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FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator
tinhead:
--- Quote from: maxwell3e10 on August 23, 2019, 09:13:03 pm --- I couldn't resist.
--- End quote ---
https://en.wikipedia.org/wiki/Idiocracy
BU508A:
--- Quote from: maxwell3e10 on August 23, 2019, 09:13:03 pm ---No one argues that a transmission line needs termination. The question is what constitutes a transmission line. The general rule is that the length of the line needs to be comparable to the wavelength of the signal transmitted. That is why impedance matching only becomes an issue at higher frequencies (shorter wavelength).
--- End quote ---
Nope. The general rule is, what the source "sees" when it is going to leave a signal. If you put a wire into thin air,
then it will see the impedance of free space, around 377Ohm. If the Impedance of the source is different from that,
which is mostly the case, then it will come to distortions caused by reflections. If you put a piece of cable with the
impedance of 50 Ohm to the source, no matter how long it is, then the source will "see" these 50 Ohms at first.
If the impedance of the source differs from these 50 Ohms, you'll have reflections, distortions etc.
That is why the impedance of the source should match the impedance of the transmission line, which is seen
by the source at first.
Assuming the transmission line is homogen all over and it hits the end, then the same will happen as at
the beginning.
Therefore: no matter, how your transmission line is looking, if you want to avoid reflection and distortion, then
you have to follow the rule:
impedance of the source = impedance of the transmission line = impedance of the target.
This is independet of the length.
For this reason we have antennas. They adapt the impedance of the source to the impedance of the target.
In ideal cases you have a SWR of 1:1 which means that nearly all of the energy coming from the source will
reach the target. No reflection. No distortion.
maxwell3e10:
This should really go to a different thread. But two questions come to mind. a) What is the impedance of your audio headphones "transmission line"? and b) If you design an RF circuit (below 1 GHz), does every node on it sees 50 Ohm?
radiolistener:
--- Quote from: maxwell3e10 on August 23, 2019, 09:13:03 pm ---It has already been shown by radiolistener that changing impedance matching only affects the ringing on pulse transitions
--- End quote ---
my example is not the worst case for impedance mismatch. It's just random mismatch. So it cannot be used as example that the difference is not so significant.
For clarification here is impedance measurement of my oscilloscope 1M input:
- at 10 MHz = 57 - j884 Ohms
- at 100 MHz = 43.9 - j96.5 Ohms
As you can see, my oscilloscope input impedance at 10 MHz is not 1 MOhm, but 57-j884 Ohm. So, you're needs to take this into account.
And don't expect that your oscilloscope will give you the same impedance mismatch, because your oscilloscope may have different input impedance at 1 MOhm mode.
BU508A:
The rule is the same, but in this frequency range the distortion is very low. The main reason for this is,
that the source has usually a very low impedance and the target in respect a high impedance. If some energy is
reflected, it will be absorbed by the source.
Most of the audio equipement which uses unbalanced connections have a source and target impedance of 47kOhm.
But as I said, in this frequency range the "wrong" impedance of the cable doesn't really matter.
Regarding the RF circuit: no one said, that it must have an impedance of 50 Ohm. The only condition is:
source Z = transmission line Z = target Z. If the signal leaves the circuit one have to make sure,
that the impedances are adapted, for example by using a Pi-network.
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