LT5520
7
5520f
Table 1 lists the differential IF input impedance and reflec-
tion coefficient for several frequencies. A 4:1 balun can be
used to transform the impedance up to about 50Ω.
Table 1. IF Input Differential Impedance
Frequency Differential Input Differential S11
(MHz) Impedance Mag Angle
10 10.1 + j0.117 0.663 180
44 10.1 + j0.476 0.663 179
70 10.1 + j0.751 0.663 178
140 10.2 + j1.47 0.663 177
170 10.2 + j1.78 0.663 176
240 10.2 + j2.53 0.663 174
360 10.2 + j3.81 0.663 171
500 10.2 + j5.31 0.663 167
LO Input Port
The simplified circuit for the LO buffer input is shown in
Figure 4. The LO buffer amplifier consists of high-speed
limiting differential amplifiers, optimized to drive the mixer
quad for high linearity. The LO
+
and LO
–
ports can be
driven differentially; however, they are intended to be
driven by a single-ended source. An internal resistor
connected across the LO
+
and LO
–
inputs provides a
broadband 50Ω impedance match. Because of the resis-
tive match, a DC voltage at the LO input is not recom-
mended. If the LO signal source output is not AC coupled,
then a DC blocking capacitor should be used at the LO
input.
Figure 3. IF Input with External Matching
C1
C2
C3
IF
IN
50Ω
T1
4:1
2
3
100Ω
0.1%
100Ω
0.1%
V
CC
18mA
18mA
5520 F03
LT5520
LO
IN
50Ω
14
15
V
CC
5520 F04
85Ω
LO
–
LO
+
5pF
5pF
220Ω
220Ω
LT5520
Figure 4. LO Input Circuit
Though the LO input is internally 50Ω matched, there may
be some cases, particularly at higher frequencies or with
different source impedances, where a further optimized
match is desired. Table 2 includes the single -ended input
impedance and reflection coefficient vs frequency for the
LO input for use in such cases.
Table 2. Single-Ended LO Input Impedance
Frequency Input S11
(MHz) Impedance Mag Angle
1300 62.8 – j9.14 0.139 –30.9
1500 62.2 – j11.4 0.148 –37.1
1700 61.5 – j13.4 0.157 –42.4
1900 60.0 – j15.2 0.164 –48.9
2100 58.4 – j16.9 0.172 –54.7
2300 56.5 – j17.9 0.176 –60.4
2500 54.9 – j18.8 0.182 –65.1
2700 53.7 – j18.8 0.182 –68.5
RF Output Port
An internal RF transformer, shown in Figure 5, reduces the
mixer-core impedance to provide an impedance of 50Ω
across the RF
+
and RF
–
pins. The LT5520 is designed and
tested with the outputs configured for single-ended opera-
tion, as shown in the Figure 5; however, the outputs can be
used differentially as well. A center-tap in the transformer
provides the DC connection to the mixer core and the
transformer provides DC isolation at the RF output. The
RF
+
and RF
–
pins are connected together through the
secondary windings of the transformer, thus a DC voltage
should not be applied across these pins.
APPLICATIO S I FOR ATIO
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