ANGLE MODULATION

1. UNIT-2-ANGLE MODULATION
Angle Modulation: Basic Definitions, Properties,
Relationship between PM and FMwaves- Narrow
band Frequency Modulation –Wide band Frequency
Modulation: Bandwidth,Generation of FM waves -
Demodulation of FM signals: Frequency
Discriminator, Phase LockedLoop, FM Stereo
Multiplexing.

2. Angle Modulation
It is defined as the process of varying the angle of the high frequency
carrier signal in accordance with the baseband or message signal keeping
the amplitude of the carrier constant.
There are two variants in anglemodulation depending on the
component of angle to be varied namely,
FrequencyModulation(FM) and
Phase Modulation (PM)

4. Mathematical Expression for Angle
Modulation:
Let (t) denote the angle of the modulated sinusoidal carrier,
assumed to be a function of the message signal.
The resulting angle modulated wave is expressed as:

11. FREQUENCY MODULATION
Frequency Modulation is defined as the process of
changing the frequency of the high frequency carrier
signal in accordance with the modulating or message
signal keeping the amplitude of the carrier constant.

12. Expression for Single Tone Frequency Modulation:
The term f is called as frequency deviation, which is defined as
the maximum deviation of the instantaneous frequency of the
FM signal from the carrier frequency fc.The frequency
deviation of the FM signal depends on the amplitude of
modulating signal Am and is independent of the modulating
frequency.

13. The angle of the FM signal is obtained as:

14. NARROWBAND FREQUENCY MODULATION
An FM signal is said to be Narrowband FM, if the modulation index is
less than one radian, i.e., FM signal with small bandwidth. The
maximum modulating frequency is usually 3 kHz and the maximum
deviation is 5 kHz.
Analysis of Narrow Band FM

16. Applications of Narrowband FM:
The narrowband FM systems requires low bandwidth, so they are
mostly employed in FM mobile communication,especially for voice
communication such as,
• Short range VHF ship to shore services
• Police radio
• Ambulances
• Taxicabs
• Radio controlled appliance repair services

17. WIDEBAND FREQUENCY MODULATION
An FM signal is said to be Wideband FM, if the modulation index
is greater than one radian, i.e., FM with wide bandwidth typically
15 times that of Narrowband FM system.
The maximum modulation index ranges from 5 to 2000. The
maximum permissible deviation is 75 kHz and modulating
frequencies range from 30 Hz to 15 kHz.

18. Analysis of Wideband FM

22. Applications of Wideband FM
The wideband FM systems requires large bandwidth,
so they are used in entertainment broadcasting
applications, such as:
• FM radio broadcasting
• Terrestrial TV broadcasting (Sound)
• Satellite TV broadcasting (Video).

23. GENERATION OF FM SIGNAL
There are essentially two methods of
generating FM signal, namely
Direct Method
Indirect Method

24. (i) Direct Method:

27. Wideband Frequency Modulator using VCO

28. (ii) Indirect Method:
The indirect method generates wideband FM signal from the
narrowband signal,unlike direct method, which generates
wideband FM signal directly.The indirect method of FM
generation is also known as “Armstrong method”.

31. Block diagram for generating narrowband FM signal

32. (i) Foster- Seeley Discriminator:
FM DISCRIMINATOR
Principle of Operation:
The phase shift between the primary and secondary voltages of
the tuned transformer is afunction of frequency.It can be shown
that the secondary voltage lags primary voltage by 900at the
carrier center frequency. This carrier frequency is the resonance
frequency of the transformer.
Foster-Seeley discriminator is also called as Phase Discriminator.
(ii) Ratio Detector:
The ratio detector is a modified version of Foster-Seeley
discriminator, in order to make it unresponsive to the
amplitude variations of the incoming FM signal.

33. Circuit Diagram of Foster-Seeley Discriminator

35. Frequency Response of Foster-Seeley Discriminator

36. Circuit Diagram of Ratio Detector

37. (a) If the input voltage is constant, C5 will charge up to potential
existing between a’ and b’.This will be a DC voltage and no current
will flow through the capacitor, i.e. the input impedance of C5 is
infinite. The total impedance for two diodes is therefore the sum of
R.The output will remain the same.
(b) For an increase in input signal level due to noise, the output
voltage follows the input.The capacitor C5 must get charged to that
level. To charge itself to the increased input level,the capacitor C5
draws charging current from the input resonant circuit, thereby
loading to agreat extent. As a result the Q factor is lowered reducing
the input signal level.
(c) For an decrease in input signal level, the capacitor must discharge
to the input level.Thus there is a discharging current through R1 and
R2. This reduces loading upon the input resonant circuit causing its Q
factor to increase. As a result, the input signal level is
increased.

38. PLLAS FM DEMODULATOR