Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 210

We know the following modulation methods:

• Amplitude modulation
• Frequency modulation
• Phase modulation
• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PSK)
• Amplitude and phase shift keying (QAM)

What we want is to reduce the bandwidth for data signal transmission.
This is possible only by using modern digital modulation methods. Our
aim is to cut the required bandwidth by several factors relative to the data
rate of the signal transmitted.

u(t)

sin

u(t)

Q

Im

A = vector length A=vector length

A

A

im

f=1/T;

f=1/T

Φ

t

t

re

Re

I

Φ

cos

T

T

Euler formula:
A e (2ft + ) = re cos (2ft) + j im(2ft);

(2Πft+ Φ)

Euler Formula: Ae = re cos(2Πft) + j im sin(2Πft);

u(t) = A sin(2 Π t/T + Φ );

u(t)=A sin(2 t/T + );

Fig. 13.1. Vector representation of a sinusoidal signal

It is obvious that this will not go without disadvantages, i.e. susceptibil-
ity to noise and interference will increase. In the following, the digital
modulation methods will be discussed.
Before entering into this subject, we should like to point out that in elec-
trical engineering it is customary to represent sinusoidal quantities by
means of vectors (s. Fig. 13.1.). Each sinusoidal quantity can be unambi-
guously described by its amplitude and zero-phase angle. Moreover, the
frequency must be known. In the vector representation, the rotating vector