Transcript PPT of FM Demodulators

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Contents
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Review of Modulation
What is demodulation
Frequency Demodulation Definition
Types of FM Demodulators
Study of Various FM Demodulators (Slope,
Balanced, Foster-Seeley, Ratio Detectors and
Phase Locked Loop)
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What is Modulation
• Modulation is the process of changing
characteristics of carrier w.r.t message signal.
Carrier
Wave
Modulator
Modulated
Signal
Information
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Types of Modulation
Common modulation methods include:
• AM :- in which the Amplitude of the carrier
is varied w.r.t message signal.
• FM;- in which the frequency of the carrier is
varied w.r.t message signal.
• PM:- in which the phase of the carrier is varied
w.r.t message signal.
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Amplitude Modulation Example
Transmitted
Signal
Modulating
Signal
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Frequency Modulation Example
Transmitted
Signal
Modulating
Signal
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What is a Demodulator?
• Demodulation is the process 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.
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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( Discriminator).
• And then original information is extracted with
the AM demodulation techniques say diode
detector.
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Types of FM Demodulators
FM
Demodulation
Direct
Indirect
Phase Lock Loop(PLL)
Detector
•Balanced Slope Detector
•Foster-Seeley Phase
Discriminator
•Ratio Detector
• Slope
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Basic FM Demodulator
Frequency
Variations
TUNED
CIRUIT
Amplitude
Variations
NOTE: Amplitude Variations are added to wave
according to frequency variations, and frequency
variations remain present in incomingwave.
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Basic FM Demodulator
• 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.
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Basic FM Demodulator
• This type of circuit converts the FM IF voltage
of constant amplitude into a voltage, that is
from FM to AM.
• The later is applied to a detector which reacts
to amplitude changes and ignores frequency
changes.
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Basic FM Demodulator
FM Wave
Output
of Tuned
Circuit
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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.
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SLOPE Detector
FM
Detector
Output
FM
Source
Tank
Circuit
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Transfer
Curve
Output
Slope Detector
Transfer
Characteristics
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Transfer
Curve
Output
Slope Detector Transfer
Characteristics
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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.
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Limitations of Slope Detector
• It is inefficient, as it is linear in very limited
frequency range.
• It reacts to all amplitude changes(input FM
signal).
• It is relatively difficult to tune, as tuned circuit
must be tuned to different frequency than
carrier frequency.
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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.
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Balanced Slope Detector
• The top secondary circuit is tuned above the
IF(carrier frequency)by an amount f, and
bottom circuit is tuned below IF by f.
• 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.
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Balanced Slope Detector
R1
C1
Vo1
C2
Vo2
R2
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Balanced Slope Detector
• When input frequency = fc
Then output of T’(+Ve)= output of T’’ (-Ve)
Vo= Zero
• When input frequency = fc+f
Then output of T’(+Ve) > output of T’’ (-Ve)
Vo= +Ve
• When input frequency = fc-f
Then output of T’(+Ve) < output of T’’ (-Ve)
So sum of outputs of T’ and T’’ (Vo) = -Ve
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Transfer Curve of Balanced slope Deter
Useful Range
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Balanced 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.
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Foster-seley 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.
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Foster-seley Phase Discriminator
As C & C4 are coupling & RF Bypass capacitors
respectively, therefore VL3 VIN So
Voltage across diode= VIN + Secondary voltage/2
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Foster-seley Phase Discriminator
• Now when Transformer voltage is induced in
the secondary as a result of current in primary.
And
jM VIN . X C 2
Vab 
L1 R2  jX 2
• Where X2= XL2-XC2
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Foster-seley Phase Discriminator
• At resonance i.e. when input frequency is fc,
X2=0
jM VIN . X C 2
Vab 
L1
R2
• i.e. Vab leads VIN by 900.
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Foster-seley Phase Discriminator
• And from the phasor diagram given below :
Vab
2
Vab

2
• That as Vao=Vbo, hence discriminator output is
zero.
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Foster-seley Phase Discriminator
• When input frequency is greater than fc, then
XL2>XC2 & hence X2 is positive.
jM VIN . X C 2 VIN X C 2 M900

Vab 
L1 Z 2 
L1 R2  jX 2
VIN X C 2 M

(900   )
L1 Z 2
• That is Vab leads VIN by less than 900.
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Foster-seley Phase Discriminator
• And from the phasor diagram given below :
Vab
2
Vab

2
• That as V ao> Vbo, hence discriminator output
is positive.
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Foster-seley Phase Discriminator
• When input frequency is less than fc, then
XL2<XC2 & hence X2 is negative.
jM VIN . X C 2 VIN X C 2 M900

Vab 
L1 Z 2 ( )
L1 R2  jX 2
VIN X C 2 M
0

(90   )
L1 Z 2
• That is Vab leads VIN by more than 900.
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Foster-seley Phase Discriminator
• And from the phasor diagram given below :
Vab
2
Vab

2
• That as Vao<Vbo, hence discriminator output is
negative.
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Foster-seley Phase Discriminator
Useful Range extends
up to half-power points
of tuned transformer.
Useful Range
Beyond which o/p falls
due to frequency
response of transformer.
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Foster-seley 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.
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Ratio- Detector
• Ratio detector demodulator is modified FosterSeeley 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.
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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.
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Ratio- Detector
Three changes
discriminator:
are
made
in
Foster-Seeley
• 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.
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Ratio- Detector
Change 1: Diode D2 is reversed so that now sum of Vao
& Vbo appears across points a’ and b’ instead of
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difference.
Ratio- Detector
+
V3
-
Change 2: A capacitor C5 with large time constant is
connected across a’-b’ in order to keep Vao+Vbo
constant.
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Ratio- Detector
Change 3: Output is taken from o-o’ as the
difference of Vao + Vbo appears there. Ground is
shifted to O’.
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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.
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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.
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Operation Above Resonance
• So output voltage in this case will be positive
as shown in vector diagram:
Vab
2
Vab

2
Output
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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
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Operation Below Resonance
• So the output in this case will be negative.
Vab
2
Vab

2
Output
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Ratio-Detector Advantages
• Amplitude limiting is possible.
• Linearity is quite good as compared to others.
So quite often used in high quality receivers.
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Ratio-Detector Disadvantages
• Under critical noise conditions, such as
satellite receivers, where demodulator noise
performance becomes very significant, even
this demodulator is found inefficient.
• Under these conditions more advanced
demodulators such as Phase Locked Loop are
used.
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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.
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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.
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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.
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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.
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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.
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Phase Locked Loop
• Control Voltage  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.
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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.
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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.
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Transfer
Curve
Output
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