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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
Angle / Exponential Modulation
Lecture-3 FM Generation and Demodulation
3.0 Introduction.
3.1 Indirect FM Generation.
3.2 Direct FM Generation / Parameter Variation Methods.
3.2.1 Reactance Modulator
3.2.2 Varactor Diode Modulator
3.2.3 Limitations of direct methods of FM generation
3.3 FM Demodulators
Direct type FM detector:
3.3.1 Single ended slope detector
3.3.2 Balanced slope detector
3.3.3 Foster-Seeley / Phase discriminator
3.3.4 Ratio detector
3.3.5 Zero crossing detector
Indirect type FM detector
3.3.6 PLL based FM detector
3.4 Performance Comparison of FM Demodulators.
3.5 FM versus PM
3.6 Angle Modulation versus Amplitude Modulation.
3.6.1: Advantages of Angle Modulation
3.6.2: Disadvantages of Angle Modulation
3.7 References.
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
Lecture-3 FM Generation and Demodulation
3.0 Introduction: There are two basic methods for generating FM waves namely ‘Indirect FM’
and ‘Direct FM’. In the indirect method (or Armstrong modulator) of producing FM is first used
to produce a narrowband FM wave, and frequency multiplication is next used to increase the
frequency deviation to the desired level. On the other hand, in the direct method of producing
FM, the carrier frequency is directly varied in accordance with the amplitude of modulating
(message) signal. The indirect method is preferred choice for frequency modulation, when the
stability of carrier frequency is of major concern as in commercial broadcasting.
3.1 Indirect (Armstrong) Method of FM Generation: The
indirect method of FM generation consists of two steps as
shown in Fig 1. A narrow band FM generation followed by
Fig 1. Block diagram of indirect FM
a frequency multiplier is used to increase the frequency deviation to the desired level.
Step1 Generation of NBFM: The block
diagram of narrowband FM generation using
phase modulator is shown in Fig 2. The NMFM
differs from an ideal FM wave in two respects.
(i) The envelop contains a residual AM and
therefore varies with time.
Fig 2. Block diagram of generation of NBFM
(ii) For sinusoidal modulating wave, the phase of the FM wave contains harmonic distortion in
the form of 3rd and higher harmonics of f m .
However by restricting  f  0.2 , the residual AM and harmonic distortion are negligible levels.
The NBFM generation is given by
t
s1 (t )  Ac cos 2 f1t  2 k1(sin 2 f1t )  m(t ) dt 
(1)
0


where f1 is the first carrier frequency and k1 is modulation sensitivity for NBFM. Then the
max | k1m(t ) |
instantaneous frequency fi  f1  k1m(t ) and 1 
, where W is the bandwidth of
W
message signal.
Step2: Frequency Multiplication: Basically the
frequency multiplier consists of a non-linear device Fig. 3 Block diagram of Frequency Multiplier
(diode or transistor) followed by a band pass filter (BPF) as shown in Fig.3.
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
Memoryless Non-linear operation: The non-linear device is assumed to be memory-less means
that there is no energy storage. The memoryless non-linear device is represented by input and
output relation
s2 (t )  a1s1 (t )  a2 s12 (t )  a3s13 (t )  .....  an s1n (t )
(2)
where a1 , a2 ,…, an are constant coefficients.
By substituting eq (1) in eq(2), then s2 (t ) contains dc component and n frequency modulated
waves with carrier frequencies, f1 , 2 f1 , . . . , n f1 and frequency deviation f1 , 2 f1 , …, n f1 .
The value of f1 is determined by the frequency sensitivity k1 of NBFM and maximum
amplitude of the m(t ) .
Band Pass Filter (BPF): The band pass filter is designed with two aims:
(i) To pass the FM wave centered at the new (desired) carrier frequency fc  nf1 and new
(desired) frequency deviation f  nf1 .
(ii) To suppress the all other spectra.
WBFM Generation: The complete block
schematic diagram of WBFM generation
is illustrated in Fig 4. Then the out of the
band pass filter (BPF) is a wideband FM
signal represented by
Fig 4. block schematic diagram of WBFM generation
t
sFM (t )  Ac cos  2 fct  2 k f  m(t ) dt  (3)
0


where fc  nf1 is new carrier frequency and new modulation sensitivity k f  nk1 . Then the
max | k f m(t ) |
instantaneous frequency fi  nf1  nk1m(t )  fc  k f m(t ) and  f 
, where W is
W
the bandwidth of message signal.
Example of Indirect FM:
Fig 5 shows the simplified block diagram of a typical FM transmitter (based on indirect method)
used to transmit audio signals containing frequencies in the range 100 Hz to 15 KHz. The NBPM
is supplied with a carrier wave of frequency f1  0.1 MHz by a crystal controlled oscillator. The
desired FM wave at the transmitter output has a carrier frequency fc  100 MHz and frequency
deviation f  75 KHz.
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
Fig 5 Block diagram of FM for example
In order to limit the harmonic distortion produced by the NBPM, we restrict the modulation
index 1 to be 0.2 rad (i.e., 1  0.2 rad).
Since 1 
f1
, and for fm  100 Hz, the frequency deviation f  20 Hz.
fm
To produce frequency deviation f  75 KHz, A frequency multiplication n 
f
 3750 is
f1
required. However, using straight frequency multiplication equal to the value would produce a
much higher carrier frequency at the transmitter output than the desired value of 100 MHz. To
generate FM wave having both the desired frequency deviation and carrier frequency, it is need
to use two stage frequency multiplier with an intermediate stage of frequency translation, as
illustrated in Fig 5.
Let n1 and n2 denote the respective frequency multiplication ratio so that n  n1 n2  3750 .
The carrier frequency at the first frequency multiplier output is translated downward in
frequency to ( f 2  n1 f1) by mixing it with a sinusoidal wave of frequency f 2  9.5 MHz, which
is supplied by a second crystal controlled oscillator. However, the carrier frequency at the input
of the second frequency multiplier is equal to fc / n2 .
f
Equating these two frequencies, we get f 2  n1 f1  c .
n2
With f1  0.1 MHz, f 2  9.5 MHz, and fc  100 MHz, we obtain n1  75 and n2  50 . Using
these frequency multiplication ratios, we get the set of values indicated below.
Parameter
Carrier frequency
Frequency deviation
At the Phase
Modulator
output
0.1 MHz
20 Hz
At the first
frequency
multiplier output
7.5 MHz
1.5 KHz
At the mixer
output
2.0 MHz
1.5 KHz
At the second
frequency
multiplier output
100 MHz
75 KHz
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
3.2 Direct Method of FM Generation / Parameter Variation Methods: In the direct method
of FM generation, the instantaneous frequency of
the carrier wave varied directly in accordance
with the message signal by means of a device
known as voltage controlled oscillator (VCO).
Fig 6 shows a schematic diagram for a simple direct
Fig 6 Simple direct FM generator
FM generator. The tank circuit ( L and Cm ) is the frequency determining section for a standard
LC oscillator. The capacitor microphone is a transducer that converts acoustical energy to
mechanical energy, which is used to vary the distance between the plates of Cm and,
consequently change in its capacitance. Thus the oscillator output frequency is changed directly
by the modulating signal, and the magnitude of the frequency change is proportional to the
amplitude of the modulating signal voltage. Reactance modulator and varactor diode method of
FM generations are discussed here.
3.2.1 Reactance Modulator: In direct FM generation, the
instantaneous frequency of the carrier is changed directly
in proportion with the message signal. For this, a device Fig 7 Illustration of Reactance Modulator
called voltage control oscillator (VCO) is used. A VCO can be implemented by using a
sinusoidal oscillator with a tuned circuit having a high
quality factor. The frequency of this
oscillator is changed by incremented variation in the reactive components involved in the tuned
circuit. If L or C of a tuned circuit of an oscillator is changed in accordance with amplitude of
modulated signal then FM can be obtained across the tuned circuit as shown in Fig.7. A two or
three terminal device placed across the tuned
circuit. The reactance of the device is varied
proportional to modulating signal voltage. This
will vary the frequency of the oscillator to produce
FM. The device used is FET, transistor or varactor
diode. Fig 8 shows a simple reactance modulator
using FET as the active device. The circuit
Fig 8 A simple reactance modulator using FET
configuration is called reactance modulator because the FET looks like a variable reactance load
to the L1C1 tank circuit. The modulating signal varies the reactance of FET which causes a
corresponding change in the resonant frequency of the tank circuit.
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
Circuit Operation:




v
v
The gate voltage v g  i g R  
 R , Then the drain current iD  g mv g  g m 
R .
 R  jX c 
 R  jX c 
where g m is the transconductance of FET and X c is the capacitive reactance.
v R  jX c
Then the impedance between drain and ground is z d 
.

iD
gm R
 jX c
j

Assuming that R  X c , the impedance zd 
.
g m R 2 f m g m RC
Here g m RC is equivalent to a variable capacitance. The impedance zd is inversely
proportional to R, the modulating frequency ( 2 f m ), and the transconductance g m .
When the modulating signal is applied, the gate to source voltage varied accordingly, causing
proportional change in g m . As a result the frequency of oscillator tank circuit is a function of the
amplitude of the modulating signal and the rate at which it changes is equal to f m .
3.2.2 Varactor Diode Method for FM Generation:
The varactor diode is a semiconductor
diode whose junction capacitance changes with dc bias voltage .The capacitance of a varactor is
inversely proportional to the reversed biased voltage amplitude. The most common frequency
modulators use a varactor to vary the frequency of an LC
circuit or crystal in accordance with the modulating
signal. This varactor diode is connected in shunt with the
tuned circuit of the carrier oscillator as shown in Fig 9.
An example of direct FM is shown in Fig 9 which
Fig 9. Varactor diode based for FM generation
uses a BJT Hartley oscillator along with a varactor diode. The varactor diode is reverse biased,
and its capacitance is dependent on the reverse voltage applied across it. This capacitance is
shown by the capacitor C(t) in Fig.9. The Frequency of oscillations of the Hartley oscillator
shown in the Fig 9 is given by
fi (t ) 
1
2 ( L1  L2 )C (t )
(4)
where C (t ) is the total capacitance C (t )  C0  Cv , where again C0 is the fixed tuning
capacitance (in the absence of modulation) and Cv is the varactor diode capacitance, L1 and
L2 are the two inductances in the frequency determining network.
Assume that for a modulating signal m(t) , the capacitance C (t ) is expressed as
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
C (t )  C0  kc m(t )
(5)
where kc is the variable capacitor’s sensitivity to voltage change.
From eq (4) and (5), we get
f i (t ) 
f0
(6)
k
1  c m(t )
C0
where f 0 is unmodulated frequency of oscillations f0 
1
2 C0 ( L1  L2 )
(7)
Provided that the maximum change in capacitance produced by the modulating wave is small
compared with the unmodulated capacitance C0 , then we formulate


k
fi (t )  f0 1  c m(t ) 
 2C0

(8)
f kc
fi (t )  f0 1  k f m(t )  where k f  0
2C0
where is the resultant frequency sensitivity of the modulator, where again k f called as the
Then the instantaneous frequency of the oscillator
frequency sensitivity of the modulator.
Frequency Stabilized FM Modulator: An FM
transmitter using the direct method as described
here has the disadvantage that the carrier frequency
is not obtained from a highly stable oscillator.
Fig 10. Frequency stabilized FM modulator
A method to provide a stabilized oscillator based FM generation is shown in Fig 10. The output
of the FM generator is applied to a mixer together with the output of a crystal-controlled
oscillator, and the difference frequency term is extracted. The mixer output is next applied to a
frequency discriminator is a device whose output voltage has an instantaneous amplitude that is
proportional to the instantaneous frequency of FM wave applied to its input. When the FM
transmitter has exactly the correct carrier frequency, the low pass filter output is zero. However
deviations of the transmitter from its assigned value will cause the frequency discriminator-filter
combination to develop a dc output voltage with a polarity determined by the sense of the
transmitter frequency drift. This dc voltage after suitable amplification is applied to the VCO in
such a way that as to modify the frequency of the oscillator in a direction that tends to restore the
carrier frequency to its required value.
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
3.2.3 Limitations of direct methods of FM generation: The direct methods of FM generation
suffer from the following limitations:
 In the direct methods of FM generation, it is difficult to obtain a high order of stability in
carrier frequency. This is because the modulating signal directly controls the tank circuit
which is generating the carrier. The crystal oscillator can be used for carrier frequency
stability, but frequency deviation is limited.
