# How does frequency multiplexing work ?

1. Jan 4, 2009

### RipClaw

I don't understand how frequency multiplexing works on one cable (For example :a cable tv network that sends 100s of TV channels to a TV)

I understand that we can mix frequencies, and then demultiplex them.

But how does demultiplexing happen ?

After mixing, we get a soup.(which contains the crusts & troughs of various channels added/subtracted)
Let us assume that the crest formed by 2 inputs signals adds to give 5 volt.

Now how does the receiver/filter know the original signals were 1V and 4V , 3V and 2V, or other various combinations ????

2. Jan 5, 2009

### Averagesupernova

They aren't really multiplexed. In a hypothetical scenario we send out one carrier wave over the air on one antenna and then send a different one on a different frequency out on a different antenna but the receiver will get both signals. In the receiver they are selected out the same way they would be in your cable scenario. Do some research on receivers and filters. As far as combining several TV channels into the same cable goes, typically they use combiners and filters to do this. Each modulator has a filter on the output that allows it to only 'see' within it's own frequency passband. All of the outputs are then connected to the cable. This process generally is not considered multiplexing. At least I have never heard it termed that way.

3. Jan 5, 2009

### Phrak

You really need an RF guy hear. I'm not one.

Say your channel information is riding on a 4.5 MHz carrier. If you mix a 4.5 MHz signal with the signal you will demodulate into base band. But if the demodulator frequency you mix into the signal is variable (slighly lower or higher than 4.5 MHz) you can adjust the channel centerband to be centered on the bandpass filter. I think this usual method used today to select channels where variable capacitance diodes are used to tune the demodulator frequency.

Edit: you can be sure I'm not an RF guy. What I've called a demodulator frequency is misnamed. It's something like intermodulation frequency.

Last edited: Jan 5, 2009
4. Jan 6, 2009

### sai_2008

First, freq multiplexing is actually time-addition of signals that are frequency separated, having been modulated with different carrier frequencies.

Basically it involves multiplying in time carrier and the signal. Then the output of such modulated channels is time added in the cable.

Without getting into math, this is the simple way of looking at what happens in a receiver.

When you tune the receiver, you select a "channel", that is, you set the center frequency of the local oscillator and you look at a freq window (= bandwidth of the signal). The local oscillator freq is multiplied to the incoming signal (demodulation) and the output is passed thru a band pass filter that has the center freq = selected channel center freq and the bandwidth = the signal bandwidth (which is fixed for all channels. For NTSC TV I think it is 6 MHz). The act of multiplying the incoming "composite" signal with a local oscillator signal gives the HIGH gain for ONLY that component of the incoming signal that was originally modulated by the same carrier freq (center freq) on the sender's side, and it spreads out the other contributors in such a way that they lie out of the passband of the band pass filter. That is if you wanted to see CNN channel, and out of 5 V crest its only 3 V of CNN and rest are contributions of other channels, the demodulation causes the energy of other channels to spread out of the passband and band pass filter suppresses them. You are left with the energy (and some noise) of CNN only.

Note that w.r.t. to any given TV channel of interest, the signal contributions from all other channels can simply be considered noise.

I dont want to write equations for you, but take any basic text book on telecommunications and it will suffice.

Hope that helps.

sai