AD9125 Analog Devices, AD9125 Datasheet - Page 35

• Manufacturer Part Number: AD9125
• Manufacturer: Analog Devices

Specifications of AD9125

• Resolution (bits): 16bit
• Dac Update Rate: 1GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

NCO MODULATION
The digital quadrature modulator 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 digital modulator is shown in
Figure 57. The fine modulation provided by the digital modulator,
in conjunction with the coarse modulation of the interpolation
filters and premodulation block, allows the signal to be placed
anywhere in the output spectrum with very fine frequency
resolution.
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 FTW[31:0]
in Register 0x30 through Register 0x33.
The NCO operating frequency, f
bypassed) or twice f
the complex carrier signal can be set from dc up to f
frequency tuning word (FTW) is calculated as
The generated quadrature carrier signal is mixed with the I and
Q data. The quadrature products are then summed into the I
and Q datapaths, as shown in Figure 57.
Updating the Frequency Tuning Word
The frequency tuning word registers do not update immediately
upon writing as other configuration registers. After loading the
FTW registers with the desired values, Bit 0 of Register 0x36
must transition from 0 to 1 for the new FTW to take effect.
Q DATA
I DATA
NCO PHASE OFFSET
FTW
Figure 57. Digital Quadrature Modulator Block Diagram
=
INTERPOLATION
INTERPOLATION
FTW[31:0]
f
CARRIER
f
NCO
INVERSION
SPECTRAL
[15:0]
DATA
×
2
–1
(HB1 enabled). The frequency of
32
COSINE
NCO
SINE
0
NCO
1
, is at either f
–
+
DATA
NCO
(HB1
I OUTPUT
Q OUTPUT
. The
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DATAPATH CONFIGURATION
Configuring the AD9125 datapath starts with the application
requirements of the input data rate, the interpolation ratio, the
output signal bandwidth, and the output signal center frequency.
Given these four parameters, the first step in configuring the
datapath is to verify that the device supports the bandwidth
requirements. The modes of the interpolation filters are then
chosen. Finally, any additional frequency offset requirements
are determined and applied with premodulation and NCO
modulation.
Determining Datapath Signal Bandwidth
The available signal bandwidth of the datapath is dependent on
the center frequency of the output signal in relation to the center
frequency of the interpolation filters used. Signal center frequencies
that are offset from the center frequencies of the half-band
filters lower the available signal bandwidth.
When correctly configured, the available complex signal band-
width for 2× interpolation is always 80% of the input data rate.
The available signal bandwidth for 4× interpolation vs. output
frequency varies between 50% and 80% of the input data rate,
as shown in Figure 58. Note that in 4× interpolation mode,
f
four times from dc to f
Configuring 4× interpolation using the HB2 and HB3 filters
can lower the power consumption of the device at the expense
of reduced bandwidth. The lower curve in Figure 58 shows that the
supported bandwidth in this mode varies from 30% to 50% of f
The available signal bandwidth for 8× interpolation vs. output
frequency varies between 50% and 80% of the input data rate,
as shown in Figure 59. Note that in 8× interpolation mode,
f
eight times from dc to f
DAC
DAC
Figure 58. Signal Bandwidth vs. Center Frequency of the Output Signal,
= 4 × f
= 8 × f
0.8
0.5
0.3
DATA
DATA
HB1 AND HB2
; therefore, the data shown in Figure 58 repeats
; therefore, the data shown in Figure 59 repeats
0.2
DAC
DAC
4× Interpolation
.
0.4
.
HB2 AND HB3
f
OUT
/
f
DATA
0.6
0.8
AD9125
1.0
DATA
.