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