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Re: Measuring self-capacitance directly (Re: flat secondary)



Original poster: "rheidlebaugh by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rheidlebaugh-at-zialink-dot-com>

Antonio; I nead your councel. I know what distributed capacitance of a coil
is. Why isnt the shunt capacitance of a plate on the side of a flat coil
nearly the same as distributed capacitance. No not exacty, but nearly the
same for practical uses ?
  Robert  H

> From: "Tesla list" <tesla-at-pupman-dot-com>
> Date: Wed, 13 Mar 2002 19:15:43 -0700
> To: tesla-at-pupman-dot-com
> Subject: Re: Measuring self-capacitance directly (Re: flat secondary)
> Resent-From: tesla-at-pupman-dot-com
> Resent-Date: Wed, 13 Mar 2002 19:48:27 -0700
> 
> Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>
> 
> Tesla list wrote:
>> 
>> Original poster: "Paul Nicholson by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>
> 
>>> The model reproduces well the input impedance, and quite well
>>> the output impedance too, even without the Ldc-L1-L3 inductance.
>> 
>> Except for the real part of the input Z, which is zero. Through most
>> of the frequency range this is negligible error, but becomes
>> significant near f1, f2, and fp.
> 
> Certainly, because it is a lossless model.
> 
>>> As a lossless model, it doesn't model the Q factor, but with
>>> a resistor added at the input (series) it would model it too.
>> 
>> Yes, but with these component values, you can choose a resistor to
>> match a given Q, or to match a given Zin, but not both together.
> 
> Ok. With one resistor only one maximum or minimum of the input or
> transfer impedances would be correctly modelled. The others would
> be approximations. With more resistors, the model would match other
> points too.
> 
>> To match both Q and Zin with a single resistor, we would have to use
>> different L and C values, derived from the energy storage behaviour of
>> the resonator.
>> 
>> Let me try to explain why, by using a simplified distributed model of
>> the base-driven unloaded secondary.
>> 
>> Consider the coil in n sections, eg
>> 
>> Base          V1     V2           Vx              Vn-1     Vn   Top
>> o---------L1--+--L2--+- ... --Lx--+-- ... --- Ln-1--+--Ln--+---o
>> |      |            |                 |      |
>> C1     C2 ...       Cx  ...           Cn-1   Cn
>> |      |            |                 |      |
>> ===    ===          ===               ===    ===
>> 
>> so that the *peak* voltages across each cap C1..Cn are V1..Vn.
>> Treat the coils L1..Ln as lumped self inductances - the mutual
>> inductance has no effect on this argument.
>> 
>> At a point in the RF cycle when there is no current flowing, the total
>> stored energy is
>> ...
> 
> I see a problem in this derivation: If the coil follows the lclclc...
> model shown, there is no point in the RF cycle where all the energy
> is stored in the capacitors. The energy is always distributed among
> the capacitances and the inductances. You can only assume that at some
> point all the energy is in the capacitance if there is just one section,
> falling then into the usual model, with a single L and a single C.
> 
>>> Fp is the frequency where the input impedance has poles, a maximum,
>>> not a minimum.
>> 
>> Yes, I thought it better to the find the Zin pole by looking for the
>> Ztop zero.
>> 
>>> Measuring from the other side, with the base open, the same poles
>>> would appear too,
>> 
>> Yes, but I meant the base to stay grounded, so that the Zin pole
>> becomes a zero at the top.  I just thought it would help to eliminate
>> the effect of instrument stray C.  But then you said
> 
> Something strange here. Zin is the base impedance with the top end
> open, or the z11 open-circuit impedance parameter of the network.
> The impedance seen at the top end with the base grounded is the
> inverse of the y22 short-circuit admittance parameter of the network.
> Some calculation shows that the poles of z11 don't appear anywhere
> in y22 (the networks are different, and can't have the same natural
> frequencies *), and that the zeros of z11 are also zeros of y22.
> Or simply:
> Looking at the top impedance with the base grounded, you see
> poles where zin has zeros, and nothing special where zin has
> poles.
> See this example, with a simple c-l-c-l-c network (values 1,2,3,4,5 from
> left to right):
> http://www.coe.ufrj.br/~acmq/tesla/z11xy22a.gif
> The plots of z11 and 1/y22 result in:
> http://www.coe.ufrj.br/~acmq/tesla/z11xy22b.gif
> The same happens with any linear circuit that has shunt impedances
> across both ports.
> Anyway, to measure at the output, with the base open, is a valid idea,
> since there is more capacitance at the top end (with top load), and so
> the measurement is less sensitive to parasitic capacitances.
> 
> * This is more complicated to demonstrate than appears to be...
> 
> Antonio Carlos M. de Queiroz
> 
> 
> 
>