Fm Demodulation Excllemt

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<ul><li><p>Punjab EDUSAT Society (PES)</p></li><li><p>What We Shall CoverReview of Modulation (with animations)What is demodulationFrequency Demodulation DefinitionTypes of FM DemodulatorsStudy of Various FM Demodulators (Slope, Balanced, Foster-Seeley, Ratio and Phase Locked Loop)Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>What is ModulationModulation is the addition of information (or the signal) to an electronic or optical signal carrier.ModulatorCarrier WaveInformationModulated SignalPunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>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 </p><p>Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Amplitude Modulation ExampleModulating SignalTransmitted SignalPunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Frequency Modulation ExampleTransmitted SignalModulating SignalPunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>What is FM DemodulatorAn 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Punjab EDUSAT Society (PES)Types of FM DemodulatorsFM DemodulationIndirectDirect Slope DetectorBalanced Slope DetectorFoster-Seeley Phase DiscriminatorRatio DetectorPhase Lock Loop(PLL)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Basic FM DemodulatorPunjab EDUSAT Society (PES)TUNED CIRUITFrequency VariationsAmplitude VariationsNOTE: Amplitude Variations are added to wave according to frequency variations, and frequency variations remain present in wave.</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Basic FM DemodulatorThe 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Basic FM DemodulatorThis 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Punjab EDUSAT Society (PES)FM WaveOutput of Tuned CircuitBasic FM Demodulator</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Basic FM DemodulatorThe 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.Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>FM SourceTank CircuitDetectorOutputFMSLOPE DetectorPunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Transfer CurveOutputSlope Detector Transfer CharacteristicsPunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Transfer CurveOutputSlope Detector Transfer CharacteristicsPunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>SLOPE DetectorThe 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Limitations of Slope DetectorIt 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Balanced Slope DetectorThis 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.</p><p>Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Balanced Slope DetectorThe 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Balanced Slope DetectorPunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Balanced Slope DetectorPunjab EDUSAT Society (PES)Output of T at fc+f (+10 to +15V)Output of T at fc-f(+5 to +10V)</p><p>Output of T at fc-f (-10 to -15V)</p><p>Output of T at fc+f (-5 to -10V)</p><p>5V10V15V-5V-10V-15V</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Combined Transfer CurvePunjab EDUSAT Society (PES)Useful Range</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Balanced Slope DetectorWhen input frequency = fcThen output of T(+Ve)= output of T (-Ve)So sum of outputs of T and T = ZeroWhen input frequency = fc+fThen output of T(+Ve) &gt; output of T (-Ve)So sum of outputs of T and T = +VeWhen input frequency = fc-fThen output of T(+Ve) &lt; output of T (-Ve)So sum of outputs of T and T = -VePunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Balance Slope Detector- DrawbacksEven 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Foster-Seeley (Phase) DiscriminatorIn 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Punjab EDUSAT Society (PES)As C &amp; C4 are coupling &amp; RF Bypass capacitors respectively, therefore VL3 VIN SoVoltage across diode= VIN + Secondary voltage/2Phase Discriminator</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase DiscriminatorNow in Transformer voltage is induced in the secondary as a result of current in primary.And</p><p>Where X2= XL2-XC2Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase DiscriminatorAt resonance i.e. when input frequency is fc, X2=0</p><p>i.e. Vab leads VIN by 900.Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase DiscriminatorAnd from the phasor diagram given below :</p><p>That as Vao=Vbo, hence discriminator output is zero.Punjab EDUSAT Society (PES)VaoVbo</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase DiscriminatorWhen input frequency is greater than fc, then XL2&gt;XC2 &amp; hence X2 is positive.</p><p>That is Vab leads VIN by less than 900.Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Punjab EDUSAT Society (PES)Phase DiscriminatorAnd from the phasor diagram given below :</p><p>That as Vao&gt;Vbo, hence discriminator output is positive.VaoVbo</p><p>Punjab EDUSAT Society (PES)</p></li><li>Phase DiscriminatorWhen input frequency is less than fc, then XL2</li><li><p>Punjab EDUSAT Society (PES)Phase DiscriminatorAnd from the phasor diagram given below :</p><p>That as Vao</p></li><li><p>Phase DiscriminatorPunjab EDUSAT Society (PES)Useful RangeBeyond which o/p falls due to frequency response of transformer.Useful Range extends upto half-power points of tuned transformer.</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase DiscriminatorIt 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 &amp; more on primary-secondary phase relation, which is quite linear.Only drawback is, there is no provision for amplitude limiting.Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio- DetectorRatio 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.Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio-detectorBut 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. Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio-DetectorThree 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio-DetectorPunjab EDUSAT Society (PES)Change 1: Diode D2 is reversed so that now sum of Vao &amp; Vbo appears across points a and b instead of difference.</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio-DetectorPunjab EDUSAT Society (PES)Change 2: A capacitor C5 with large time constant is connected across a-b in order to keep Vao+Vbo constant.</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio-DetectorPunjab EDUSAT Society (PES)Change 3: Output is taken from o-o as the difference of Vao + Vbo appears there. Ground is shifted to O.</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Operation at ResonanceNo phase shift occurs at resonance and both Vao &amp; 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. Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Operation Above ResonanceWhen 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.Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Operation Above ResonanceSo output voltage in this case will be positive as shown in vector diagram:</p><p>Punjab EDUSAT Society (PES)Output</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Operation Below ResonanceWhen 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 voltagePunjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Operation Below ResonanceSo the output in this case will be negative.Punjab EDUSAT Society (PES)Output</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio-Detector AdvantagesAmplitude limiting is possible.Linearity is quite good as compared to others. So quite often used in high quality receivers.</p><p>Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Ratio-Detector Dis-advantagesUnder 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase Locked LoopA 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase Locked LoopLoop 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase Locked LoopPunjab 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. </p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase Locked LoopAs 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase Locked LoopControl Voltage rate of input freq change</p><p>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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase Locked LoopFree 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.</p><p>Punjab EDUSAT Society (PES)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Phase Locked LoopLinearity 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)</p><p>Punjab EDUSAT Society (PES)</p></li><li><p>Punjab EDUSAT Society (PES)Transfer CurveOutputkulwinderpannu@gmail.com</p><p>Punjab EDUSAT Society (PES)</p></li></ul>

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