Review of Modulation What is demodulation Frequency Demodulation Definition Types of FM Demodulators Study of Various FM Demodulators Slope Balanced FosterSeeley Ratio Detectors and Phase Locked Loop ID: 725889
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1
FM DEMODULATORSSlide2
Contents
Review of Modulation
What is demodulation
Frequency Demodulation DefinitionTypes of FM DemodulatorsStudy of Various FM Demodulators (Slope, Balanced, Foster-Seeley, Ratio Detectors and Phase Locked Loop)
2Slide3
What is Modulation
Modulation is the process of
changing characteristics of carrier
w.r.t message signal.
3
Modulator
Carrier Wave
Information
Modulated SignalSlide4
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.
4Slide5
Amplitude Modulation Example
5
Modulating Signal
Transmitted SignalSlide6
Frequency Modulation Example
Transmitted Signal
Modulating Signal
6Slide7
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.
7Slide8
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.
8Slide9
9
Types of FM Demodulators
FM Demodulation
Indirect
Direct
Slope Detector
Balanced Slope Detector
Foster-Seeley Phase Discriminator
Ratio Detector
Phase Lock Loop(PLL
)Slide10
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
incomingwave
.Slide11
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.
11Slide12
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.
12Slide13
13
FM Wave
Output of Tuned Circuit
Basic FM DemodulatorSlide14
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 andThe
entire frequencies
should fall on
linear region of transfer curve
of tuned circuit.
14Slide15
FM Source
Tank Circuit
Detector
Output
FM
SLOPE Detector
15Slide16
Voltage
Voltage
f
t
Input
t
Transfer Curve
Output
Slope Detector Transfer Characteristics
16Slide17
Voltage
Voltage
f
t
Input
t
Transfer Curve
Output
Slope Detector Transfer Characteristics
f
c
f
c
+
f
f
c
-
f
f
17Slide18
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.
18Slide19
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.
19Slide20
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
180
0
out of phase.
20Slide21
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.
21Slide22
Balanced Slope Detector
22
D1
FMIN
f
c
f
c
+
f
f
c
-
f
T’
T’’
D2
Vo
+
-
+
-
V
o1
V
o2
C
1
R
1
R
2
C
2Slide23
Balanced Slope Detector
When input frequency =
f
c Then output of T’(+Ve)= output of T’’ (-Ve)
V
o
= Zero
When input frequency =
f
c+
f
Then output of T’(+Ve) > output of T’’ (-Ve) V
o= +VeWhen input frequency =
f
c
-
f
Then output of T’(+Ve) < output of T’’ (-Ve)
So sum of outputs of T’ and T’’
(V
o
) = -Ve
23Slide24
24
Useful Range
f
c
f
c
-
f
f
c
+
f
V
o
Transfer Curve of Balanced slope DeterSlide25
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
.
25
Balanced Slope Detector-DrawbacksSlide26
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.
26
Foster-seley Phase DiscriminatorSlide27
27
D
1
V
IN
D
2
V
a’b’
+
-
+
-
L
3
R
3
R
4
C
3
C
4
L
2
L
1
a
b
a’
b’
o
C
1
C
As C & C4 are coupling & RF Bypass capacitors respectively, therefore V
L3
V
IN
So
Voltage across diode= V
IN
+ Secondary voltage/2
Foster-seley Phase DiscriminatorSlide28
Foster-seley Phase Discriminator
Now when Transformer voltage is induced in the secondary as a result of current in primary.
And
Where X
2
= X
L2
-X
C2
28Slide29
Foster-seley Phase Discriminator
At resonance i.e. when input frequency is f
c
, X2
=0
i.e. V
ab
leads V
IN
by 90
0
.
29Slide30
Foster-seley Phase Discriminator
And from the phasor diagram given below :
That as V
ao
=V
bo
, hence discriminator output is zero.
30
V
ao
V
boSlide31
Foster-seley Phase Discriminator
When input frequency is greater than f
c
, then XL2
>X
C2
& hence X
2
is positive.
That is V
ab
leads V
IN by less than 90
0.
