AD6652 Datasheet, PDF (37/76 Pages) Analog Devices – 12-Bit, 65 MSPS IF to Baseband Diversity Receiver

FIFTH-ORDER CIC FILTER
The fourth signal processing stage, CIC5, implements a sharper
fixed-coefficient, decimating filter than rCIC2. The input rate to
this filter is fSAMP2. The maximum input rate is given by the
following equation. NCH equals two for diversity channel real
input mode; otherwise, NCH equals one. To satisfy this equation,
increase MrCIC2 or reduce NCH.
f _SAMP2
≤
f _CLK
N _CH
The decimation ratio, MCIC5, can be programmed from 2 to 32
(all integer values). The frequency response of the filter is given
by the following equations. Use these equations to calculate the
gain and pass-band droop of CIC5. Both parameters can be
compensated for in the RCF stage.
H(z)
=
1
2 ^{SCIC5 +5}
×
⎜⎜⎝⎛
1
− z ^{− MCIC}
1 − z ^−1
5
⎟⎟⎠⎞ ⁵
H(
f
)
=
1
2 ^{SCIC5 +5}
×
⎜⎛
⎜
⎜
sin
⎜⎜⎝⎛
π
M_CIC5 ×
f _SAMP2
f
⎟⎟⎠⎞
⎟⎞
⎟
⎟
5
⎜
⎜
⎝
sin _{⎜⎜⎝⎛} π
f
f _SAMP2
⎟⎟⎠⎞
⎟
⎟
⎠
The scale factor, SCIC5, is a programmable unsigned integer
between 0 and 20. It serves to control the attenuation of the data
into the CIC5 stage in 6 dB increments. For the best dynamic
range, set SCIC5 to the least attenuation without creating an
overflow condition. This can be safely accomplished using the
following equation, where OLrCIC2 is the largest fraction of full
scale possible at the input to this filter stage. This value is output
from the rCIC2 stage, then pipelined into the CIC5.
( ( )) S_CIC5 = ceil log ₂ M _CIC55 × OL_rCIC2 − 5
( ) _{OLCIC5 =}
M _CIC5 ⁵
2 ^{SCIC5 +5}
× OL_rCIC2
The output rate of this stage is given by the following equation:
f _SAMP5
=
f _SAMP2
M _{CIC 5}
CIC5 REJECTION
Table 15 lists the amount of bandwidth in percentage of the
input rate that can be protected with various decimation rates
and alias rejection specifications. The maximum input rate into
the CIC5 is 65 MHz when the rCIC2 decimates by 1. As in the
previous rCIC2 table (Table 14), these are the single-sideband
bandwidth characteristics of the CIC5.
AD6652
The CIC5 stage can protect a much wider band to any given
rejection than rCIC2.
Given the desired filter characteristics, Table 15 can help in the
calculation of an upper bound on decimation, MCIC5.
Table 15. SSB CIC5 Alias Rejection Table (fSAMP2 = 1)
MCIC5 −50 dB −60 dB −70 dB −80 dB −90 dB −100 dB
2
10.277 8.078 6.393 5.066 4.008 3.183
3
7.924 6.367 5.11
4.107 3.297 2.642
4
6.213 5.022 4.057 3.271 2.636 2.121
5
5.068 4.107 3.326 2.687 2.17
1.748
6
4.267 3.463 2.808 2.27
1.836 1.48
7
3.68
2.989 2.425 1.962 1.588 1.281
8
3.233 2.627 2.133 1.726 1.397 1.128
9
2.881 2.342 1.902 1.54
1.247 1.007
10 2.598 2.113 1.716 1.39
1.125 0.909
11 2.365 1.924 1.563 1.266 1.025 0.828
12 2.17
1.765 1.435 1.162 0.941 0.76
13 2.005 1.631 1.326 1.074 0.87
0.703
14 1.863 1.516 1.232 0.998 0.809 0.653
15 1.74
1.416 1.151 0.932 0.755 0.61
16 1.632 1.328 1.079 0.874 0.708 0.572
17 1.536 1.25
1.016 0.823 0.667 0.539
18 1.451 1.181 0.96
0.778 0.63
0.509
19 1.375 1.119 0.91
0.737 0.597 0.483
20 1.307 1.064 0.865 0.701 0.568 0.459
21 1.245 1.013 0.824 0.667 0.541 0.437
22 1.188 0.967 0.786 0.637 0.516 0.417
23 1.137 0.925 0.752 0.61
0.494 0.399
24 1.09
0.887 0.721 0.584 0.474 0.383
25 1.046 0.852 0.692 0.561 0.455 0.367
26 1.006 0.819 0.666 0.54
0.437 0.353
27 0.969 0.789 0.641 0.52
0.421 0.34
28 0.934 0.761 0.618 0.501 0.406 0.328
29 0.902 0.734 0.597 0.484 0.392 0.317
30 0.872 0.71
0.577 0.468 0.379 0.306
31 0.844 0.687 0.559 0.453 0.367 0.297
32 0.818 0.666 0.541 0.439 0.355 0.287
+10
–10
–30
–50
–70
–90
–110
–130
–150
–2
–1
0
1
2
MULTIPLES OF CIC5 OUTPUT RATE
Figure 48. Double Side-Band Graph Showing CIC5 Filter Response
and Alias Rejection of −100 dB
Rev. 0 | Page 37 of 76