LT5528 LINER [Linear Technology], LT5528 Datasheet - Page 9

• Manufacturer Part Number: LT5528
• Description: 1.5GHz to 2.4GHz High Linearity Direct Quadrature Modulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
and 12pF to ground is incorporated (see Figure 1), which
limits the baseband bandwidth to approximately 330MHz
(–1dB point). The common-mode voltage is about 0.52V
and is approximately constant over temperature.
It is important that the applied common-mode voltage level
of the I and Q inputs is about 0.52V in order to properly
bias the LT5528. Some I/Q test generators allow setting
the common-mode voltage independently. In this case, the
common-mode voltage of those generators must be set
to 0.26V to match the LT5528 internal bias, because for
DC signals, there is no –6dB source-load voltage division
(see Figure 2).
+
–
Figure 2. DC Voltage Levels for a Generator Programmed at
0.26V
GENERATOR
0.52V
50Ω
DC
DAC
DC
for a 50Ω Load and the LT5528 as a Load
0.26V
0mA TO 20mA
0mA TO 20mA
Figure 3. LT5528 5
50Ω
DC
U
GND
U
–
+
0.5V
0.5V
GENERATOR
0.52V
R5, 50Ω
R6, 50Ω
50Ω
th
Order Filtered Baseband Interface with Common DAC (Only I-Channel is Shown)
DC
W
C1
0.52V
DC
L1A
L1B
C2
5528 F02
LT5528
U
0.52V
45Ω
DC
L2A
L2B
C3
+
–
It is recommended that the part be driven differentially;
otherwise, the even-order distortion products will de-
grade the overall linearity severely. Typically, a DAC will
be the signal source for the LT5528. To prevent aliasing,
a fi lter should be placed between the DAC output and the
LT5528’s baseband inputs. In Figure 3, an example interface
schematic shows a commonly used DAC output interface
followed by a passive 5
this example sources a current from 0mA to 20mA. The
interface may be DC coupled. This allows adjustment of
the DAC’s differential output current to minimize the LO
feed-through. Optionally, transformer T1 can be inserted
to improve the current balance in the BBPI and BBMI pins.
This will improve the second-order distortion performance
(OIP2).
The maximum single sideband CW RF output power at
2GHz using 20mA drive to both I and Q channels with the
confi guration shown in Figure 3 is about –2.5dBm. The
maximum CW output power can be increased by con-
necting resistors R5 and R6 to –5V instead of GND, and
changing their values to 550Ω. In that case, the maximum
single sideband CW RF output power at 2GHz will be about
2.3dBm. In addition, the ladder fi lter component values
require adjustment for a higher source impedance.
RF = –2.5dBm, MAX
OPTIONAL
1:1
T1
BBMI
BBPI
C
45Ω
R1
BALUN
th
R3
order ladder fi lter. The DAC in
V
REF
LOMI
GND
CM
= 0.52V
V
CC
= 5V
R4
45Ω
R2
LT5528
LT5528
LOPI
5528 F03
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