AMPLITUDE MODULATION AND DEMODULATION

OVERVIEW OF MODULATION

Computer Modem

Serial linkRS -232

PhoneLineRJ-11

Digital Analog

3

AMPLITUDE MODULATION TECHNIQUES

Modulation is a process of mixing a signal with a sinusoid to produce a new signal. This new signal,

conceivably, will have certain benefits over an un-modulated signal, especially during transmission.

In other words we can say that modulation is used because the some data signals are not always

suitable for direct transmission, but the modulated signal may be more suitable.

Amplitude modulation

1. Double sideband-suppressed carrier (DSB-SC)

2. Conventional

3. Single sideband (SSB)

4. Vestigial sideband (VSB)

4

AMPLITUDE MODULATION

AM is formally defined as a process in which the amplitude of the carrier wave c(t) is varied about a mean value linearly with a message signal m(t).

Message Signal:

Sinusoidal Carrier:

Modulated:

( )m t

( ) cos(2 )c cc t A f t

( ) 1 ( ) cos(2 )c a cs t A k m t f t

If m(t) has a peak positive values of +1 and a peak negative value of -1

AM signal 100% modulated

Envelope detection can be used if % modulation is less than 100%.

An Example of a message signal m(t) Waveform for Amplitude modulation of the message signal m(t)

DEMODULATION

Demodulation is the process of recovering the signal intelligence from a modulated carrier wave. This process, also called detection, is the reverse process of modulation.

WHY DO WE NEED DEMODULATION?

• The wireless signal consists of radio frequency (high frequency) carrier wave modulated by audio frequency (low frequency).

• The diaphragm of a telephone receiver or a loud speaker cannot vibrate with high frequency. Moreover, this frequency is beyond the audible range of human ear. So, it is necessary to separate the audio frequencies from radio- frequency carrier waves.

AMPLITUDE DEMODULATION PROCESS

IMPORTANCE OF RECTIFICATION IN DEMODULATION

The modulation enveloped on the positive and negative sides are 180o out of phase.

If rectification is not done, then after filtering, the positive and negative envelopes would cancel each other.

So, the original modulating signal cannot be recovered. Thus, rectification is necessary in demodulation.