Study of PCM Transmitter and Receiver

Aim

Study of PCM Transmitter and Receiver.

(a)Study of Pulse Amplitude Modulation using Natural & Flat top Sampling.

Apparatus Required:

1. ST2103 trainer with power supply cord
2. Oscilloscope with connecting probe
3. Connecting cords.

Theory

The ST2103 & 2104, TDM PCM transmitter & receiver trainer demonstrates the basic scheme used to transmit an information signal using coding technique. It covers very basic concepts like role of sample Amplifier, Analog to digital conversion Pseudo random synch code generator, Digital to analog conversion, Pseudo random synch code detector of sampling pulse while transmitting a signal. It also demonstrates signal recovery using low pass filters of different orders.

Steps in Pulse Code Modulation:-

Sampling:-

The signals which are required to be transmitted as information is known as information signal and in the case of voice communication this will be a continuously changing signal containing speech information. The aim of the kit is to transmit the signals in digital form and is to reproduce this information signal in analog form at the receiving end of the communication system with the help of sampling and reconstruction trainer. In the exercises to follow, you will simulate audio signal by a 1 KHz test signal provided On-board. The repetitive, non-changing waveform does not contain information. Provided the frequency of the test-signal lies within the frequency range which an information signal will occupy, a test signal of this type can be extremely helpful in system analysis and testing The voice signals are limited to the range 300 Hz to 3.4 KHz, a 1 KHz frequency fits conveniently in this range and can be used to demonstrate and test many techniques used in communication system. Theory of sampling: The signals we use in the real world, such as our voice, are called "analog" signals. To process these signals for digital communication, we need to convert analog signals to "digital" form. While an analog signal is continuous in both time and amplitude, a digital signal is discrete in both time and amplitude. To convert continuous time signal to discrete time signal, a process is used called as sampling. The value of the signal is measured at certain intervals in time. Each measurement is referred to as a sample.

Principle of Sampling:-

Consider an analogue signal x(t) that can be viewed as a continuous function of time, as shown in figure. We can represent this signal as a discrete time signal by using values of x(t) at intervals of nTs to form x(nTs) as shown in figure. We are "grabbing" points from the function x(t) at regular intervals of time, Ts, called the sampling period.

Figure depicts the sampling of a signal at regular interval (period) t= nTs where n is an integer. The sampling signal is a regular sequence of narrow pulses δ (t) of an amplitude. Figure shows the sampled output of narrow pulses δ (t) at regular interval of time.

Circuit Diagram:-

Procedure:-

Initial set up for trainer ST2103:

Mode Switch Position: FAST position.

Function generator setting:

DC l & DC 2 amplitude controls: fully clockwise direction.

1 KHz & 2 KHz signal levels: 10 V peak -peak.

Pseudo random sync code generator switch: OFF position.

Error check code selector switches A & B: A = 0 & B =0 Position ('Off' Mode).

All switched faults: OFF position.

1. Make the following connections as shown in figure.
2. I. DC 1 To CH 0
3. II. DC 2 To CH 1
4. Turn ‘On’ the power supply and oscilloscope. Adjust the DC1 amplitude control such that the voltage measured at TP10 (CH 0) with the help of DMM / oscilloscope is + 3 Volts. Adjust the DC 2 amplitude control so that the voltage at TP12 (CH 1) is 2 V.
5. The LED outputs of A/D Converter & shift register are a combination of the two input voltages. Also since the trainer is working in fast mode, it is impossible to detect the code.
6. As stated earlier, the two channels are sampled at different time. Approximately, after 10 seconds, when the system has settled down to slow mode, observe the LEDs of A/D converter Block. Notice that a particular combination of LEDs is lit in the A/D converter Block for approximately 7 seconds. These LEDs represent the latched output from the A/D Converter for every sample of CH 0 & CH 1 Channels. Note the output of the A/D Converter, Note: You may find the A/D Converter's output may not be identical every time you switch the circuit from fast to slow mode for the same DC Control setting. This is due to the slight change in voltage at Sample / Hold circuit at the time of switching. However the change in code will only be 1 Bit.
7. The parallel data from the A/D Converter is then loaded in the shift register which converts in serial output. Connect the oscilloscope at following points :
8. Oscilloscope channel 1 to TX. Clock output (TP3)
9. Oscilloscope channel 2 to S/L test point (TP9)
10. External trigger to TX. to output (TP4) You may have to adjust the oscilloscope trigger levels to obtain a stable display.
11. Observe the interdependence of S/L, TX clock output and the shift register outputs as shown by their respective LEDs. Record the waveforms. The timing diagram for the process is shown in figure.

Conclusion:

As the controlling signals are properly synchronized the output of the two input waveforms are also synchronized.