 The non linearity produces a frequency variation due to harmonics of the modulating
signal hence there are distortions in the output FM signal.
3.3 FM DEMODULATORS: Frequency demodulation is the process that enables one to extract
the original modulating signal (baseband signal) from the frequency modulated wave. This can
be achieved by a system which has a transfer characteristic just inverse of voltage controlled
oscillator (VCO). In other words a frequency demodulator produces an output voltage whose
instantaneous frequency of input FM signal. The overall transfer function for an FM demodulator
V (Volts)
is nonlinear but when operated over its linear range is kd 
, where kd is transfer
f ( Hz )
function.
The output from an FM demodulator is expressed as
Vout (t )  kd f
where Vout (t ) = demodulated output signal (Volts)
kd = demodulator transfer function (Volts per Hertz)
f = difference between the input frequency and the center frequency
of the demodulator (Hertz).
Several circuits are used for demodulating FM signals. Basically there are two types of FM
demodulators, frequency discriminators and PLL based demodulator. The slope detector,
Balanced slope detector, Foster-Seeley discriminator, and Ratio detector are tuned circuit
frequency discriminators.
3.3.1: Slope Detector: Fig 11 shows the
schematic diagram of a single ended slope
detector which is a simplest form of
frequency discriminator. The single ended
slope detector circuit consists of a tuned circuit tuned to a
Fig 11. Simple Slope Detector
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
frequency f 0 slightly below the carrier frequency fc and followed by an envelope detector.
Tuned Circuit: The tuned circuit transfer function is shown in Fig 12. As the instantaneous
frequency
fi , of the incoming FM wave swings above or
below fc , the amplitude ratio of tuned circuit converts the
frequency variation to an amplitude variation (FM to AM
conversion) as shown in Fig 13(b). The resulting signal sc (t ) is
basically a hybrid FM-AM modulated wave.
Fig 12.Tuned circuit Transfer function
Envelope Detector: This hybrid FM-AM modulated
wave is applied to a peak / envelope detector with
R1C1 load of suitable time constant. The circuit is in fact
to that of an AM detector. The envelope detector
produces the demodulated signal (baseband signal) as
shown in Fig 13(c).
Advantages: The only advantage of the basic slope
detector circuit is its simplicity.
Limitations:
(i). The range of linear slope of tuned circuit is quite
small.
(ii) The detector also responds to spurious amplitude
variations of the input FM.
These drawbacks are overcome by using balanced
slope detector.
Fig 13. FM Slope Detector and waveforms
3.3.2: Balanced Slope Detector: Fig 14
shows the circuit diagram of the balanced
slope detector. The circuit shows that the
balanced slope detector consists of two slope
detector circuits. The input transformer has a
center tapped secondary. Hence the input
voltages to the slope detectors are 1800 out of
phase.
Fig 14. Balanced Slope detector
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
There are three tuned circuits. Out of them, primary tank is tuned to carrier frequency fc . The
upper tuned circuit of secondary is tuned to above fc by V i.e., its resonant frequency
fc  f . Similarly the lower tuned circuit of secondary is tuned below fc by V , i.e., fc  f .
R1C1 and R2C2 are the filters used to bypass the RF ripple. Vo1 and Vo 2 are the output voltages
of the two slope detectors. The final output
voltage Vo is obtained by taking Vo  Vo1  Vo2 .
Working Operation of the Circuit: It can be
understand the circuit operation by dividing the
input frequency into three ranges as follows:
fin  fc : When the input frequency is
(i)
instantaneously equal to fc , the induced
voltage in the T1 winding of secondary is
exactly equal to that induced in the
winding T2 . Thus the input voltages to both
Fig 15. Characteristics of balanced slope detector
diodes are equal and the net out voltage is zero.
(ii)
fin  fc  fc  f : In this range of input frequency, the induced voltage in the winding
T1 is higher than that induced in T2 . Therefore the input to D1 is higher than D2 . Hence the
positive output Vo1 is higher than that of Vo 2 . The resultant output voltage Vo is positive. As
the input frequency increases towards fc  f , the positive output voltage increases as
shown in Fig 15.
If the output frequency goes outside the range of fc  f to fc  f , the output voltage will fall
due to the reduction in tuned circuit response.
