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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
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 1 / 16
Outline
1 Modulation, Demodulation
2 Spectra
3 AM Receivers
4 Vector Voltmeter
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 2 / 16
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?
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 3 / 16
Modulation, Demodulation
Receiving AM and simple detection
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 4 / 16
Modulation, Demodulation
Diode Detection
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 5 / 16
Modulation, Demodulation
Selecting a Station
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 6 / 16
Modulation, Demodulation
AM modulation sidebands around the Carrier
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 7 / 16
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
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 8 / 16
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
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 9 / 16
Spectra
Diodes as Detectors and as a Mixer
• how is "detection" like "peak detection"?• What is a Homodyne detector?
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 10 / 16
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??
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 11 / 16
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?
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 12 / 16
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?
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 13 / 16
Vector Voltmeter
Vector Voltmeter - measures magnitude and phase
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 14 / 16
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
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 15 / 16
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?
John D. Fox (SLAC) Modulation and Demodulation of Signals January 2016 16 / 16