EXPERIMENT NO. - 2

OBJECT:

Study of PAM generation and detector and observe characteristics of both single and dual

pulse amplitude modulation.

COMPONENT REQUIRED:-

- ±9V D.C. at 100 mA

-IC regulated power supply internally connected

- Variable frequency sampling pulse generator

-Sine wave audio frequency modulating signal generator

- PAM modulator circuit based on operational amplifier

- PAM demodulator circuit based on a point connected diode and operational amplifier

-The unit is operative on 230V ±10% at 50Hz A.C. Mains,

THEORY-

Modulation is the process of varying one or more properties of a high-frequency

periodic waveform, called the carrier signal, with respect to a modulating signal. This is

done in a similar fashion as a musician may modulate a tone (a periodic waveform) from a

musical instrument by varying itsvolume, timing and pitch. The three key parameters of a

periodic waveform are its amplitude ("volume"), its phase ("timing") and

its frequency ("pitch"), all of which can be modified in accordance with a low frequency

signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used

as carrier signal, but a square wave pulse train may also occur.

In telecommunications, modulation is the process of conveying a message signal, for example

a digital bit stream or an analog audio signal, inside another signal that can be physically

transmitted. Modulation of a sine waveform is used to transform a baseband message signal

into a passband signal, for example low-frequency audio signal into a radio-frequency signal

(RF signal). In radio communications, cable TV systems or the public switched telephone

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network for instance, electrical signals can only be transferred over a limited passband

frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies.

Types of Modulation:-

Analog communication

Digital communication

Ana log communication:

An analog or analogue communication consist of any continuous signal for which the time

varying feature (variable) of the signal is a representation of some other time varying

quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms

of small fluctuations in the signal which are meaningful. Analog is usually thought of in an

electrical context; however, mechanical, pneumatic, hydraulic, and other systems may also

convey analog signals. An analog signal uses some property of the medium to convey the

signal's information. For example, an aneroid barometer uses rotary position as the signal to

convey pressure information. Electrically, the property most commonly used is voltage

followed closely by frequency, current, and charge. Any information may be conveyed by an

analog signal; often such a signal is a measured response to changes in physical phenomena,

such as sound, light, temperature, position, or pressure, and is achieved using a transducer.

Digital communication:

In digital communication the term digital signal is used, to refer a discrete-time signal that

have a discrete number of levels, for example a sampled and quantified analog signal, or to

the continuous-time waveform signals in a digital system, representing a bit-stream. In the

first case, a signal that is generated by means of a digital modulation method which is

considered as converted to an analogue signal, while it is considered as a digital signal in the

second case.

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Modulation

Continuous wave Pulse

Amplitude Pulse amplitude

Pulse time

Pulse code

Pulse position

Pulse width

Angle

Frequency Phase

Need For Modulation

1. Frequency Multiplexing

Multiplexing may be achieved by translating each one of the original signal to a different

frequency range. Suppose one signal is translated to the frequency range f1’ to f2’. The

second of the range f1’’ to f2’’. And so on. If these new frequency ranges do not overlap,

then the signal may be separated at the receiving end by appropriate band pass filter.

2. Practicability of Antennas

When free space is the communication channel radiate and receive the signal. It turns out that

antennas operate effectively only when their dimensions are of the order of magnitude of the

wavelength of the signal being transmitted. A signal strength 1 kHz corresponds to a wave

length of 300,000m, an entirely impractical length. The required length may be reduced to the

point of practicability by translating the audio tone to a higher frequency.

3. Narrow Banding

The process of the frequency translation may be used to change wideband signal into a

narrow band signal which may well be more conveniently processed. The term wideband and

narrowband are being used here to refer not to an absolute range of frequencies but rather to

the fractional change in frequency from one band edge to other.

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4. Common Processing

It may happen that we may have to process, in turn a number of signals similar

in general character but occupying different spectral range. It will then be

necessary as we go from signal to signal, to adjust frequency range of our

processing apparatus to correspondence to the frequency range of the signal to

be processed. If processing apparatus is rather elaborate, it may well be wiser

to leave the processing apparatus to operate in some fixed frequency range and

instead to translate the frequency range of each signal in turn to correspond to

fixed frequency.

