TIME DIVISION MULTIPLEXING

AIM: To study the 4 – channel analog multiplexing and demultiplexing technique.

APPARATUS: TDM Trainer kit, 

                    Power chords, 

                    20 MHz Dual trace CRO,

                    Power supply.
THEORY:  

        A time division multiplex system enables the joint  utilization of a common communication channel by a plurality of independentmessage sources without mutual interference among them.The block diagram as shown  in figure illustrates the concept of TDM. Each input signal is first restricted in bandwidth by a low pass anti aliening filter to remove the frequencies that are non-essential to an adequate signal representation. The low pass filter outputs are then applied to commentator, which is usually implanted using electronic switching circuitry the function of the commutator is two fold.
To take a narrow sample of each of the N input messages at rate Fst hat are slightly higher than
2w where W is the cutoff frequency of the anti-aliening. 

           To sequentially interleave there N samples inside the sampling interval TSIn deed this later function is the essence of the time division multiplexing operation following the communication process the multiplied signal is  applied to pulse modulator, the purpose of which is to transform the multiplied signal into a  form suitable for transmission over the communication channel it is clear that the use of time division multiplying introduces a band width expansion factor N because the scheme must squeeze N samples derived form N independent message sources into a time slot equal  to one sampling interval at the receiving end of the system, the receive signal is applied to pulse demodulator, which performs the reverse operation of the pulse modulator. The narrow samples produced at the pulse demodulator output are distributed to the appropriate low pass reconstruction filter by means of a dissimulators which operates in synchronism with the commutator in the transmitter the is synchronization is essential for a satisfactory operation of the system. The way this synchronization is implemented depends naturally on the method of pulse modulationuse to transmit the multiplied sequence of samples.

          The TDM systems are highly sensitive to dispersion  in the common channel .so accurate
equalization of both magnitude3 and phase response  of the channel is necessary to ensure a
satisfactory operation of the system.

BLOCK DIAGRAM:

PROCEDURE:

1)  Connect the power supply to the trainer kit, turn it ON.
2)  Observe each one of the 4 outputs of AF QUAD generator,one by one using CRO and
adjust the amplitude of each signal to be with in 2Vp-p and the frequency to 1KHz.
3)  Adjust the frequency of 555 timer to 32 KHz so thateach of the 4 signals is sampled at a
requency greater than Nyquist rate.(Initially keep the clock frequency low so as to observe
the multiplexed signals on CRO).
4)  Connect the 4 outputs of the signal generator to the respective inputs of TDM multiplexer
and connect the clock to TDM multiplexer.
5)  Connnect the O/P of the clock to CH1 of CRO and theTDM output to CH2 of CRO.It can
be observed that the 4 multiplexed signals are sampled,multiplexed and sent during each
clock cycle.
6)  Connect the TDM output to the TDM input of the demultiplexer and observe the
individual outputs using CRO(compare these outputs with I/P signals on CRO).
7)  Take an output of the demultiplexer,connect it to the filter I/P and observe the output of
the filter using CRO,Compare this O/P with respective I/P signal on CRO.
8)  Vary the Sampling frequency and observe the effect on output.
9)  Repeat steps 7 & 8 with remaining O/P signals.
10)  Once it is verified that TDM multiplexing is working,adjust the clock to high frequency
such as 32KHz.
11)  Then observe and plot the waveforms at every stage on graph paper.

MODEL GRAPH: