Datasheet

AD9122
Rev. B | Page 41 of 60
DATAPATH CONFIGURATION EXAMPLES
8× Interpolation Without NCO
For this example, the following parameters are given:
•
f
DATA
= 100 MSPS
•
8× interpolation
•
f
BW
= 75 MHz
•
f
CENTER
= 100 MHz
The desired 75 MHz of bandwidth is 75% of f
DATA
. In this case,
the ratio of f
OUT
/f
DATA
= 100/100 = 1.0. From Figure 59, the band-
width supported at f
DATA
is 0.8, which verifies that the AD9122
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
.
Choosing the third row (highlighted by the superscripted
number 2) of the Interpolation Factor column from Tabl e 20
selects filter modes that give 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 one-half into each consecutive stage. The
output signal band spans 0.15 to 0.35 of f
IN3
(400 MHz). There-
fore, 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
For this example, the following parameters are given:
•
f
DATA
= 250 MSPS
•
4× interpolation
•
f
BW
= 140 MHz
•
f
CENTER
= 175 MHz
The desired 140 MHz of bandwidth is 56% of f
DATA
. 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. This verifies that the AD9122 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
. Choosing the second row in the Interpolation Factor
column in the 4× interpolation section of Table 2 0 selects the filter
modes that give a center frequency of f
DATA
/2 with 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 as follows:
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
08281-030
Figure 60. Signal Propagation for 8× Interpolation (f
DATA
Modulation)