Peter Chadwick wrote:
>Steve said:
>>In a finite universe I figure there's always going to be some
>capacitance and/or inductance and/or resistance between the 'open' ends
>of a series L and C, so everything is a parallel circuit. The issue is
>whether it's significant.<
>
>All that does is complicate the circuit. The point I'm trying to get
>across is that the current induced in the coil produces an EMF in
>series with the coil. Not in parallel with it - if it was, a non
>resonant coil in a strong RF field would get hot. So the circuit is a
>series circuit of the EMF generator, the coil and the capacitor.
Surely the important thing is whether there is a CIRCUIT, in the strict
literal sense of that word - a closed loop around which current can
circ-u-late.
Without a closed loop, no current can flow from one part of the circuit
to another, so there can be no exchange of stored energy between
electric and magnetic fields, and therefore no possibility of resonance,
no coupling with the coil of a GDO, and no "dip".
Also the resonance belongs to the whole circuit - not just the single
component that you're trying to test. This is where the problems begin,
when the circuit is completed by unknown factors like distributed
capacitance, or stray inductance in other parts of the loop.
The grid in a "grounded grid" amplifier is a case in point. It is
behaving as an inductive length of wire, terminated inside the tube by
some distributed capacitance to the anode and the cathode... which in
turn are connected somehow back to ground. Certainly you have a circuit
there, with inductance and capacitance in the loop, so it will have a
resonance; and if you can couple a GDO into it, you'll see a dip.
But that resonance belongs to the whole circuit, involving the anode and
cathode and all their associated components. There are too many unknowns
in that loop to understand what the frequency of the observed dip might
be telling us about reverse feed-through from the anode back to the
cathode.
>I must admit that I've never had a lot of use for GDO
Me neither, because it's so difficult to understand what a dip actually
means, except in the very simplest cases.
The GDO is deceptively simple instrument. The trick is to tell when it
stops being simple, and starts being deceptive.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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