LT5518
11
5518f
Table 1. LO Port Input Impedance vs Frequency for EN = High
Frequency Input Impedance S
11
MHz Ω Mag Angle
1000 44.5 + j18.2 0.197 95
1400 60.3 + j6.8 0.112 30
1600 62.8 – j0.6 0.113 –2.4
1800 62.4 – j9.0 0.136 –32
2000 56.7 – j15.6 0.157 –58
2200 50.9 – j16.5 0.161 –77
2400 46.6 – j15.2 0.159 –94
2600 42.9 – j13.9 0.165 –109
The input impedance of the LO port is different if the part
is in shut-down mode. The LO input impedance for EN =
Low is given in Table 2.
Table 2. LO Port Input Impedance vs Frequency for EN = Low
Frequency Input Impedance S
11
MHz Ω Mag Angle
1000 42.1 + j43.7 0.439 75
1400 121 + j34.9 0.454 15
1600 134 – j31.6 0.483 –11
1800 91.3 – j68.5 0.510 –33
2000 56.4 – j66.3 0.532 –53
2200 37.7 – j54.9 0.544 –70
2400 27.9 – j43.6 0.550 –87
2600 22.1 – j33.9 0.553 –104
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 3 shows the RF
port output impedance vs frequency.
Table 3. RF Port Output Impedance vs Frequency for EN = High
and P
LO
= 0dBm
Frequency Input Impedance S
22
MHz Ω Mag Angle
1000 21.3 + j9.7 0.421 153
1400 29.8 + j20.3 0.348 121
1600 39.1 + j23.5 0.280 100
1800 50.8 + j18.4 0.180 77.1
2000 52.1 + j5.4 0.057 65.5
2200 43.2 – j0.1 0.073 –179
2400 36.0 + j2.0 0.164 171
2600 32.1 + j5.6 0.228 159
Figure 6. Equivalent Circuit Schematic of the RF Output
RF
OUTPUT
20pF
21pF 3nH
52.5Ω
5518 F06
V
CC
The RF output S
22
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
Frequency Input Impedance S
22
MHz Ω Mag Angle
1000 21.7 + j9.9 0.416 153
1400 32.3 + j19.5 0.312 119
1600 42.2 + j18.5 0.214 102
1800 46.8 + j9.6 0.104 103
2000 41.8 + j3.7 0.098 154
2200 36.1 + j4.3 0.170 160
2400 32.8 + j7.4 0.226 152
2600 31.2 + j10.5 0.264 144
For EN = Low the S
22
is given in Table 5.
Table 5. RF Port Output Impedance vs Frequency for EN = Low
Frequency Input Impedance S
22
MHz Ω Mag Angle
1000 20.9+j9.6 0.428 154
1400 28.5 + j20.2 0.365 123
1600 36.7 + j24.5 0.311 103
1800 48.7 + j23.1 0.229 80.2
2000 55.7 + j11.0 0.116 56.7
2200 48.9 + j0.6 0.013 158.9
2400 39.8 – j0.02 0.115 –179
2600 34.2 + j3.2 0.193 167
To improve S
22
for lower frequencies, a shunt capacitor
can be added to the RF output. At higher frequencies, a
shunt inductor can improve the S
22
. Figure 6 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 linearity performance if the 50Ω termination imped-
ance is connected directly to ground. To prevent this, a
APPLICATIO S I FOR ATIO
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