Understanding Frequency, Amplitude, and Phase Modulation

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

1. AM – has only 3 frequency (carrier and the first two sidebands). FM- has infinite number of sidebands and carrier
2. 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
3. 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.
4. 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.
5. 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

1. Amplitude modulation
2. Angle modulation

Angle Modulation is divided into

1. Frequency modulation
2. Phase modulation – not used in practical analog transmission systems

It is possible to obtain FM from PM by “Armstrong System.”

FM	PM
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	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 ᶲm Some form of frequency changes take place

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

	FM	AM
Reception	More immune to noise.	More prone to noise
Noise Reduction	It is possible to reduce noise still further by increasing deviation.	It is not possible to exceed 100% modulation without causing severe distortion.
Standard Frequency Allocation (allocated worldwide by INTERNATIONAL RADIO CONSULTATIVE COMMITTEE of the ITU)	Provided a guard band between commercial FM stations, so that there is less adjacent channel interface than in AM	Has more adjacent channel interface
Frequency Operation	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.	Operates at MF and HF ranges (more noise)
Power Transmitted	All of the transmitted power is useful.	Most of its transmitted carrier, contains no useful information.
Modulation	Amplitude of the FM wave is constant (independent of modulation depth).	AM modulation depth governs the transmitted power.
Channel Required		A much wider channel is required by FM (10x as large as AM required).
Transmitting and Receiving device		FM transmitting and receiving equipment tends to be more complex, particularly for modulation and demodulation.
Area of reception		Area of reception for FM is much smaller than for AM.

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.

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