LT5571 데이터 시트보기 (PDF) - Linear Technology
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LT5571 Datasheet PDF : 16 Pages
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LT5571
APPLICATIONS INFORMATION
Table 3. LO Port Input Impedance vs Frequency for EN = Low
and PLO = 0dBm
FREQUENCY INPUT IMPEDANCE
S11
(MHz)
(Ω)
Mag
Angle
500
35.6 + j42.1
0.467
83
600
65.5 + j70.1
0.531
46
700
163 + j76.3
0.602
14
800
188 – j95.2
0.654
–13
900
72.9 – j114
0.692
–36
1000
34.3 – j83.5
0.715
–56
1100
21.6 – j63.3
0.726
–73
1200
16.4 – j50.5
0.727
–86
For EN = Low the S22 is given in Table 6.
Table 6. RF Port Output Impedance vs Frequency for EN = Low
FREQUENCY
(MHz)
OUTPUT IMPEDANCE
(Ω)
S22
Mag
Angle
500
21.5 + j5.0
0.403
166
600
26.9 + j11.8
0.333
144
700
36.5 + j16.0
0.239
120
800
48.8 + j11.2
0.113
89
900
52.8 – j2.2
0.035
–38
1000
46.6 – j11.5
0.123
–99
1100
39.7 – j13.9
0.191
–117
1200
35.0 – j13.0
0.232
–130
RF Section
After up-conversion, the RF outputs of the I and Q mixers are
combined. An on-chip balun performs internal differential
to single-ended output conversion, while transforming the
output signal impedance to 50Ω. Table 4 shows the RF
port output impedance vs frequency.
Table 4. RF Port Output Impedance vs Frequency for EN = High
and PLO = 0dBm
FREQUENCY OUTPUT IMPEDANCE
S22
(MHz)
(Ω)
Mag
Angle
500
22.2 + j5.2
0.390
165
600
28.4 + j11.7
0.311
143
700
38.8 + j14.3
0.202
119
800
49.4 + j6.8
0.068
91
900
49.4 – j5.8
0.058
–92
1000
42.7 – j11.7
0.149
–115
1100
36.9 – j12.6
0.207
–128
1200
33.2 – j11.3
0.241
–138
The RF output S22 with no LO power applied is given in
Table 5.
Table 5. RF Port Output Impedance vs Frequency for EN = High
and No LO Power Applied
FREQUENCY
(MHz)
OUTPUT IMPEDANCE
(Ω)
S22
Mag
Angle
500
22.9 + j5.3
0.377
165
600
30.0 + j11.2
0.283
143
700
40.6 + j11.2
0.160
123
800
47.3 + j1.9
0.034
145
900
44.2 – j7.4
0.099
–123
1000
38.4 – j10.4
0.175
–131
1100
34.2 – j10.2
0.221
–140
1200
31.7 – j8.7
0.246
–148
To improve S22 for lower frequencies, a series capacitor
can be added to the RF output. At higher frequencies, a
shunt inductor can improve the S22. Figure 5 shows the
equivalent circuit schematic of the RF output.
Note that an ESD diode is connected internally from the
RF output to ground. For strong output RF signal levels
(higher than 3dBm) this ESD diode can degrade the lin-
earity performance if an external 50Ω termination imped-
ance 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.
47Ω
VCC
21pF
RF
OUTPUT
1pF 7nH
5571 F05
Figure 5. Equivalent Circuit Schematic of the RF Output
Enable Interface
Figure 6 shows a simplified schematic of the EN pin inter-
face. The voltage necessary to turn on the LT5571 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 VCC by more than 0.5V. If this should occur, the
5571f
11
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