What is a Multiplexer
A multiplexer is a device that is used to combine a number of digital input streams in to a single output stream. The configuration of a multiplexer depends upon the type of multiplexing that it is being used for. Input signals can be combined in the frequency domain or the time domain depending upon the design of the system. The output stream obtained from the multiplexer is then modulated and transmitted through the communication channel.
In the frequency domain, multiplexers are nothing but frequency combiners that combine the different narrowband frequency input streams in to a single output stream consisting of a wide frequency band in which the input signals occupy different spectral ranges. In fact in frequency domain multiplexing, most of the times the different frequency signals are just transmitted in to the channel without any combining and the receiver monitoring the channel can select the desired signal by tuning itself to its particular frequency. It is in the time division multiplexing, that multiplexers are of vast importance.
Time Division multiplexers combine a number of different input signals usually of same frequency in to a single output signal by transmitting the input signals in different time slots. For example:- If three signals A,B,C are to be combined in a single output signal D, then D will consist of time slots say of t seconds i.e. in the first t seconds A is transmitted, in the next t seconds B is transmitted and in the next t seconds C is transmitted and the process repeats itself. The clock frequency at which the multiplexer works in this system must be three times the frequency of the input signals.
A time division multiplexer can easily be made by first of all developing a digital selection logic between the input signals and the output signal with the help of your basic AND, OR and NOT gates and then connecting the selection lines of the logic to the outputs of a counter whose highest output matches the number of input signals of the multiplexer. For e.g. For a multiplexer with 3 input signals, commonly known as 3:1 MUX ; output lines of a MOD-3 counter will be connected to the select lines. These counters can be easily made by the use of J-K flip flops. A single NAND gate can be used to control the MOD of the counter. Though it is very easy and cheap to make these multiplexers because of vast availability of the IC’s of various gates and flip flops used in them, these are also available as already synthesized IC’s in the market.