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ECE4371, Fall, 2009

ECE4371, Fall, 2009. Zhu Han Department of Electrical and Computer Engineering Class 6 Sep. 10 th , 2007. FM Modulator and Demodulator. FM modulator Direct FM Indirect FM FM demodulator Direct: use frequency discriminator (frequency-voltage converter) Ratio detector

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ECE4371, Fall, 2009

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  1. ECE4371, Fall, 2009 Zhu Han Department of Electrical and Computer Engineering Class 6 Sep. 10th, 2007

  2. FM Modulator and Demodulator • FM modulator • Direct FM • Indirect FM • FM demodulator • Direct: use frequency discriminator (frequency-voltage converter) • Ratio detector • Zero crossing detector • Indirect: using PLL • Superheterodyne receiver • FM broadcasting and Satellite radio

  3. FM Direct Modulator • Direct FM • Carrier frequency is directly varied by the message through voltage-controlled oscillator (VCO) • VCO: output frequency changes linearly with input voltage • A simple VCO: implemented by variable capacitor • Capacitor Microphone FM generator

  4. FM Direct Modulator cont. • Direct method is simple, low cost, but lack of high stability & accuracy, low power application, unstable at the carrier frequency • Modern VCOs are usually implemented as PLL IC • Why VCO generates FM signal?

  5. Indirect FM • Generate NBFM first, then NBFM is frequency multiplied for targeted Δf. • Good for the requirement of stable carrier frequency • Commercial-level FM broadcasting equipment all use indirect FM • A typical indirect FM implementation: Armstrong FM • Block diagram of indirect FM

  6. Indirect FM cont. • First, generate NBFM signal with a very small β1 m(t)

  7. Indirect FM cont. • Then, apply frequency multiplier to magnify β • Instantaneous frequency is multiplied by n • So do carrier frequency, Δf, and β • What about bandwidth?

  8. Analysis of Indirect FM … …

  9. A simple electronic implementation of frequency multiplier 30 MHz output. X3 (x5) C1:100pF, L1:2.7μH. D:1N914 L2:.22μH, L3:1.8μH, L4:330μH C2:120pF, C3:10pF.

  10. Armstrong FM Modulator • Invented by E. Armstrong, an indirect FM • A popular implementation of commercial level FM • Parameter: message W=15 kHz, FM s(t): Δf=74.65 kHz. • Can you find the Δf at (a)-(d)?

  11. FM Demodulator • Four primary methods • Differentiator with envelope detector/Slope detector • FM to AM conversion • Phase-shift discriminator/Ratio detector • Approximates the differentiator • Zero-crossing detector • Frequency feedback • Phase lock loops (PLL)

  12. FM Slope Demodulator • Principle: use slope detector (slope circuit) as frequency discriminator, which implements frequency to voltage conversion (FVC) • Slope circuit: output voltage is proportional to the input frequency. Example: filters, differentiator

  13. FM Slope Demodulator cont. • Block diagram of direct method (slope detector = slope circuit + envelope detector) so(t) linear with m(t)

  14. Slope Detector Magnitude frequency response of transformer BPF.

  15. Hard Limiter • A device that imposes hard limiting on a signal and contains a filter that suppresses the unwanted products (harmonics) of the limiting process. • Input Signal • Output of hard limiter • Bandpass filter • Remove the amplitude variations

  16. Ratio Detector • Foster-Seeley/phase shift discriminator • uses a double-tuned transformer to convert the instantaneous frequency variations of the FM input signal to instantaneous amplitude variations. These amplitude variations are rectified to provide a DC output voltage which varies in amplitude and polarity with the input signal frequency. • Example • Ratio detector • Modified Foster-Seeley discriminator, not response to AM, but 50%

  17. Zero Crossing Detector

  18. FM Demodulator PLL • Phase-locked loop (PLL) • A closed-loop feedback control circuit, make a signal in fixed phase (and frequency) relation to a reference signal • Track frequency (or phase) variation of inputs • Or, change frequency (or phase) according to inputs • PLL can be used for both FM modulator and demodulator • Just as Balanced Modulator IC can be used for most amplitude modulations and demodulations

  19. PLL FM • Remember the following relations • Si=Acos(wct+1(t)), Sv=Avcos(wct+c(t)) • Sp=0.5AAv[sin(2wct+1+c)+sin(1-c)] • So=0.5AAvsin(1-c)=AAv(1-c) • Section 2.14

  20. Phase and Frequency Acquisition

  21. Phase-Locked Loop Demodulator (a) Block diagram for a PLL FM demodulator; (b) PLL FM demodulator using the XR-2212 PLL 32-38

  22. Nonlinear Channel (device) vi(t) v0(t) Nonlinear Effects in FM Systems 1.Strong nonlinearity, e.g., square-law modulators , hard limiter, frequency multipliers. 2.Weak nonlinearity, e.g., imperfections Nonlinear input-output relation An FM system is extremely sensitive to phase nonlinearity. Common types of source: AM-to-PM conversion 52

  23. Electronic Amplifier A: low power B: high distortion C: need a filter but narrow band 52

  24. Superheterodyne Receiver • Radio receiver’s main function • Demodulation  get message signal • Carrier frequency tuning  select station • Filtering  remove noise/interference • Amplification  combat transmission power loss • Superheterodyne receiver • Heterodyne: mixing two signals for new frequency • Superheterodyne receiver: heterodyne RF signals with local tuner, convert to common IF • Invented by E. Armstrong in 1918. • AM: RF 0.535MHz-1.605 MHz, Midband 0.455MHz • FM: RF 88M-108MHz, Midband 10.7MHz

  25. Advantage of superheterodyne receiver • A signal block (of circuit) can hardly achieve all: selectivity, signal quality, and power amplification • Superheterodyne receiver deals them with different blocks • RF blocks: selectivity only • IF blocks: filter for high signal quality, and amplification, use circuits that work in only a constant IF, not a large band

  26. FM Broadcasting • The frequency of an FM broadcast station is usually an exact multiple of 100 kHz from 87.5 to 108.5 MHz . In most of the Americas and Caribbean only odd multiples are used. • fm=15KHz, f=75KHz, =5, B=2(fm+f)=180kHz • Pre-emphasis and de-emphasis • Random noise has a 'triangular' spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver.

  27. Fc=19KHz.(a) Multiplexer in transmitter of FM stereo. (b) Demultiplexer in receiver of FM stereo. FM Stereo Multiplexing Backward compatible For non-stereo receiver

  28. TV FM broadcasting • fm=15KHz, f=25KHz, =5/3, B=2(fm+f)=80kHz • Center fc+4.5MHz

  29. Satellite Radio • WorldSpace outside US, XM Radio and Sirius in North America

  30. XM vs. Sirus

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