FM Demodulator

WHAT WE SHALL COVER
• Review of Modulation (with animations)
• What is demodulation
• Frequency Demodulation Definition
• Types of FM Demodulators
• Study of Various FM Demodulators (Slope, Balanced,
Foster-Seeley, Ratio and Phase Locked Loop)
2Umera Anjum - M.Tech

WHAT IS MODULATION
• Modulation is the addition of information (or the
signal) to an electronic or optical signal carrier.
3
Modulator
Carrier
Wave
Information
Modulated
Signal
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TYPES OF MODULATION
Common modulation methods include:
• Amplitude modulation (AM), in which the voltage applied
to the carrier is varied over time
• Frequency modulation (FM), in which the frequency of the
carrier waveform is varied in small but meaningful
amounts
• Phase modulation (PM), in which the natural flow of the
alternating current waveform is delayed temporarily

AMPLITUDE MODULATION
EXAMPLE
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Modulating
Signal
Transmitted
Signal
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Frequency Modulation Example
Transmitted
Signal
Modulating
Signal

WHAT IS A DEMODULATOR?
• Demodulation is the act of extracting the original
information-bearing signal (modulating signal) from a
modulated carrier wave.
• A demodulator is an electronic circuit used to recover the
information content from the modulated carrier wave.

WHAT IS FM DEMODULATOR
• An electronic circuit in which frequency variations of
modulated signals are converted to amplitude
variations first, with the help of tuned circuit
• And then original information is extracted with the
AM demodulation techniques say diode detector.

9
Types of FM Demodulators
FM
Demodulation
IndirectDirect
• Slope Detector
•Balanced Slope Detector
•Foster-Seeley Phase
Discriminator
•Ratio Detector
Phase Lock Loop(PLL)

BASIC FM DEMODULATOR
10
TUNED
CIRUIT
Frequency
Variations
Amplitude
Variations
NOTE: Amplitude Variations are added to wave
according to frequency variations, and frequency
variations remain present in wave.

• The function of FM demodulator is to change the
frequency deviation of the incoming carrier into an
AF amplitude variation.
• The detection circuit should be insensitive to
amplitude changes.

• This type of circuit converts the FM IF voltage of
constant amplitude into a voltage that is both FM
and AM.
• The later is applied to a detector which reacts to
amplitude changes and ignores frequency changes.

FM
Wave
Output
of Tuned
Circuit
Basic FM Demodulator

• The most basic circuit employed as FM
demodulator is parallel tuned LC circuit, often
known as slope detector.
• The carrier frequency should fall on one side of
resonant frequency and
• The entire frequencies should fall on linear region of
transfer curve of tuned circuit.

FM
Source
Tank
Circuit
Detector
Output
FM
SLOPE DETECTOR

Transfer
Curve
Output
SLOPE
DETECTOR
TRANSFER
CHARACTERISTI
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Transfer
Curve
Output
Slope Detector
Transfer
Characteristics
Punjab EDUSAT Society

SLOPE DETECTOR
• The output is then applied to a diode detector with
RC load of suitable time constant.
• The circuit is, in fact, identical to that of AM
detector.

LIMITATIONS OF SLOPE
DETECTOR
• It is inefficient, as it is linear in very limited
frequency range.
• It reacts to all amplitude changes.
• It is relatively difficult to tune, as tuned circuit must
be tuned to different frequency than carrier
frequency.

BALANCED SLOPE DETECTOR
• This circuit uses two slope detectors, connected in
back to back fashion, to opposite ends of center-
tapped transformer.
• And hence fed 1800
out of phase.

BALANCED SLOPE DETECTOR• The top secondary circuit is tuned above the IF by an
amount δf, and bottom circuit is tuned below IF by δf.
• Each circuit is connected to diode detectors with suitable
RC loads.
• The output is taken across series combination of loads,
so that it is sum of the individual outputs.

