Impedence?

rjo__

What I like about Chip is that he talks to us ... just like we understood exactly what he is saying.
It makes me feel much better about myself:)

Last weekend Chip was describing the architecture of a P2 pin, which as I understand is an infinite state machine, described in terms of finite state, some of which
require selecting from a limited set of impedence states. Then I remembered... I never closed the book on impedence. Something hung in my craw.
It still bothers me, like an itch I can't reach. I did my due diligence and came up empty... asked questions and remained confused.

This morning it came to me... impedence is simply a way of indicating the current limit of a signal source at a particular voltage.


Is it?

Thanks

Mike Green

Electrical impedance is "simply" the equivalent of resistance, but in the AC realm. See the Wikipedia here.

LoopyByteloose

Mike Green has said what it is short and simple. But that may be a bit hard to comprehend unless you understand the jargon.

When using pure unchanging DC, you have Ohm's Law where E = I x R, and R is resistance. For all of that, it seems quite easy.

But when you have cycles of change in your Voltage, the factor of frequency changes the formula to E = I x (Z+R) [Many writers just say E = I x Z, but there is the plain old resistance included.]

>>> Mostly impedance is a concern with small signal pulsating DC or AC that is used at a high rate, but is does increase the overall resistance (the combination of resistance and impedance) of the circuit in predictable ways.

We use capacitors and coils to create two kinds of impedance (resisting anchange in input voltage and resistring a change in output voltage). And we use the two kinds to create filters and oscillations.

Try to understand how capacitors and coils are used in applications. With pure resistance, AC behaves much as DC for a power source, that is why with have 60 cycle AC power for homes.

But as you increase the frequency to thousands or millions of cycles per second, the impedance will go way up.. even with small coils and small capacitors.

At times impedance can cause problems if it is unwanted; but at other times it is very hand if needed. Microcontrollers are very dependent on an oscillator to provide a clock to measure time and to control rates. That is a useful application of impedance. Building a radio antenna and no matching the right size of everything will waste a lot of energy and affect your neighbor's TV reception, that is an example of an improper application of impedance (But generally this is not an issue in small microcontrollers as they use very little power).

With digital hobby electronics, most of the problems of impedance are resolved inside the chips. So we generally have very little to worry about outside the chip unless your are building a motherboard in the Ghz range. The i/o you use may operate at unpredictable intervals, so it just has been designed to overcome any problems associated with impedance mismatch. But you can see signs of impedance in the imperfect shape of the square wave on a scope. The rounded corners are due to impedance creeping into the signal.

rjo__

Thanks guys. I have seen these explanations before... and of course didn't remember them, because
I can only vaguely relate to them.

Let's take a typical video signal... output on 3 pins from a Propeller. The usual circuit
shows a resistor hooked up to each pin but with different values... 1.1k, .560K, and .270k. and then all of those lines terminating in the video line.

How would we know that this line would have 75 Ohm impedence? how would we measure it?

LoopyByteloose

Well, the total resistance should be very close to 75 ohms of load. But there have been some discussions on improving the color of the Propeller out by tweaking those values to get everything to provide better color and a closer fit to the 75 ohms.

I guess what I am saying in this case is that the load resistance is seen as the total impedance.

If you want to look at something else, op amps inputs are supposed to have near infinite impedances, but when you want an output to drive something that expects a certain load, a resistor is inserted to ground to fool that output into seeing what it wants... and not the infinite impedance.

Try Elliot Sound Products http://sound.westhost.com/ for good discussion on audio impedances. And remember that as the frequency increases, impedance becomes more and more of an issue. So most of the really important applications are in radio frequencies, not audio.

Often audio impedance discussion get confused as the audio range is a bit wide, from 20hz to 30,000hz maximum. That is a wide bandwidth and very sloppy for dealing with making impedance work perfectly.

Audio is just a good playground to begin to learn. And op amps are very informative. So is antenna construction, data transmission, and RADAR.

rjo__

Nice link... popular site 10,000,000 visitors in 12 years.
http://sound.westhost.com/impedanc.htm

This should keep me busy for a while.

Thank you very much.

Hal Albach

"We use capacitors and coils to create two kinds of impedance (resisting anchange in input voltage and resistring a change in output voltage). And we use the two kinds to create filters and oscillations."

I believe it should be 'capacitors resist a change in voltage and inductors resist a change in current'.