Impedance

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

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.

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

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

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

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).

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

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.

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.

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

the

If you want to learn more about audio equipment, I'd start with the prosoundweb's study hall. Impedance:

ttyl,

--buddy

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

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

Cheers! Rich