Advantage of digital communication:-

1. Digital communication is more rugged then analog communication

because it can withstand channel noise and distortion is within limits.

2. We can use regenerative repeaters in the digital communication. The

distance over which an analog message can be transmitted is limited by

transmitted power.

3. Digital hardware implementation is flexible and permits the use of

microprocessor, miniprocessor, digital switching and large scale

integrated circuit.

4. Digital signal can be coded to yield extremely low error rate and highly

feudality and as well as privacy.

5. Digital signal is inherently more efficient then analog in realizing the

exchange of SNR and bandwidth.

6. Digital signal storage is relatively easy and inexpensive.

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PULSE AMPLITUDE MODULATION (PAM):

This training is concerned with this type of pulse modulation; therefore let’s

look at it in further detail. Here the amplitude of the pulse is varied according to

the modulating signal. This corresponds to amplitude modulation of the

continuous wave. The PAM waveform can be two types. DUAL POLARITY

PAM If we sample the modulating signal by dual polarity sampling pulses as

then resulting PAM waveform is also dual polarity types .

SINGLE POLARITY PAM If we sample the modulating single whose DC level

has been raised to a positive value say +V1 from 0 volts and sample it with

singal polarity pulses Then the resulting waveform is also a single polarity

type. In this training board we produce single polarity PAM.

Fig .2.1

The frequency spectrum of the sampling pulsesshowsPAM waveform. The frequency

spectrum of PAM signal shows that the modulating frequency fm and various sidebands

fc±fm maintain their individuality, that is, fm does not spread into the lower sideband region

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around fc or fc+fm does not spread into the lower sideband region around 2fc if: fm≤fc-fm

or fc≥2fm.

Fig: 2.2

Demodulation of pulse amplitude modulation

We can conclude that the modulating signal can be extracted from the PAM simply by a

diode and a low pass filter with its cut of frequency at Fm.

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BLOCK DIAGRAM OF PAM MODUATOR AND DEMODULATOR

FIG: 2.3 circuit diagram of PAM

2. 7

Practical circuit of pulse amplitude modulation

FIG: 2.4

Here V1is modulating signal.

V2 is the carrier signal.

The modulating and the demodulated output is obtained on the 1st and 2nd channel of

oscilloscope respectively.

WORKING:-

The block diagram of pulse amplitude modulator based on operational amplifier

(ic-741) is show on in fig. we know that the train of pulses corresponding to the

samples of each signal are modulated in amplitude in accordance with the signal

itself. Accordingly, the scheme of sampling is called pulse amplitude

modulation. In this, the modulating signal is feds to pin-3 of op-amp. through

sine wave generator. The sampling pulses are feds to pin-6 of op-amp. 10k

resistor.+9v supply is feds to pin-7 through 100µf capacitor. The pin-2 of 741

connected to two resistor of 2k2 , 4k7 & four diodes (1n4148). The pin-2

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Q1

SMBT2222A

R1

22kΩ

R24.7kΩ

R3

3.3kΩ

C1100nF

1

V1

120 Vrms 60 Hz 0°

40

V2

-1 V 1 V 0.5msec 1msec

5

0

XSC1

A B

Ext Trig+

+

_

_ + _

0

2 3

0

provides trigger pulse to 741 through diode and resistor. The o/p of 741 depends

on the amplitude of the external trigger pulse applied to this pin. Thus, we

obtained PAM waveform.

When we feds the Pam waveform to the i/p of demodulator, we obtained

modulating signal. The block diagram of Pam demodulator is shown in fig. The

Pam demodulator is low pass filter which passes only low frequency

modulating signal shows the effects of the frequency of the frequency of

sampling pulses on the recovered modulating signal after flow frequency

sampling pulses.

RESULT:-

Hence, I have studied and performed pulse amplitude modulation and

demodulation.

IMPORANTS POINT:-

Frequency of carrier must be higher than message.

Amplitude of pulse must be varied according to sinusoidal wave, due to

some distortions in the negative half change in amplitude of pulse appears

less.

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