Re: absorbing reactance into series LC


<maxfoo@xxxxxxxxxxx> wrote in message
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On Feb 19, 6:14 pm, "john jardine" <j...@xxxxxxxxxxxxxxxxxxxxx> wrote:
"Phil Newman" <phillenium2...@xxxxxxxxxxx> wrote in message

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If your X is frequency invariant then it is a resistor.
If you have L C R in series (where R can be the resistance of the
L),
then in resonance you will see R (real).
R also sets the Q factor of your series circuit, so its bandwidth.
http://en.wikipedia.org/wiki/RLC_circuit

thanks for your answer.

However, I beleive the job of the susceptance/reactance is to shift
the resonant frequency of the LC series from w = 1 to another
frequency, which denotes the transmission zero of the filter.

Phil

It's very difficult to understand the wording.
It is possible to drop the resonant frequency to say 0.5 Rads, using
just a
resistance. Maybe that's what's wanted in this case.
john

To drop the res freq you increase the inductance and/or capacitance.
Increasing resistance increases the bandwidth not the resonant
(center) frequency. Reactance is XL=2*pi*F*L, XC=1/(2*pi*F*C), so
changing the reactance requires change L, C, or F.

Yes. But ... It's not the size of that resistor that's the problem, it's
knowing where to put it!.

As example, for a parallel tuned circuit we always go ...
Fres=1/2*pi*root (L/C). Fine, no problem!.

It's not correct. The true formula is ...
Fres=1/2*pi* root(1/L*C-R^2/L^2). [a ballache to use so we don't]

But ... look how that coil resistance "R" has slimed itself in and
ingratiated itself with the resonant frequency. (only rears it's head at
very very low Q values ). That "R" can become big enough to noticeably
poison the reactive effect of the inductor and drop the Fres.

Idle thought could suggest maybe it's not unreasonable that a series tuned
circuit, which is a kind of inversion of a parallel one, might for a similar
reason also suffer an "R" caused resonant frequency shift. I.e same lousy Q
and lower Fres.
In this case (as you note), the "R" can have no Fres effect if in series
with the inductor, so maybe it needs to go in ... with the ... :)

(Dragged out I know but I'm constantly surprised at the number of ways a few
RCL components can be put together yet offer distinct features)
john



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