AM wave demodulation, Envelop detector, Square law detection, Difference between AM modulation and FM modulation

1. DEMODULATION OF AMPLITUDE
MODULATED SIGNAL
Submitted to:
Dr. Rekha Mehra
Submitted by:
Lokesh Parihar
15EC48

2. WHAT IS AMPLITUDE MODULATION?
It is a system of modulation in which the
amplitude of the carrier signal is made
proportional to the instantaneous
amplitude of the modulating message
signal.

3. WHAT IS DEMODULATION?
Demodulation is the process of recovering original message
signal from modulated wave. It also known as Detection.
DETECTION OF AMPLITUDE MODULATED SIGNAL
1. Envelop Detector or Average Detector
2. Square Law Detector
3. Synchronous Detector or Coherent Detection

4. Envelop Detector
An envelope detector is an electronic circuit that takes a high-frequency
signal as input and provides an output which is the envelop of the original
signal. The capacitor in the circuit stores up charge on the rising edge, and
releases it slowly through the resistor when the signal falls. The diode in
series rectifies the incoming signal, allowing current flow only when the
positive input terminal is at a higher potential than the negative input terminal.

5. Capacitor discharge between positive peaks produces a ripple signal of frequency ωc
in the output. By increasing the time constant RC, ripple can be reduced so that the
capacitor discharges very little between the positive peaks.
The envelop voltage is given by Ac [1+m cosωm t]
Time rate of decrease of this voltage is
-
𝒅𝒗
𝒅𝒕
= Ac mωm sinωm t
Rate of Capacitor discharge =
Ac [1+m cosωm t]
𝑹𝑪
In order to avoid carrier ripple the rate of capacitor discharge that should be greater
than the rate of drop of voltage
Ac [1+m cosωm t]
𝑹𝑪
≥ Ac m ωm sinωm t
1
𝑹𝑪
≥
mωm sinωm t
1+m cosωm t
1
𝑹𝑪
= mωm ≅ 0.3 or 0.4

6. SQUARE-LAW DEMODULATOR
Square law demodulator is used to demodulate low level AM wave.
Square Law
Device
Low Pass
Filter
AM wave Demodulated
Output
Ac 2 (1+m cosωm t) 2 cos 2 ωc t
Ac 2 (1+m 2 cos 2 ωm t+ 2m cosωm t) cos 2 ωc t
Ac 2 (cos 2 ωc t +m 2 cos 2 ωc t cos 2 ωm t+ 2m cos 2 ωc t cosωm t)
We get by above equation:-
2ωc , ωc – ωm , ωc + ωm , ωc – 2ωm and ωm
We will use a Low Pass Filter with frequency ‘fm’ to extract only ωm. As shown in
above figure.

7. Amplitude Modulation
1. The equation for AM wave is
s(t)= Ac [1+µ cosωm t] cosωc t
2. The value of modulation index is always between
zero and one.
3. Transmitted power is dependent upon
modulation index PT = Pc [ 1+
µ2
2
]
4. In an AM signal, only two sidebands are
produced, for any value of modulation index.
5. The amplitude of the sidebands is dependent on
the modulation index, and is always less than the
amplitude of carrier.
Frequency Modulation
1. The equation for FM wave is
s(t)= Ac cos[ωc t+β sinωm t]
2. The modulation index can have value either less
than one or more than one.
3. Since in FM, amplitude of carrier is constant, the
transmitted power is constant, independent of the
modulation index.
4. The modulation index determines the number of
significant pairs of sidebands in an FM signal.
5. The amplitude of the carrier and sidebands vary
with the modulation index and can be calculated
with Bessel functions.
Difference Between Amplitude Modulation
and Frequency Modulation

8. Amplitude Modulation
6. The sideband amplitude is never zero for any value of
modulation greater than zero.
7. The bandwidth of an AM signal is twice the highest
modulating frequency.
8. For AM, % of modulation is the ratio of amplitude of
modulating voltage to the amplitude of the carrier
multiplied by 100.
9. The AM system is more susceptible to noise and more
affected by noise than FM.
10. When two AM signals occupy the same frequency,
both signals will generally be heard regardless of
relative signal strength.
11. The efficiency of AM is less than that of FM due to use
of class-B amplifier.
12. The bandwidth required to transmit AM signal is much
less than that of FM typically 10KHz in AM
broadcasting.
Frequency Modulation
6. The carrier of sideband amplitudes are zero at some
modulation index.
7. The bandwidth of an FM signal is proportional to the
modulation index.
8. For FM, the % of modulation is the ratio of the actual
frequency deviation and the maximum permissible
frequency deviation multiplied by 100.
9. The main advantage of FM over AM is its noise
immunity, as limiter stage in FM receiver clips off noise
signals.
10. The capture effect in FM allows the strongest signal on
a frequency to dominate without interference from the
other signal
11. In FM, greater transmitter efficiency can be realized
using class-C amplifiers, as amplitude of FM signal is
constant.
12. The bandwidth of FM signal is much more than the
bandwidth of AM. The bandwidth of a typical FM
channel is 200KHz.

9. Sources:
• Internet
• Electronic Communication Systems (4th Edition)
• Lecture Notes