Angle Modulation.pdf

COMPARISON NBFM&WBFM
WBFM NBFM
Modulation index greater than 10 less than 1
Freq deviation 75 kHz 5 kHz
Modulation
frequency
30 Hz- 15 kHZ 3 kHz
Spectrum Infinite no of sidebands and
carrier
Two sidebands and carrier
Bandwidth 15 x NBFM
2(δ*fm (max))
2 fm
Noise More suppressed Less suppressed
Application Entertainment &
Broadcasting
Mobile communication
1

ANGLE MODULATION
PART 2
◼GENERATION & DEMODULATION OF FM
◼NOISE IN FM
◼FM THRESHOLD EFFECT
◼NONLINEAR EFFECT
◼APPLICATION OF FM
2

GENERATION OF FM
• TWO MAJOR FM GENERATION:
i) DIRECT METHOD:
i) STRAIGHT FORWARD, REQUIRES A VCO WHOSE OSCILLATION
FREQUENCY HAS LINEAR DEPENDENCE ON APPLIED VOLTAGE.
ii) ADVANTAGE: LARGE FREQUENCY DEVIATION
iii) DISADVANTAGE: THE CARRIER FREQUENCY TENDS TO DRIFT
AND MUST BE STABILIZED.
iv) COMMON METHODS:
i) FM REACTANCE MODULATORS
ii) VARACTOR DIODE MODULATORS
3

1) Reactance modulator
Generation of FM (cont’d)
4

• THIS CIRCUIT USES A TRANSISTOR AMPLIFIER THAT ACTS AS EITHER A
VARIABLE CAPACITOR OR AN INDUCTOR.
• WHEN THE CIRCUIT IS CONNECTED ACROSS TUNED CIRCUIT OF AN
OSCILLATOR, THE OSCILLATOR FREQUENCY CAN BE VARIED BY
APPLYING THE MODULATING SIGNAL TO THE AMPLIFIER.
• USING RC PHASE SHIFT CIRCUIT.
• VOLTAGE ACROSS R THAT IS APPLIED TO THE BASE OF TRANSISTOR
LEADS VOLTAGE FROM OSCILLATOR BY 90°.
• REACTANCE MODULATOR LOOKS LIKE CAPACITOR TO OSCILLATOR
TUNED CIRCUIT.
• MODULATING SIGNAL VARIES THE BASE VOLTAGE & CURRENT OF
TRANSISTOR; COLLECTOR CURRENT VARIES PROPORTIONALLY.
• COLLECTOR CURRENT AMPLITUDE VARIES, PHASE SHIFT ANGLE
CHANGE WITH RESPECT TO OSCILLATOR VOLTAGE.
• AS MODULATING SIGNAL CHANGES, EFFECTIVE CAPACITANCE
VARIES, OSCILLATOR FREQUENCY VARIES.
5

2) Varactor diode modulator
Generation of FM (cont’d)
6

• DC VOLTAGE REVERSE BIASES VARACTOR DIODE & DETERMINES
THE REST FREQUENCY OF THE OSCILLATOR (CARRIER).
• MODULATING SIGNAL VOLTAGE ADDS TO & SUBTRACTS FROM
DC BIAS, CHARGES THE CAPACITANCE OF DIODE & FREQUENCY
OF OSCILLATION.
• POSITIVE GOING SIGNAL INCREASE REVERSE BIAS VOLTAGE,
DECREASE THE CAPACITANCE, INCREASE FREQUENCY OF
OSCILLATION (CARRIER FREQ).
• POSITIVE GOING SIGNAL INCREASE REVERSE BIAS VOLTAGE,
DECREASE THE CAPACITANCE, INCREASE FREQUENCY OF
• NEGATIVE GOING SIGNAL DECREASE REVERSE BIAS VOLTAGE,
INCREASE THE CAPACITANCE, DECREASE FREQUENCY OF
7

GENERATION OF FM (CONT’D)
II) INDIRECT METHOD:
i. FREQUENCY-UP CONVERSION.
ii. TWO WAYS:
a. HETERODYNE METHOD
b. MULTIPLICATION METHOD
iii. ONE MOST POPULAR INDIRECT METHOD IS THE
ARMSTRONG MODULATOR
8

