# How are multiple signals propagated in a coaxial cable?

I am reading that coaxial cables can carry hundreds of MHz signals at How Stuff Works. How are these signals propagated and how are they deconstructed once they reach their destination?

One idea that comes to my mind is that the RF signals of each frequency are translated into voltage waves that are then superimposed onto one, thereby creating one electrical signal that is sent through the cable. At the destination this signal is deconstructed using some form of correlation, to get the corresponding frequency?

the RF signals of each frequency are translated into voltage waves that are then superimposed onto one, thereby creating one electrical signal that is sent through the cable.

On the one hand, I think you've got it. On the other, I'm not sure you understand what's going on well enough to get it for the right reasons.

If we want to broadcast an rf signal through space, we have to emit that signal from an antenna. To get the signal from our generating equipment to the antenna we use a transmission line, typically a coaxial cable. The voltage signal on the transmission line excites the antenna, which generates EM waves that travel to the receiving antenna.

At the receiving antenna, the EM waves are delivered to another transmission line (again, typically coax) to get them to the receiving equipment.

Sending signals over coax is just like that, but without the antennas. The generating equipment is just connected to the receiving equipment by the cable. The same voltage signals in the cable that would have excited the antenna, travelled through space, been collected by the other antenna, etc, are just delivered straight to the coax that feeds the receiving equipment.

My point is there's no fundamental difference between the signals in a coax cable and "rf signals". Both are just voltages (or electric fields) varying with time.

And there's no need to "translate" rf signals into voltages. The RF signals already are (or started out as) voltages before they were broadcast as EM waves.

In fact you can also look at the signal in the coaxial cable as an EM wave, but it just happens to be travelling in a dielectric sandwiched between two conductors instead of travelling in free space.

• Great, now how about the the deconstruction of this signal that is done at the receiving end, how does the TV or modem choose a specific frequency of super signal and decode the data that rides on this carrier? – user1084113 Nov 28 '13 at 6:01
• The same way it would do if the signal came over the air. It's not related to the medium the signal traveled over to get there, except that some modulation schemes might be preferred for one medium or the other. – The Photon Nov 28 '13 at 6:14

Funny - I wrote (a version of) the text you refer to, many many years ago when I was designing cable modems (not sure why I am not credited, but I don't really care).

If you look at downstream (data coming from the internet towards the modem), the signal is first constructed at baseband (a low frequency) using a modulation technique called QAM. The baseband signal will occupy a 6 or 8 MHz band of frequencies (say from 2MHz to 8MHz) and will look almost like a box filling the band on an amplitude versus frequency plot.

The baseband signal is then up-converted by mixing the baseband signal with a higher frequency to produce a mirror of the signal at a higher frequency. When filtered we have only the intended frequency shifted version of the signal left (say at 350-356 MHz).

This signal is then mixed with all the other TV channel signals and sent into the CATV network.

On the modem side, this specific channel is picked out by a tuner. The tuner basically mixes the incoming signal with a frequency close to the desired channel to get a mirror of the signal at a low (IF) frequency. The IF is filtered usually using a very sharp SAW filter to output just that channel, but shifted to a low (near DC) frequency.

After the downconversion comes a QAM demodulator chip, which reconstructs the bits and clock inserted initially.

There are a lot of other details, but I hope this was what you were looking for. Feel free to ask more questions.