lt5558 Linear Technology Corporation, lt5558 Datasheet - Page 11

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

APPLICATIONS INFORMATION
The RF output S22 with no LO power applied is given in
Table 4.
Table 4. RF Port Output Impedance vs Frequency for EN = High
and No LO Power Applied
For EN = Low the S
To improve S
can be added to the RF output. At higher frequencies, a
shunt inductor can improve the S
equivalent circuit schematic of the RF output.
Table 5. RF Port Output Impedance vs Frequency for EN = Low
FREQUENCY
FREQUENCY
(MHz)(
Figure 5. Equivalent Circuit Schematic of the RF Output
(MHz)
1000
1100
1200
1000
1100
1200
500
600
700
800
900
500
600
700
800
900
22
for lower frequencies, a series capacitor
52Ω
OUTPUT IMPEDANCE (Ω)
OUTPUT IMPEDANCE (Ω)
22
is given in Table 5.
31.7 + j10.7
46.8 – j11.1
40.8 – j13.5
36.6 – j12.6
34.3 – j10.5
28.4 + j11.8
40.2 + j15.4
49.2 – j15.8
41.9 – j17.0
37.3 – j15.3
23.4 + j5.0
44.1 + j9.5
50.9 – j1.7
21.8 + j4.8
54.3 + j8.3
56.7 – j7.2
1pF
7nH
21pF
22
5558 F05
. Figure 5 shows the
V
CC
RF
OUTPUT
0.367
0.257
0.118
0.019
0.118
0.178
0.209
0.222
0.398
0.311
0.200
0.090
0.092
0.158
0.203
0.225
MAG
MAG
S
S
22
22
ANGLE
–115.5
–128.1
–139.0
ANGLE
165.5
142.0
116.1
–60.8
–99.3
–105.0
–120.0
166.5
142.9
112.9
–43.3
–83.8
58.1
Note that an ESD diode is connected internally from the
RF output to the ground. For strong output RF signal
levels (higher than 3dBm), this ESD diode can degrade
the linearity performance if an external 50Ω termination
impedance is connected directly to ground. To prevent this,
a coupling capacitor can be inserted in the RF output line.
This is strongly recommended during 1dB compression
measurements.
Enable Interface
Figure 6 shows a simplifi ed schematic of the EN pin inter-
face. The voltage necessary to turn on the LT5558 is 1V.
To disable (shut down) the chip, the enable voltage must
be below 0.5V. If the EN pin is not connected, the chip is
disabled. This EN = Low condition is guaranteed by the
75kΩ on-chip pull-down resistor.
It is important that the voltage at the EN pin does not
exceed V
full-chip supply current could be sourced through the EN
pin ESD protection diodes, which are not designed for this
purpose. Damage to the chip may result.
Evaluation Board
Figure 7 shows the evaluation board schematic. A good
ground connection is required for the LT5558’s Exposed
Pad. If this is not done properly, the RF performance will
degrade. Additionally, the Exposed Pad provides heat sink-
ing for the part and minimizes the possibility of the chip
overheating. R1 (optional) limits the EN pin current in the
event that the EN pin is pulled high while the V
are low. The application board PCB layouts are shown in
Figures 8 and 9.
CC
by more than 0.5V. If this should occur, the
EN
Figure 6. EN Pin Interface
V
CC
75k
25k
5558 F06
LT5558
CC
11
inputs
5558fa