lt5558 Linear Technology Corporation, lt5558 Datasheet - Page 9

• Manufacturer Part Number: lt5558
• Description: 600mhz To 1100mhz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
If the I/Q signals are DC-coupled to the LT5558, it is
important that the applied common-mode voltage level
of the I and Q inputs is about 2.1V in order to properly
bias the LT5558. Some I/Q generators allow setting the
common-mode voltage independently. In this case, the
common-mode voltage of those generators must be set to
1.05V to match the LT5558 internal bias where the internal
DC voltage of the signal generators is set to 2.1V due to
the source-load voltage division (See Figure 2).
The LT5558 baseband inputs should be driven differen-
tially, otherwise, the even-order distortion products will
degrade the overall linearity severely. Typically, a DAC
will be the signal source for the LT5558. A pulse-shaping
fi lter should be placed between the DAC outputs and the
LT5558’s baseband inputs.
An AC-coupled baseband interface with the LT5558 is
drawn in Figure 3. Capacitors C1 to C4 will introduce a
V
CC
Figure 2. DC Voltage Levels for a Generator Programmed at
1.05VDC for a 50Ω Load and the LT5558 as a Load
BBMI
BBPI
= 5V
RF
Figure 1. Simplifed Circuit Schematic of the LT5558
(Only I-Half is Drawn)
GENERATOR
+
–
2.1V
50Ω
200
DC
200
1.05V
CC
1.8P
1.8P
50Ω
1.3k
1.3k
GENERATOR
+
–
C
2.1V
50Ω
DC
BALUN
2.1V
V
REF
LOMI
DC
CM
= 0.5V
1.5kΩ
2.1V
LT5558
GND
DC
+
–
5558 F02
LT5558
FROM Q
LOPI
5558 F01
low-frequency high-pass corner together with the LT5558’s
differential input impedance of 3kΩ. Usually, capacitors
C1 to C4 will be chosen equal and in such a way that the
–3dB corner frequency f
lower than the lowest baseband frequency.
DC coupling between the DAC outputs and the LT5558
baseband inputs is recommended, because AC coupling
will introduce a low-frequency time constant that may affect
the signal integrity. Active level shifters may be required to
adapt the common mode level of the DAC outputs to the
common mode input voltage of the LT5558. Such circuits
may, however, suffer degraded LO leakage performance
as small DC offsets and variations over temperature
accumulate. A better scheme is shown in Figure 16, where
feedback is used to track out these variations.
LO Section
The internal LO input amplifi er performs single-ended to
differential conversion of the LO input signal. Figure 4
shows the equivalent circuit schematic of the LO input.
The internal, differential LO signal is split into in-phase
and quadrature (90° phase shifted) signals that drive
LO buffer sections. These buffers drive the double bal-
anced I and Q mixers. The phase relationship between
Figure 4. Equivalent Circuit Schematic of the LO Input
SOURCE
Figure 3. AC-Coupled Baseband Interface
BB
V
CC
C1
C2
2, 4, 6, 9, 10,
INPUT
2.1V
12, 15, 17
2.1V
LO
C5
DC
14
16
DC
8, 13 11
V
–3dB
CC
BBPI
BBMI BBMQ
5558 F04
GND
V
CC
LT5558
RF
20pF
= 1/(π • R
BBPQ
1
EN
LO
3
2.1V
7
2.1V
5
4.5V TO 5.25V
≈ 50Ω
DC
DC
RF OUT
C3
C4
IN
,
DIFF
SOURCE
LT5558
BB
• C1) is much
5558 F03
5558fa
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