AD6654/PCB Analog Devices Inc, AD6654/PCB Datasheet - Page 47

• Manufacturer Part Number: AD6654/PCB
• Description: BOARD EVALUATION FOR AD6654
• Manufacturer: Analog Devices Inc

Specifications of AD6654/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6554
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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
C1, C11
C3, C9
C5, C7
C6
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.
Normalized
Coefficient
0.02734375
−0.12890625
0.603515625
1
Decimal Coefficient
(10-Bit)
14
−66
309
512
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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.
–100
–20
–40
–60
–80
0
0
Figure 54. Interpolating Half-Band Frequency Response
FILTER RESPONSE
INTERPOLATING
0.2
HALF-BAND
FREQUENCY AS FRACTION OF INPUT RATE
0.4
0.6
0.8
0.75
1.0
1.2
1.25
1.4
1.6
1.8
AD6654
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