AD6620SSLASHPCB datasheet(22/43 Pages) AD

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AD6620

–22–

REV. 0

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 other allowable sample rate up to 65 MHz in

Single Channel Mode or 32.5 MHz in Diversity Channel Mode.

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

decimation between CIC2, CIC5 and the RCF.

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

The decimation ratio, MCIC2, is an unsigned integer that may

be between 1 and 16. This stage may be bypassed under certain

conditions by setting, MCIC2 equal to 1. For this to happen the

processing clock rate, fCLK must be two or more times the input

data rate, fSAMP. This is because the I and Q data is processed

in parallel within the CIC2 filter, and the I and Q output data is

then multiplexed through the same data pipe before it enters the

CIC5 filter.

The frequency response of the CIC2 filter is given by the follow-

ing equations.

CIC

()

–

=×









CIC

SAMP

CIC

()

sin

=×





























The scale factor, SCIC2 is a programmable unsigned integer

between 0 and 6. This serves as an attenuator that can reduce

the gain of the CIC2 in 6 dB increments. For the best dynamic

range, SCIC2 should be set to the smallest value possible (i.e.,

lowest attenuation) without creating an overflow condition.

This can be safely accomplished using the equation below,

where input_level is the largest fraction of full scale possible at

the input to this AD6620 (normally 1). The CIC2 scale factor is

not ignored when the CIC2 is bypassed.

ceil

input level

CIC

SCIC

=×

















log (

) –

MASKED

COUNT = 0?

SYNC

MASK

SYNC_NCO

PHASE

ACCUMULATOR

PHASE

OFFSET

PHASE

DITHER

NCO FREQ

AMPLITUDE

DITHER

COS

SIN

Figure 40. NCO Block Diagram

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