Datasheet

AD9125
Rev. 0 | Page 37 of 56
DATAPATH CONFIGURATION EXAMPLE
8× Interpolation Without NCO
Given the following conditions, the desired 75 MHz of
bandwidth is 75% of f
DATA
:
•
f
DATA
= 100 MSPS
•
8× interpolation
•
f
BW
= 75 MHz
•
f
CENTER
= 100 MHz
In this case, the ratio of f
OUT
/f
DATA
= 100/100 = 1.0. From Figure 59,
the bandwidth supported at f
DATA
is 0.8, which verifies that the
AD9125 supports the bandwidth required in this configuration.
The signal center frequency is f
DATA
, and assuming the input
signal is at baseband, the frequency shift required is also f
DATA
.
Using the settings detailed in the third row of the IF column
from Table 21 (these settings use the configuration in the 8×
interpolation without NCO example) selects filter modes that
result in a center frequency of f
DATA
and a frequency translation
of f
DATA
. The selected modes for the three half-band filters are
HB1, Mode 2; HB2, Mode 2; and HB3, Mode 1. Figure 60 shows
how the signal propagates through the interpolation filters.
Because 2 × f
IN1
= f
IN2
and 2 × f
IN2
= f
IN3
, the signal appears
frequency scaled by ½ into each consecutive stage. The output
signal band spans 0.15 to 0.35 of f
IN3
(400 MHz). Therefore,
the output frequency supported is 60 MHz to 140 MHz, which
covers the 75 MHz bandwidth centered at 100 MHz, as desired.
4× Interpolation with NCO
Given the following conditions, the desired 140 MHz of
bandwidth is 56% of f
DATA
:
•
f
DATA
= 250 MSPS
•
4× interpolation
•
f
BW
= 140 MHz
•
f
CENTER
= 175 MHz
As shown in Figure 58, the value at 0.7 × f
DATA
is 0.6. This is
calculated as 0.8 − 2(0.7 − 0.6) = 0.6. Therefore, the AD9125
supports a bandwidth of 60% of f
DATA
, which exceeds the
required 56%.
The signal center frequency is 0.7 × f
DATA
, and assuming the
input signal is at baseband, the frequency shift required is also
0.7 × f
DATA
. Using the settings detailed in the second row in the IF
column in the 4× interpolation section in Table 21 selects the
filter modes that give a center frequency of f
DATA
/2 and no
frequency translation. The selected modes for the three half-
band filters are HB1, Mode 1; HB2, Mode 1; and HB3,
bypassed.
Because Mode 1 of HB1 was selected, the premodulation block
should be enabled. This provides f
DATA
/2 modulation, which
centers the baseband input data at the center frequency of HB1.
The digital modulator can be used to provide the final frequency
translation of 0.2 × f
DATA
to place the output signal at 0.7 × f
DATA
,
as desired.
The formula for calculating the FTW of the NCO is
32
2×=
NCO
CARRIER
f
f
FTW
where:
f
CARRIER
= 0.2 × f
DATA
.
f
NCO
= 2 × f
DATA
. Therefore, FTW = 2
32
/10.
0
1
2
3
0
–0.5 0.5
HB1
0.1 0.4 0.6
1.5 2.0
×
f
IN1
1.00
–0.5 0.5
HB3
0.2–0.2 0.3 0.7
0.15 0.35
1.5 2.0
×
f
IN3
1.00
753
4
0
62
1
–0.5 0.5
HB2
0.25 0.75
0.3 0.7
1.25 1.75
1.5 2.0
×
f
IN2
1.00
753
6
40
1
2
09016-030
Figure 60. Signal Propagation for 8× Interpolation (f
DATA
Modulation)