31Slide32
32
Foster-seley Phase Discriminator
And from the phasor diagram given below :
That as V
ao
>
V
bo
, hence discriminator output is positive.
V
ao
V
boSlide33
Foster-seley Phase Discriminator
When input frequency is less than f
c
, then X
L2
<X
C2
& hence X
2
is negative.
That is V
ab
leads V
IN
by more than 90
0
.
33Slide34
34
Foster-seley Phase Discriminator
And from the phasor diagram given below :
That as V
ao
<
V
bo
, hence discriminator output is negative.
V
ao
V
boSlide35
Foster-seley Phase Discriminator
35
Useful Range
f
c
V
o
Beyond which o/p falls due to frequency response of transformer.
Useful Range extends up to half-power points of tuned transformer.Slide36
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.
36Slide37
Ratio-
Detector
Ratio detector demodulator is modified Foster-Seeley circuit in order to incorporate amplitude limiting.In Foster-Seeley discriminator that sum of voltages V
ao
+V
bo
Should remain constant,
and their difference should vary due to variation in input frequency.
37Slide38
Ratio-
Detector
But practically speaking any variation in the amplitude of input signal, also has impact on sum of Vao+V
bo
, leading to distortion.
Ratio-detector circuit eliminates this variation of V
ao
+V
bo
, and performs the function of amplitude limiter also.
38Slide39
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.
39Slide40
Ratio-
Detector
40
D
1
V
IN
D
2
V
o
+
-
+
-
L
3
R
3
R
4
C
3
C
4
L
2
L
1
a
b
a’
b’
o
C
1
C
C
5
R
5
R
6
Change 1: Diode
D2
is reversed so that now sum of
V
ao
& V
bo
appears across points
a’ and b’
instead of difference
.
SUMSlide41
Ratio-
Detector
41
D
1
FM input
D
2
V
o
+
-
+
-
L
3
R
3
R
4
C
3
C
4
L
s
L
P
Vo1
Vo2
C
P
C
C
5
R
5
R
6
Change 2: A capacitor C
5
with large time constant is connected across a’-b’ in order to keep
V
ao
+V
bo
constant.
+ V
3
-Slide42
Ratio-
Detector
42
D
1
V
IN
D
2
V
o
+
+
-
L
3
R
3
R
4
C
3
C
4
L
2
L
1
a
b
a’
b’
o
C
1
C
C
5
R
5
R
6
Change 3: Output is taken from o-o’ as the difference of
V
ao
+ V
bo
appears there. Ground is shifted to O’.
V
1
V
2
o’
-Slide43
Operation at Resonance
No phase shift occurs at resonance and both
V
ao & 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 C
5
with large time constant maintains voltage at constant level.
43Slide44
Operation Above Resonance
When a tuned circuit operates at a frequency higher than resonance, the tank is inductive.
Secondary voltage V
1
is nearer in phase with primary voltage, while V
2
is shifted further out of phase with primary.
44Slide45
Operation Above Resonance
So output voltage in this case will be positive as shown in vector diagram:
45
V
ao
V
bo
OutputSlide46
When a tuned circuit operates below resonance, it is capacitive. Secondary current leads the primary voltage and
secondary voltage V
2
is nearer in phase with primary voltage and voltage V1 is shifted away in phase from primary voltage
46
Operation Below ResonanceSlide47
Operation Below Resonance
So the output in this case will be negative.
47
V
ao
V
bo
OutputSlide48
Ratio-Detector Advantages
Amplitude limiting is possible.
Linearity is quite good as compared to others. So quite often used in high quality receivers.
48Slide49
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.
49Slide50
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.
50Slide51
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.
51Slide52
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.
52Slide53
Phase Locked Loop
53
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. Slide54
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.
54Slide55
Phase Locked Loop
Control Voltage
rate of input freq changeHence this signal can be directly used as output.
PLL must have low time constant so that it can follow modulating signal.
55Slide56
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.
56Slide57
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.
57Slide58
58
Voltage
f
Input
t
Transfer Curve
Output
THANKSSlide59
59
FM DEMODULATORS
By
P.Lakshmi Prasanna