Music Man said:
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