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Home Electrical and Electronics Analog & Digital Communication Study Time Division Multiplexing (TDM) and De multiplexing
Analog & Digital Communication Lab Experiments

Study Time Division Multiplexing (TDM) and De multiplexing



Aim

Study Time Division Multiplexing (TDM) and De multiplexing.

Apparatus Required:

  1. ST2102 trainer with power supply cord
  2. Oscilloscope with connecting probe
  3. Connecting cords.

Theory

Multiplexing: A sampled signal consists of a train of pulses, where each pulse corresponds to the amplitude of the signal at the corresponding sampling time. The signal sent to line is modulated in amplitude and hence the name Pulse Amplitude Modulation (PAM). Multiplexing is the process of combining signals from different information sources so that they can be transmitted over a common channel. Multiplexing is advantageous in cases where it is impracticable and uneconomical to provide separate links for the different information sources. The price that has to be paid to acquire this advantage is in the form of increased system complexity and bandwidth. The two most commonly used methods of multiplexing are:

  1. Frequency division multiplexing (FDM)
  2. Time division multiplexing (TDM)

Time Division Multiplexing:-

Time division multiplexing is the process of combining the samples from different information signals, in time domain so that they can be transmitted over the common channel. The fact utilized in TDM technique is that there are large intervals between the message samples. The samples from the other sources can be placed within these time intervals. Thus every sample is separated from other in time domain. The time division multiplexing system can be simulated by two rotating switches, one at transmitter and the other at receiver. (See figure) The two wipers rotate and establish electrical contact with one channel at a time.

Each signal is sampled over one sampling interval and transmitted one after the other along a common channel. Thus part of message 1 is transmitted first followed by part of message 2, message 3 and then again message 1 so on. The switches connect the transmitter and the receiver to each of the channels in turn for a specific interval of time. In effect each channel is sampled and the sample is transmitted when the switches are in the channel 1 position, channel 1 forms a PAM channel with an LPF for reconstruction, and so on for channels 2 and 3. The result is that the amplitudes samples from each channel share the line sequentially, becoming interleaved to form a complex PAM wave, as shown above. A major problem in any TDM system is the synchronization of the transmitter and receiver timing circuits. The transmitter and receiver must switch at the same time and frequency. Also SW1 must be in the channel 1 position when SW2 is in the channel 1 position, so that the switches must be synchronized in position also. In a system that uses analogue modulation (PAM) the time slots are separated by guard slots to prevent crosstalk between channels.

principle-operation-of-tdm-pam-system
timing-waveform-of-tdm-pam-system

Circuit Diagram:-

circuit-diagram

Procedure:

Initial Setup of Trainer.

Function Generator pot direction.

Anti clock wise Duty cycle Position.

Delay control.

Anti clockwise Comparator Threshold level.

Anti clockwise.

  1. Connect the power cord to the trainer. Keep the power switch in ‘Off’ position.
  2. Switch ‘On’ the trainer's power supply & Oscilloscope.
  3. Connect BNC connector to the CRO and to the trainer’s output port.
  4. Observe the clock signal (TP5) provided on Transmitter Timing Logic block on CRO. It will be a train of pulses having clock frequency of 64 KHz.
  5. Display the clock signal (TP5) along with channel 0(TP6). Observe the relation between two signals.
  6. Vary the Duty Cycle Selector switch and observe the variation in both signals (TP5 & TP6).
  7. Observe the waveforms at TP7, 8, 9 & 10. Sketch the relative time graph between the waveforms observed at TP 5, 7, 8, 9 & 10.
  8. With the same links, observe the waveform at Transmitter CH0 output (TP6) on channel 1 of the oscilloscope.
  9. Observe the waveforms at TP7, 8, 9, & 10 on the other channel.
  10. Plot the wave forms in time domain with reference to the Transmitter CH0 Signal.

Conclusion:

  1. The ‘On’ and ‘Off” time of Sync signal is changed as we vary the Duty Cycle Selector Switch.
  2. The switching time is varying as we change duty cycle, so accordingly we can change sampling period of input signals.