Guys all this is first year circuit theory. We all know and come across series to paarllel convertions of reactive networks that introduce FDR effects where the real part of the immitance function varies with freq and these are LINEAR circuits. And as no real device is devoid of inductive/capacitive parasitics ,no matter how small, freq dependancy should not be a mistery.
Nonlinear effects were the reason I started this thread for modeling such things as ferrite cores. Manufactures rountinely supply spice parameters for these devices which involve freq dependant effects related to nonlinear loss mechanisms ( hysterisis etc ) .
George
_____
From: Hubert Hagadorn [mailto:huhaga@...]
To: LTspice@...
Sent: Thu, 15 Sep 2011 21:20:01 +0100
Subject: Re: [LTspice] Re: Freqeucny Dependent resistor
I believe it is impossible to have a physically realizable resistor that is frequency dependent and has no reactive component. A transmission line comes close in that its resistance is almost constant for a range of frequencies, but as you know at very low frequencies, neglecting any series resistive components, its reactance is largely capacitive.
Hubert
----- Original Message -----
From: Tony Casey
To: LTspice@...
Sent: Thursday, September 15, 2011 12:43 PM
Subject: [LTspice] Re: Freqeucny Dependent resistor
<snip>
--- In LTspice@..., Christian Thomas <ct.waveform@...> wrote:
Well, that's a question AG.
Might we not be looking at a naive question here? Ie. Can I please have a
resistor that changes with frequency but with none of those nasty reactive
elements? If that's the case then looking in the s-plane is not the answer
being sought.
In which case the answer needed is "No, you can't. Or at least you can't
have a full solution. (I think that must be right). But we do have some
useful reactive components that perform that function, and that's what
everyone else uses. C and L in LTSpice; and their s-plane behaviour is
built in."
CT
</snip>
Hello Christian,
I'm sure you ask the question tongue-in-cheek, because you surely must be aware of instances where the real part of an impedance changes with frequency without significant change in the reactance.
What about the resistance of straight length of wire? This increases due to the skin effect, whereby as the frequency rises more and more of the current travels closer to the outer (indeed for circular cross-section, the only) surface of the wire, so in effect reducing the cross-sectional area of the wire. In the limit, there is also a change in the inductance per unit length too, but it is not significant compared to the change in resistance.
And although not strictly a "component", there is the free space acoustic radiation resistance of a diaphragm, which also rises with frequency up to the frequency where the circumference is approximately equal to the wavelength. I will concede in this example that the reactive part of the impedance also changes at a fair rate of knots over the same frequency interval.
I'm sure you already knew all that. But it does illustrate why it is perfect legitimate to seek frequency-dependent resistance models.
Regards,
Tony
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