AMPLITUDE MODULATION AND FREQUENCY MODULATION

1. AMPLITUDE MODULATION AND FREQUENCY
MODULATION

3. INTRODUCTION
 Communication is the process of transfer information from one place to other through a communication channel. Therefore, transmitter
modifies the message signal into a suitable form for transmission over channel. This modification is done by a process known as
modulation. If we consider the human voice signal that has frequency up to 3KHz or the entire audible spectrum that is 20Hz to 20KHz,
which is a low frequency signal. So this low frequency message signal called baseband signal cannot travel long distance without any loss,
with the help of high frequency periodic signal massage can be transmitted over a long distance. This high frequency periodic signal is called
carrier signal and the process is called modulation.
 When the modulated signal reaches to the receiver it is necessary to separate the massage signal from the carrier signal. This process of
recovering the massage signal from carrier signal is called demodulation.
NEED OF
MODULATION
Narrow Band Signal
Avoid Mixing Of
Signal
Effective Power
Radiated By Antenna

4. TYPES OF MODULATION
MODULATION
Amplitude
modulation
Frequency
modulation
Phase
modulation

5. AMPLITUDE MODULATION
Let carrier wave be 𝑪(𝒕) = 𝑽𝒄 𝒄𝒐𝒔 𝝎𝒄𝒕 and signal wave be m(t)= 𝑽𝒎 𝒄𝒐𝒔 𝝎𝒎𝒕
Then the amplitude modulated wave will be
For𝑚𝑎=0.4, carrier wave is 40%
modulated.
For𝑚𝑎=1, carrier wave is 100%
modulated.
For 𝑚𝑎=2, carrier wave is over
modulated.
[Amplitude modulated waves]
𝑺(𝒕)𝑨𝑴 = 𝑽𝒄 𝟏 + 𝒎𝒂𝒄𝒐𝒔𝝎𝒎𝒕 𝒄𝒐𝒔𝝎𝒄𝒕
Where 𝒎𝒂=𝑲𝒂𝑽𝒎/𝑽𝒄 is called modulation index.

6. DETECTION OF AM WAVE
SQUARE LAW DETECTOR:
The mathematical relation between the input and the
output of square law device is
S 𝒕 = 𝒂𝟏𝑺(𝒕)𝑨𝑴 𝒕 + 𝒂𝟐𝑺(𝒕)𝑨𝑴
𝟐
𝒕
To neglect distortion in signal
𝑆𝑖𝑔𝑛𝑎𝑙 𝑝𝑜𝑤𝑒𝑟
𝑁𝑜𝑖𝑠𝑒 𝑝𝑜𝑤𝑒𝑟
≫ 1 ⇒ 𝒎𝒂 ≪ 𝟐
[Circuit diagram of Square law detector]
𝑽𝒐 𝒕 =
𝒂𝟐𝑽𝒄
𝟐𝒎𝒂
𝟐𝒄𝒐𝒔𝟐(𝝎𝒎𝒕)
𝟐
+ 𝒂𝟐𝒎𝒂𝑽𝒄
𝟐
𝒄𝒐𝒔𝝎𝒎𝒕
[Demodulated AM wave]
After passing through Lowpass filter with cutoff
frequency 𝑓𝑚, output 𝑉
𝑜(𝑡)becomes

7. FREQUENCY MODULATION
Let carrier wave be C(t) = 𝑽𝒄 𝒄𝒐𝒔 𝝎𝒄𝒕and signal wave be m(t)= 𝑽𝒎 𝒄𝒐𝒔 𝝎𝒎𝒕
Then the frequency modulated wave will be 𝑺(𝒕)𝑭𝑴 = 𝑽𝒄 𝒄𝒐𝒔 𝝎𝒄𝒕 + 𝒎𝒇 𝒔𝒊𝒏 𝝎𝒎𝒕
Where 𝒎𝒇 =
∆𝒇
𝒇𝒎
called frequency modulation index.
Where∆𝒇 = [𝒇 𝒕 − 𝒇𝒄]𝒎𝒂𝒙
𝑲𝒇𝑽𝒎
𝟐𝝅
≡frequency deviation.
Amplitude
(Volt)
Time (sec)
Time (sec) Time (sec)
When frequency of message signal increases, the
rate of frequency deviation increases.
When amplitude of message signal increases,
the amount of frequency deviation increases.
Amplitude
(Volt)
[Frequency Modulated Waves]

8. DETECTION OF FM WAVE
a) For an unmodulated carrier input,𝑉
𝑠 leads 𝑉1 by
90°. Then𝑉6 = 𝑉7
⇒ 𝑉8=0.
b) For a modulated carrier,
(i) when input signal frequency >𝑓𝑐, 𝑉
𝑠 leads 𝑉1 by
less than 90°. Then 𝑉6>𝑉7
⇒ 𝑉8 will be positive.
(ii)when input signal frequency <𝑓𝑐, 𝑉
𝑠 leads𝑉1
by more than 90°. Then 𝑉6<𝑉7
⇒ 𝑉8 will be negative.
Amplitude
(Volt)
[Circuit diagram of Foster Seeley Discriminator]
[Demodulated FM wave]
FOSTER SEELEY DISCRIMINATOR:
It is a phase discriminator.

9. CONCLUSION
The modulation properties can be used to generate amplitude modulated waveform and to
demodulate carrier. This principle is used in commercial AM broadcasting. It has serious deficiencies
in dynamics range and its noise immunity has to improve so we need FM. FM and demodulation has
been utilised by the broadcasting industry is the reduction of noise. It does not suffer audio
amplitude variations as the signal level varies. AM is preferred for picture transmission in TV since the
distortion which arises due to interference between multiple signals is more in FM than AM because
the frequency of FM signal continuously changes.
Thank

10. Thank You