9
Wideband Armstrong Modulator

• A complete Armstrong modulator is supposed to provide
a 75kHz frequency deviation. It uses a balanced modulator
and 90o phase shifter to phase-modulate a crystal
oscillator.
• Required deviation is obtained by combination of
multipliers and mixing, raise the signal from
suitable for broadcasting.
• With the Armstrong transmitter, the phase of the carrier is
directly modulated in the combining network, producing
indirect FM.
•Magnitude of phase deviation is directly proportional to the
amplitude of modulating signal but independent of its
frequency.
ARMSTRONG MODULATOR
10
kHz
MHz
to
Hz
kHz 75
2
.
90
47
.
14
400 


GENERATION OF FM AND PM
11
2 f
p
k
k

2
p
f
k
k


FM DETECTION/DEMODULATION
• FM DEMODULATION
• IS A PROCESS OF GETTING BACK OR REGENERATE
THE ORIGINAL MODULATING SIGNAL FROM THE
MODULATED FM SIGNAL.
• IT CAN BE ACHIEVED BY CONVERTING THE
FREQUENCY DEVIATION OF FM SIGNAL TO THE
VARIATION OF EQUIVALENT VOLTAGE.
• THE DEMODULATOR WILL PRODUCE AN OUTPUT
WHERE ITS INSTANTANEOUS AMPLITUDE IS
PROPORTIONAL TO THE INSTANTANEOUS
FREQUENCY OF THE INPUT FM SIGNAL.
12

FM DETECTION (CONT’D)
• TO DETECT AN FM SIGNAL, IT IS NECESSARY TO HAVE A
CIRCUIT WHOSE OUTPUT VOLTAGE VARIES LINEARLY
WITH THE FREQUENCY OF THE INPUT SIGNAL.
• THE MOST COMMONLY USED DEMODULATOR IS THE PLL
DEMODULATOR. CAN BE USE TO DETECT EITHER NBFM OR
WBFM.
13

PLL DEMODULATOR
14
Phase
detector
VCO
Low pass
filter
Amplifier
FM input
Vc(t)
fvco
V0(t)
fi

PLL DEMODULATOR
• THE PHASE DETECTOR PRODUCES AN AVERAGE OUTPUT
VOLTAGE THAT IS LINEAR FUNCTION OF THE PHASE
DIFFERENCE BETWEEN THE TWO INPUT SIGNALS. THEN LOW
FREQUENCY COMPONENT IS PASS THROUGH THE LPF TO
GET A SMALL DC AVERAGE VOLTAGE TO THE AMPLIFIER.
• AFTER AMPLIFICATION, PART OF THE SIGNAL IS FED BACK
THROUGH VCO WHERE IT RESULTS IN FREQUENCY
MODULATION OF THE VCO FREQUENCY. WHEN THE LOOP IS
IN LOCK, THE VCO FREQUENCY FOLLOWS OR TRACKS THE
INCOMING FREQUENCY.
15

PLL DEMODULATOR
• LET INSTANTANEOUS FREQ OF FM INPUT,
FI(T)=FC +K1VM(T),
AND THE VCO OUTPUT FREQUENCY,
F VCO(T)=F0 + K2VC(T);
F0 IS THE FREE RUNNING FREQUENCY.
• FOR THE VCO FREQUENCY TO TRACK THE INSTANTANEOUS
INCOMING FREQUENCY,
FVCO = FI;
16

PLL DEMODULATOR
• F0 + K2VC(T)= FC +K1VM(T), SO,
• IF VCO CAN BE TUNED SO THAT FC=F0, THEN
• WHERE VC(T) IS ALSO TAKEN AS THE OUTPUT VOLTAGE,
WHICH THEREFORE IS THE DEMODULATED OUTPUT
)
(
)
( 1
0 t
v
k
f
f
t
V m
c
c +
−

)
(
)
( 1 t
v
k
t
V m
c 
17

NOISE IN FM
• NOISE IS INTERFERENCE GENERATED BY LIGHTNING, MOTORS,
AUTOMOTIVE IGNITION SYSTEMS, AND POWER LINE
SWITCHING THAT PRODUCES TRANSIENT SIGNALS.
• NOISE IS TYPICALLY NARROW SPIKES OF VOLTAGE WITH HIGH
FREQUENCIES.
• NOISE (VOLTAGE SPIKES) ADD TO A SIGNAL AND INTERFERE
WITH IT.
• SOME NOISE COMPLETELY OBLITERATES SIGNAL INFORMATION.
18

