Measuring DVB-T Signals - Digital Video and Audio Broadcasting Technology - page 401

If the same signal is fed into the I branch and into the Q branch, but with
a phase difference of 90 degrees from one another, a vector diagram as
shown in Fig. 21.16. ('single sideband modulation') is obtained. It can be
seen clearly that two sideband vectors are added and two sideband vectors
cancel (are subtracted). One sideband is thus suppressed, resulting in sin-
gle sideband amplitude modulation. A COFDM modulator can thus be in-
terpreted as being a single sideband modulator for many thousands of sub-
carriers. In an ideal COFDM modulator, there is no crosstalk from the
upper COFDM band to the lower one and vice versa.
Since the IFFT is a purely mathematical process, it can be assumed to be
ideal. The I/Q mixer, however, can be implemented as a digital (ideal)
mixer or as an analog mixer and there are and will be in future analog I/Q
mixers in DVB-T modulators (direct modulation).
If then an I/Q amplitude imbalance exists, this means that the upper or
lower sideband no longer cancel completely, leaving an interference com-
ponent. The same applies to an I/Q phase error. It is clear, therefore, that
all the subcarriers are subject to noise-like interference, with the exception
of the center carrier. It is also clear why a residual carrier will push the
constellation pattern away from the center at the center carrier and only in-
terferes with the latter.
Fig. 21.17. Spectrum of a DVB-T signal
This can also be shown impressively in the spectrum of the DVB-T sig-
nal if the DVB-T modulator has the test function of switching off e.g. the
lower carrier band in the spectrum. This can be done, for example, with a
DVB test transmitter. In the center of the band (center carrier), an existing
residual carrier can be seen clearly. If the I/Q modulator is then adjusted to
produce an amplitude imbalance, crosstalk from the upper to the lower
sideband is clearly apparent. The same applies to an I/Q phase error.