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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 l.jpg

ECE4371, Fall, 2009

Zhu Han

Department of Electrical and Computer Engineering

Class 6

Sep. 10th, 2007

Fm modulator and demodulator l.jpg
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 l.jpg
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 l.jpg
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?

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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

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Indirect FM cont.

  • First, generate NBFM signal with a very small β1


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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?

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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.

Armstrong fm modulator l.jpg
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 l.jpg
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)

Fm slope demodulator l.jpg
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

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FM Slope Demodulator cont.

  • Block diagram of direct method (slope detector = slope circuit + envelope detector)

so(t) linear with m(t)

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Slope Detector

Magnitude frequency

response of

transformer BPF.

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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

Ratio detector l.jpg
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%

Fm demodulator pll l.jpg
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

Pll fm l.jpg

  • 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

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Phase-Locked Loop Demodulator

(a) Block diagram for a PLL FM demodulator; (b) PLL FM demodulator using the XR-2212 PLL


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Channel (device)



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


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Electronic Amplifier

A: low power

B: high distortion

C: need a filter

but narrow band


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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

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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

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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.

Fc 19khz a multiplexer in transmitter of fm stereo b demultiplexer in receiver of fm stereo l.jpg
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

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TV FM broadcasting

  • fm=15KHz, f=25KHz, =5/3, B=2(fm+f)=80kHz

  • Center fc+4.5MHz

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Satellite Radio

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