Digital Signal Processing Reference
In-Depth Information
analog IQ modulator is therefore normally used in VSB modulation which
directly converts the baseband signal to RF. As an analog component, the
IQ modulator no longer operates as perfectly as does a digital device. It
must therefore be ensured that the gain in the I and Q paths is identical,
and that the phase of the carrier supplied to the Q path is exactly 90 °. Oth-
erwise the unwanted part of the lower sideband will be inadequately sup-
pressed. After RF conversion, the signal passes through the pre-
equalization and power amplifier stages and is then taken to the antenna. A
passive bandpass filter in the antenna feeder line suppresses out-of-band
components.
Pilot addition
DC +1.25
+
+
Hilbert
trans
former
90°
Carrier
Fig. 23.20.
Typical 8VSB modulator with Hilbert transformer
23.2 8VSB Gross Data Rate and Net Data Rate
The symbol rate employed in 8VSB is calculated as follows:
symbol_rate = 4.5/286 • 684 MS/s = 10.76223776 MS/s;
This yields the following gross data rate:
gross_data_rate = 3 bit/symbol • 10.76 MS/s = 32.2867 Mbit/s;
The net data rate is then:
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