Electronics > Metrology

Measurement and direction of information

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John Heath:
I wish to measure the current between one op-amp output at + 1 volt 1 u second pulse impedance 0 and another op-amp at + 1 volt 1 u second pulse impedance 0. On the surface the answer would be 0 current before and after the 1 u second pulse as both op-amps are in agreement voltage wise. However the 1 u second + and + pulses have to travel through 100 feet of coax cable before it reaches the other op-amp. This places the meeting of the + and + 1 volt 1 u second pulse midway in the coax cable not the op-amp outputs. The burning question. When the two pulses meet mid way in the coax cable do they reflect off each other then return to their source or do they pass each other like ships in the night. This is a very tricky question that gets to the heart of the direction of information. Can identical information , pulse , propagating in a coax cable traveling  from A to B and B to A make it there? The end result of a reflection , identical information can not travel from A to B , in a coax cable or ships passing in the night , identical information can travel from A to B. The answer has no relevance as either way the end result is the same as I should measure high current on the op-amp outputs when the time displaced pluse arrives. The real question is high current caused by the op-amps own pulse reflected back later in time or high current caused by the pulse from the other op-amp. Put in other words can identical information be conveyed through a coax cable from A to B and B to A or is this physically impossible as they will always reflect off each other in the coax cable?

sarepairman2:
they had ascii diagrams in 1970

But to answer your question, try approaching the phenomenon from a different angle with non electrical waves that have a slower propagation delay, i.e. mechanical shock waves.

Reflections happen from impedance mismatch, does the impedance of a transmission line change as a pulse is moving through it? Electrical signals superimpose on each other, if you had a slotted wave guide you can measure this.

I think though, on a advanced level, your question reaches into areas of physics that we don't understand too well (if you look at it from a low enough level). i.e. some kind of distortion n energy transfer between waves?

edavid:

--- Quote from: John Heath on May 14, 2016, 02:35:54 pm ---Put in other words can identical information be conveyed through a coax cable from A to B and B to A...

--- End quote ---

Sure.  It doesn't matter if the information is "identical".  If the transmission line is linear, it can be used bidirectionally.

Reflections only occur at discontinuities.

edavid:
Telephone pairs aren't used as transmission lines, so they aren't exactly relevant to the OP's question.

John Heath:

--- Quote from: sarepairman2 on May 14, 2016, 03:47:38 pm ---they had ascii diagrams in 1970

But to answer your question, try approaching the phenomenon from a different angle with non electrical waves that have a slower propagation delay, i.e. mechanical shock waves.

Reflections happen from impedance mismatch, does the impedance of a transmission line change as a pulse is moving through it? Electrical signals superimpose on each other, if you had a slotted wave guide you can measure this.

I think though, on a advanced level, your question reaches into areas of physics that we don't understand too well (if you look at it from a low enough level). i.e. some kind of distortion n energy transfer between waves?

--- End quote ---

In electronics I always think in terms of physics. It just seems easier that way. Inductors are fly wheels , voltage pressure and so on. For this reason when speaking of physics I am really speaking of electronics in the only way my pea brain can understand it :)

To your thoughts. Say the coax has an impedance of 50 Z ohms. For a pulse the cable feels like 50 Z as it propagates down the cable. If that pulse runs into another identical pulse moving the other way does the cable still feel like 50 Z ??.  both pulses go to 1 volt .If they are both 1 volt then there is not a reason for electrons to flow in either direction. This being the case the cable would feel like infinity impedance for the pulses. It would be as if the two pulses ran into a brick wall so they would reflect and reverse direction ,, i think.  I have a video of this. Just before 50 percent of the video you can see two pulses colliding into each other. What do you think? Are the pulses passing each other like ships in the night or did they hit a brick wall reflecting off each other reversing direction.

To diuligent minds I hope this video meets your requirement for a visualization.
 

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