FM Demodulators

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Presentation Description

This lecture explains the basic working of FM Demodulation i.e. extraction of information from frequency modulated signal. The demodulators discussed here are Slope, Balanced, Foster-Seeley, Ratio and Phase Locked Loop, Including animation of transfer characteristics of 'LC tuned circuit' or 'Tank circuit'

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PowerPoint Presentation:

Punjab EDUSAT Society (PES) Presented By Kulwinder Singh Lecturer ECE GPCG, Ludhiana Contact info: email: kulwinderpannu@gmail.com Phone: 97813-00151 FM DEMODULATORS

What We Shall Cover:

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:

What is Modulation Modulation is the addition of information (or the signal) to an electronic or optical signal carrier. Modulator Carrier Wave Information Modulated Signal Punjab EDUSAT Society (PES)

Types of Modulation:

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 Punjab EDUSAT Society (PES)

Amplitude Modulation Example:

Amplitude Modulation Example Modulating Signal Transmitted Signal Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Frequency Modulation Example Transmitted Signal Modulating Signal Punjab EDUSAT Society (PES)

What is a Demodulator?:

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. Punjab EDUSAT Society (PES)

What is FM Demodulator:

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. Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Punjab EDUSAT Society (PES) Types of FM Demodulators FM Demodulation Indirect Direct Slope Detector Balanced Slope Detector Foster-Seeley Phase Discriminator Ratio Detector Phase Lock Loop(PLL )

Basic FM Demodulator:

Basic FM Demodulator Punjab EDUSAT Society (PES) TUNED CIRUIT Frequency Variations Amplitude Variations NOTE: Amplitude Variations are added to wave according to frequency variations, and frequency variations remain present in wave.

Basic FM Demodulator:

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. Punjab EDUSAT Society (PES)

Basic FM Demodulator:

Basic FM Demodulator 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. Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Punjab EDUSAT Society (PES) FM Wave Output of Tuned Circuit Basic FM Demodulator

Basic FM Demodulator:

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. Punjab EDUSAT Society (PES)

SLOPE Detector:

FM Source Tank Circuit Detector Output FM SLOPE Detector Punjab EDUSAT Society (PES)

Slope Detector Transfer Characteristics:

Voltage Voltage f t Input t Transfer Curve Output Slope Detector Transfer Characteristics Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Voltage Voltage f t Input t Transfer Curve Output Slope Detector Transfer Characteristics Punjab EDUSAT Society (PES) f c f c + f f c - f f

SLOPE Detector:

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. Punjab EDUSAT Society (PES)

Limitations of Slope 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. Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Punjab EDUSAT Society (PES)

PowerPoint Presentation:

FM Detector O/P FM Wave Punjab EDUSAT Society (PES)

Balanced Slope Detector:

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. Punjab EDUSAT Society (PES)

Balanced Slope Detector:

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. Punjab EDUSAT Society (PES)

Balanced Slope Detector:

Balanced Slope Detector Punjab EDUSAT Society (PES) D1 IN f c f c + f f c - f T’ T’’ D2 Vo + - + -

Balanced Slope Detector:

Balanced Slope Detector Punjab EDUSAT Society (PES) Output of T’ at f c + f (+10 to +15V) Output of T’ at f c - f (+5 to +10V) Output of T’ at f c - f (-10 to -15V) Output of T’’ at f c + f (-5 to -10V) 5V 10V 15V -5V -10V -15V f c f c + f f c - f + Ve - Ve T’ T’’

Combined Transfer Curve:

Combined Transfer Curve Punjab EDUSAT Society (PES) Useful Range f c f c - f f c + f V o

Balanced Slope Detector:

Balanced Slope Detector When input frequency = f c Then output of T’(+ Ve )= output of T’’ (- Ve ) So sum of outputs of T’ and T’’ = Zero When input frequency = f c + f Then output of T’(+ Ve ) > output of T’’ (- Ve ) So sum of outputs of T’ and T’’ = + Ve When input frequency = f c - f Then output of T’(+ Ve ) < output of T’’ (- Ve ) So sum of outputs of T’ and T’’ = - Ve Punjab EDUSAT Society (PES)

Balance Slope Detector- Drawbacks:

