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

90
AUX1_P
500Ω
AD9779A
250Ω
93
OUT1_P
RBIP
82pF
50Ω
C1I
RBIN
92
50Ω
OUT1_N
82pF
250Ω
C3I
89
AUX1_N
500Ω
87
AUX2_N
500Ω
250Ω
84
OUT2_N
RBQN
82pF
50Ω
C1Q
RBQP
83
50Ω
OUT2_P
82pF
250Ω
C3Q
86
AUX2_P
500Ω
Figure 79. Typical Use of Auxiliary DACs AC Coupling to
Quadrature Modulator

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 78. To achieve LO feedthrough
compensation in a circuit, each of four outputs of these auxiliary
DACs can be connected through a 500 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 of the quadrature modulator signal inputs.
To achieve LO feedthrough compensation, the user should start
with the default conditions of the auxiliary DAC sign registers,
and then increment the magnitude of one or the other auxiliary
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 auxiliary DAC being adjusted or
adjusting the output current of the other auxiliary DAC. It may
take practice before an effective algorithm is achieved.
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.
LPI
390nH
21
IBBP
RSLI
39pF
100Ω
C2I
22
IBBN
LNI
390nH
LNQ
390nH
9
QBBN
39pF
RSLQ
C2Q
100Ω
10
QBBP
LPQ
390nH

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-
rature demodulator before gain and offset correction. Figure 81
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.
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 2.1 GHz,
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,
Rev. B | Page 45 of 56
AD9776A/AD9778A/AD9779A
RBW 3kHz REF ATT
VBW 3kHz MIXER
SWT 56s UNIT
20MHz
No Gain or LO Compensation
RBW 20kHz REF ATT
VBW 20kHz MIXER
SWT 1.25s UNIT
20MHz
Gain and LO Compensation Optimized
30dB
–40dBm
dBm
SPAN 200MHz
20dB
–40dBm
dBm
SPAN 200MHz