Modulation and Demodulation of Signals - @let@token Contributed ...
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Modulation and Demodulation of SignalsContributed to the ACAS 2016 School
John D. Fox
Applied Physics DepartmentStanford University
Stanford Linear Accelerator CenterWork supported by the DOE under contract # DE-AC02-76SF00515 and the DOE LHC
Accelerator Research Program LARP
January 2016
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Outline
1 Modulation, Demodulation
2 Spectra
3 AM Receivers
4 Vector Voltmeter
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Modulation, Demodulation
Modulation, Demodulation
• A signal is often represented as a modulation on a carrier• AM ( Amplitude Modulation)• FM ( Frequency Modulation)• PM ( Phase Modulation)
• The signal itself and information is often called the basebandinformation
• To process and recover the baseband signal, a receiver oftenheterodynes a modulated signal with local oscillator to translatethe information to processing IF ( intermediate Frequency)
• Local Oscillator• Mixer• Detection method ( Diodes? PLL? Slope detector?
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Modulation, Demodulation
Receiving AM and simple detection
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Modulation, Demodulation
Diode Detection
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Modulation, Demodulation
Selecting a Station
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Modulation, Demodulation
AM modulation sidebands around the Carrier
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Spectra
Spectra for FM and PM modulation
Frequency Modulation
Acos(ω′t + Φ)ω′ = ωc(1 + kcos(ωmt))
(1)
k is modulation indexΦ Modulation
Acos(ωc t + Φ′)Φ′ = Φ0(1 + kcos(ωmt))
(2)Bessel Functions describe amplitudes of
sidebandsFor small modulation, only ±ωm sidebands
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Spectra
Demodulation and Detection
• AM Detection• Baseband Diode Detector• Synchronous ( Homodyne) detection
• FM and PM detection• Slope Detection ( tune circuit off frequency, FM to AM converter)• PLL ( Phase Locked Loop)• Foster-Seely Detector
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Spectra
Diodes as Detectors and as a Mixer
• how is "detection" like "peak detection"?• What is a Homodyne detector?
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AM Receivers
AM receivers beyond the Crystal radio
• What is the advantage of the Heterodyned receiver?• How do you choose the IF Frequency and bandwidth??
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AM Receivers
Spectrum Analyzer as a Tuned Receiver
• how do you set the instrument to be a tuned receiver?• How do you choose the IF filter, video filter bandwidths?
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AM Receivers
A real multi-stage Spectrum Analyzer
• how do you set the instrument to be a tuned receiver?• How do you choose the IF filter, video filter bandwidths?
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Vector Voltmeter
Vector Voltmeter - measures magnitude and phase
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Vector Voltmeter
Vector Voltmeter - is a Sampling (heterodyning)instrument
• Why are there TWO probe leads??• Sampling pulses are VERY narrow( 1 ns or less)• What is in the sampling probe? a sample and hold circuit
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Vector Voltmeter
Vector Voltmeter - Detailed system
• How is sampling related to heterodyning?• How is the phase at the RF frequency related to the phase at the IF frequency?
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