lt5558 Linear Technology Corporation, lt5558 Datasheet - Page 14

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

APPLICATIONS INFORMATION
LT5558
The common-mode voltage on the LT5558 is sampled
using resistors R7, R8, R17 and R18 and shifted up to
about 2.5V using resistor R9. Op amp U4 compensates
for the gain loss in the resistor networks and provides a
low-ohmic drive to steer the common-mode input pins
of U2 and U3. Resistors R20 and R21 improve op amp
14
0.1µF
Figure 14. LO Feedthrough and Image Rejection vs
Baseband Drive Voltage After Calibration at 25°C
2.5V
C3
R20
249Ω
SOURCE
BB
DC
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
0
1
2
3
4
0
+IN
–IN
GND
V–
85°C
–40°C
I AND Q BASEBAND VOLTAGE (V
LTC1565-31
Figure 15. Baseband Interface Schematic of the LTC1565 with the LT5558 for RFID applications.
U2
1
SHDN
85°C
+OUT
–OUT
25°C
V
V+
CC
2
8
7
6
5
–40°C
–40°C
25°C
2.5V
R5
3.57k
3
DC
499Ω
499Ω
P
R1
R2
RF
IR
R6
3.57k
P-P,DIFF
4
C1, C2
2 × 0.1µF
LOFT
R3
3.01k
R4
3.01k
2.1V
2.1V
5558 F14
88.7k
)
R9
DC
DC
5
V
EN = HIGH
f
f
f
f
P
LO
BBI
BBQ
RF
CC
LO
= 900MHz,
= f
= 5V
= 2MHz, 0°
= 0dBm
= 2MHz, 90°
R7
49.9k
R8
49.9k
2, 4, 6, 9, 10
BB
12, 15, 17
+ f
R22
22.1k
R22
22.1k
14
16
8, 13
LO
BBPI
BBMI
GND
V
CC
11
LT5558
RF
U1
BBMQ
BBPQ
1
U4’s stability while driving the large supply decoupling
capacitors C3 and C4. This corrected common-mode
voltage is applied to the common-mode input pins of U2
and U3 (pins 3). This results in a positive feedback loop
for the common mode voltage with a loop gain of about
-10dB. This technique ensures that the current compliance
on the baseband input pins of the LT5558 is not exceeded
under supply voltage or temperature extremes, and internal
diode voltage shifts or combinations of these. The core
current of the LT5558 is thus maintained at its designed
level for optimum performance. The recommended com-
mon-mode voltage applied to the inputs of the LTC1565
is about 2V. Resistor tolerances are recommended 1%
accuracy or better. The total current consumption is about
160mA and the noise fl oor at 20MHz offset is –147dBm/Hz
with 3.7dBm RF output power. For a 2V
input swing, the output power at f
and the third harmonic at f
2.6V
and the third harmonic at f
EN
LO
3
7
5
RF = 3dBm MAX
3.32k
PP, DIFF
R24
49.9k
49.9k
R17
R18
4
3
–
+
U4
2
5
2.1V
2.1V
input, the output power at f
1
3.01k
3.01k
DC
DC
R13
R14
LT1797
3.57k
R16
499Ω
R11
499Ω
R12
3.57k
2.5V
R15
DC
LO
LO
– 3f
– 3f
8
7
6
5
4.5V to 5.25V
+OUT
–OUT
V+
SHDN
BB
LTC1565-31
BB
is –40.5dBm.
LO
U3
is –48.6dBm. For a
LO
+ f
PP, DIFF
GND
+IN
–IN
V–
+ f
BB
1
2
3
4
BB
SOURCE
is 1.6dBm
is 3.8dBm
2.5V
baseband
BB
249Ω
R21
5558 F16
C4
0.1µF
DC
5558fa