Lo Feedthrough Compensation; Results Of Gain And Offset Correction - Analog Devices AD9776A Manual

0.1µF
AD9779A
I DAC
0.1µF
25Ω TO 50Ω
Figure 78. Typical Use of Auxiliary DACs AC Coupling to
AD9779A
I OR Q DAC
25Ω TO 50Ω
Figure 79. Typical Use of Auxiliary DACs DC Coupling to Quadrature

LO FEEDTHROUGH COMPENSATION

The LO feedthrough compensation is the most complex of all
three operations. This is due to the structure of the offset aux-
iliary DACs, as shown in Figure 77. To achieve LO feedthrough
compensation in a circuit, each of four outputs of these AUX
DACs must be connected through a 50 Ω resistor to ground
and through a 250 Ω resistor to one of the four quadrature
modulator signal inputs. The purpose of these connections is
to drive a very small amount of current into the nodes at the
quadrature modulator inputs, therefore adding a slight dc bias
to one or the other of the quadrature modulator signal inputs.
This can be seen in the schematics for the AD9776A/AD9778A/
AD9779A evaluation board (see Figure 106).
To achieve LO feedthrough compensation, the user should start
with the default conditions of the AUX DAC sign registers, and
then increment the magnitude of one or the other AUX DAC
output currents. While this is being done, the amplitude of the
LO feedthrough at the quadrature modulator output should be
sensed. If the LO feedthrough amplitude increases, try either
changing the sign of the AUX DAC being adjusted, or adjusting
the output current of the other AUX DAC. It may take practice
before an effective algorithm is achieved.
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QUADRATURE
MODULATOR V+
AD9779A
AUX
DAC1
QUADRATURE
MODULATOR V+
OPTIONAL
QUAD MOD
PASSIVE
I INPUTS
FILTERING
0.1µF
OPTIONAL
AD9779A
PASSIVE
Q DAC
FILTERING
0.1µF
25Ω TO 50Ω
Quadrature Modulator
QUADRATURE
MODULATOR V+
AD9779A
AUX DAC1
QUAD MOD
OR
I OR Q INPUTS
AUX DAC2
OPTIONAL
PASSIVE
FILTERING
25Ω TO 50Ω
Modulator with DC Shift
Using the AD9776A/AD9778A/AD9779A evaluation board, the
LO feedthrough can typically be adjusted down to the noise
floor, although this is not stable over temperature.

RESULTS OF GAIN AND OFFSET CORRECTION

The results of gain and offset correction can be seen in Figure 80
and Figure 81. Figure 80 shows the output spectrum of the quad-
AD9779A
AUX
rature demodulator before gain and offset correction. Figure 81
DAC2
shows the output spectrum after correction. The LO feedthrough
spur at 2.1 GHz has been suppressed to the noise level. This
result can be achieved by applying the correction, but the correc-
tion needs to be repeated after a large change in temperature.
QUAD MOD
Q INPUTS
Note that the gain matching improved the negative frequency
image rejection, but there is still a significant image present.
The remaining image is now due to phase mismatch in the
quadrature modulator. Phase mismatch can be distinguished
from gain mismatch by the shape of the image. Note that the
image in Figure 80 is relatively flat and the image in Figure 81
slopes down with frequency. Phase mismatch is frequency
dependent, so an image dominated by phase mismatch has
this sloping characteristic.
REF LVL
0dBm
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
CENTER 2.1GHz
Figure 80. AD9779A and ADL5372 with a Multitone Signal at 2.1GHz, No
REF LVL
0dBm
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
CENTER 2.1GHz
Figure 81. AD9779A and ADL5372 with a Multitone Signal at 2.1 GHz, Gain
Rev. A | Page 41 of 60
AD9776A/AD9778A/AD9779A
RBW 3kHz REF ATT
VBW 3kHz MIXER
SWT 56s UNIT
20MHz SPAN 200MHz
Gain or LO Compensation
RBW 20kHz REF ATT
VBW 20kHz MIXER
SWT 1.25s UNIT
20MHz SPAN 200MHz
and LO Compensation Optimized
30dB
–40dBm
dBm
20dB
–40dBm
dBm