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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

ECE4371, Fall, 2009

Zhu Han

Department of Electrical and Computer Engineering

Class 6

Sep. 10th, 2007

fm modulator and demodulator
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
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
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
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
Indirect FM cont.
  • First, generate NBFM signal with a very small β1


indirect fm cont7
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?
a simple electronic implementation of frequency multiplier
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
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
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
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
FM Slope Demodulator cont.
  • Block diagram of direct method (slope detector = slope circuit + envelope detector)

so(t) linear with m(t)

slope detector
Slope Detector

Magnitude frequency

response of

transformer BPF.

hard limiter
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
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
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
  • 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
Phase-Locked Loop Demodulator

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



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


Electronic Amplifier

A: low power

B: high distortion

C: need a filter

but narrow band


superheterodyne receiver
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
advantage of superheterodyne receiver
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
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
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
TV FM broadcasting
  • fm=15KHz, f=25KHz, =5/3, B=2(fm+f)=80kHz
  • Center fc+4.5MHz
satellite radio
Satellite Radio
  • WorldSpace outside US, XM Radio and Sirius in North America