Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 218

ing much mathematics, this can be explained quite simply by the fact that
01 alternations represent the highest frequency; i.e. the period has a length
of 2 bits and is thus 2 Μs in the case of a data rate of 1 Mbit/s. The recipro-
cal of 2 Μs is then 500 kHz and the minimum baseband bandwidth for
transmitting an NRZ code is then:

f baseband_NRZ [Hz] ≥ 0.5 • data rate NRZ [bits/s];

If such a filtered NRZ code (Fig. 13.8.) is then supplied, e.g. without
DC to a mixer as in the I path of this IQ modulator, two sidebands having
each the bandwidth of the input baseband signal (Fig. 13.9.) are produced
at RF. The minimum bandwidth required at RF is thus:

f RF_NRZ [Hz] ≥ data rate NRZ [bits/s];

Q

BPSK

I

1
s

RF
bandwidth
>=1 MHz

1 1 1 1

0 000 0

600
MHz

Example: NRZ 1 Mbit/s

LO
600 MHz

Symbol rate BPSK = 1/symbol duration BPSK =
1/bit duration BPSK = 1/1s = 1 MSymbols/s;
1 MSymbols/s Î RF bandwidth >= 1 MHz

Fig. 13.9. BPSK modulation

In this type of modulation, therefore, the ratio between data rate and
minimum bandwidth required at RF is 1:1. This type of modulation is
called binary phase shift keying, or biphase shift keying, BPSK. With
BPSK, a data rate of 1 Mbit/s requires a minimum bandwidth of 1 MHz at
RF level. The duration of one stable state of the carrier is called a symbol