08-16-2017, 09:41 PM
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TIME-DIVISION MULTIPLEX1NG
OBJECTIVE
To perform Time division multiplexing and de-multiplexing using PAM Signals.
1.2. HARDWARE REQUIRED
1. TDM Trainer Kit ST2102
2. CRO
3. Patch Chords
4. Probes
INTRODUCTION
An important feature of pulse-amplitude modulation is a conservation of time. That is, for a given message signal, transmission of the associated PAM wave engages the communication channel for only a fraction of the sampling interval on a periodic basis. Hence, some of the time interval between adjacent pulses of the PAM wave is cleared for use by the other independent message signals on a time-shared basis. By so doing, we obtain a time-division multiplex system (TDM), which enables the joint utilization of a common channel by a plurality of independent message signals without mutual interference. Each input message signal is first restricted in bandwidth by a low-pass pre-alias filter to remove the frequencies that are nonessential to an adequate signal representation. The pre-alias filter outputs are then applied to a commutator, which is usually implemented using electronic switching circuitry. The function of the commutator is two-fold: (1) to take a narrow sample of each of the N input messages at a rate fs that is slightly higher than 2W, where W is the cutoff frequency of the pre-alias filter, and (2) to sequentially interleave these N samples inside a sampling interval Ts 1/fs. Indeed, this latter function is the essence of the time-division multiplexing operation. Following the commutation process, the multiplexed signal is applied to a pulse-amplitude modulator, the purpose of which is to transform the multiplexed signal into a form suitable for transmission over the communication channel. At the receiving end of the system, the received signal is applied to a pulse- amplitude demodulator, which performs the reverse operation of the pulse amplitude modulator. The short pulses produced at the pulse demodulator output are distributed to the appropriate low-pass reconstruction filters by means of a decommutator, which operates insynchronism with the commutator in the transmitter. . This synchronization is essential for satisfactory operation of the TDM system, and provisions have to be made for it.