NOISE IN FM
• IN AM SYSTEMS, NOISE EASILY DISTORTS THE
TRANSMITTED SIGNAL HOWEVER, IN FM SYSTEMS ANY
ADDED NOISE MUST CREATE A FREQUENCY DEVIATION IN
ORDER TO BE PERCEPTIBLE.
19
θ

NOISE IN FM(CONT’D)
 THE MAXIMUM FREQUENCY DEVIATION DUE TO RANDOM NOISE
OCCURS WHEN THE NOISE IS AT RIGHT ANGLES TO THE RESULTANT
SIGNAL. IN THE WORST CASE THE SIGNAL FREQUENCY HAS BEEN
DEVIATED BY:
Δ = ΘFM
 THIS SHOWS THAT THE DEVIATION DUE TO NOISE INCREASES AS THE
MODULATION FREQUENCY INCREASES. SINCE NOISE POWER IS THE
SQUARE OF THE NOISE VOLTAGE, THE SIGNAL TO NOISE RATIO CAN
SIGNIFICANTLY DEGRADE.
 NOISE OCCURS PREDOMINANTLY AT THE HIGHEST FREQUENCIES
WITHIN THE BASEBAND
20

NOISE-SUPPRESSION EFFECTS OF FM
 FM SIGNALS HAVE A CONSTANT MODULATED CARRIER AMPLITUDE.
 FM RECEIVERS CONTAIN LIMITER CIRCUITS THAT DELIBERATELY
RESTRICT THE AMPLITUDE OF THE RECEIVED SIGNAL.
 ANY AMPLITUDE VARIATIONS OCCURRING ON THE FM SIGNAL ARE
EFFECTIVELY CLIPPED BY LIMITER CIRCUITS.
 THIS AMPLITUDE CLIPPING DOES NOT AFFECT THE INFORMATION
CONTENT OF THE FM SIGNAL, SINCE IT IS CONTAINED SOLELY
WITHIN THE FREQUENCY VARIATIONS OF THE CARRIER.
21

22
Figure 5-11: An FM signal with noise.

PREEMPHASIS
• NOISE CAN INTERFERE WITH AN FM SIGNAL AND
PARTICULARLY WITH THE HIGH-FREQUENCY COMPONENTS
OF THE MODULATING SIGNAL.
• NOISE IS PRIMARILY SHARP SPIKES OF ENERGY AND
CONTAINS A LOT OF HARMONICS AND OTHER HIGH-
FREQUENCY COMPONENTS.
• TO OVERCOME HIGH-FREQUENCY NOISE, A TECHNIQUE
KNOWN AS PREEMPHASIS IS USED.
• A SIMPLE HIGH-PASS FILTER CAN SERVE AS A
TRANSMITTER’S PRE-EMPHASIS CIRCUIT.
• PRE-EMPHASIS PROVIDES MORE AMPLIFICATION OF ONLY
HIGH-FREQUENCY COMPONENTS.
23

24
Preemphasis circuit.

PREEMPHASIS
• A SIMPLE LOW-PASS FILTER CAN OPERATE AS A
DEEMPHASIS CIRCUIT IN A RECEIVER.
• A DEEMPHASIS CIRCUIT RETURNS THE FREQUENCY
RESPONSE TO ITS NORMAL FLAT LEVEL.
• THE COMBINED EFFECT OF PREEMPHASIS AND
DEEMPHASIS IS TO INCREASE THE SIGNAL-TO-NOISE
RATIO FOR THE HIGH-FREQUENCY COMPONENTS
DURING TRANSMISSION SO THAT THEY WILL BE
STRONGER AND NOT MASKED BY NOISE.
25

26
Deemphasis circuit.

FM THRESHOLD EFFECT
 IN FM SYSTEMS WHERE THE SIGNAL LEVEL IS WELL ABOVE NOISE
RECEIVED CARRIER-TO-NOISE RATIO AND DEMODULATED SIGNAL-TO-
NOISE RATIO ARE RELATED BY:
= SIGNAL-TO-NOISE RATIO AT OUTPUT OF FM DEMODULATOR
= MODULATION INDEX
= CARRIER-TO-NOISE RATIO AT INPUT OF FM DEMODULATOR
 DOES NOT APPLY WHEN THE CARRIER-TO-NOISE RATIO DECREASES
BELOW A CERTAIN POINT. BELOW THIS CRITICAL POINT THE SIGNAL-
TO-NOISE RATIO DECREASES SIGNIFICANTLY.
 KNOWN AS THE FM THRESHOLD EFFECT
27

