Digital Signal Processing Reference
In-Depth Information
Normally, as the baseband signal will have both positive and
negative frequency components. For example, the complex QPSK
modulator trajectory can move both clockwise and counter-
clockwise depending upon the input data sequence. When
upconverted, the positive and negative baseband components
will no longer overlay each other, but will be unfolded on either
side of the carrier frequency. For example, a baseband signal with
a frequency spectrum of 0 to 10 kHz will occupy a total of 20 kHz,
with 10 kHz on either side of the carrier frequency. The baseband
signal represents the positive and negative frequencies using
quadrature form, which is why it is in the form of two dimen-
sional I and Q signals, each with a frequency spectrum from 0 to
10 kHz.
A typical transmit chain in a cable video-distribution system
would have many channels digitally modulated and placed upon
different carrier frequencies. In this way, many video channels
could be carried on the same coax or fiber. The NCO generates
the complex exponential signal. This process is shown in
Figure 17.18
.
In this implementation, there are N video channels with
6 MHz bandwidth being sampled at 16 MHz. The next step is to
4096 Msps
16 Msps
256 Msps
Sample Rate >> Clock Rate
Sample Rate < = Clock Rate
Figure 17.18. Digital transmit circuit path.
