Digital Signal Processing Reference
In-Depth Information
Polyphase Decomposition
Figure 17.19. Polyphasing of high rate transmit paths.
pulse shape and interpolate, or up-sample each video signal by
16, to a sample rate of 256 MHz. Each of these baseband channels
is placed upon a different carrier frequency, multiplying by the
output of a numerically controlled oscillator (NCO), also running
at 256 MHz, which is programmed to the desired carrier
frequency.
The video carriers are all at separate intermediate frequencies,
and so can be summed together. At this point, the composite
signal can be digitally upconverted to RF frequency. Modern
DACs can run at several GHz, and this design uses a DAC running
at 4.096 GHz. This can allow for an RF signal in the multi-GHz
range out of the DAC, which is filtered and amplified before being
transmitted over the cable or broadcast over the air.
Most digital chips cannot clock at 4 GHz. Here, the portions of
the circuit with data rates above 256 MHz are built using poly-
phasing techniques, which involve creating parallel paths in order
to process a signal being sampled at rates above the digital clock
rate. The sampling rate is stepped up from 256 MHz to 4096 MHz
in four stages, each doubling the sample rate, for an overall
interpolation of 16.
