Re: Series resonance/Was: Waveguide TC

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Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


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Sent: Saturday, December 14, 2002 8:51 AM
Subject: Series resonance/Was: Waveguide TC


 > Original poster: "Jolyon Vater Cox by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
 >
 > Jim,
 >  >From what I understand an open-ended 1/4 wave transmission line is
 > series-resonant circuit; series resonant tuned circuit have 90 degree
phase
 > shift between current at
 > the driven end (low impedance) and current at the terminal end (high
 > impedance).
 >
Except that by Kirchoffs Current Law, the current in and out of the
terminals must sum to zero. Therefore the current at the bottom of the
secondary has to be equal to the current at the top of the secondary.
EXCEPT.. there is some parasitic capacitance (not insignificant) from the
secondary winding to "ground", and some current flows through that
capacitance.  But, a number of folks have made measurements of the current
at top and bottom and find they are only a few degrees apart.

 > A lumped series-resonant circuit can be visualised as an inductor L and a
 > capacitor C
 > in series with a signal source or "generator". Surely there is a 90 degree
 > phase shift between current at the driven end and the current at the
 > "terminal" end i.e. current flowing "through" the capacitor?

Nope.. in a series circuit, the currents all have to be equal.  The 90
degree phase shift is between the voltage across the device and the current
through the device.

 >
 > There is a common belief (incorrect?) that the secondary of a TC is a
 > series-resonant
 > circuit - I might be wrong but would this not presume a 90 degree phase
 > shift between input and output-
 > whereas observations made on real TCs suggest little or no phase shift
 > between currents at the grounded and the terminal ends.

Indeed the secondary of a TC IS a series resonant LC circuit. Actually, a
bit more complex, the TC is a pair of coupled series resonant circuits,
coupled via the flux linked between secondary and primary.

 > Is the "series-resonant secondary" like the quarterwave resonant theory
 > incorrect
 > -or is it possible to have a series-resonant tuned circuit with
 > little or NO phase shift between low-impedance input and high-impedance
 > output?

 > Or does the series-resonant voltage boosting effect involve not just the
 > secondary but
 > the primary as well -with the primary leakage inductance resonating with a
 > "transformed" value of the secondary capacitance via the magnetic
coupling?
 >

In a "disruptive" TC (i.e. not a CW oscillator), there's no "boosting" due
to resonance (perhaps you're thinking of the voltage rise due to resonance
in a tuned circuit?), because you're only putting in one "lump" of energy,
rather than a new burst on every cycle (as in a CW coil, where resonant rise
IS the mechanism, or, in a transmission line transformer, which is an
inherently narrow band device).