Angle Modulation in Analog Communication

Angle modulation is a class of carrier modulation that is used in telecommunications transmission systems. The class comprises frequency modulation (FM) and phase modulation (PM), and is based on altering the frequency or the phase, respectively, of a carrier signal to encode the message signal.

1. ANGLE MODULATION

2. Introduction
•Anglemodulationistheprocessbywhichtheangle
(frequencyorphase)ofthecarriersignalischangedin
accordancewiththeinstantaneousamplitudeofmodulating
ormessagesignal.

3. Cont’d…
•ANGLE MODULATION is classified into
two types such as
•Frequency modulation (FM)
•Phase modulation (PM)
•Used for :
•Commercial radio broadcasting
•Television sound transmission
•Two way mobile radio
•Cellular radio
•Microwave and satellite communication system

4. PHASE MODULATION(PM)
•Theprocessbywhichchangingthephaseofcarriersignal
inaccordancewiththeinstantaneousofmessagesignal.
Theamplituderemainsconstantafterthemodulation
process.

5. Phase Modulation
PM (phase modulation) signal( ) cos 2 ( )
c c p
s t A f t k m t
 where c
A is a constant, ()mt is the message signal, and p
k is a parameter that specifies how
much change in the angle occurs for every unit of change of ()mt .
The phase and instantaneous frequency of this signal are ( ) 2 ( ), : phase sensitivity
(2 )() ()
,
()
instantanous frequency ( )
2
PM c p p
cPM
p
p
ic
t f t k m t k
d f tdt dm t
k
dt dt dt
kdm t
f t f
dt







6. Phase Modulation

7. Forms of phase modulation
•Although phase modulation is used for some analogue
transmissions, it is far more widely used as a digital form of
modulation where it switches between different phases.
•This is known as phase shift keying, PSK, and there are many
flavours of this. It is even possible to combine phase shift
keying and amplitude keying in a form of modulation known
as quadrature amplitude modulation, QAM.

8. FREQUENCY MODULATION
PRINCIPLES
•InFMthecarrieramplituderemainsconstant,thecarrier
frequencyvarieswiththeamplitudeofmodulatingsignal.

9. Frequency Modulation
•FM signal0
( ) cos 2 2 ( )
t
c c f
s t A f t k m t dt


  where f
k is a parameter that specifies how much change in the frequency occurs for every unit
change of ()mt When there is no message or the message signal is zero, the carrier wave is at its center frequency, fc.
When a message signal exists, the instantaneous frequency of the output signal varies above and below
the center frequency and is expressed by




The instantaneous phase of the output signal is equal to 2π multiplied by the integral of the
instantaneous frequency as shown below
0
0
angle ( ) 2 ( )
2 2 ( )
t
ii
t
cf
t f t dt
f t k m t dt






: frequency sensitivity
instantanous frequency ( ) ( )
f
i c f
k
f t f k m t

11. Resting f
c
Increasing f
c
Increasing f
c
Decreasing f
c
Resting f
c
Modulating signal
Carrier
FM

12. Frequency Deviation
•The amount of change in carrier frequency produced by the modulating
signal is known as frequency deviationΔf
•The frequency deviation of a radio is of particular importance in
relation to bandwidth, because less deviation means that more
channels can fit into the same amount of frequency spectrum.
•Calculate the maximum frequency deviation for the FM signal v(t)= 10 cos(6000t+ 5sin2200t)
•Explanation:
A standard FM signal is represented by
v(t)= A
ccos(2πf
ct+ k
fsin2πf
mt)
A
c= carrier amplitude
f
c= carrier frequency
k
f= modulation index
f
m= modulating frequency = 2200/2π = 350 Hz
k
f= frequency deviation/modulating frequency
5= freqdeviation/ 350
Therefore, deviation= 5 *350
= 1750Hzmax | ( ) |
f m f
f k A k m t  

13. Modulation Index
•In FM, modulation index is the ratio of frequency deviation(Δf) to
the modulating frequency(Fm). It is expressed as follows.
•m =
•Example:
•Max. frequency deviation of carrier = +/-25KHz
•Max. modulating frequency = 10 KHz
•modulation index = 25/10 = 2.5
•It is known as deviation ratio when both and are used with
maximum value in the calculation.m
f
f
 f m
f

14. Compare Angle Modulation and
Amplitude Modulation
•properties of amplitude modulation
•Amplitude modulation is linear
•just move to new frequency band, spectrum shape does not change.
No new frequencies generated.
•Spectrum: S(f) is a translated version of M(f)
•Bandwidth ≤ 2W
•Properties of angle modulation
•They are nonlinear
•spectrum shape does change, new frequencies generated.
•S(f) is not just a translated version of M(f)
•Bandwidth is usually much larger than 2W

15. Angle Modulation Pro/Con Application
•Why need angle modulation?
•Better noise reduction
•Improved system fidelity
•Disadvantages
•Low bandwidth efficiency
•Complex implementations
•Applications
•FM radio broadcast
•TV sound signal
•Two-way mobile radio
•Cellular radio
•Microwave and satellite communications

16. Advantages of frequency
modulation
•Resilient to noise:One of the main advantages of frequency
modulation that has been utilised by the broadcasting industry is the
reduction in noise. As most noise is amplitude based, this can be
removed by running the signal through a limiter so that only frequency
variations appear. This is provided that the signal level is sufficiently high
to allow the signal to be limited.
•Resilient to signal strength variations:In the same way that amplitude
noise can be removed, so too can any signal variations. This means that
one of the advantages of frequency modulation is that it does not suffer
audio amplitude variations as the signal level varies, and it makes FM
ideal for use in mobile applications where signal levels constantly vary.
This is provided that the signal level is sufficiently high to allow the
signal to be limited.
•Does not require linear amplifiers in the transmitter:As only
frequency changes are required to be carried, any amplifiers in the
transmitter do not need to be linear.
•Enables greater efficiency than many other modes:The use of non-
linear amplifiers, e.g. class C, etcmeans that transmitter efficiency levels
will be higher -linear amplifiers are inherently inefficient.

17. Disadvantages of frequency
modulation
•Requires more complicated demodulator: One of the minor dis-
advantages of frequency modulation is that the demodulator is a
little more complicated, and hence slightly more expensive than the
very simple diode detectors used for AM. Also requiring a tuned
circuit adds cost. However this is only an issue for the very low cost
broadcast receiver market.
•Some other modes have higher data spectral efficiency: Some
phase modulation and quadrature amplitude modulation formats
have a higher spectral efficiency for data transmission that
frequency shift keying, a form of frequency modulation. As a result,
most data transmission system use PSK and QAM.
•Sidebands extend to infinity either side: The sidebands for an FM
transmission theoretically extend out to infinity. To limit the
bandwidth of the transmission, filters are used, and these introduce
some distortion of the signal.