I'm studying op-amp circuit combined with rf mixer.
While I was reading articles on the circuit,I found that the term like
"50 ohm load"
or "50 ohm ( )" occurs frequently.
I also remember that one of the input options of Digital oscilloscope
was 50 ohm load.
Because I'm a newcomer to analog electronics, I don't know
significance of that term.
Why should we think about the specific value 50 ohm?
How do we get such value?
I'll add to the already excellent answers, and we'll assume 50 ohms as the
Value of the Gods. Transmitters and other signal sources will have an
output impedance of 50 ohms. It is expected that they will drive a 50 ohm
load, as, after all, that is the Value of the Gods. As John Fields hinted,
when the source impedance matches the load impedance, maximum power is
transferred. There is an engineering theorem addressing that phenomenon,
and is called, appropriately, the Maximum Power Transfer theorem.
Another topic that was hinted at was "reflections." A reflection is a
return of the transmitted signal back into the transmitter. If a
reflection happens, it indicates that the power transfer is not perfect,
and hence that source and load impedances are not perfectly matched.
Reflections can be measured and compensated for.
Here's an interesting phenomenon, and one that bears learning: if you
disconnect the 50 ohm load and drive an RF or audio source into practical
infinity, the observed voltage at the output will double. That might seem
like a good thing on the surface, but it's impractical and potentially
damaging to the source. One thing that happens with such a driven source
is that reflections are maximized (this in fact is one part of a reference
test for a transmission line and antenna - driving an open line, and
driving a shorted line). When reflections are maximized, all sorts of
harmonic distortion can be introduced on the waveform.
When testing RF transmitters, a "dummy load" is always attached to the
output if there is no antenna path into free space, in order to prevent
reflections and possible damage to the transmitter.
The concept of matching source to load applies at lower audio frequencies
as well, such as when fitting loudspeakers to a stereo system. The best
arrangement is to match source and load as precisely as possible. The
common example is 4-ohm auto systems vs. 8-ohm home systems - ideally, one
shouldn't mix components between the two.