Chapter 2: Amplitude Modulation Transmission EET-223: RF - - PowerPoint PPT Presentation

Chapter 2: Amplitude Modulation Transmission

EET-223: RF Communication Circuits Walter Lara

Introduction

• As see before, modulation is needed to:

– Avoid interference since intelligence signals are at approximately the same frequency – Avoid impractical large antennas since intelligence signals have low frequencies

• Problem: how to put intelligence signal onto a

carrier (high frequency) signal for transmission

• Simplest solution: put intelligence into carrier’s

amplitude

AM Fundamentals

• Combining (“mixing”) the intelligence and carrier

signals can be done:

– Using linear device (e.g. resistor) – simple addition, but not suitable for transmission (receiver cannot detect intelligence) – Non-linear device (e.g., BJT or OpAmp) – method used in practice

• Non-linear mixing results on:

– DC Component – Components at original frequencies (intelligence & carrier) – Components at sum & difference of original frequencies – Harmonics of original frequencies

AM Fundamentals – Cont’d

• Only the following components resulting from non-

linear mixing are used on an AM waveform:

– Carrier frequency (fc) – Lower-side frequency (fc - fi) – Upper-side frequency (fc + fi)

Figure 2-1 Linear addition of two sine waves.

Figure 2-2 Nonlinear mixing.

AM Waveforms

• An AM modulated signal can be expressed as:

e(t) = (Ec + Ei sin wit) sin wct where: Ec = peak value of carrier signal Ei =peak value of intelligence signal wc= angular frequency of carrier signal wi= angular frequency of intelligence signal

• It can be demonstrated that:

e(t)= Ec sin wct + (Ei/2)cos (wc - wi)t - (Ei/2)cos (wc + wi)t

Figure 2-3 AM waveform under varying intelligence signal (ei) conditions.

Figure 2-4 Carrier and side-frequency components result in AM waveform.

Figure 2-5 Modulation by a band of intelligence frequencies.

Figure 2-6 Solution for Example 2-1.

Percentage Modulation

• Aka Modulation Index or Modulation Factor
• Measure of extend to which carrier voltage is varied

by intelligence

• Defined as: %m = Ei / Ec * 100

– Ei: Peak value of intelligence signal – Ec: Peak value of carrier signal

• Can also be computed using the peak-to-peak value
• f the AM waveform (see Fig. 2-8)

– Convenient in graphical (oscilloscope) solutions.

Figure 2-8 Percentage modulation determination.

Overmodulation

• Overmodulation is a condition that occurs when an

excessive intelligence signal overdrives an AM modulator making %m > 100% (because Ei> Ec)

• Modulated carrier amplitude reach value greater than

double of unmodulated value

• It produces a distortion known as sideband splatter,

which results on transmission at frequencies outside the allocated range

• It is unacceptable because it causes severe interference

with other stations and causes a loud splattering sound to be heard at the receiver.

Figure 2-9 Overmodulation.

AM Analysis

• Recall:

e(t) = (Ec + Ei sin wit) sin wct = Ec sin wct + (Ei/2)cos (wc - wi)t + (Ei/2)cos (wc + wi)t

• Since Ei = m Ec , then:

e(t) = Ec sin wct + (mEc/2) cos (wc - wi)t + (mEc/2) cos (wc + wi)t

• Therefore, the side-frequency amplitude is:

ESF = mEc/2

Why is important to use a high %m?

• The higher m, the more transmitted power gets to
• ur sidebands, which contain the intelligence.
• The total power can be computed as:

PT = PC + 2PSF = PC (1 + m2 / 2) Where: PC : carrier power PSF : single sideband power

• The total current can be computed as:

IT = Ic 𝟐 + 𝒏𝟑/𝟑

• The power efficiency can be computed as:

Efficiency = 2PSF / PT = m2 / (2 + m2)

AM Transmitter System

• Refer to block diagram at Fig. 2-18 (next slide).
• Main components are:

– Oscillator: generates carrier signal at high accuracy (crystal- controlled) – Buffer Amplifier: provides high impedance load to oscillator to minimize drift – Intelligence Amplifier: amplifies the signal from input transducer – Modulated Amplifier (aka Modulator): generates modulated/mixed signal – Linear Power Amplifier: amplifies modulated signal on high- power (commercial) systems

Figure 2-18 Simple AM transmitter block diagram.

Trapezoidal Patterns

• Method to check proper modulation of AM signal

– More revealing than viewing signal on scope

• Procedure:

– Put scope in XY Mode – Put AM signal on vertical – Put intelligence signal on horizontal (through RC phase-shift network

• Possible Results (see Fig 2-23):

– Top & bottom straight lines: proper modulation – Single vertical line: no intelligence (carrier only) – Concave curvature: poor linearity on modulation stage – Convex curvature: improper bias or low carrier signal – Half oval with inner Y : improper phase relationships

Figure 2-23 Trapezoidal pattern connection scheme and displays.

Spectrum Analyzers

• Show plot of amplitude vs frequency
• Swept-tuned (superhetereodyne) Analyzer – uses

analog frequency sweep, can go up to GHz range

• Fourier Analyzer – digitizes waveform and uses FFT
• algorithms. Limited to ~40 MHz (EET Labs)
• Vector Signal Analyzer (VSA) – uses analog front-

end and digitizes after down-convertion.

– Best of both worlds, but expensive – Can measure Total Harmonic Distortion (THD)

Figure 2-24 Spectrum analysis of AM waveforms.

Figure 2-25 Spectrum analyzer and typical display. (Courtesy of Tektronix, Inc.)

Figure 2-25 (continued) Spectrum analyzer and typical display. (Courtesy of Tektronix, Inc.)

Relative Harmonic Distortion (RHD)

• Ratio of fundamental with respect to the largest

undesired harmonic

– The greater, the better

• Can be computed (in dB) as:

RHD = 𝟑𝟏 𝒎𝒑𝒉 𝑾𝟐/𝑾𝟑 Where: V1: desired component (fundamental frequency) V2: largest undesired harmonic component

Figure 2-26 Relative harmonic distortion.

Total Harmonic Distortion (THD)

• Ratio of power from unwanted harmonics to

desired frequency components

– The greater, the worst

– More descriptive distortion spec than RHD

• Occurs in amplifiers and non-linear devices
• Can be computed as:

THD = (𝑾𝟑

𝟑 + 𝑾𝟒 𝟑 + 𝑾𝟓 𝟑 + … )/𝑾𝟐 𝟑

Where: V1: desired component (fundamental frequency) V2, V3, … : undesired harmonic components