Datasheet

AD9122
Rev. B | Page 36 of 60
DIGITAL DATAPATH
The block diagram in Figure 50 shows the functionality of the
digital datapath. The digital processing includes a premodulation
block, three half-band (HB) interpolation filters, a quadrature
modulator with a fine resolution NCO, phase and offset adjust-
ment blocks, and an inverse sinc filter.
PREMOD
PHASE
AND
OFFSET
ADJUST
HB1 HB2 HB3
SINC
–1
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Figure 50. Block Diagram of Digital Datapath
The digital datapath accepts I and Q data streams and processes
them as a quadrature data stream. The signal processing blocks can
be used when the input data stream is represented as complex data.
The digital datapath can also be used to process an input data
stream representing two independent real data streams, but the
functionality is somewhat restricted. The premodulation block
and any of the nonshifted interpolation filter modes can be used
for an input data stream representing two independent real data
streams. See the Coarse Modulation Mixing Sequences section
for more information.
PREMODULATION
The half-band interpolation filters have selectable pass bands
that allow the center frequencies to be moved in increments of
one-half their input data rate. The premodulation block provides
a digital upconversion of the incoming waveform by one-half the
incoming data rate, f
DATA
. This can be used to frequency-shift base-
band input data to the center of the interpolation filter pass band.
INTERPOLATION FILTERS
The transmit path contains three interpolation filters. Each of the
three interpolation filters provides a 2× increase in output data
rate. The half-band (HB) filters can be individually bypassed or
cascaded to provide 1×, 2×, 4×, or 8× interpolation ratios. Each
half-band filter stage 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 described 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.
Half-Band Filter 1 (HB1)
HB1 has four modes of operation, as shown in Figure 51. The
shape of the filter response is identical in each of the four modes.
The four modes are distinguished by two factors: the filter center
frequency and whether the input signal is modulated by the filter.
0
–20
–40
–60
–80
–100
021.81.61.41.21.00.80.60.40.2
MAGNITUDE (dB)
FREQUENCY (× f
IN1
) (Hz)
.0
MODE 0
MODE 1 MODE 3
MODE 2
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Figure 51. HB1 Filter Modes
As shown in Figure 51, the center frequency in each mode is
offset by one-half the input data rate (f
IN1
) of the filter. Mode 0
and Mode 1 do not modulate the input signal. Mode 2 and
Mode 3 modulate the input signal by f
IN1
. When operating in
Mode 0 and Mode 2, the I and Q paths operate independently
and no mixing of the data between channels occurs. When oper-
ating in Mode 1 and Mode 3, mixing of the data between the
I and Q paths occurs; therefore, the data input into the filter is
assumed to be complex. Table 15 summarizes the HB1 modes.
Table 15. HB1 Filter Modes
Mode f
CENTER
f
MOD
Input Data
0 DC None Real or complex
1 f
IN
/2 None Complex
2 f
IN
f
IN
Real or complex
3 3f
IN
/2 f
IN
Complex