AD6654SLASHPCB datasheet(47/88 Pages) AD

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AD6654

Rev. 0 | Page 47 of 88

INTERPOLATING HALF-BAND FILTER

The AD6654 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.

Consequently, 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 23. Other coefficients are 0.

Table 23. 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.

The image rejection of this filter is about 55 dB, but is sufficient

because the image is from the desired signal, not an interfering

signal. Note that the interpolating half-band filter can be

enabled by writing Logic 1 to Bit 9 of the MRCF control

registers.

The frequency response of the interpolating half-band FIR is

shown in Table 23 with respect to the chip rate. The input rate

to this filter is 2× the chip rate, and the output rate is 4× the

chip rate.

FREQUENCY AS FRACTION OF INPUT RATE

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

–20

–40

–60

–80

–100

–53

0.75

1.25

INTERPOLATING

HALF-BAND

FILTER RESPONSE

Figure 54. Interpolating Half-Band Frequency Response

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