L64777 LSI Logic Corporation, L64777 Datasheet - Page 41

• Manufacturer Part Number: L64777
• Description: DVB Qam Modulator
• Manufacturer: LSI Logic Corporation

2.11 Square Root Nyquist Filter
Figure 2.19 Pulse Shaper Structure
C0 C1 C2 C3 C4 C5 C6 C7 C8 C9
Numbers represent coefficient
word width of MAC structures
11
11 11 11
11 11 11 11 11
Differential encoding has an infinite error propagation. You can switch it
off independently from the mapping function (see bit 3 of Register 6, on
page 4-8).
This pulse-shaper module implements a programmable square-root
raised cosine filtering function with a default 15% roll-off factor. The
precision of the internal Nyquist filter computations, and the width of the
output data bus, are sufficient for QAM modulations up to 256. The filter
operates at four times the oversampling rate. Figure 2.19 illustrates the
structure of the pulse shaper.
Note that the 20 bits per channel is a result of the length of each
coefficient register (2
stages (30 = 2
coefficient set for each of the four phases multiplied by the MAC
structure, which contains 31 multipliers).
Each of the two I and Q branches has one filter, realized as polyphase
structures. Each filter consists of four filter branches, which compute
1-phase filter results at the symbol rate. Thus, the L64777 Nyquist filter
module generates the desired pulse shape by combining the outputs of
four identical filter branches for I and Q.
Square Root Nyquist Filter
From Coefficient Register (Register 0)
The big adder
5
). The total number of coefficients is 124 (there is one
11
11
C23 C24 C25 C26 C27 C28 C29 C30
) plus the length of INQ (2
11 11 11
QS[19:0]
IS[19:0],
to Shifter
11 11 11 11 11
4
) plus the number of
11 bit
offset
2-27