Output of T’ at fc+δf
(+10 to +15V)
Output of T’ at fc-δf
(+5 to +10V)
Output of T’ at fc-δf
(-10 to -15V)
Output of T’’ at fc+δf
(-5 to -10V)
5V
10V
15V
-5V
-10V
-15V

COMBINED TRANSFER CURVE
Useful Range

• When input frequency = fc
• Then output of T’(+Ve)= output of T’’ (-Ve)
• So sum of outputs of T’ and T’’ = Zero
• When input frequency = fc+δf
• Then output of T’(+Ve) > output of T’’ (-Ve)
• So sum of outputs of T’ and T’’ = +Ve
• When input frequency = fc-δf
• Then output of T’(+Ve) < output of T’’ (-Ve)
• So sum of outputs of T’ and T’’ = -Ve

BALANCE SLOPE DETECTOR-
DRAWBACKS
• Even more difficult to tune, as there are three
different frequencies to be tuned.
• Amplitude limiting still not provided.
• Linearity, although better than single slope detector,
is still not good enough.

FOSTER-SEELEY (PHASE)
DISCRIMINATOR
• In this all the tuned circuits are tuned to the same
frequency.
• Balanced Slope Detector circuit with some changes
is used.
• This circuit yields far better linearity than slope
detection.

As C & C4 are coupling & RF Bypass capacitors
respectively, therefore VL3≈ VIN So
Voltage across diode= VIN + Secondary voltage/2
Phase Discriminator

PHASE DISCRIMINATOR
• Now in Transformer voltage is induced in the
secondary as a result of current in primary.
And
• Where X2= XL2-XC2
22
2
1
.
jXR
XV
L
jM
V CIN
ab
+
=

PHASE DISCRIMINATOR
• At resonance i.e. when input frequency is fc, X2=0
• i.e. Vab leads VIN by 900
.
2
2
1
.
R
XV
L
jM
V CIN
ab =

PHASE DISCRIMINATOR
• And from the phasor diagram given below :
• That as Vao=Vbo, hence discriminator output is zero.
2
abV
2
abV
−
Vao
Vbo

PHASE DISCRIMINATOR
• When input frequency is greater than fc, then XL2>XC2 &
hence X2 is positive.
• That is Vab leads VIN by less than 900
.
22
2
1
.
jXR
XV
L
jM
V CIN
ab
+
=
θ∠
∠
=
21
0
2 90
ZL
MXV CIN
)90( 0
21
2
θ−∠=
ZL
MXV CIN

Phase Discriminator
• That as Vao>Vbo, hence discriminator output is
positive.
2
abV
2
abV
−
Vao
Vbo

Phase Discriminator
• When input frequency is less than fc, then
XL2<XC2 & hence X2 is negative.
• That is Vab leads VIN by more than 900
.
22
2
1
.
jXR
XV
L
jM
V CIN
ab
−
=
)(
90
21
0
2
θ−∠
∠
=
ZL
MXV CIN
)90( 0
21
2
θ+∠=
ZL
MXV CIN

Phase Discriminator
• That as Vao<Vbo, hence discriminator output is
negative.
2
abV
2
abV
−
Vao
Vbo

PHASE DISCRIMINATOR
Useful Range
Beyond which o/p falls
due to frequency
response of
transformer.
Useful Range extends
upto half-power points
of tuned transformer.

PHASE DISCRIMINATOR
• It is much easier to align, as there are now two tuned
circuits and both are tuned to the same frequency.
• Linearity is quite better, as circuit relies less on frequency
& more on primary-secondary phase relation, which is
quite linear.
• Only drawback is, there is no provision for amplitude
limiting.

RATIO- DETECTOR
• Ratio detector demodulator is modified Foster-
Seeley circuit in order to incorporate amplitude
limiting.
• In Foster-Seeley discriminator that sum of voltages
Vao+Vbo Should remain constant,
• and their difference should vary due to variation in
input frequency.

RATIO-DETECTOR
• But practically speaking any variation in the amplitude of
input signal, also has impact on sum of Vao+Vbo, leading to
distortion.
• Ratio-detector circuit eliminates this variation of Vao+Vbo,
and performs the function of amplitude limiter also.

