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Recently I've been trying to understand how multiple signals can travel along a coax cable. Indeed, I have already read "How are multiple signals propagated in a coaxial cable?", but it doesn't seem to be exactly the same question, nor is it specific enough, so please don't mark this as a dupe.

So how exactly are multiple signals transmitted on different frequencies? For example, I'd like to send data between two devices using a 50-Ohm cable, and I have a 7MHz carrier and a 11MHz carrier. Do I simply couple the signals to the cable using 50-Ohm resistors? Or capacitors? Transformers? Or some kind of push-pull line driver?

In addition, what are the typical voltage levels for coax? I can only find maximum ratings. If I were to design a device

Finally, how are signals sent bidirectionally? I suppose I could implement an EIA RS-232 or RS-422 where I could simply send binary data as negative and positive signals. However, I'd imagine that it would only suffice for sending data in one direction. If the two devices is is trying to exchange data, and one device is trying to output +1V while the other device is trying to send -1V back, won't there be some sort of contention, so to speak? I could do an I2C and have the "master" device coordinate the whole thing to send signals both ways, but I really want to do it the right way and send RF signals.

Thanks for your help.

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fuzzyhair2
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  • That's a lot of questions together there. Some of them don't relate to coaxial cable at all, but any cable at all. I'd suggest splitting them into several separate questions, you'd be likely to get higher quality answers to the individual parts then. – Majenko Oct 16 '14 at 19:30
  • I was thinking that all these different aspects affect each other. That is, the type of driver I use might affect the method I send the signals bidrectionally. – fuzzyhair2 Oct 16 '14 at 19:32
  • Not really, no - the same principles affect all similar transmission media. There are ways that could be construed as coax-only, but they are more akin to RF transmission than normal data lines with discrete voltages. – Majenko Oct 16 '14 at 20:12
  • See also, [How does “bidirectional” transmission on gigabit ethernet work?](http://electronics.stackexchange.com/questions/132486/how-does-bidirectional-transmission-on-gigabit-ethernet-work/) – The Photon Oct 16 '14 at 20:34
  • With regards to coupling to RF carriers, a cap would do the trick. If you start pushing the frequency up you might need something a bit more sophisticated like a directional coupler. – Oliver Oct 17 '14 at 08:09

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Sending full duplex signals (biderectional) is not difficult but you have to have localized cancellation circuits at each end to remove the signal being locally transmitted leaving (mainly) the received signal. Telephones work in the same way - you can send and receive audio simultaneously and because of the cancellation circuits you can barely hear in your earpiece what you are transmitting thru the microphone. It's called a telephone hybrid: -

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It doesn't need a transformer but one can be used to provide isolation and line impedance matching. Basically it's a wheatstone bridge where the line forms one limb and the driver (local transmission) is the "excitation". If the line impedance matches Zref, the receiver circuit gets 50% of the line signal coming down the line to it but, more importantly, it receives zero signal from the local transmission i.e. total cancellation of the locally transmitted signal.

When driving a 50 ohm coax with two signals at one end, each has to drive thru a 100 ohm resistor in order to maintain the impedance seen by the coax as 50 ohms. This prevents reflections and the normal problems associated with reflections.

Voltage levels are whatever you wish them to be. The higher the level the less possible chance of corruptions due to external influences but the bigger the possibility is that you might interfere with another system. Coax aint perfect and it can create EMI due to the data edges or carrier frequencies.

Andy aka
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