AD6620SSLASHPCB datasheet(22/44 Pages) AD

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AD6620

–22–

REV. A

ceil

input

level

input

level

CIC

SCIC

=×









=×

log (

)

The equations for calculating CIC2 output level is correct when

stage is not bypassed (normal operation). However, when by-

passed, the following equations should be used instead.

OLCIC2 = Input Level

The gain and pass band droop of the CIC2 should be calculated

by the equations above, as well as the filter transfer equations

that follow. If these are unacceptable, they can be compensated

for in subsequent stages.

CIC2 Rejection

The table below illustrates the amount of bandwidth in percent

of the data rate into the CIC2 stage. The data in this table may

be scaled to any allowable sample rate up to 67 MHz in Single

Channel Mode or 33.5 MHz in Diversity Channel Mode. The

table can be used as a tool to decide how to distribute the deci-

mation between CIC2, CIC5 and the RCF.

The data in this table may be scaled to any allowable sample

rate up to 67 MHz in Single Channel Mode or 33.5 MHz in

Diversity Channel Mode.

Table III. SSB CIC2 Alias Rejection Table (fSAMP = 1)

Bandwidth Shown in Percentage of fSAMP

MCIC2

–50 dB –60 dB

–70 dB

–80 dB –90 dB

–100 dB

1.79

1.007

0.566

0.318

0.179

0.101

1.508

0.858

0.486

0.274

0.155

0.087

1.217

0.696

0.395

0.223

0.126

0.071

1.006

0.577

0.328

0.186

0.105

0.059

0.853

0.49

0.279

0.158

0.089

0.05

0.739

0.425

0.242

0.137

0.077

0.044

0.651

0.374

0.213

0.121

0.068

0.038

0.581

0.334

0.19

0.108

0.061

0.034

0.525

0.302

0.172

0.097

0.055

0.031

0.478

0.275

0.157

0.089

0.05

0.028

0.439

0.253

0.144

0.082

0.046

0.026

0.406

0.234

0.133

0.075

0.043

0.024

0.378

0.217

0.124

0.07

0.04

0.022

0.353

0.203

0.116

0.066

0.037

0.021

0.331

0.19

0.109

0.061

0.035

0.02

Example Calculations

Goal: Implement a filter with an Input Sample Rate of 10 MHz

requiring 100 dB of Alias Rejection for a

±7 kHz pass band.

Solution: First determine the percentage of the sample rate that

is represented by the pass band.

kHz

MHz

FRACTION

=×

100

007

Find the –100 dB column on the right of the table and look

down this column for a value greater than or equal to your

pass band percentage of the clock rate. Then look across to the

extreme left column and find the corresponding decimation

rate. Referring to the table, notice that for a decimation of 4, the

frequency having –100 dB of alias rejection is 0.071 percent

which is slightly greater than the 0.07 percent calculated. There-

fore, the maximum bound on CIC2 decimation for this condi-

tion is four. Additional decimation means less alias rejection

than the 100 dB required.

Note that although an MCIC2 less then four would still yield the

required rejection, overall power consumption is reduced by

decimating as much as possible in this stage. Decimation in

CIC2 lowers the data rate and thus reduces power consumed in

subsequent stages.

The plot below shows the CIC2 transfer function using a deci-

mation of four. The first plot is referenced to the input sample

rate, the complex spectrum from –fSAMP/2 to fSAMP/2. The sec-

ond plot is referenced to the CIC2 output rate, the complex

spectrum from –fSAMP2/2 to fSAMP2/2. The aliases of the CIC2

can be seen to be “folding back” in toward the edge of the

desired filter pass band. It is the level of these aliases as they

move into the desired pass band that are important.

–0.5

–120

–100

–80

–60

–40

–20

f/fSAMP

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

–0.5

–120

–100

–80

–60

–40

–20

f/fSAMP2

–0.4 –0.3

–0.2

–0.1

0.1

0.2

0.3

0.4

0.5

Figure 36. CIC2 Alias Rejection, MCIC2 = 4

The set of plots below show a decimation of 16 in the CIC2

filter. The lobes of the filter drop as the decimation rate

increases, but the amplitudes of the aliased frequencies increase

because the output rate has been reduced.

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