lt5579 Linear Technology Corporation, lt5579 Datasheet - Page 14

• Manufacturer Part Number: lt5579
• Description: 1.5ghz To 3.8ghz High Linearity Upconverting Mixer
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LO Input Interface
The simplifi ed schematic for the single-ended LO input port
is shown in Figure 4. An internal transformer provides a
broadband impedance match and performs single-ended
to differential conversion. An internal capacitor also aids
in impedance matching and provides DC isolation to the
primary transformer winding. The transformer secondary
feeds the differential limiting amplifi er stages that drive
the mixer core.
The measured return loss of the LO input port is shown
in Figure 5 for an LO input power of –1dBm. The imped-
ance match is acceptable from about 1.1GHz to beyond
4GHz, with a minimum return loss across this range of
about 9dB at 2300MHz. If desired, the return loss can
be improved below 1.1GHz by external components as
shown in Figure 4. The return loss can also be improved
by reducing the LO drive level, though performance will
degrade if the level is too low.
LT5579
14
INPUT
LO
FREQUENCY
MATCHING
EXTERNAL
FOR LOW
ONLY
C13
L6
Figure 4. LO Input Circuit
22
LO
V
CC
V
BIAS
5579 F04
While external matching of the LO input is not required
for frequencies above 1.1GHz, external matching should
be used for lower LO frequencies for best performance.
Table 3 lists the input impedance and refl ection coeffi cient
vs frequency for the LO input for use in such cases.
Table 3. Single-Ended LO Input Impedance
(at Pin 22, No External Match)
FREQUENCY
(MHz)
1000
1500
1900
2000
2150
2400
3050
3150
4000
750
–10
–25
–15
–20
–5
0
500 1000
Figure 5. LO Input Return Loss
IMPEDANCE
20.3||– j1120
78.4||– j1250
63.3||– j30.5
74.7||– j96.3
66.8||– j81.5
53.8||– j69.8
79.1||– j113
33.7||– j115
33.0||– j146
43.9||+ j173
INPUT
1500
FREQUENCY (MHz)
2000
2500 3000
REFLECTION COEFFICIENT
MAG
0.68
0.42
0.22
0.34
0.35
0.36
0.35
0.26
0.24
0.15
3500 4000
5579 F05
ANGLE
–65.2
–74.7
–87.0
–125
–179
–105
–148
–154
–123
–7.7
5579f