Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 231

It is difficult to implement an ideal 90° phase shifter for all harmonics of
a baseband signal as an analog circuit. Digital implementation is no prob-
lem - thanks to the Hilbert Transform. A Hilbert transformer is a 90°
phase shifter for all components of a real time-domain signal.
Fig. 13.23. shows the suppression of a sideband by means of an IQ
modulator and a Hilbert transformer. A real baseband signal is directly fed
to the I path of an IQ modulator and to the Q path via a Hilbert trans-
former. The continuous lines at f=0 represent the spectrum of the baseband
signal, the dashed lines at f=0 the spectrum of the Hilbert Transform of the
baseband signal.
It can be seen clearly that, under the Hilbert Transform, the half-turn
symmetrical imaginary component becomes a mirror symmetrical real
component and the mirror symmetrical real component becomes a half-
turn symmetrical imaginary component at the baseband.
If the unmodified baseband signal is then fed into the I path and the Hil-
bert Transform of the baseband signal into the imaginary path, spectra
about the IQ modulator carrier like those shown in Fig. 13.22. are ob-
tained. It can be seen that in this case the lower sideband is suppressed.

Information

Source coding

e.g. compression

forward error
correction (FEC)

Channel coding

Modulation

Interferences

Transmission
link

Demodulation

Bit errors

Channel decoding

Source decoding

Information

Fig. 13.24. Information transmission