RATIO-DETECTOR
Three changes are made in Foster-Seeley discriminator:
• One of The diodes has been reversed.
• A large capacitor has been placed between points, from
where output was taken.
• Output now is taken from elsewhere.

RATIO-DETECTOR
Change 1: Diode D2 is reversed so that
now sum of Vao & Vbo appears across
points a’ and b’ instead of difference.

RATIO-DETECTOR
Change 2: A capacitor C5 with large time
constant is connected across a’-b’ in order
to keep Vao+Vbo constant.

RATIO-DETECTOR
Change 3: Output is taken from o-o’ as the
difference of Vao + Vbo appears there.
Ground is shifted to O’.

OPERATION AT RESONANCE
• No phase shift occurs at resonance and both Vao & Vbo
are equal. Hence their difference (output) is zero.
• During negative part of cycle of input signal, polarity
across secondary also changes and both diodes get
reverse biased.
• But C5 with large time constant maintains voltage at
constant level.

OPERATION ABOVE
RESONANCE
• When a tuned circuit operates at a frequency higher than
resonance, the tank is inductive.
• Secondary voltage V1 is nearer in phase with primary
voltage, while V2 is shifted further out of phase with
primary.

OPERATION ABOVE
RESONANCE• So output voltage in this case will be positive as shown in
vector diagram:
2
abV
2
abV
−
Vao
Vbo
Output

OPERATION BELOW
RESONANCE
• When a tuned circuit operates below resonance, it
is capacitive. Secondary current leads the primary
voltage and
• secondary voltage V2 is nearer in phase with
primary voltage and voltage V1 is shifted away in
phase from primary voltage

OPERATION BELOW
RESONANCE
• So the output in this case will be negative.
2
abV
2
abV
−
Vao
Vbo
Output

RATIO-DETECTOR
ADVANTAGES
• Amplitude limiting is possible.
• Linearity is quite good as compared to others. So
quite often used in high quality receivers.

RATIO-DETECTOR DIS-
ADVANTAGES• Under critical noise conditions, such as satellite receivers,
where demodulator noise performance becomes very
significant, even this demodulator is found wanting.
• Under these conditions more advanced demodulators
such as Phase Locked Loop are used.

PHASE LOCKED LOOP (PLL)
• It is the best frequency demodulator.
• A phase-locked loop (PLL) is an electronic circuit
with a voltage- or current-driven oscillator that is
constantly adjusted to match in phase (and thus
lock on) with the frequency of an input signal.

PHASE LOCKED LOOP
• A basic phase Locked Loop consists of Three
components:
• Phase discriminator: compares phase of two
signals and generates a voltages according to
phase difference of two signals.

PHASE LOCKED LOOP
• Loop Filter: A low pass filter to filter the output of
phase discriminator.
• Voltage controlled Oscillator(VCO): generates RF
signals whose frequency depends upon voltage
generated by phase discriminator.

PHASE LOCKED LOOP
compare the two input signals and
generate an output signal that, when
filtered, will control the VCO.
adjusts the VCO frequency in
an attempt to correct for the
original frequency or phase
difference.

PHASE LOCKED LOOP
• As incoming frequency changes, The phase
discriminator generates a voltage to control the
frequency and phase of VCO.
• This control voltage varies at the same rate as the
frequency of the incoming signal.

PHASE LOCKED LOOP
Control Voltage α rate of input freq change
Hence this signal can be directly used as output.
PLL must have low time constant so that it can follow
modulating signal.

PHASE LOCKED LOOP
• Free running frequency of VCO is set equal to the
carrier frequency of the FM wave.
• The lock range must be at least twice the maximum
deviation of the signal.

PHASE LOCKED LOOP
• Linearity is governed by voltage to frequency
characteristics of VCO.
• As it swings over small portion of its bandwidth, the
characteristic can be made relatively linear.
• Hence the distortion levels of PLL demodulators are
normally very low.

Transfer
Curve
Output

FM Demodulator

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