-----Original Message-----
From: Don Kessler <ki6sz@compuserve.com>
To: amps <amps@contesting.com>
To: <amps@contesting.com>
Date: 07 September 2000 03:40
Subject: Re: [AMPS] Blown TL922A... What to do?
>
>Tom wrote,
>
>"Sounds like you have outgassed. You need to take more nichrome...
>
>The tank is typically a virtual short for VHF and UHF energy,
>because it has a capacitor shunting the input.
That's not quite right, Don. The tune C has inherent inductance, so you get
resonances. The resonance frequencies vary with shape, size and capacitance
setting, but in all that I have put on the network analyser, the lowest
frequency resonance is 'series', or low impedance and the next resonance is
'parallel' or high impedance. I'm using the term resonance to mean points
where the impedance goes purely resistive.
The one I looked at yesterday was 100pF max, .06" spacing and about 2.3"
cube overall. The first, series, resonance came in at 90-150MHz, depending
on C setting, and the higher one was in the 200-300MHz region. The higher
frequency one was low Q.
Looking back to previous comments and threads:
The impedance at the capacitor terminal is low either side of the series
resonant point. How low depends on how far you move in frequency, but it
doesn't suddenly go high impedance at any frequency 'close' to the resonant
point.
Adding inductance in series with the capacitor (to simulate the lead from an
anode) changes the resonant frequencies, but doesn't change the overall
characteristic.
Adding a L/R parallel circuit in series with the capacitor introduces loss
as the frequency increases, but does not introduce multiple resonances. As I
understood what he said, Rich suggested that the separate current paths
through the L and the R should produce multiple resonances. I said then I
thought his analysis was wrong. I still do.
If anyone is interested in putting them on a web page, I will run off some
pictures.
Steve
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