impedance

[markw at mohawksoft.com]

> From: Tom Metro <blu at vl.com>
> Subject: Re: impedance
> To: L-blu <discuss at blu.org>
> Message-ID: <45CA8EB4.7080109 at vl.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> markw at mohawksoft.com wrote:
>>> (he meant impedance, rather than inductance).
>>
>> Actually no, pedant point here, however, and inductor has inductance. A
>> cable (any wire, really) acts like an inductor.
>
> It acts like a resistor and a capacitor as well, and thus impedance[1]
> is more accurately descriptive.

No, "A" wire does not act like a capacitor, but two wires next to each
other may. A capacitor works on the differential of electrical charge.

An inductor works on the principle of electromagnetic force that is
generated as current passed through wire. The creation of the EMF is
"inductance."

In fact, this discussion is getting WAY beyond anything that can be
effectively discussed in this medium, it is pretty grounded in basic
electrical theory, and the high speed transmission discussion is pretty
advanced, without knowing how versed the participants are with the theory
and the math involved, it just becomes a frustrating series of
explanations that could be found just as easily in books like Horowitz and
Hill.

That being said, using the term "impedance" in such a discussion is, IMHO,
wrong. "Impedance" is an effect caused by inductance, capacitance, and
frequency of regularly repeating sine wave.

Digital signals typically do not have "impedance," per se' because the
leading edge of state change is not a gradual change, it is a change that
is as fast as can be driven, followed by a non-changing period. I guess
"ringing" can be seen as a sine, but those are artifacts.

If you are talking about data transmission over a carrier signal, say like
a cable modem, that is different than something like USB or SATA. That is
an RF discussion.

Since "impedance" in a circuit is usually measured based on frequency,
what is the impedance of a circuit to the leading edge of a square wave,
and how does that apply to the frequency of the signal?

Like I said, its starting to get involved, and its getting late.



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