Frequency Modulation – the amplitude of the carrier is kept
constant, frequency and rate of change are varied by the modulating signal
Phase modulation – the amplitude of the carrier is kept
constant, phase of the carrier is varied.
Deviation of the carrier is proportional to the amplitude of
the modulating signal
Deviation ratio – the shift in the carrier frequency from
its resting point compared to the amplitude of the modulation voltage
Maximum deviation ration – 8 significant sidebands
Deviation – the amount by which the carrier frequency is
varied from its unmodulated value of the modulating voltage. The rate at which
this frequency variation changes or takes place is equal to the modulating
frequency
“The amplitude of the frequency-modulated wave remains
constant at all times. (Greatest advantage of FM)”
Observations from frequency spectrum of the FM Wave
- AM – has only 3 frequency (carrier
and the first two sidebands). FM- has infinite number of
sidebands and carrier
- J coefficients decrease in value
as n increases. J coefficients represents the amplitude of a particular pair of
sidebands.The Modulation index determines
how many sidebands components have significant amplitudes
- Sidebands at equal distances from
fc have equal amplitudes, so that sidebands distortion is symmetrical about
carrier frequency. J Coefficients have negative
values, signifies a 180 phase change for a particular sidebands.
- AM – increased depth of modulation
increases the sideband power and therefore the total transmitted power. FM – the total transmitted power
always remains constant, but with increased depth of modulation the required
bandwidth is increased.
- FM – the amplitude of the carrier
component does not remain constant. It is possible for the carrier
component of the FM wave to disappear completely. This happen for certain value
of modulation index “EIGENVALUES”
Two types of continuous wave modulation
- Amplitude modulation
- Angle modulation
Angle Modulation is divided into
- Frequency modulation
- Phase modulation – not used in
practical analog transmission systems
It is possible to obtain FM from PM by “Armstrong System.”
FM
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PM
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Modulation index – inversely proportional to the modulating frequency
Frequency modulation is proportional to the amplitude of the modulating
voltage
Fm is a form of PM
The larger the frequency deviation, the larger the phase modulation
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Modulation index – inversely proportional to the modulating voltage
only.
Phase deviation is proportional to the amplitude of the modulating
signal.
Independent of frequency
Instantenous angular velocity is continually changing between the
limits imposed by į¶²m
Some form of frequency changes take place
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FM and PM are indistinguishable for a single modulating
frequency.
When Modulating frequency is changed, the PM modulation
index will remain constant while the FM modulating index will increase as
modulation frequency is reduced.
PM received by FM system would appear to be lacking in bass.
This deficiency is corrected by bass boosting the modulating signal prior to
phase modulation. (difference of PM and FM)
FM VS AM
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FM
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AM
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Reception
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More immune to noise.
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More prone to noise
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Noise Reduction
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It is possible to reduce noise still further by increasing deviation.
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It is not possible to exceed 100% modulation without causing severe
distortion.
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Standard Frequency Allocation (allocated worldwide by INTERNATIONAL
RADIO CONSULTATIVE COMMITTEE of the ITU)
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Provided a guard band between commercial FM stations, so that there
is less adjacent channel interface than in AM
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Has more adjacent channel interface
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Frequency Operation
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Operates in the upper VHF and UHF frequency range(less noise)
It is possible to operate several independent transmitter on the same
frequency with considerably less interference than AM.
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Operates at MF and HF ranges (more noise)
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Power Transmitted
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All of the transmitted power is useful.
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Most of its transmitted carrier, contains no useful information.
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Modulation
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Amplitude of the FM wave is constant (independent of modulation
depth).
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AM modulation depth governs the transmitted power.
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Channel Required
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A much wider channel is required by FM (10x as large as AM required).
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Transmitting and Receiving device
|
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FM transmitting and receiving equipment tends to be more complex,
particularly for modulation and demodulation.
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Area of reception
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Area of reception for FM is much smaller than for AM.
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Commercial FM broadcast began in 1940. Decade after AM
A single noise frequency will affect the output of a
receiver only if it falls within its bandpass.
The noise to carrier voltages remains constant, so does the
modulation index remain constant.
The noise voltage modulates the carrier.
Modulation index due to noise remains constant, the
modulation index caused by the signal will go on increasing in proportion to
the reduction in frequency.
Triangular noise distribution for FM is called NOISE
TRIANGLE.
Noise has a greater effect on the
higher modulating frequencies than on the lower ones.
If the higher frequencies were
artificially boosted at the transmitter and correspondingly cut at the
receiver, an improvement in noise immunity could be expected, thereby
increasing the signal-to-noise ratio. This is termed as pre-emphasis.
Compensation at the receiver is
called DE-EMPHASIS.
75us – amount of pre-emphasis in
US FM and in sound transmission accompanying television.
50us – used in EU and Australian
Wideband FM has been defined as
that in which the modulation index normally exceeds unity.
Narrowband FM is used by FM
mobile communication.