# Impedance

Discussion in 'Electronic Basics' started by Music Man, May 6, 2005.

1. ### Music ManGuest

How does impedance mismatches alter signals?
I know you lose HF but always wanted to know how different frequencies
are affected by different circuitry.

Thanks

2. ### John PopelishGuest

Impedance is an expansion of the concept of resistance to include
time. Resistance relates voltage across something to the current
through it. Another way to say ohms is volts per ampere. But that
description of the relationship between voltage and current does not
involve time. It describes stuff that only consumes energy when
voltage is connected across it, and does so, regardless of how long
the voltage is applied, or how fast it changes.

When you include the possibility of energy storage as well as
consumption, time becomes an important factor. For example,
capacitance stores energy in proportion to the square of the voltage
across it (E=(1/2)*(V^2)*C), but it takes time for a given current to
build up that voltage (I=C*(dv/dt) or the capacitive current is
proportional to the time rate of change of voltage across the
capacitor). So once energy storage is involved, you need a two
dimensional description of the relationship between voltage and current.

Back to the frequency effects: If an impedance has a capacitive
component, then the current that passes through it will increase when
the voltage changes faster. If the current through that impedance
arrives through a series resistance, then as frequency rises, more of
the total applied voltage will get used up across the resistance
(because the current is rising) and less will appear across the
capacitive impedance (because the total of the resistive and
capacitive voltages must add up to the applied voltage). This forms a
basic low pass filter if the signal across the capacitor is the output.

3. ### cedirx.metalixGuest

Easy, first of all this applys only if the dimensions of your circuit
are much much smaller than the wavelenght... High Frecuency... on easy
words. When you send a wave trough a transmission line (e.g. coaxial
cable) and the other end is not impedance matched, you get a stationary
wave patern on you transmission line, this makes some of the transmited
power to return to your transmitter. On High Power RF trasnmitter you
burn the Tx. On your little circuit it just reduces the power you get
at the other end. The stationary wave patern depends of the frecuency
and the lenght of the line.
You can find anything about it on a "Transmission Lines" book.

4. ### Bob EldredGuest

First off, maximum power is transferred when impedances match. A mismatch
will cause less power to be transmitted to a load than when matched. This is
especially important when the power is extremely low like signals from an
antenna where every little bit of signal counts. Also getting the maximum
power to a load from an amplifier or transmitter can be an important reason
to have matched impedances.

Secondly, part any signal moving down a line will reflect off of an
impedance mismatch and reverse direction back toward where it came from. It
reflects back because maximum power is not transferred across the mismatch
and that not transferred gets reflected. The reflected signal will
interfere with the forward signal and produce peaks and valleys of
amplitude. These peaks and valleys forms a standing wave (doesn't move in
position) on the line that can interfere with the forward transmission of
signal. A measurement in voltage of the peaks and valleys is called the
Voltage Standing Wave Ratio, VSWR and is an indication of the amount of
mismatch.

One consequence of a mismatch condition can be seen on a TV screen when
sharp vertical edges like lettering show ghosting or multiple lines near
each other where there should only be one line. This ghosting is caused by
multiple reflections back and forth on a line or in the air. Similarly, data
transmissions can be compromised by reflections that smear out the data
timing.

These effects occur at all frequencies and are not frequency dependant.
However, if the wavelength on a line is large compared to the dimensions of
the line, or length of mismatch spacing, there is rarely a problem. That's
why this usually becomes an issue at RF frequencies but not audio
frequencies or below.

However there are many times when an impedance mismatch is desirable even
necessary. For example when you plug a light into the wall, the impedance
must be mismatched because you do not want maximum power transferred, you
want all of the power required to light the lamp transferred but not all of
the power the generator can deliver. The impedance of the wall is near zero
and the lamp is, maybe 140 ohms, a definite mismatch, but it's the only
practical way to deliver power without the source wasting half of it as
happens when impedances are matched. This is called constant voltage and is
the way most circuits work. In other words, most connections are not
impedance matched.
Bob

5. ### Tom MacIntyreGuest

I may be wrong, but, in strictly technical terms, isn't it when the
resistive components are equal and the reactive components are equal
but opposite, complex conjugates (?) of one another?

Tom

6. ### Bob EldredGuest

Yes, you are correct but I'm trying to keep it simple and not bring
reactance into it at this level. First things first is a basic understanding
of the underlying principles and why line reflections occur and are
important. If the basic concepts are not there, complex conjugates are
going to fall on deaf ears.
Bob

7. ### Music ManGuest

So are you saying,basically,that the high capacitance can't deal with the
faster frequencies?
Thanks

8. ### John PopelishGuest

Capacitance deals with high frequencies just fine, by sucking current
out of it. ;-)

Seriously, the effect is that the higher the frequency, the lower the
impedance (volts per ampere) the capacitor exhibits. This can be a
bad thing if you are trying to avoid it (like sending audio through a
long chunk of high capacitance cable from a high impedance source like
a guitar pickup, and the cable soaks up the higher frequencies) or
real handy if you are making use of the effect (like you do in a tone
control circuit).

9. ### Music ManGuest

So what is the relation to frequency and capacitance John?
Could you explain on how audio signals are "expressed" in electronic
circuits.
What is need to creat a clean signals?How resistors creat noise and add to
signal?
What I mean is why would a Neve mixing console sound better than a cheapo
desk.

Thanks

11. ### John PopelishGuest

The effect of capacitance (the ability of the capacitance to pass
current per volt across it) is proportional to frequency. Pick any
capacitance and this formula tells you the volts needed to drive 1
ampere through it. But volts per ampere are called ohms, so the
formula is Xc = 1/(2*pi*f*C)
where Xc is the capacitive impedance in ohms (or volts per ampere), pi
is 3.14159, f is frequency in hertz, and C is capacitance in farads.
Either as a voltage that represents the signal or as a current that
represents the signal. If a capacitor is in series with the signal,
it passes it better as the frequency goes up. If the capacitor is
between the signal and ground, it drains more and more of the signal
to ground as the frequency goes up.
Whole different subject. Resistors (the gas of electrons in
resistors, actually) make noise just to be in thermal equilibrium with
their surroundings much like gas molecules bang around just from
thermal energy. Then when you pass current through resistors, it
bumps and bangs and surges a bit, because the current is composed of
finite charges, not a smooth fluid, adding a different spectrum of
noise to that from the unbiased resistor. You should probably read a
bit on this and come back with questions.
http://zone.ni.com/devzone/conceptd.nsf/webmain/8DE42E13BD089D8B86256816006545CD?OpenDocument
There are noisy resistors (noisier than can be explained by thermal
noise), low quality capacitors that pick up vibrations and change
capacitance as the signal voltage swings, opamps that are noisier or
quieter, well shielded and poorly shielded designs, etc.
Understanding all that, including the etceteras can take a lot of
study and experience.

12. ### Rich GriseGuest

Xc = 1 / (2 * pi * f * C)

Xc = capacitive reactance in Ohms
f = frequency in Hertz
C = capacitance in Farads.

Cheers!
Rich

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