Lecture 16: FM Modulators and Demodulators

1 10/23/20191 Lecture 16: FM Modulators and DemodulatorsDr. Mohammed HawaElectrical Engineering DepartmentUniversity of JordanEE421: Communications I: Lecture 16. For more information read Chapter 5 in your textbook or visit Dr. Mohammed HawaElectrical Engineering Department, University of JordanFM modulator : VCOc(t) = cos(ct)c(t) = cos(ct)Vcc(t); i m(t)Vccm(t)210/23/20192 Copyright Dr. Mohammed HawaElectrical Engineering Department, University of Jordan3 Oscillator To build an oscillator, we require three components: Amplifier Positive feedback LC tank The frequency of the oscillator is controlled by the LC tank resonant circuit. Many implementations available: Colpittsoscillator, Hartley oscillator, Ring oscillator, Dr. Mohammed HawaElectrical Engineering Department, University of Jordan4 Varactor Diode (Varicap)10/23/20193 Copyright Dr.

2 Mohammed HawaElectrical Engineering Department, University of Jordan5 Example VCO: uses Colpitts oscillator (NOTin the exam)Copyright Dr. Mohammed HawaElectrical Engineering Department, University of JordanFM Demodulators FM Discriminator (also called Slope Detector or Ratio Detector): Convert frequency variations into amplitude variations, then use an envelope detector. Quadrature Detector: Convert frequency variations into phase variations, then use a phase-difference detector. Phase-Locked Loop: A phase-difference feedback Dr. Mohammed HawaElectrical Engineering Department, University of Jordan7FM Discriminator(aka Slope Detector or Ratio Detector) Copyright Dr. Mohammed HawaElectrical Engineering Department, University of Jordan(FM) into (FM+AM)810/23/20195 Copyright Dr.

3 Mohammed HawaElectrical Engineering Department, University of JordanQuadrature Detector9 Copyright Dr. Mohammed HawaElectrical Engineering Department, University of Jordanres|H( )|H( )090+90res1010/23/20196 Copyright Dr. Mohammed HawaElectrical Engineering Department, University of JordanPhase-Locked Loop (PLL)11 Copyright Dr. Mohammed HawaElectrical Engineering Department, University of Jordan12 PLL Terminology A PLL is said to be in-lockif the frequency of the inner VCO is exactly the sameas the frequency of the received signal . If the PLL stays in-lock, then the output voltage is proportional to the baseband message signal . We want the PLL to stay in-lock. See the datasheet for MC4046 for Dr. Mohammed HawaElectrical Engineering Department, University of JordanPLL: Step 1 (In Lock)13 Copyright Dr.

4 Mohammed HawaElectrical Engineering Department, University of JordanPLL: Step 2 (Input changes)1410/23/20198 Copyright Dr. Mohammed HawaElectrical Engineering Department, University of JordanPLL: Step 3 (PLL follows)newFM(t)(PLL Input)vVCO(t)(VCO Output)newChangeContinuous Change15 Copyright Dr. Mohammed HawaElectrical Engineering Department, University of JordanPLL Applications Carrier recovery for Synchronous detectors. Clock recovery for digital baseband receivers. Stabilizing VCO frequencies in FM transmitters. FM Dr. Mohammed HawaElectrical Engineering Department, University of JordanPM Demodulator: Synchronous17