-1
\$\begingroup\$

I have another question about transmission line behaviour. People always tell (at least I read it multiple times in various articles while googling) that when one is measuring the voltage along a TL (assume an ideal scope probe, i.e. the probe has no influence), the reflections will always be worst in the middle of the TL. So it is said to be best to measure voltage at either the receiver or the transmitter side, but the "eye pattern" is said to be worst in the middle. Now why is this? If there are NO reflections (ideal case), i should measure the "true" and undistorted signal everywhere along the line. If i have some reflections, why are the IN GENERAL always worst in the middle between TX and RX? One thought I had is that maybe it is because of the nonideal scope probe that represents some capacitance to GND and causes reflections. Still, why are those reflections more or less evil, depending on the location of measurement? Is this generalisation true? I will try to simulate that stuff but will have no time until the weekend ;-)

Thanks!

\$\endgroup\$
  • 3
    \$\begingroup\$ When asking about something you've been told or have read, it's a good idea to provide a reference. \$\endgroup\$ – Dmitry Grigoryev Apr 28 '16 at 6:46
  • 1
    \$\begingroup\$ I'm not doubting that you thought you were told this but, your lack of understanding of what you believe you were told means that you have represented it here incorrectly. This is why you should provide a reference as per the @DmitryGrigoryev comment. \$\endgroup\$ – Andy aka Apr 28 '16 at 7:31
  • \$\begingroup\$ The generalization is not strictly true in all cases. That is for sure. Also, you are correct that in the ideal case, where the line is terminated in its characteristic impedance. When that happens, there is no reflection, so the signal will appear the same anywhere on the line. But this case very seldom if ever applies to digital circuits. Usually digital loads have high input impedance, so the best place to measure is at the load (where the voltage is being sensed). It doesn't really matter how it looks anywhere else. \$\endgroup\$ – mkeith Apr 28 '16 at 16:37
1
\$\begingroup\$

The problem is that the reflections will set up standing waves in the transmission line so that the amplitude varies with position. The end result is that you will get different measurements at different positions on the line.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.