Advantages: (i) This circuit is more efficient than simple slope detector.
(ii) It has better linearity than the simple slope detector.
Limitations: (i) Even though linearity is good, it is not good enough.
(ii) This circuit is difficult to tune since the three tuned circuits are to be tuned at
different frequencies, fc , fc  f and fc  f .
(iii) Amplitude limiting is not provided.
10
Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
3.3.3 Foster-Seeley Discriminator (Phase Discriminator): A Foster-Seeley discriminator is a
tuned circuit frequency discriminator whose operation is very similar to that of the balanced
slope detector as shown in Fig 16. The capacitance values Cc , C1 and C 2 are chosen such that
they are short circuits for the center frequency (carrier frequency f c ). Therefore the input voltage
of FM Vin is fed directly (in phase) across L3 ( VL3 ). At the resonant frequency, the secondary
current I s is in phase with the secondary voltage Vs , and 1800 out of phase with VL3 . VLa and
VLb are 1800 out of phase with each other and in quadrature or 900 out of phase with VL3 . The
voltage across VD1 is the vector sum of VL3 and VLa . Similarly, The voltage across VD 2 is the
vector sum of VL3 and VLb . The corresponding vector diagrams are shown in Fig 17.
Principle of Operation: Even though the primary and secondary tuned circuits are tuned to the
same center frequency, the voltages applied to the two diodes D1 and D2 are not constant. They
are very depending on the frequency of the input signal. This is due to change in phase shift
between the primary and secondary windings depending on the input frequency.
The results are described as below:
(i) For input frequency fin  fc , the individual output voltages of the two diodes will be
equal and opposite. Then the resultant output voltage is zero. That is Vout  VC1  VC 2  0 .
The corresponding phasor diagram shown in Fig 17(a).
(ii) For fin  fc , the phase shift between the primary and secondary windings is such that the
output of D1 is higher than D2 . That is VD1  VD 2 , and total output voltage Vout is
positive. The corresponding phasor diagram is shown in Fig 17(b).
(iii) For fin  fc , the phase shift between the primary and secondary windings is such that
output of D2 is higher than that output of D1 making the output voltage Vout is negative.
The corresponding phasor diagram is shown in Fig 17(c).
A Foster-Seeley discriminator is tuned by injecting a frequency equal to the center frequency and
tuning C0 for 0 volts out. Fig 18 shows a typical voltage-versus-frequency response curve for a
Foster-Seeley discriminator. For obvious reasons, it is often called an S-curve. It can be seen that
the output voltage-versus-frequency deviation curve is more linear than of a slope detector, and
because there is only one tank circuit, it is easier to tune.
11
Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
For a distortionless demodulation, the frequency deviation should be restricted to the linear
portion of the secondary tuned circuit frequency response curve. As with the slope detector, a
Foster-Seeley discriminator responds to amplitude as well as frequency variations and therefore
must be preceded by a separate limiter circuit.
Fig 16. Foster-Seeley discriminator (Phase discriminator)
Phase Diagrams: The phasor diagrams at different input frequencies are shown below.
Fig 17 Phasor diagrams at different input frequencies (a) fin  f o , (b) fin  f o , and (c) fin  f o
Frequency Response of Phase Discriminator: The frequency response of phase discriminator is
shown in Fig18.
Advantages:
(i) Tuning procedure is simpler than balanced slope
detector, because it contains only two tuned circuits
and both are tuned to the same frequency fc .
(ii) Better linearity, because the operation of the circuit is
Fig 18 The discriminator response
dependent more on the primary to secondary phase relationship which is very much linear.
Limitations: It does not provide amplitude limiting. So in the presence of noise or any other
spurious amplitude variations, the demodulator output respond to them and produce errors.
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
3.3.4 Ratio Detector: Ratio detector is another frequency demodulator circuit is illustrated in
Fig.19. The ratio detector has one major advantage over slope detector and Foster-Seeley
discriminator is that, the ratio detector is relatively immune to amplitude variations in its input
signal. As with the Foster-Seeley discriminator, the ratio detector has single tuned circuit in the
transformer secondary. The circuit diagram is similar to the Foster-Seeley discriminator with
minor modifications as described below.
(i) The direction of diode D2 is reversed.
(ii) A large capacitance Cs is included in the circuit.