 BELOW THE FM THRESHOLD POINT THE NOISE SIGNAL (WHOSE AMPLITUDE
AND PHASE ARE RANDOMLY VARYING), MAY INSTANTANEOUSLY HAVE AN
AMPLITUDE GREATER THAN THAT OF THE WANTED SIGNAL.
 WHEN THIS HAPPENS THE NOISE WILL PRODUCE A SUDDEN CHANGE IN THE
PHASE OF THE FM DEMODULATOR OUTPUT.
28
• In an audio system this
sudden phase change
makes a "click". In video,
the term "click noise" is
used to describe short
horizontal black and
white lines that appear
randomly over a picture.

NONLINEAR EFFECT IN FM
1. STRONG NONLINEARITY; INTENTIONALLY INTRODUCED IN A
CONTROLLED MANNER. IT IS INTRODUCED FOR PARTICULAR
APPLICATION E.G. SQUARE LAW MODULATORS, HARD-LIMITERS AND
FREQUENCY MULTIPLIERS.
2. WEAK NONLINEARITY; INTRODUCED BECAUSE OF IMPERFECTIONS IN
THE COMMUNICATION CHANNEL. SUCH LINEARITIES REDUCE THE USEFUL
SIGNAL LEVELS.
• IN NEXT SLIDE, WE WILL EXAMINE THE EFFECTS OF WEAK
NONLINEARITIES ON FM SIGNAL
29

TRANSFER CHARACTERISTIC OF COMMUNICATION
CHANNEL IS GIVEN BY
WHERE
WE KNOW THAT
30
)
(
)
(
)
(
)
(
3
3
2
2
1 t
e
a
t
e
a
t
e
a
t
e i
i
i
o +
+
=
)]
(
)
(
cos[
)
( t
t
w
E
t
e c
c
i 
+
=
))
(
)
(
(
cos
))
(
)
(
(
cos
))
(
)
(
cos(
)
(
3
3
3
2
2
2
1
t
t
w
E
a
t
t
w
E
a
t
t
w
E
a
t
e
c
c
c
c
c
c
o



+
+
+
+
+
=
4
3
cos
cos
3
cos
;
2
cos
2
1
cos 3
2 x
x
x
x
x
+
=
+
=

• AFTER FILTERING THROUGH BANDPASS FILTER, THE FM
SIGNAL OUTPUT
• EFFECT OF NONLINEARITIES: NONLINEAR NATURE OF
CHANNEL CHANGES THE AMPLITUDES OF THE FM SIGNAL
31
))
(
3
)
(
3
2
cos(
4
1
))
(
2
)
(
2
2
cos(
2
1
))
(
)
(
(
cos
))
(
)
(
2
cos(
)
4
3
(
2
1
)
(
3
1
2
2
2
3
1
1
3
3
t
t
fc
E
a
t
t
f
E
a
t
t
w
E
a
t
t
f
E
a
E
a
E
a
t
e
c
c
c
c
c
c
c
c
c
o







+
+
+
+
+
+
+
+
+
=
))
(
)
(
2
cos(
)
4
3
(
)
(
3
1 3 t
t
f
E
a
E
a
t
e c
c
c
o 
 +
+
=

APPLICATION OF FM
• FM IS COMMONLY USED AT VHF RADIO FREQUENCIES
FOR HIGH-FIDELITY BROADCASTS OF MUSIC AND SPEECH
(FM BROADCASTING). NORMAL (ANALOG) TV SOUND IS
ALSO BROADCAST USING FM. THE TYPE OF FM USED IN
BROADCAST IS GENERALLY CALLED WIDE-FM, OR W-FM
• A NARROWBAND FORM IS USED FOR VOICE
COMMUNICATIONS IN COMMERCIAL AND AMATEUR
RADIO SETTINGS. IN TWO-WAY RADIO, NARROWBAND
NARROW-FM (N-FM) IS USED TO CONSERVE BANDWIDTH.
IN ADDITION, IT IS USED TO SEND SIGNALS INTO SPACE.
32

FREQUENCY MODULATION VERSUS
AMPLITUDE MODULATION
Major applications of AM and FM 33

