Pentek 6230 Operating Manual page 152

GC4016 MULTI-STANDARD QUAD DDC CHIP
ADDRESS 21: Decimation Ratio Byte 0, Suggested default = 0x07
BIT TYPE
0-7 R/W
ADDRESS 22: Decimation Ratio Byte 1, Suggested default = 0x70
BIT TYPE
0-3 R/W
4-6 R/W
7 MSB R/W
The CIC decimation is N. When processing in normal mode (not SPLITIQ) DEC should be set to N-1 where N ranges from
8 to 4096. When processing in SPLITIQ mode DEC should be set to 2N-1 where N ranges from 4 to 2048.
ADDRESS 23: CIC Scale, Suggested default = 0x79
BIT TYPE
0-2 R/W
3-5 R/W
6 R/W
7 R/W
The CIC filter has a gain which is equal to N
(62-SHIFT-SCALE-6*BIG_SCALE) bits before filtering. The values of SHIFT, SCALE and BIG_SCALE must be such that:
Overflows due to improper gain settings will go undetected if this relationship is violated. For example, this restriction means
that for N equal to 8, and SHIFT equal to 4, BIG_SCALE and SCALE should be less than or equal to 7 and 1 respectively.
ADDRESS 24: SplitIQ, Suggested default = 0x00
BIT TYPE
0-3 R/W
4 R/W
5 R/W
6 R/W
7 R/W
Sections 3.3 and 3.4 describe how to use these controls.
© GRAYCHIP,INC.
This document contains information which may be changed at any time without notice
NAME DESCRIPTION
DEC[0:7] The LSBs of the decimation control
NAME DESCRIPTION
DEC[8:11] The 4 MSBs of the decimation control
GAIN_SYNC The new fine gain takes effect on this sync.
Unused
NAME DESCRIPTION
SCALE SCALE ranges from 0 to 5.
BIG_SCALE BIG_SCALE ranges from 0 to 7.
MIX20B Round mixer output to 20 bits if MIX20B=1 or to 16 bits if MIX20B=0. If
MIX20B=1 then SHIFT (see address 16) must be at least 4 to shift the extra 4
bottom bits into the datapath. For very large decimations (>3104) unity gain
through the CIC can only be accomplished by using a smaller SHIFT and hence
MIX20B must be zero. The default is MIX20B=1.
unused
( ×
SHIFT + SCALE + 6 BIG_SCALE
NAME DESCRIPTION
NEG_CTL A four bit pattern that multiplies the output of PFIR by +/- 1. This is used to
perform complex to real conversion (Fs/4 upconvert), flip the spectrum, or
perform an Fs/2 frequency shift.
SPLITIQ Process only the real data (at twice the throughput). Used to allow two channels
to be ganged together for wider output bandwidth.
IONLY All samples output by this channel are real. Used in the SPLITIQ mode to
identify the channel generating the I-half of the complex output.
QONLY All samples output by this channel are imaginary. Used in the SPLITIQ mode to
identify the channel generating the I-half of the complex output. When
performing complex to real conversion this bit must be set (amongst others).
Unused
5
. To remove this gain the CIC inputs are pre-scaled down by
) ≤ (
62 5log – N + log
2
- 39 -
DATA SHEET REV 1.0
)
(NZERO+1)
2
August 27, 2001