(iii) The output is taken different locations.
Fig 19 Ratio detector (a) Circuit diagram (b) frequency response curve
Operation: After several cycles of input signal, shunt capacitance Cs charges to approximately
to the peak voltage across the secondary winding. The reactance of Cs is low, and Rs simply
provides a dc path for diode current. Therefore the time constant Rs and Cs is sufficiently long so
that rapid changes in the amplitude of input signal due to thermal noise or other interfering
signals are shorted to ground and have no effect on the average voltage across Cs . Consequently
C1 and C2 charge and discharge proportional to frequency changes in the input signal and are
relatively immune to amplitude variations.
Also the output voltage from ratio detector is taken with respect to ground, and for the
diode polarities shown in Fig.19, the average output voltage is positive. At resonance the output
voltage is divided equally between C1 and C2 , and redistributed as the input frequency is divided
above and below resonance. Therefore changes in Vout are due to the changing ratio of voltage
across C1 and C2 , while the total voltage is clamped by Cs .
13
Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
Fig 19(b) shows the output frequency response curve for the ratio detector shown in Fig 19
(a). It can be seen that at resonance, Vout is not equal to zero, but retain the one half of the
voltage across the secondary. Because a ratio detector is relatively immune to amplitude
variations, it is often selected over discriminator. However a discriminator produces a more
linear output voltage-versus-frequency response curve.
Advantages:
(i) Easy to align.
(ii) Good linearity due to linear phase relationship between primary and secondary.
(iii)Amplitude limiting is provided inherently. Hence additional limiter is not required.
3.3.5 Zero Crossing Detector: The zero crossing detector operator on the principle that the
instantaneous frequency of an FM wave approximately given by
C1 
1
2 t
where t is the time difference between the adjacent zero crossover points of the FM wave as
shown in Fig. Let us consider a time duration T as shown in figure. The time T is chosen such
that it satisfies the following two conditions:
(i) The interval T is small compared to the reciprocal of the message band width ‘W’.
(ii) The interval T is large compared to the reciprocal of the carrier frequency of the FM f c
wave .
Condition 1 means that the message signal m(t) is essentially constant inside the interval T.
Condition 2 ensures that a reasonable number of
zero crossings of the FM wave occurs inside the
interval T. Fig 20 illustrates these two conditions.
Let n0 denote the number of zero crossings inside
the interval T. We may then express the time t
between adjacent zero crossings as
t 
T
n0
Fig 20 FM wave illustrating interval T
n
Hence fi  0 . Since by definition, the instantaneous
2T
frequency is linearly related to the message signal
Fig 21. Block diagram of zero crossing detector
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
m(t), the message signal can be recovered from a knowledge of n0 . Fig 21 is the block diagram
of a simplified form of the zero-crossing detector based on this principle. The limiter produces a
square-wave version of the input FM wave. The pulse generator produces short pulses at the
positive going as well as negative going edges of the limiter output. Finally, the integrator
performs the averaging over interval T, thereby reproducing the original message signal m(t) at
its output.
3.4
S.No.
Performance Comparison of FM Demodulators
Parameter of
Comparison
Balanced Slope
detector
Foster-Seeley
(Phase)
discriminator
Ratio Detector
Not Critical
Not Critical
Primary and
secondary phase
relation.
Primary and
secondary phase
relation.
Very good
Good
(i)
Alignment/tuning
Critical as three circuits
are to be tuned at
different frequencies
(ii)
Output characteristics
depends on
Primary and secondary
frequency relationship
(iii)
Linearity of output
characteristics
Poor
(iv )
Amplitude limiting
Not providing inherently
Not Provided
inherently
Provided
Not used in practice
FM radio,
satellite station
receiver etc.
TV receiver
sound section,
narrow band FM
receivers.
(v)
Amplifications
3.5 FM versus PM: From a purely theoretical point of view, the difference between FM and PM
is quite simple. The modulation index for FM is defined differently than for PM. With PM, the
modulation index is directly proportional to the amplitude of the modulating signal and
independent of its frequency. With FM, the modulation index is directly proportional to the
amplitude of the modulating signal and inversely proportional to its frequency.
