SUBJECT: ANALOG COMMUNICATION LAB

CLASS: III ECE – A & B

TOPIC: PULSE AMPLITUDE MODULATION AND

DEMODULATION and PULSE WIDTH MODULATION

AND DEMODULATION

By

S. PRASAD

ECE Dept

PULSE AMPLITUDE MODULATION & DEMODULATION

Aim

To study Pulse Amplitude modulation and demodulation

Apparatus

1.  Physitech’s Pulse Amplitude modulation and demodulation,

2.  CRO, Signal generator,

3.  BNC probes, connecting wires.

THEORY

Pulse Modulation is used to transmit analog information. In this system continuous wave forms are sampled at regular intervals. Information regarding the signal is transmitted only at the sampling times together with syncing signals.

At the receiving end, the original waveforms may be reconstituted from the information regarding the samples.

The Pulse Amplitude Modulation is the simplest form of the pulse modulation. PAM is a pulse modulation system in which the signal is sampled at regular intervals, and each sample is made proportional to the amplitude of the signal at the instant of sampling. The pulses are then sent by either wire or cable are used to module division multiplexing is used.

CIRCUIT DIAGRAM:

PROCEDURE

1.  The 4016 integrated circuit is a CMOS bilateral switch which is used as a sampling switch. A positive voltage on pin 13 closes the CMOS transistor switch between pins 1 & 2. When pin 13 is a as zero volts, the switch is open.

2.  Switch on the trainer.

3.  Connect a 10K Hz sine wave of 5V P-P From an audio generator at the point marked ‘SINE’. (A.F. Input)

4.  Connect the oscilloscope to pin 2 of 4016 IC, adjust the 1K potentiometer (R1) to vary the amplitude of the modulating signal. Also adjust the frequency of the modulating signal to obtain stable display on the oscilloscope. The wave form obtained is a dual polarity PAM.

5.  Vary the amplitude and frequency of the sine wave signal and observe the change in the output waveform.

6.  Connect the modulated output to the input of the demodulator.

7.  Connect channel 1 of the dual trace oscilloscope to the demodulator output and channel 2 to the input sine wave. Compare the two waveforms you will find that they are 180 out of phase.

OBSERVATIONS :

Ac Fc / Modulating Signal / Modulated Signal
Am in volt / Fm in Khz / Amax / Amin

RESULT: Pulse Amplitude modulation and demodulation is studied and verified.

PULSE WIDTH MODULATION & DEMODULATION

Aim

To study Pulse Width Modulation

APPARATUS

1.  Physitech’s pulse Width Modulation Kit

2.  CRO, Signal Generator

3.  BNC probes, connecting wires.

THEORY

In pulse time modulation (PTM), the modulating signal is sampled, just as it is in PAM. However, in PTM, the amplitude of the sample is indicated by a timing variation of the modulated pulse, rather than an amplitude variation. The variable timing characteristics may be the duration, position, or frequency of the pulses. Therefore, there are three basic types of PTM: pulse duration modulation, pulse position modulation, and pulse frequency modulation .

This type of PTM is also called pulse width or pulse length modulation, however, pulse duration modulation (PDM) is the preferred term. There are three different classifications of PDM: symmetrical PDM, leading edge PDM, and trailing edge PDM. These are shown in Figure 1 along with the sine wave modulating signal.

Figure 1 (a) shows a symmetrical PDM waveform. Here the modulation signal as sampled and both the leading and trailing edges of the pulse are varied in accordance with the sample amplitude. When the sample is a high negative difference duration, the spacing between the center of the pulses remains constant as shown.

Leading edge PDM is shown in Figure 1 (b). In this type of PDM, the sample amplitude various the leading edge of the pulse. The trailing edge of each pulse is fixed and, therefore, the spacing or timing between each pulse’s trailing edge is constant.

Fig. 1 (c) shows trailing edge PDM. Here, the sample amplitude varies the trailing edge of the pulse, with the leading edge remaining fixed.

CIRCUIT DIAGRAM:

PROCEDURE:

1.  Study the circuit configuration given on the front panel of the trainer.

2.  Connect the circuit for PWM as shown in Figure 2 with RA = 10k and C = 0.01 mfd.

3.  Apply a sine wave modulating signal of 500Hz, 5V p-p to pin 5.

4.  Apply a square wave of 20 KHz frequency as trigger to pin 2.

5.  Connect your oscilloscope to pin 3.

6.  Vary the frequency and the amplitude of the modulating signal and notice the corresponding change in the width of the output pulses.

OUTPUT WAVEFORMS:

RESULT: Pulse Width Modulation is studied and verified.