Principles of Communication Systems Chapter 3 (part 2): FM ...
Transcript of Principles of Communication Systems Chapter 3 (part 2): FM ...
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Principles of Communicartion System by N Hasan
Principles of Communication Systems
Chapter 3 (part 2): FM Transmitter & Receiver
byNurulfadzilah Hasan
Faculty of Electrical & Electronics [email protected]
For updated version, please click on http://ocw.ump.edu.my
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Principles of Communicartion System by N Hasan
Lesson Outcomes
• By the end of this topics students should be able to:– Explain how FM transmitters and receivers
operate– Describe how FM overcomes noise
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Principles of Communicartion System by N Hasan
FM Transmitter
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Principles of Communicartion System by N Hasan
Description
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Principles of Communicartion System by N Hasan
FM Receiver
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Principles of Communicartion System by N Hasan
Description (cont.)
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Principles of Communicartion System by N Hasan
Discriminator
Discriminator (or slope detection) converts FM signal to AM signal and then by using AM demodulation circuit is to get back the information signal.
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Principles of Communicartion System by N Hasan
Frequency modulators
Two common types of frequency modulator
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Principles of Communicartion System by N Hasan
Varactor Modulator
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Principles of Communicartion System by N Hasan
Reactance Modulator
• Reactance modulator works by using transistoramplifier as variable capacitor or an inductor.
• When the circuit is connected across the tunedcircuit of an oscillator, the oscillator frequencycan be varied by applying the modulating signalto the amplifier.
• Reactance modulators can produce frequencydeviation over a wide range.
• Reactance modulators are highly linear, sodistortion is minimal.
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Principles of Communicartion System by N Hasan
A Reactance Modulator
Image source: https://et.wikipedia.org/wiki/Fail:Reaktiivmodulaator.png
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Principles of Communicartion System by N Hasan
Frequency Demodulators
Slope Detector
Pulse-Averaging Discriminators
Quadrature Detector
Phase-Locked Loop Detector
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Principles of Communicartion System by N Hasan
Slope Detector
• Slope detector uses tuned circuit and a diode detector toconvert frequency variations into voltage variations.
• The main difficulty with slope detectors is tuning them.
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Principles of Communicartion System by N Hasan
Pulse-Averaging Discriminators
• Consists of a zero crossing detector, a one shot multi vibratorand a low-pass filter.
• A very high-quality frequency demodulator.• This discriminator is used in many electronic products.
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Principles of Communicartion System by N Hasan
Quadrature Detector
• Most widely used FM demodulator.• Uses a phase-shift circuit to produce a phase shift of 90
degrees at the unmodulated carrier frequency.
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Principles of Communicartion System by N Hasan
Phase-Locked Loops
• Phase-locked loop (PLL) is a frequency-sensitive feedbackcontrol circuit used in frequency demodulation
• PLLs have three basic elements: Phase detector, Low-passfilter, Voltage-controlled oscillator (VCO)
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Principles of Communicartion System by N Hasan
Noise in FM
• Noise is interference that affect information signal.
• In FM, modulated signals have a constant amplitude.
• At receiver, limiter circuits clipped any amplitude variationsthat may exist on the received signal.
• Information content of the FM signal is not affected since it iscontained by the frequency variations, not amplitude of thereceived signal.
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Principles of Communicartion System by N Hasan
Noise and Angle Modulation
• Noise distribution in FM is non-uniform. • Higher frequencies suffers more noise than the lower frequencies. • Higher frequencies also have lower amplitudes compared to lower
frequencies.• Consequently, the Signal to noise ratio is also non-uniform.• The SNR for higher-modulating frequencies are lower than the SNR for the
lower frequencies.
Without preemphasis
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Principles of Communicartion System by N Hasan
• Solution: the high-frequency modulating signals are emphasized or boosted in amplitude of the transmitter prior to performing modulation and then deemphasized or attenuated during demodulation at receiver.
With preemphasis
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Principles of Communicartion System by N Hasan
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Principles of Communicartion System by N Hasan
Pre-emphasis
De-emphasis
Pre-emphasis & De-emphasis circuits
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Principles of Communicartion System by N Hasan
FM Application
Radio broadcasting
Two way mobile radio
Microwave communication
TV sound transmission
Cellular radio communication
Satellite communication
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Principles of Communicartion System by N Hasan
Collaborative authors:
Nurulfadzilah Binti Hasan Noor Zirwatul Ahlam Binti Naharuddin
Norhadzfizah Binti Mohd RadiMohd Hisyam Bin Mohd Ariff
Faculty of Electrical & Electronics Engineering, UMP