EEE 330Introduction to

Communication Systems

Lecture # 3

Modulation and DetectionAmplitude Modulation

Overview

� The Objectives of Today’s Lecture

� Amplitude Modulation

� Read Carlson et al. pp. 152–159, 176–177

To Study Communication Systemsyou must understand…

� Signals and Systems

� Fourier Analysis

� Modulation Theory

� We will study this in detail

� Detection Theory

� Given that this signal is corrupt at the receiver, how do we determine the original signal?

� Probability Theory

� Since the transmit signal and noise are both unknown to the receiver, we can use probability theory to study communications systems

Baseband Communication

� In communication, baseband is used the band of frequencies where the transmitter and the receiver communicates� telephony – audio band (0 - 3.5 kHz)

� television – video band (0 - 4.3 MHz)

� In baseband communication, the baseband signals are transmitted without modulation� short distance communication

� coaxial cable, optical fibers

� Local telephone, short-haul PCM, long-distance PCM over optical fibers

Carrier Communication

� In carrier communication, the baseband signal is shifted to higher frequencies by modulation and transmitted to long distances

� In continuous-wave (CW) modulation, the carrier is a sinusoid of frequency ωc.

� This is the traditional mode for all-analogue communications.

c(t)= Accos(ωct +θc)

� In pulse modulation, the carrier is a

CW Modulation

� Modulation means the change of one of the parameters (amplitude, phase or frequency) of the carrier signal in proportion to the baseband signal(information signal)

� Amplitude (Ac) – Amplitude Modulation (AM)

� Phase (θc) – Phase Modulation (PM)

� Frequency (ωc)– Frequency Modulation (FM)

CW Modulation

(a) Carrier wave.

(b) Sinusoidal modulating signal.

(c) Amplitude-modulated signal.

(d) Angle-modulated signal.

Benefits of Modulation

There are three practical benefits that result from modulation:1. Modulation can shift the spectral content of a message

signal into a band which is better suited to the channel.� Antennas only efficiently radiate and admit signals whose

wavelength is similar to their physical aperture.� Hence, to transmit and receive, say, voice, by radio we need

to shift the voice signal to a much higher frequency band.

2. Modulation permits the use of multiplexing.� Multiplexing means allowing simultaneous communication by

multiple users on the same channel.� For instance, the radio frequency spectrum must be shared

and modulation allows users to separate themselves into bands.

3. Modulation can provide some control over noise/interference.� As we will see, frequency modulation (FM) permits a tradeoff

between bandwidth and noise.

Amplitude Modulation (AM)

� Amplitude modulation (AM) is a technique from the very beginning of CW radio transmission.

� Also called “Large carrier (LC)” AM or “Double Sideband Large Carrier (DSB-LC) AM

� It is still in use today because of its simplicity.

Definitions

� Message signal – information-bearing signal that is to be recovered at the receiver [x(t) ]

� Carrier – the sinusoid with frequency

ωc that is used to “carry” the

information signal

� Envelope – the time-varying magnitude of the sinusoidal signal(modulated signal)

Amplitude Modulation (AM)

� A message signal x(t) is amplitude modulated as follows:

g(t)= Ac (1 + µ x(t)) cos(ωct +θc)

� The modulation index µ > 0 is chosen to ensure that (1 + µ x(t)) > 0 and to conserve power

� Also |µ x(t)| < 1. When this is violated, we call this “over-modulation”.

Envelope Variation

� The envelope of the transmit signal g(t)has the same shape as the message signal provided that1. Over-modulation doesn’t occur. In other

words as long as |µ x(t)| < 1.� This is the same as saying that (1 + µ x(t)) must be

positive. Since this represents the amplitude of the carrier, we say that the amplitude cannot be “negative”

� Negative amplitude corresponds to a phase reversal

2. The carrier frequency is much greater than the message bandwidth ( fc >> W )� This is the same as saying that the carrier signal

changes much more quickly than the message signal

Frequency-Domain Analysis of an AM Signal

envelope

A System for Amplitude Modulation

� Basic AM requires only an amplifier, a summer and a mixer.

Amplitude Modulation

Demodulation

� Definition: The recovery of the message signal from the modulated signal is called the demodulation or detection.

� Demodulation occurs at the receiver.

Amplitude Demodulation

A System for Amplitude Demodulation

� To demodulate the received signal, i.e., to recover the original messagesignal, we can use an envelope detector circuit.

� A diode is used to half-wave rectify the received signal.� The R1C1 filter then smooths to recover an approximation of the

original envelope.� R2C2 removes the bias.

Double Sideband Suppressed-Carrier (DSB-SC) AM

� A problem with AM is that it is inefficient with power.Most of the transmitted power is wasted during the transmission of the carrier component.

� We can improve the power efficiency of AM by removing the unmodulated carrier component.

� This is termed Double Sideband Suppressed-Carrier (DSB-SC) AM

g(t)= Ac m(t) cos(ωct +θc)

� Now, all of the power is devoted to the message –more power efficient

� However, a simple envelope detector is not possible, a product detector is needed.

DSB-SC AM Modulator

Ac cos(ωct +θc)

m(t) g(t)= Ac m(t) cos(ωct + θc)

Baseband signal Modulated Signal

Carrier

DSB-SC AM

� message signal m(t)

Bandwidth = B

Baseband signalBaseband spectrum

DSB-SC AM

� Modulated Signal A m(t) cos(ωct +θc)

Let θc = 0 (since it is a constant) and A = 1

Bandwidth = 2B

Modulated Signal SpectrumModulated Signal

Notes on DSB-SC

� Multiplying by cos(ωct) relocates the baseband spectrum to ±ωc .

� There is no discrete component of the frequency ωc . “Suppressed Carrier”

DSB-SC AM

DSB AM

Suppressed carrier

Discrete carrier component

Notes on DSB-SC

� Evenif we view the baseband spectrum as having + frequencies, the modulated signal spectrum shows upper and lower parts.

� The modulated signal spectrum centered at ωc is composed of two parts� USB – Upper Sideband (ω> ωc)

� LSB – Lower Sideband (ω< ωc)

� Similarly, the spectrum centered at –ωc is composed of two parts� USB – Upper Sideband (ω< -ωc)

� LSB – Lower Sideband (ω> -ωc)

Notes on DSB-SC

� Each ± copy of the baseband spectrum loses ½ amplitude relative to the baseband.

� To have non-overlapping spectra centered at +ωc and –ωc , 2πB≤ ωc.

� Nyquist criterion

� If two spectra overlaps, the message signal cannot be recovered (Aliasing)

Notes on DSB-SC

� When m(t) crosses zero, the envelope is momentarily estinguished, and it appears as m(t) goes negative.

� An accurate phase information is needed at the receiver.

� The envelope of DSB-SC reflects |m(t)|

� Envelope detector cannot be used.

m(t) crosses zero

Demodulation of DSB-SC(Product Detector)

Local Osscilator

As stated previously, one can no longer use a simple envelope detector as a receiverO Envelope doesn’t follow message signalO However we can still recover the message through the use of a product detector

Product Detector

LPF

Suppressed by LPF

Notes on DSB-SC Demodulation

� Multiply the modulated signal by the “same”carrier signal of the transmitter used.

“same” : same frequency

: same phase

� This type of demodulation is called the synchronous or coherent detection

� We need very precise local oscillator tuning or automatic carrier recovery circuits