Lecture 22 Threshold effects in FM.pptx

1. Modulation: Need & Types
Lecture-3

2. Need for modulation
Frequency=distance/time
If the frequency of the signal is increased, it will travel
for a longer distance

3. Need for modulation
2. Antenna height:
where  is antenna height. Hence, to minimize the height of antenna,
frequency must be high.
3. Hardware Limitations:
The use of L is mandatory in oscillators. But, the size of L must be low for
minimizing the circuit complexity. As per the above equation, the frequency
must be high for low values of L.

c
f 
LC
f

2
1


4. Need for modulation
4.Interference minimization & multiplexing:
• Suppose you have two signals with frequencies 21 Hz and
22 Hz respectively. If you transmit as such, both the signals will
mix with each other and information is lost.
• So, modulate 21 Hz with 100 KHz and modulate 22 Hz with
1000 KHz. So, now the o/p of 1st signal becomes 100.21
KHz(100+21) and the o/p of 2nd signal becomes 1000.22 KHz
(1000+22).
• Now, if these signals are transmitted, they will not mix with
each other. Thus, by modulating message signals with different
carrier frequencies, interference can be minimized.

5. Need for modulation
5. Adjustment of bandwidth (BW):
• BW is directly proportional to
Signal to noise ratio (SNR).
• BW is also dependent on
frequencies
• Thus, increasing the frequencies
(by modulation) increases the BW
which in turn increases the SNR.
• SNR is the measure of the quality of
the signal and thus the quality is
improved by modulation

6. Types of Modulation
• Analog = both message and carrier are analog
• Digital = both message and carrier are digital
• Pulse = one signal is analog and other is digital
Analog M Digital M Pulse M (Hybrid)
Amplitude M (DSBFC) Angle M
FM PM
DSBSC SSBSC VSB ISBSC
ASK FSK PSK PAM PPM PWM PCM
TYPES

7. Qs