modulation

1. AMPLITUDE MODULATION
AND DEMODULATION

2. OVERVIEW OF MODULATION
Computer Modem
Serial link
RS -232
Phone
Line
RJ-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)
3

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   
4

5.  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%.

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

7. 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.

8. 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.

9. AMPLITUDE DEMODULATION PROCESS

10. 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.