ECE 4371 Fall 2017 Introduction to Telecommunication EngineeringTelecommunication

ECE 4371, Fall, 2017 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 4 Sep. 13 th, 2017

FM Modulator and Demodulator l l l Review of FM FM modulator – Direct FM – Indirect FM FM demodulator – Direct: use frequency discriminator (frequency-voltage converter) – Ratio detector – Zero crossing detector – Indirect: using PLL l l l Superheterodyne receiver FM broadcasting and Satellite radio Project 1

Review of last class l PLL and math l Instantaneous frequency l FM and PM l Modulation index l Narrow band FM characteristics l Carson’s rule LPF VCO

FM Direct Modulator l 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. l Direct method is simple, low cost, but lack of high stability & accuracy, low power application, unstable at the carrier frequency l Modern VCOs are usually implemented as PLL IC l Why VCO generates FM signal?

Indirect FM l Generate NBFM first, then NBFM is frequency multiplied for targeted Δf. l Good for the requirement of stable carrier frequency l Commercial-level FM broadcasting equipment all use indirect FM l A typical indirect FM implementation: Armstrong FM l Block diagram of indirect FM

Indirect FM cont. l First, generate NBFM signal with a very small β 1 m(t)

Indirect FM cont. l Then, apply frequency multiplier to magnify β – Instantaneous frequency is multiplied by n – So do carrier frequency, Δf, and β – What about bandwidth?

Analysis of Indirect FM

Armstrong FM Modulator l Invented by E. Armstrong, an indirect FM l A popular implementation of commercial level FM l Parameter: message W=15 k. Hz, FM s(t): Δf=74. 65 k. Hz. l Can you find the Δf at (a)-(d)?

FM Demodulator l Four primary methods – Differentiator with envelope detector/Slope detector u FM to AM conversion – Phase-shift discriminator/Ratio detector u Approximates the differentiator – Zero-crossing detector – Frequency feedback u Phase lock loops (PLL)

FM Slope Demodulator l 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. l Block diagram of direct method (slope detector = slope circuit + envelope detector) so(t) linear with m(t)

Slope Detector Magnitude frequency response of transformer BPF.

Bandpass Limiter l A device that imposes hard limiting on a signal and contains a filter that suppresses the unwanted products (harmonics) of the limiting process. l Input Signal l Output of bandpass limiter l Bandpass filter l Remove the amplitude variations

Ratio Detector l 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 l Ratio detector – Modified Foster-Seeley discriminator, not response to AM, but 50%

Zero Crossing Detector

FM Demodulator PLL l Phase-locked loop (PLL) – A closed-loop feedback control circuit, make a signal in fixed phase (and frequency) relation to a reference signal u u 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 u Just as Balanced Modulator IC can be used for most amplitude modulations and demodulations

PLL FM l Remember the following relations – Si=Acos(wct+ 1(t)), Sv=Avcos(wct+ c(t)) – Sp=0. 5 AAv[sin(2 wct+ 1+ c)+sin( 1 - c)] – So=0. 5 AAvsin( 1 - c)=AAv( 1 - c)

Superheterodyne Receiver l Radio receiver’s main function – – l 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.

Advantage of superheterodyne receiver l A signal block (of circuit) can hardly achieve all: selectivity, signal quality, and power amplification l Superheterodyne receiver deals them with different blocks l RF blocks: selectivity only l IF blocks: filter for high signal quality, and amplification, use circuits that work in only a constant IF, not a large band

FM Broadcasting l The frequency of an FM broadcast station is usually an exact multiple of 100 k. Hz from 87. 5 to 108. 5 MHz. In most of the Americas and Caribbean only odd multiples are used. l fm=15 KHz, f=75 KHz, =5, B=2(fm+ f)=180 k. Hz l 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. l Block diagram and spectrum l Relation of stereo transmission and monophonic transmission

FM Stereo Multiplexing Fc=19 KHz. (a) Multiplexer in transmitter of FM stereo. (b) Demultiplexer in receiver of FM stereo. Backward compatible For non-stereo receiver

TV FM broadcasting l fm=15 KHz, f=25 KHz, =5/3, B=2(fm+ f)=80 k. Hz l Center fc+4. 5 MHz l Eye cells structure

XM vs. Sirus

Frequency Allocation ECE 4371 Fall 2008

Project 1 l Project 1 – AM/FM/Real voice – Due 10/1/17