Datasheet

Data Sheet AD9148
Rev. B | Page 51 of 72
Table 18. Programmable Inverse Sinc Filter Coefficient Widths and Ranges
Coefficient Width Minimum Maximum
c
0
in-phase (real) 3 100b 011b
−4 3
c
0
quadrature (imaginary) 3 0100b 011b
−4 3
c
1
in-phase (real) 4 1000b 0111b
−8 7
c
1
quadrature (imaginary) 4 1000b 0111b
−8 7
c
2
in-phase (real) 5 10000b 01111b
−16
15
c
2
quadrature (imaginary) 5 10000b 01111b
−16 15
c
3
in-phase (real) 7 1000000b 0111111b
−64 63
c
3
quadrature (imaginary) 7 1000000b 0111111b
−64
63
c
4
in-phase (real) 10 1000000000b 0111111111b
−1024 1023
c
4
quadrature (imaginary) 10 1000000000b 0111111111b
−1024 1023
The real and imaginary filters are implemented using the
structure described in Figure 62 and Figure 63.
INPUT
n
OUTPUT
n
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
+
c
0REAL
c
1REAL
c
2REAL
c
3REAL
c
4REAL
c
5REAL
+
+
+
+
+
+
+
+
+
+
+
+
++
+
+
+
+
+
08910-061
Figure 62. Real Filter implementation
INPUT
n
OUTPUT
n
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
z
–1
+
–
c
0IMG
c
1IMG
c
2IMG
c
3IMG
c
4IMG
c
5IMG
+
–
+
+
+
–
+
+
+
–
+
+
+
–
+
+
+
+
08910-062
Figure 63. Imaginary Filter implementation
The AD9148 evaluation tools provide software that allows for the
processing of the filter coefficients based on the DAC sampling
frequency, the amount of interpolation used (combination of
HB1, HB2, and HB3), and the desired center frequency. This
center frequency is limited to
[−0.4 × f
DAC
+ 0.5 × signalBW, 0.4 × f
DAC
− 0.5 × signalBW]
The bandwidth of the inverse sinc filter equals the maximum
allowable signal bandwidth of the interpolation filters (0.8 × f
DATA
).
When there is no interpolation used, the real filter coefficients
can be fixed at (no imaginary coefficients)
C
0
= 2
;
C
8
= 2
C
1
= −4
;
C
7
= −4
C
2
= 10
;
C
6
= 10
C
3
= −35
;
C
5
= −35
C
4
= 401
INTERPOLATION FILTERS
The transmit path contains three interpolation filters. Each of
the three interpolation filters provides a 2× increase in output
data rate. The filters can be cascaded to provide 2×, 4×, or 8×
interpolation ratios. Each of the half-band filter stages offers a
different combination of bandwidths and operating modes.
The bandwidth of the three half-band filters with respect to the
data rate at the filter input is as follows:
• Bandwidth of HB1 = 0.8 × f
IN1
• Bandwidth of HB2 = 0.5 × f
IN2
• Bandwidth of HB3 = 0.4 × f
IN3
The usable bandwidth is defined as the frequency over which
the filters have a pass-band ripple of less than ±0.001 dB and an
image rejection of greater than +85 dB. As is discussed in the
Half-Band Filter 1 (HB1) section, the image rejection usually sets
the usable bandwidth of the filter, not the pass-band flatness.
The half-band filters operate in several modes, providing
programmable pass-band center frequencies as well as signal
modulation. The HB1 filter has four modes of operation, and
the HB2 and HB3 filters each have eight modes of operation.