LT5568 데이터 시트보기 (PDF) - Linear Technology
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LT5568 Datasheet PDF : 16 Pages
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LT5568
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 0.54V
and is approximately constant over temperature.
It is important that the applied common mode voltage level
of the I and Q inputs is about 0.54V in order to properly
bias the LT5568. 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 0.27V to match the LT5568 internal bias, because for
DC signals, there is no –6dB source-load voltage division
(see Figure 2).
50Ω
–+ 0.54VDC
GENERATOR
0.27VDC
50Ω
50Ω
48Ω
–+ 0.54VDC
GENERATOR
0.54VDC
0.54VDC –+
LT5568
5568 F02
Figure 2. DC Voltage Levels for a Generator Programmed at
0.27VDC for a 50Ω Load and the LT5568 as a Load
The baseband inputs should be driven differentially; other-
wise, the even-order distortion products will degrade the
overall linearity severely. Typically, a DAC will be the signal
source for the LT5568. Reconstruction filters should be
placed between the DAC output and the LT5568’s baseband
inputs. In Figure 3, an example interface schematic shows a
commonly used DAC output interface followed by a passive
5th order ladder filter. The DAC in 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 feedthrough. Optionally,
transformer T1 can be inserted to improve the current
balance in the BBPI and BBMI pins. This will improve the
2nd order distortion performance (OIP2).
The maximum single sideband CW RF output power at
850MHz using both I and Q channels with the configura-
tion shown in Figure 3 is about –3dBm. The maximum
CW output power can be increased by connecting load
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 850MHz will be about
+2dBm. In addition, the ladder filter component values
require adjustment for a higher source impedance.
VCC = 5V
RF = –3dBm, MAX
BALUN
C
LOMI
LT5568
LOPI
0.5V
0mA to 20mA
R5, 50Ω
DAC
C1
0mA to 20mA GND R6, 50Ω
L1A
C2
L1B
L2A
C3
L2B
BBPI
T1
1:1
BBMI
R1
45Ω
CM
R3
33Ω
VREF = 500mV
R2
45Ω
R4
33Ω
15mA
0.5V
GND
5568 F03
Figure 3. LT5568 5th Order Filtered Baseband Interface with Common DAC (Only I-Channel is Shown)
5568f
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