Considering FM as a form of phase modulation, the larger the frequency deviation, the
larger the phase deviation. Therefore, the latter depends, at least to a certain extent, on the
amplitude of the modulating signal, just as with PM. With PM, the modulation index is
proportional to the amplitude of the modulating signal voltage only, where as with FM, the
modulation index is also inversely proportional to the modulating signal frequency. If FM
15
Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
transmissions are received on a PM receiver, the bass frequencies would have considerably more
phase deviation than a PM modulator would have given them. Because the output voltage from a
PM demodulator is proportional to the phase deviation, the signal appears excessively bass
boosted. In more practical situation, PM demodulated by an FM receiver produces an
information signal in which the higher frequency modulating signals are boosted.
3.6 Angle Modulation versus Amplitude Modulation: Various advantages and disadvantages
of FM over AM are illustrated as below.
3.6.1: Advantages of Angle Modulation: Angle modulation has several inherent advantages
over amplitude modulation.
(a) Noise Immunity: Probably the most significant advantages of angle modulation
transmission (FM and PM) over amplitude modulation transmission is noise immunity.
Most noise results in unwanted amplitude variations in the modulated wave (i.e., AM
noise). FM and PM reveivers include limiters that remove most of the AM noise from
the received signal before the final demodulation process occurs- a process that cannot
be used with AM receivers because the information is also contained in amplitude
varations, and removing the noise would also remove the information.
(b) Noise performance and Signal-to-Noise Improvement: With the use of limiters, FM
and PM demodulators can actually reduce the noise level and improve the signal-tonoise ratio during the demodulation process. This is called FM thresholding. With AM,
the noise has contaminated the signal, it cannot be removed.
(c) Capture effect: With the FM and PM, a phenomenon is known as capture effect allows a
receiver to differentiate between two signals received with the same frequency.
Providing one signal at least twice as high in amplitude as the other, the receiver will
capture the stronger signal and eliminate the weaker signal. With the AM, two or more
signals are received with same frequency; both will be demodulated and produce audio
signals. One may be larger in amplitude than the other, but both can be heard.
(d) Power Utilization and efficiency: With AM transmission, most of the transmitted power
is contained in the carrier while the information is contained in the much lower
sidebands. With angle modulation, the total power remains constant regardless of the
modulation is present. With AM, the carrier power remains constant with modulation,
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Dr. M. Venu Gopala Rao, Professor, Dept. of ECE, KL University
and the sideband power simply adds to the carrier power. With angle modulation, power
is taken from the carrier with modulation and redistributed in the sidebands; thus angle
modulation puts most of power in the information.
3.6.2 Disadvantages of Angle Modulation: Angle modulation also has several inherent
disadvantages over amplitude modulation.
(a) Bandwidth: High quality angle modulation produces many side frequencies, thus
necessitating a much wider bandwidth than is necessary for AM transmission.
Narrowband FM utilizes a low modulation index and, consequently, produces only one
set of sidebands. Those sidebands, however, contain an even more disproportionate
percentage of the total power than a comparable AM system. For high quality
transmission, FM and PM require much more bandwidth than AM. Each station in the
commercial AM radio band is assigned 10 kHz of bandwidth, whereas in the
commercial FM broadcast band. 200 kHz is assigned each station.
(b) Circuit Complexity and Cost: PM and FM modulators, demodulators, transmitters
and receivers are more complex to design and build than their AM counterparts. At one
time, more complex meant more expensive. Today, however, with the advent of
inexpensive, large scale integration ICs, the cost of manufacturing FM and PM circuits
is comparable to their AM counterparts.
3.7 References:
1. H Taub & D. Schilling, Gautam Sahe, ”Principles of Communication Systems, TMH, 2007,
3rd Edition.
2. Simon Haykin ,”Principles of Communication Systems “,John Wiley, 2nd Ed.
3. John G. Proakis, Masond, Salehi ,”Fundamentals of Communication Systems “,PEA, 2006.
4. B.P. Lathi and Zhi Ding, “Modern Digital and Analog Communication Systems”,
International, 4th
Edition, Oxford University Press, 2010.
5. George Kennedy, “ Electronic Communication Systems”, 3rd edition, Tata McGraw-Hill
Edition.
6. Wayne Tomasi, ‘Electronic Communication Systems- fundamentals through advanced’,
5th edition, Pearson Education Inc, 2011.
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