Datasheet
AD9148 Data Sheet
Rev. B | Page 54 of 72
Figure 69 shows the pass-band filter response for HB3. In most
applications, the usable bandwidth of the filter is limited by the
image suppression provided by the stop-band rejection and not
by the pass-band flatness. Table 24 shows the pass-band flatness
and stop-band rejection the HB3 filter supports at different
bandwidths.
0.02
0
–0.02
–0.04
–0.06
–0.08
–0.10
00.280.240.200.160.120.080.04
MAGNITUDE (dB)
(×
f
IN3
)
08910-068
Figure 69. Pass-Band Detail of HB3
Table 24. HB3 Pass-Band and Stop-Band Performance by
Bandwidth
Bandwidth (% of f
IN3
)
Pass-Band
Flatness (dB)
Stop-Band
Rejection (dB)
40 0.001 85
40.8 0.0014 80
42.4 0.002 70
45.6 0.0093 60
49.8 0.03 50
55.6 0.1 40
The maximum bandwidth can be achieved if the signal carrier
frequency is placed directly at the center of one of the filter
pass bands. In this case, the entire quadrature bandwidth of the
interpolation filter (0.8 × f
DATA
) is available. The available signal
bandwidth decreases as the carrier frequency of the signal moves
away from the center frequency of the filter. The worst-case
carrier frequency is one that falls directly between the center
frequency of two adjacent filters. Figure 70 shows how the
signal bandwidth changes as a function of placement in the
spectrum and interpolation rate.
0.4
0.3
0.2
0.1
COMPLEX BW (×
f
DAC
)
DC–1/2 1/2–3/8 3/8–1/4 1/4–1/8 1/8
f
C
(×
f
DAC
)
×2 MODE
×4 MODE
×8 MODE
CARRIER FREQUENCY
0.075
0.0375
0.15
0
08910-069
Figure 70. Complex Signal Bandwidth as a Function of Output Frequency
FINE MODULATION
The fine modulation makes use of a numerically controlled oscillator,
a phase shifter, and a complex modulator to provide a means for
modulating the signal by a programmable carrier signal. A block
diagram of the fine modulator is shown in Figure 71. The fine
modulator allows the signal to be placed anywhere in the output
spectrum with very fine frequency resolution.
INTERPOLATION
INTERPOLATION
NCO
1
0
–1
COSINE
SINE
I DATA
Q DATA
FTW[31:0]
SPECTRAL
INVERSION
OUT_I
OUT_Q
+
–
NCO PHASE OFFSET
WORD [15:0]
08910-070
Figure 71. Fine Modulator Block Diagram
The quadrature modulator is used to mix the carrier signal
generated by the NCO with the I and Q signal. The NCO produces
a quadrature carrier signal to translate the input signal to a new
center frequency. A complex carrier signal is a pair of sinusoidal
waveforms of the same frequency, offset 90° from each other. The
frequency of the complex carrier signal is set via the FTW[31:0]
value in Register 0x54 through Register 0x57.
The NCO operating frequency, f
NCO
, is at the DAC rate. The
frequency of the complex carrier signal can be set from dc up
to f
DAC
/2. The frequency tuning word (FTW) is calculated as
32
2
DAC
CENTER
f
f
FTW
The generated quadrature carrier signal is mixed with the I and Q
data. The quadrature products are then summed into the I and
Q data paths, as shown in Figure 71.