Impedance
in [Basics]
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From: Music Man on 6 May 2005 12:13 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
From: John Popelish on 6 May 2005 13:05 Music Man wrote: > How does impedance mismatches alter signals? > I know you lose HF but always wanted to know how different frequencies > are affected by different circuitry. 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.
From: cedirx.metalix on 6 May 2005 23:50 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.
From: Bob Eldred on 9 May 2005 17:43 "Music Man" <j.michael(a)cwcom.net> wrote in message news:427b97b1$0$335$cc9e4d1f(a)news.dial.pipex.com... > 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 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
From: Tom MacIntyre on 9 May 2005 18:15
On Mon, 9 May 2005 14:43:33 -0700, "Bob Eldred" <nsmontassoc(a)yahoo.com> wrote: > >"Music Man" <j.michael(a)cwcom.net> wrote in message >news:427b97b1$0$335$cc9e4d1f(a)news.dial.pipex.com... >> 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 > >First off, maximum power is transferred when impedances match. 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 |