AD6636PCB datasheet(35/72 Pages) AD

35 / 72 page

AD6636

Rev. 0 | Page 35 of 72

CHANNEL RAM COEFFICIENT FILTER (CRCF)

Following the DRCF is the programmable decimating CRCF

FIR filter. The only difference between the DRCF and CRCF

filters is the coefficient bit width. The DRCF has 14-bit

coefficients, while the DRCF has 20-bit coefficients.

This filter can calculate up to 64 asymmetrical filter taps or up

to 128 symmetrical filter taps. The filter is capable of a

programmable decimation rate from 1 to 16. The flexible

coefficient offset feature allows loading multiple filters into the

coefficient RAM and changing the filters on the fly. The

decimation phase feature allows for a polyphase implementa-

tion in which multiple AD6636 channels are used to process a

single carrier.

The CRCF filter has 20-bit input and output data and 14-bit

coefficient data. The number of filter taps to calculate is

programmable and is set in the CRCF taps register. The value of

the number of taps minus one is written to this register. For

example, a value of 19 in the register corresponds to 20 filter

taps. The decimation rate is programmable using the 4-bit

CRCF decimation rate word in the CRCF control register.

Again, the value written is the decimation rate minus one.

Bypass

The CRCF filter can be used in normal operation or bypassed

using the CRCF bypass bit in the CRCF control register. When

the CRCF filter is bypassed, no scaling is applied and the output

of the filter is the same as the input to the CRCF filter.

Scaling

The output of the CRCF filter can be scaled using the 2-bit

CRCF scaling word in the CRCF control register. Table 20

shows the valid values for the 2-bit word and the corresponding

settings. | ∑COEFF | is the sum of all coefficients (in normalized

form) used to calculate the FIR filter.

Table 20. CRCF Scaling Factor Settings

CRCF Scale Word [1:0]

Scaling Factor

18.06 dB attenuation

12.04 dB attenuation

6.02 dB attenuation

No scaling, 0 dB

Symmetry

The CRCF filter does not require symmetrical filters. However,

if the filter is symmetrical, then the symmetry bit in the CRCF

control register should be set. When this bit is set, only half the

impulse response needs to be programmed into the CRCF

coefficient memory registers. For example, if the number of

filter taps is equal to 15 or 16 and the filter is symmetric, then

only eight coefficients need to be written into the coefficient

memory. Because a total of 64 taps can be written into the

memory registers, the CRCF can perform 64 asymmetrical filter

taps or 128 symmetrical filter taps.

Coefficient Offset

More than one set of filter coefficients can be loaded into the

coefficient RAM at any time (given sufficient RAM space). The

coefficient offset can be used in this case to access the two or

more different filters. By changing the coefficient offset, the

filter coefficients being accessed can be changed on the fly. This

decimal offset value is programmed in the CRCF coefficient

offset register. When this value is changed during the calcula-

tion of a particular output data sample, the sample calculation is

completed using the old coefficients and the new coefficient

offset is brought into effect from the next data sample

calculation.

Decimation Phase

When more than one channel of the AD6636 is used to process

one carrier, polyphase implementation of the corresponding

channels’ DRCF or CRCF is possible using the decimation

phase feature. This feature can be used only under certain

conditions. The decimation phase is programmed using the

4-bit CRCF decimation phase word of the CRCF control

Maximum Number of Taps Calculated

The output rate of the CRCF filter is given by

CRCF

DRCF

CRCF

where:

fDRCF is the data rate out of the DRCF filter and into the CRCF

filter.

MCRCF is the decimation rate in the CRCF filter.

The CRCF filter consists of two multipliers (one each for the I

and Q paths). Each multiplier, working at the high speed clock

rate (PLL clock), can multiply (or tap once). Therefore, the

maximum number of filter taps that can be calculated

(symmetrical or asymmetrical filter) is given by

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

CRCF

PLLCLK

ceil

Taps

Number

Maximum

where:

fPLLCLK is the high speed internal processing clock generated by

the PLL clock multiplier.

fCRCF is the output rate of the CRCF filter as calculated

previously.

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