Author Topic: Can you send more then one voltage down the same pair of wires?  (Read 593 times)

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Offline Beamin

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I as watching w2aew videos and he talks about using a diode to block rf on a wire that goes to a powered antenna. Will this only work with DC AND AC? Could you have 1V DC and 60 Hz 120VAC and 200MHz going down a pair of wires?  NO...? Or 100Mhz and 2GHz? I was trying to figure out how those internet ac adapters work or how you could send signals down rail road tracks or power lines to switch transformers or lights on a railroad crossing.
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Offline Vtile

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Re: Can you send more then one voltage down the same pair of wires?
« Reply #1 on: March 10, 2018, 06:43:12 am »
Yes. It could be done and it is done it great many places. The optimal mix of signals or voltages as you say, of course varies by case to case.

One common example of such activity is ie. radio signal of FM (frequency modulation) and especially AM (amplitude modulation) transmissions.

From attached picture you can see what happens when you combine to waveforms. XCas is free and you can get it here: https://www-fourier.ujf-grenoble.fr/~parisse/giac.html
« Last Edit: March 10, 2018, 07:10:58 am by Vtile »
 

Offline Beamin

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Re: Can you send more then one voltage down the same pair of wires?
« Reply #2 on: March 10, 2018, 07:25:59 am »
Thats not how I expected those two signals to look but if you line them up it makes sense.

What if you tried to send a battery voltage down mains lines? DC and DC wouldn't work if you used both conductors? But: use one conductor of an outlet to the + of the battery then had a wire going from the - to another room outlet?

A unrelated question: when a battery is not hooked up to anything there is no potential on the electrodes because no chemistry is going on but a generator is always at potential because the magnetic field is present? What about a charged capacitor? Tricky like the act of observing causes a change to the observed object.
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Online rstofer

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Re: Can you send more then one voltage down the same pair of wires?
« Reply #3 on: March 10, 2018, 07:36:40 am »
From attached picture you can see what happens when you combine to waveforms. XCas is free and you can get it here: https://www-fourier.ujf-grenoble.fr/~parisse/giac.html

Thanks for that!  That is a nice way to show Fourier series.
 

Online T3sl4co1l

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Re: Can you send more then one voltage down the same pair of wires?
« Reply #4 on: March 10, 2018, 07:45:16 am »
You might read up on telecom multiplex.  This was introduced in the 30s, using RF modulator techniques, and some very well constructed filters, to take a single telephone line (running between exchanges) that might be 100kHz useful bandwidth, and deliver 30 or more phone conversations simultaneously along it.  And with electronic switching (or at least, as an option -- instead of electromechanical exchanges).

This was a basic enabling technology for transatlantic phone cables -- a single line for a single conversation would've been preposterously expensive.

The basic breakdown is this: each phone line is filtered to its exact specified bandwidth (typically 500-3000Hz, giving that classic muffled telephone sound quality; the skirts were quite steep, something like >60dB attenuation at 4kHz), and using balanced mixers and precision oscillators, the channel is shifted up or down to its assigned carrier frequency.  Then that is filtered (to remove the unneeded sideband), and finally all the signals are mixed together for transmission.  On the receiving end, the opposite happens: a channel is selected with a filter, and mixed down to baseband (the baseband filter only needs to remove the carrier pilot tone, frequency sum, and harmonics, which saves a little).

Note that the phone line is full duplex on both ends, so a precision hybrid is required to separate rx/tx waves.  And for that precision to matter, the line itself also has to be trimmed to very flat impedance and frequency characteristics (some guy at Bell, with the name of Zobel, proved very good at this).

Tim
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Offline Vtile

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Re: Can you send more then one voltage down the same pair of wires?
« Reply #5 on: March 10, 2018, 07:47:08 am »
Thats not how I expected those two signals to look but if you line them up it makes sense.
One picture tells more than thousand words or how it did go..  :)

What if you tried to send a battery voltage down mains lines? DC and DC wouldn't work if you used both conductors? But: use one conductor of an outlet to the + of the battery then had a wire going from the - to another room outlet?
Yes that would work, but as you can guess it is extremely dangerous because of small mistakes you can make, so I don't continue further on that example.

