AD6636PCB datasheet(36/72 Pages) AD

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AD6636

Rev. 0 | Page 36 of 72

Programming CRCF Registers for an Asymmetrical Filter

To program the CRCF registers for an asymmetrical filter:

Write NTAPS – 1 in the CRCF taps register, where NTAPS

is the number of filter taps. The absolute maximum value

for NTAPS is 64 in asymmetrical filter mode.

Write 0 for the CRCF coefficient offset register.

Write 0 for the symmetrical filter bit in the CRCF control

In the CRCF start address register, write the start address

for the coefficient RAM, typically equal to the coefficient

offset register.

In the CRCF stop address register, write the stop address

for the coefficient RAM, typically equal to the following:

Coefficient Offset + NTAPS – 1

Write all coefficients in reverse order (start with last

coefficient) to the CRCF coefficient memory register. In

8-bit microport mode or serial port mode, write the lower

byte of the memory register first and then the higher byte.

In 16-bit microport mode, write the lower 16-bits of the

CRCF memory register first and then the high four bits.

After each write access to the CRCF coefficient memory

with the start address and ending with the stop address.

Note that each write or read access increments the internal

RAM address. Therefore, all coefficients should be read first

before reading them back. Also, for debugging purposes, each

RAM address can be written individually by making the start

and stop addresses the same. Therefore, to program one RAM

location, the user writes the address of the RAM location to

both the start and stop address registers, and then writes the

coefficient memory register.

Programming CRCF Registers for a Symmetrical Filter

To program the CRCF registers for a symmetrical filter:

Write NTAPS – 1 in the CRCF taps register, where NTAPS

is the number of filter taps. The absolute maximum value

for NTAPS is 128 in symmetrical filter mode.

Write ceil(64 – NTAPS/2) for the CRCF coefficient offset

greater than or equal to the argument.

Write 1 for the symmetrical filter bit in the CRCF control

In the CRCF start address register, write the start address

for the coefficient RAM, typically equal to the coefficient

offset register.

In the CRCF stop address register, write the stop

address for the coefficient RAM, typically equal to

ceil(NTAPS/2) – 1.

Write all coefficients to the CRCF coefficient memory

towards the end of the filter. When coefficients are

numbered 0 to NTAPS – 1, the middle coefficient is given

by the coefficient number ceil(NTAPS/2). In 8-bit

microport mode or serial port mode, write the lower byte

of the memory register first and then the higher byte. In

16-bit microport mode, write the lower 16-bits of the

CRCF memory register first and then the high four bits.

After each write access to the CRCF coefficient memory

with the start address and ending with the stop address.

Note that each write or read access increments the internal

RAM address. Therefore, all coefficients should be read first

before reading them back. Also, for debugging purposes, each

RAM address can be written individually by making the start

and stop addresses the same. Therefore, to program one RAM

location, the user writes the address of the RAM location to

both the start and stop address registers, and then writes the

coefficient memory register.

INTERPOLATING HALF-BAND FILTER

The AD6636 has interpolating half-band FIR filters that

immediately follow the CRCF programmable FIR filters and

precede the second data router. Each interpolating half-band

filter takes 22-bit I and 22-bit Q data from the preceding CRCF

and outputs rounded 22-bit I and 22-bit Q data to the second

data router. A 10-tap fixed-coefficient filter is implemented in

this stage.

The maximum input rate into this block is 17 MHz. Conse-

quently, the maximum output is constrained to 34 MHz. The

normalized coefficients used in the implementation and the

10-bit decimal equivalent value of the coefficients are listed in

Table 21. Other coefficients are 0.

Table 21. Interpolating HB Filter Fixed Coefficients

Coefficient

Number

Normalized

Coefficient

Decimal Coefficient

(10-Bit)

C1, C11

0.02734375

C3, C9

−0.12890625

−66

C5, C7

0.603515625

309

512

The half-band filters interpolate the incoming data by 2×. For a

channel running at 2× the chip rate, the half-band can be used

to output channel data at 4× the chip rate. The interpolation

operation creates an image of the baseband signal, which is

filtered out by the half-band filter.

AD6636PCB Datasheet(PDF) 36 Page - Analog Devices

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