FREQUENCY MODULATION VS.
AMPLITUDE MODULATION
ADVANTAGES OF FM
NOISE IMMUNITY
✓ MOST NOISE RESULTS IN UNWANTED AMPLITUDE VARIATIONS IN
THE MODULATED WAVE, BUT FM AND PM RECEIVERS HAVE LIMITERS
THAT REDUCE THE NOISE
✓ THIS PROCESS CANNOT BE USED WITH AM RECEIVERS BECAUSE
REMOVING THE NOISE WOULD REMOVE THE INFORMATION.
CAPTURE EFFECT
✓ CAPTURE EFFECT ALLOWS A RECEIVER TO DIFFERENTIATE BETWEEN
TWO SIGNALS RECEIVED WITH THE SAME FREQUENCY.
✓ THE RECEIVER WILL CAPTURE THE STRONGER SIGNAL AND
ELIMINATE THE WEAKER SIGNAL
✓ WITH AM, BOTH SAME FREQUENCY SIGNALS WILL BE
DEMODULATED AND PRODUCE AUDIO SIGNALS. ONE MAY BE
LARGER IN AMPLITUDE THAN THE OTHER, BUT BOTH CAN BE HEARD.
34

POWER UTILIZATION AND EFFICIENCY
✓ WITH FM, ALL THE TRANSMITTED POWER IS USEFUL BECAUSE
POWER IS REDISTRIBUTED IN THE SIDEBANDS I.E. MOST OF ITS
POWER IN THE INFORMATION. HOWEVER, IN AM MOST OF THE
POWER IS IN THE TRANSMITTED CARRIER, WHICH CONTAINS NO
USEFUL INFORMATION
35

DISADVANTAGES OF FM
WIDER BANDWIDTH
✓ ANGLE MODULATION WILL PRODUCE MANY SIDE FREQUENCIES
AND THUS, NECESSITATING A WIDER BANDWIDTH.
CIRCUIT COMPLEXITY AND COST
✓ TODAY, HOWEVER, WITH THE ADVENT OF INEXPENSIVE, LARGE-
SCALE INTEGRATION ICS, THE COST OF MANUFACTURING FM
AND PM CIRCUITS IS COMPARABLE TO AM.
36

SUMMARY OF ANGLE MODULATION
-WHAT YOU NEED TO BE FAMILIAR
WITH
37

PHASE MODULATION (PM) SIGNAL
ANALYSIS
✓ PHASE MODULATION IS A SYSTEM IN WHICH THE PHASE OF THE
CARRIER
SIGNAL IS VARIED BY THE INFORMATION SIGNAL.
✓ THE AMPLITUDE OF THE CARRIER SIGNAL IS KEPT CONSTANT.
✓ THE PHASE () IN THE EQUATION 3.1 IS VARIED SO THAT ITS
MAGNITUDE IS
PROPORTIONAL TO INSTANTANEOUS AMPLITUDE OF THE
MODULATING SIGNAL.
38

PM WAVEFORMS
✓FIGURE 3.2 ILLUSTRATES HOW THE PM WAVEFORM GENERATED
DEPENDS ON THE PHASE CHANGE OF THE INFORMATION SIGNAL. IT
IS BEST ILLUSTRATED USING A SQUARE INFORMATION SIGNAL OR
MODULATING SIGNAL.
39

PHASE DEVIATION AND
MODULATION INDEX
✓ FOR A CARRIER THAT IS BEING PHASE MODULATED, IT CAN BE EXPRESSED
BY,
✓ THE MODULATION INDEX FOR A PHASE-MODULATED CARRIER IS EXPRESSED
MATHEMATICALLY AS,
WHERE = MODULATION INDEX FOR PM OR PEAK PHASE
DEVIATION(=)
= DEVIATION SENSITIVITY
= PEAK MODULATING-SIGNAL AMPLITUDE (VOLT)
)]
cos(
cos[
)
( t
m
t
V
t
v m
p
c
c
PM 
 +
=
m
p
p V
K
m =
p
m
p
K
m
V
(3.7)
41

EXAMPLE :
DETERMINE THE PEAK PHASE DEVIATION (M) FOR A PM
MODULATOR WITH A DEVIATION SENSITIVITY K = 2.5 RAD/V
AND A MODULATING SIGNAL,
SOLUTION:
PEAK PHASE DEVIATION FOR PM WAVE IS THE MODULATION
INDEX.
)
t
2000
2
cos(
2
)
t
(
vm 
=
rad
V
K
m m
p
p
5
2
5
.
2
=

=
=
42

Angle Modulation.pdf

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Angle Modulation.pdf