CommunicationElectronics Principles & ApplicationsChapter 5Frequency ModulationCircuits

subtopicsFrequency modulators:VVC, frequency modulating a crystal oscillator, reactance modulator, voltage controlled oscillatorPhase modulators:Basic Phase shift cct, practical PM, Using tuned cct for PMFrequency demodulators:Foster Seeley discriminator, Ratio detector, Pulse averaging Discriminator, Quadrature Detector, Differential Peak Detector, PLL demodulator

DIRECT FREQUENCY MODULATORSDirect frequency modulators vary the frequency of a carrier oscillator.A voltage variable capacitor (VVC) or varactor is widely used to vary the frequency of an LC or crystal oscillator.Older FM transmitters used a reactance modulator to vary the carrier frequency.

VOLTAGE VARIABLE CAPACITOR (VVC)A VVC or varactor is a silicon diode that when reversed biased acts as a variable capacitor.Varying the reverse bias voltage changes the capacitance. Increasing the voltage decreases the capacitance and visa versa.The useable capacitance range is from about 5 pF to 100 pF.

+VRFCModulatingsignal inFMoutputCrystalFM WITH A VVCVVC

FM WITH A VVC4. The crystal and oscillator are pulled higher in frequency.

PM MODULATORSA PM modulator generates indirect FM.A fixed carrier frequency from a crystal oscillator is passed through a phase shifter whose phase shift is determined by the modulating signal.A transistor can be used as a variable resistor or a VVC can be used as a variable capacitor to implement a phase modulator.

+VDDRFC2OutputPHASE MODULATORRFC1ModulatingsignalCrystal-controlledcarrieroscillatorQ1R2R1C3C2C1

Using Tuned Circuit for PM

C1

RF Amplifier

Carrier input

C2

L

PM Output

R3

R4

C5

C4

C3

RFC

+V

R1

R2

D1

Modulating signal

FREQUENCY DEMODULATORSA demodulator converts the carrier frequency variations into the original modulating signal.There are six common FM demodulators:Foster Seeley DiscriminatorRatio DetectorPulse Averaging DiscriminatorQuadrature DetectorDifferential Peak DetectorPhase-Locked Loop (PLL)The quadrature detector and PLL are the most common.

OutputQUADRATURE DETECTORRC1CFMinput90 B0 APulse averaginglow-pass filter

QUADRATURE DETECTOR OPERATIONA phase shift circuit, usually a parallel tuned circuit and a capacitor, produces a 90 phase shift at the carrier frequency.The varying carrier frequency is compared to the phase shifter output in a phase detector circuit.The phase detector produces a pulse width modulated output that is averaged in a low pass filter to recover the original modulation.

PHASE-LOCKED LOOPInInVCOThe VCO locks onto the input signal.

PLL OPERATIONA PLL is a feedback control circuit that compares an input signal to the output of a voltage controlled oscillator (VCO) and adjusts the VCO frequency if it differs from the input signal.If the VCO and input frequencies are different, the phase detector produces an error signal that is filtered into a DC voltage that controls the VCO frequency.

InTHE OUTPUT VOLTAGE VERSUS THE INPUT FREQUENCYOutOutput voltageInput frequency

PLL DEMODULATORIf the input to a PLL is an FM signal, the VCO will track or follow the input.The low pass filter output will be the original modulating signal.The PLL demodulator is the best FM detector as it serves as a filter for good selectivity and noise reduction.

In PLL PROVIDES FM DETECTIONOut