I understand what impedance is. Help me understand why it matters.

[nobodysweasel]

Okay, so there's a lot of talk in on the forum about impedance.  I've done my research to try to figure this out before coming to post on here.  I've read about it in several threads, basic electronics books, even watched a few youtube videos.  If you give me a capacitance and resistance I can calculate Z at whatever frequency.  The problem is, all of this information was in a general electronics context.  I have no idea how it relates to signal processing.  It seems I see the term "input impedance" a lot, so I'm guessing it's important, but I don't know why.  Can anyone help me?

My hunch is that there's some really great article or forum post out there that explains this, but for all my searching I just can't find it...

[markeebee]

As always, Geofex nailed it for me.  Though I still haven't quite got my head around it.

So I'll chip in this link and continue watching this thread for enlightement:

http://www.geofex.com/article_folders/impednc.htm

[lopsided]

there is also a nice long reply on impedance and why it matters from R.G. in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=647.msg5282#msg5282

[nobodysweasel]

Thanks for the speedy replies!  Time to get reading....

[Ripthorn]

I have not read those links recently, but also remember that impedance is independent of the type of energy being transferred.  What I mean by that is that there is electrical impedance, acoustical, mechanical, thermodynamic, etc.  The mathematics that governs them is all basically the same (there are specialized formulations that make it easier or that have particulars for a certain area, but they are all governed by the same generalized equations).  For example, all my research in college was in acoustics, but now I work in radar and the math is all the same for propogation, transfer, etc. so it was a relatively easy transition to make.  In fact, when I think of electrical transfer, I still picture it as acoustics in my head.

I'm sure the links cover it quite well, as R.G. is the man, but essentially impedance governs how the energy gets transferred and in what form (reactive, resistive).

[R.G.]

Impedance of a circuit element or combination of them represents how much current flows for a given voltage and frequency applied. It's the generalization of Ohm's Law to AC and variable frequency.

It matters because we use impedances to control currents and voltages to get what we want to happen in a circuit; and it matters because every circuit element comes with a slew of impedances built in.

Let's take the simple (but important to pedal builders) example of amplifying a signal from a magnetic guitar pickup. The pickup itself is a coil of wire in a magnetic field. It has a resistance of about 4000 to 18,000 ohms. This hardly matters at all compared to the inductance of that coil, which can be as high as 4 Henries, and the self-capacitance of the coil, which shunts the signal to nearly nothing above perhaps 7kHz. The sensing of the string's motion is linear up to megahertz and beyond; but the self-impedance of the pickup coil means that it cannot put out a voltage signal much above 6-8kHz.

But we now want to amplify that signal. The input impedance of the amplifier matters because the output impedance of the pickup (and the controls on the guitar, and their settings...) limit how much current can flow and at what frequencies. As a simple example, a Fender single coil pickup has an inductance on the low side, about 2H. That means that the pickup has an internal limiter on how much voltage gets outside the pickup of Z = 2*pi*F*2H. This rises linearly with frequency, of course. And if the input impedance of the amplifier is much less than a megohm or so, you get moderate-to-severe loss of treble. This has been enshrined in muzik-speak as "tone sucking". So impedance matters in the input of the amplifier because it can not only result in loss of the input signal, it can result in losses which affect the frequency response in unfortunate ways.

This little dance of output impedance to input impedance happens at every place in the circuit where you connect signals. And it is the fundamental way we have to control what signals pass, and how big they are, and what *frequencies* pass and are amplified/attenuated.

So impedance matters as a mental tool; it is the fundamental way we control what happens in a circuit. I've mentioned it before, but it's worth saying again: it is entirely possible to have a complete, successful electrical engineering career with only a full and complete understanding of Ohm's law and its various applications.

[nobodysweasel]

Great replies, thanks!  I'm already understanding this much, much better.  Not to mention it's helping me understand buffers, which never made sense to me before.

Ripthorn, now you've got me thinking about where else I might have learned something equivalent to electrical impedance.  I'm studying hydrogeology right now....I know Darcy's law is the hydrologic equivalent of Ohm's law, so I wouldn't be surprised if I can find an equivalent to impedance also.  I've also done a lot of math regarding ground penetrating radar and seismic surveys (propagation, reflection, and refraction of radar and sound waves), so I'm positive I've learned it there, just haven't made the connection yet.  Time to dig through the old notes!

And R.G., as always your posts are extremely helpful.  Your work has been extremely valuable to me in my awkward noob stage.  Thanks again.

[R.G.]

One of the things we learned in controls is that there are analogies to resistance, impedance, inductance, capacitance, etc. in many mechanical and thermal systems. Yep, hydraulics and most likely hydrogeology should have the analogical items to electrical, as would acoustics and other wave propagation phenomena. Some systems don't have all the analogous items - for instance, there is no thermal analog to inductance, although there exist analogs to resistance, capacitance, voltage, current, etc.

[frank_p]

analogies to resistance, impedance, inductance, capacitance, etc. in many mechanical and thermal systems.

Very good:

http://www.ece.utah.edu/~ece3510/Notes_Mech_to_Elect.pdf