A unrelated question: when a battery is not hooked up to anything there is no potential on the electrodes because no chemistry is going on
I really suck at chemistry, but in real world there is always active chemistry because of 'microscopic scale' leakage through air, electrolyte and materials of battery. [/quote]

but a generator is always at potential because the magnetic field is present?
The generators doesn't always have magnetic field on them even if they rotate, inductive generator is a good example.
What about a charged capacitor?
There is electron difference between plates so there is also tension  .. as simplified answer. I'm sure T3sl4co1l or some other physic buff will correct my crude answer soon.  ;D
Tricky like the act of observing causes a change to the observed object.
That is the reason that in certain level measurement is a science itself.
[/quote] :)

From attached picture you can see what happens when you combine to waveforms. XCas is free and you can get it here: https://www-fourier.ujf-grenoble.fr/~parisse/giac.html

Thanks for that!  That is a nice way to show Fourier series.
Nice to give something back for a while.  For a fourier I would add a third wave... something like below.
« Last Edit: March 10, 2018, 08:03:47 am by Vtile »
 

Offline Beamin

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Re: Can you send more then one voltage down the same pair of wires?
« Reply #6 on: March 10, 2018, 07:23:06 pm »
You might read up on telecom multiplex.  This was introduced in the 30s, using RF modulator techniques, and some very well constructed filters, to take a single telephone line (running between exchanges) that might be 100kHz useful bandwidth, and deliver 30 or more phone conversations simultaneously along it.  And with electronic switching (or at least, as an option -- instead of electromechanical exchanges).

This was a basic enabling technology for transatlantic phone cables -- a single line for a single conversation would've been preposterously expensive.

The basic breakdown is this: each phone line is filtered to its exact specified bandwidth (typically 500-3000Hz, giving that classic muffled telephone sound quality; the skirts were quite steep, something like >60dB attenuation at 4kHz), and using balanced mixers and precision oscillators, the channel is shifted up or down to its assigned carrier frequency.  Then that is filtered (to remove the unneeded sideband), and finally all the signals are mixed together for transmission.  On the receiving end, the opposite happens: a channel is selected with a filter, and mixed down to baseband (the baseband filter only needs to remove the carrier pilot tone, frequency sum, and harmonics, which saves a little).

Note that the phone line is full duplex on both ends, so a precision hybrid is required to separate rx/tx waves.  And for that precision to matter, the line itself also has to be trimmed to very flat impedance and frequency characteristics (some guy at Bell, with the name of Zobel, proved very good at this).

Tim

So that has to be modulated with a carrier either am or FM? Would it work just by shifting up freq. filtering then shifting down? Would that work with sound waves in air down a tube trying to do the same thing (assuming the tube doesn't have things like echo's; a mathematical tube)?

Also could you use AM or FM modulation in air or other sound carrying fluid? Might have some novel use.
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Online T3sl4co1l

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Re: Can you send more then one voltage down the same pair of wires?
« Reply #7 on: March 11, 2018, 02:30:27 am »
Almost AM, specifically, SSB, unless they left both sidebands, I don't remember exactly.  FM would be more bothersome for multiple reasons.

Acoustic waveguide isn't great, because of dispersion: the variation of propagation velocity with frequency.  If you ever happen across a long pipe sitting on the ground, have a friend go to the other end and clap.  The impulse turns into a "pew" sound, because the frequencies have separated into a chirp.

You can use the same principle to listen to bats and other ultrasonic phenomena: filter above 20kHz, downshift and listen.  Now, you can't pitch-shift as well -- that's a very different operation (usually done in DSP) -- so you can only listen to each 20kHz chunk (or whatever your hearing range is) at a time.

Back in the 50s, mercury was used to store pulses, as delay line memory; apparently the dispersion in such an assembly (mercury inside a steel pipe inside air) has good enough dispersion to maintain a few hundred bits with a feedback (regeneration) loop.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 


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