Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 215

13.4 IQ Modulator

In colour television, quadrature modulation or IQ modulation has been
used for a long time for the transmission of colour information. With PAL
or NTSC colour subcarriers, the chrominance information is contained in
the phase of the subcarrier and the colour saturation, or colour intensity, in
the amplitude of the subcarrier. The colour subcarrier is superimposed on
the luminance signal.
The modulated colour subcarrier is generated by means of an IQ modu-
lator or quadrature modulator, where "I" stands for in-phase and "Q" for
quadrature phase.

I

i(t)

+

data(t) Mapper

iqmod(t)

q(t)

Q

90°

Carrier lo(t)

Fig. 13.6. IQ modulator

An IQ modulator (see Fig. 13.6.) has an I path and a Q path. The I path
incorporates a mixer which is driven with 0° carrier phase. The mixer in
the Q path is driven with 90° carrier phase. This means that I stands for 0°
and Q for 90° carrier phase. I and Q are orthogonal to each other. In the
vector diagram, the I axis coincides with the real axis and the Q axis with
the imaginary axis.
PAL or NTSC modulators, too, incorporate an IQ modulator. For digital
modulation, a mapper is connected ahead of the IQ modulator. The mapper
is fed with the data stream data(t) to be transmitted; the output signals i(t)
and q(t) of the mapper are the modulation signals for the I and the Q
mixer. i(t) and q(t) are no longer data signals but signed voltages.
If i(t)=0, the I mixer produces no output signal, if q(t)=0, the Q mixer
produces no signal. If i(t) is at 1 V, for example, the I mixer will output a