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. Punjab EDUSAT Society (PES)

Foster-Seeley (Phase) Discriminator:

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. Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Punjab EDUSAT Society (PES) 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 Phase Discriminator

Phase Discriminator:

Phase Discriminator Now in Transformer voltage is induced in the secondary as a result of current in primary. And Where X 2 = X L2 -X C2 Punjab EDUSAT Society (PES)

Phase Discriminator:

Phase Discriminator At resonance i.e. when input frequency is f c , X 2 =0 i.e. V ab leads V IN by 90 0 . Punjab EDUSAT Society (PES)

Phase Discriminator:

Phase Discriminator And from the phasor diagram given below : That as V ao =V bo , hence discriminator output is zero. Punjab EDUSAT Society (PES) V ao V bo

Phase Discriminator:

Phase Discriminator When input frequency is greater than f c , then X L2 >X C2 & hence X 2 is positive. That is V ab leads V IN by less than 90 0 . Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Punjab EDUSAT Society (PES) Phase Discriminator And from the phasor diagram given below : That as V ao > V bo , hence discriminator output is positive. V ao V bo

PowerPoint Presentation:

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 . Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Punjab EDUSAT Society (PES) Phase Discriminator And from the phasor diagram given below : That as V ao < V bo , hence discriminator output is negative. V ao V bo

Phase Discriminator:

Phase Discriminator Punjab EDUSAT Society (PES) Useful Range f c V o Beyond which o/p falls due to frequency response of transformer. Useful Range extends upto half-power points of tuned transformer.

Phase Discriminator:

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. Punjab EDUSAT Society (PES)

Ratio- Detector:

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. Punjab EDUSAT Society (PES)

Ratio-detector:

Ratio-detector But practically speaking any variation in the amplitude of input signal, also has impact on sum of V ao +V bo , leading to distortion. Ratio-detector circuit eliminates this variation of V ao +V bo , and performs the function of amplitude limiter also. Punjab EDUSAT Society (PES)

Ratio-Detector:

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. Punjab EDUSAT Society (PES)

Ratio-Detector:

Ratio-Detector Punjab EDUSAT Society (PES) 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. SUM

Ratio-Detector:

Ratio-Detector Punjab EDUSAT Society (PES) 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 2: A capacitor C 5 with large time constant is connected across a’-b’ in order to keep V ao +V bo constant.

Ratio-Detector:

Ratio-Detector Punjab EDUSAT Society (PES) 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

Operation at Resonance:

Operation at Resonance No phase shift occurs at resonance and both V ao & V bo 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. Punjab EDUSAT Society (PES)

Operation Above Resonance:

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. Punjab EDUSAT Society (PES)

Operation Above Resonance:

Operation Above Resonance So output voltage in this case will be positive as shown in vector diagram: Punjab EDUSAT Society (PES) V ao V bo Output

Operation Below Resonance:

Operation Below Resonance 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 V 1 is shifted away in phase from primary voltage Punjab EDUSAT Society (PES)

Operation Below Resonance:

Operation Below Resonance So the output in this case will be negative. Punjab EDUSAT Society (PES) V ao V bo Output

Ratio-Detector Advantages:

Ratio-Detector Advantages Amplitude limiting is possible. Linearity is quite good as compared to others. So quite often used in high quality receivers. Punjab EDUSAT Society (PES)

Ratio-Detector Dis-advantages:

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. Punjab EDUSAT Society (PES)

Phase Locked Loop (PLL):

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. Punjab EDUSAT Society (PES)

Phase Locked Loop:

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. Punjab EDUSAT Society (PES)

Phase Locked Loop:

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. Punjab EDUSAT Society (PES)

Phase Locked Loop:

Phase Locked Loop Punjab EDUSAT Society (PES) 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:

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. Punjab EDUSAT Society (PES)

Phase Locked Loop:

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. Punjab EDUSAT Society (PES)

Phase Locked Loop:

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. Punjab EDUSAT Society (PES)

Phase Locked Loop:

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. Punjab EDUSAT Society (PES)

PowerPoint Presentation:

Punjab EDUSAT Society (PES) Voltage f Input t Transfer Curve Output THANKS kulwinderpannu@gmail.com

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