Fm Demodulation Excllemt

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)

Punjab EDUSAT Society (PES)

What is Modulation

• Modulation is the addition of information (or the signal) to an electronic or optical signal carrier.

ModulatorCarrier Wave

Information

Modulated Signal


Types of ModulationCommon 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

Modulating Signal

Transmitted Signal


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.



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


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



• 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

DetectorOutput

FMSLOPE Detector


Transfer Curve

Output

Slope Detector Transfer

Characteristics


Transfer Curve

Output

Slope Detector Transfer Characteristics


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 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.






Output of T’ at fc+f (+10 to +15V)

Output of T’ at fc-f(+5 to +10V)

Output of T’ at fc-f (-10 to -15V)

Output of T’’ at fc+f (-5 to -10V)

5V

10V

15V

-5V

-10V

-15V

Combined Transfer Curve


Useful Range

Balanced Slope Detector• 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+f– Then output of T’(+Ve) > output of T’’ (-Ve)– So sum of outputs of T’ and T’’ = +Ve

• When input frequency = fc-f– 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 SoVoltage 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

jMV CINab

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

jMV CINab

Phase Discriminator

• And from the phasor diagram given below :

• That as Vao=Vbo, hence discriminator output is zero.


2abV

2abV

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.Punjab EDUSAT Society (PES)

22

2

1

.

jXR

XV

L

jMV CINab

21

02 90

ZL

MXV CIN

)90( 0

21

2 ZL

MXV CIN


Phase Discriminator


• That as Vao>Vbo, hence discriminator output is positive.

2abV

2abV

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

jMV CINab

)(

90

21

02

ZL

MXV CIN

)90( 0

21

2 ZL

MXV CIN


Phase Discriminator


• That as Vao<Vbo, hence discriminator output is negative.

2abV

2abV

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:


2abV

2abV

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.


2abV

2abV

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 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

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Fm Demodulation Excllemt