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 –Zero crossing detector –Indirect: using PLL Superheterodyne receiver FM broadcasting and Satellite radio

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

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?

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

Indirect FM cont. First, generate NBFM signal with a very small β 1 m(t)

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?

Analysis of Indirect FM … …

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

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)?

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

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

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

Slope Detector Magnitude frequency response of transformer BPF.

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.limitingsignal filter Input Signal Output of hard limiter Bandpass filter Remove the amplitude variations

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%

Zero Crossing Detector

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 u Track frequency (or phase) variation of inputs u Or, change frequency (or phase) according to inputs –PLL can be used for both FM modulator and demodulator u Just as Balanced Modulator IC can be used for most amplitude modulations and demodulations

PLL FM Remember the following relations –Si=Acos(w c t+  1 (t)), Sv=A v cos(w c t+  c (t)) –Sp=0.5AA v [sin(2w c t+  1 +  c )+sin(  1 -  c )] –So=0.5AA v sin(  1 -  c )=AA v (  1 -  c ) –Section 2.14

Phase and Frequency Acquisition

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

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 Nonlinear Channel (device) vi(t)vi(t)v0(t)v0(t) 52 Nonlinear Effects in FM Systems

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

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 –AM: RF 0.535MHz MHz, Midband 0.455MHz –FM: RF 88M-108MHz, Midband 10.7MHz

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

FM Broadcasting The frequency of an FM broadcast station is usually an exact multiple of 100 kHz from 87.5 to MHz. In most of the Americas and Caribbean only odd multiples are used.the AmericasCaribbean f m =15KHz,  f=75KHz,  =5, B=2(f m +  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.noisespectralfrequenciesbasebandtransmission

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

TV FM broadcasting f m =15KHz,  f=25KHz,  =5/3, B=2(f m +  f)=80kHz Center f c +4.5MHz

Satellite Radio WorldSpace outside US, XM Radio and Sirius in North America XM Satellite RadioSirius Company infoXMSR, $2billion, DCSIRI, $5 billion, NYC Current Subscribers7,000,000+4,000,000+ Monthly rate12.95/month Total channel170+, 90+streams of music165+, 80+streams of music Satellite2 Boeing geostationary satellites 3 Loral satellites at high- elevation geosynchronous orbit

XM vs. Sirus