LT5558 Просмотр технического описания (PDF) - Linear Technology
Номер в каталоге
Компоненты Описание
Список матч
LT5558 Datasheet PDF : 16 Pages
| |||
LT5558
APPLICATIONS INFORMATION
RF
VCC = 5V
C
BALUN
LOMI
LT5558
FROM Q
LOPI
BBPI
BBMI
200
1.3k
1.8P
1.3k
1.8P
200
VREF = 0.5V
CM
GND
5558 F01
Figure 1. Simplifed Circuit Schematic of the LT5558
(Only I-Half is Drawn)
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
filter 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
VCC
BB
SOURCE
4.5V TO 5.25V
C5
RF OUT
8, 13 11 1
C1
VCC RF EN
C3
14 BBPI BBPQ 7
2.1VDC
2.1VDC
LT5558
C2
C4
16 BBMI BBMQ 5
2.1VDC
2.1VDC
GND LO
BB
SOURCE
2, 4, 6, 9, 10,
3
12, 15, 17
5558 F03
Figure 3. AC-Coupled Baseband Interface
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–3dB = 1/(π • RIN,DIFF • C1) is much
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 amplifier 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
GENERATOR
50Ω 1.05VCC
50Ω
+– 2.1VDC
GENERATOR
LT5558
50Ω 2.1VDC 1.5kΩ
+– 2.1VDC
2.1VDC +–
5558 F02
Figure 2. DC Voltage Levels for a Generator Programmed at
1.05VDC for a 50Ω Load and the LT5558 as a Load
VCC
20pF
LO
INPUT
≈ 50Ω
5558 F04
Figure 4. Equivalent Circuit Schematic of the LO Input
5558fa
9
Share Link: 