Square Root Nyquist Filter
2-27
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).
2.11 Square Root Nyquist Filter
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.
Figure 2.19 Pulse Shaper Structure
Note that the 20 bits per channel is a result of the length of each
coefficient register (2
11
) plus the length of INQ (2
4
) plus the number of
stages (30 = 2
5
). The total number of coefficients is 124 (there is one
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.
From Coefficient Register (Register 0)
to Shifter
Numbers represent coefficient
word width of MAC structures
IS[19:0],
11 bit
QS[19:0]
C0 C1 C2 C3 C4 C5 C6 C7 C8 C9
C23 C24 C25 C26 C27 C28 C29 C30
offset
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
11
The big adder
Summary of Contents for L64777
Page 1: ...L64777 DVB QAM Modulator Order Number I14031 A Technical Manual June 2000...
Page 10: ...x Contents...
Page 14: ...1 4 Introduction...
Page 90: ...5 10 Signals...
Page 110: ...A 8 Programming the L64777 in Serial Host Interface Mode...
Page 116: ...C 2 Monitoring Device Internal Signals...
Page 124: ......
