* Ian was right after all, Peter - you awakened the monster and now
he's on the rampage again.
Rich, I think you have your Dick's mixed. The first short posting below was
mine - a basic observation straight out of "standard AC circuit analysis"
with no reference to "Q" and nothing to do with "Wes' measurements."
It's harmless to talk about VHF "Q" all you want, but with respect to
suppressing VHF parasitics, the only Q in the anode circuit that really
counts is the loaded "Q" of the entire anode parasitic circuit, on whatever
frequency it ends up resonant. In that practical context, it's the complex
impedance of the parasitic suppressor that's directly relevant, not its
"Q."
Even then, overall feedback loop gain determines whether or not oscillation
will occur, so loaded Q of the parasitic-resonant circuit is only part of
the equation. Hence the practical need for some cut-and-try (or "successive
approximation" to you engineers!) to get best results, at least when you're
not precisely cloning an existing amp.
I think the above is consistent with Ian's discussion, which gets to
practical issues without bringing in distracting and irrelevant concepts.
And BTW, I also think those old ARRL Handbooks that John mentions, cc.
1947-56 or so, covered the subject in a much more practical and helpful way
than more recent publications. Guess it's because lots of us were in high
school back then, had no choice but to brew our own amps, and (blush!) had
barely even heard of "Q."
73, Dick W0ID
-----Original Message-----
From: Rich Measures [SMTP:measures@vc.net]
Sent: Tuesday, April 06, 1999 7:46 PM
To: amps@contesting.com
Subject: RE: [AMPS] parasitic suppressors
>
>Thanks, Ian...
>
>Extremely well put! All should read and try to comprehend.
>
>Both Peter's posting and yours should remind us all of the fundamental
fact
>that equivalence of any pair of simple series and parallel X-R circuits is
>valid ONLY at a single frequency.
>
>73, Dick
>
>-----Original Message-----
>From: Ian White, G3SEK [SMTP:G3SEK@ifwtech.demon.co.uk]
>Sent: Tuesday, April 06, 1999 6:31 AM
>To: amps@contesting.com
>Subject: Re: [AMPS] parasitic suppressors
>
>
>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
>>thebove 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.
>
? Has standard AC Circuit Analysis been obviated? Has Dick looked at
the numbers in Wes' measurements?
.
>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.
>
? agreed. It also helps to have a current transient to ring #1 and get
the ball rolling. .
>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.
? Fortunately, #2 can be found on the mfg's technical specification
sheets. It is the tube's internal feedback C from the output element to
the input element.
> That's why you always have to develop the suppressor by
>cut-and-try methods.
>
? If R-supp burns out on 10m, the vhf suppressor is out of business.
>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.
? well put
>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.
? yes, and adding R to the "stray" inductance decreases the parallel
equivalent resistance, yadda, yadda. // In Wes' suppressor
measurements, as vhf suppressor Q decreases, vhf Rp decreases. Less Rp
means that the tube's vhf amplification decreases. At 100MHz, the
copper-wire suppressor Wes measured an Rp of 166 ohms (and Q = 2.2),
while the resistance-wire suppressor had an Rp of101 ohms (and Q =1.5).
Sure, resistance-wire does not offer a spectacular improvement over
copper and silver wire suppressors, but 40% is nothing to sneeze at.
>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.
? In Parasiteville, every conductor counts between the anode and C1.
There's also the sticky wicket of clandestine vhf resonances in C1.
>
>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!
>
? Calculating Q at the anode resonance freq. is throwing away any
chance of truly understanding? Q is inexorably related to Rp and Rp is
a major determiner of vhf gain. // To me, Dick's dire proscription to
ignore Q sounds like what prestidigitators call "misdirection".
- In the Wizard of Oz, wasn't there a proscription to ignore the man
behind the curtain?
>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.
? Suppressor Q is undoubtedly related to Rp. More Rp comes with more Q.
More Rp means more vhf amplification.
"I know that most men, including those who are at ease with problems of
the greatest complexity, can seldom accept even the simplest and most
obvious truth if it be such as would oblige them to admit the falsity of
conclusions which they had delighted in explaining to colleagues, which
they had proudly taught to others, and which they had woven, thread by
thread, into the fabric of their lives." - Tolstoy
- cheers, Dick.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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