LT5579
12
5579fa
applicaTions inForMaTion
The LT5579 uses a high performance LO buffer amplifier
driving a double-balanced mixer core to achieve frequency
conversion with high linearity. Internal baluns are used to
provide single-ended LO input and RF output ports. The
IF input is differential. The LT5579 is intended for opera
-
tion in
the 1.5GHz to 3.8GHz frequency range, though
operation outside this range is possible with reduced
performance.
IF Input Interface
Th
e IF inputs are tied to the emitters of the double-balanced
mixer transistors, as shown in Figure 2. These pins are
internally biased to a common mode voltage of 570mV.
The optimum DC current in the mixer core is approximately
50mA per side, and is set by the external resistors, R1 and
R2. The inductors and resistors must be able to handle
the anticipated current and power dissipation. For best
LO leakage performance the board layout must be sym
-
metrical and
the input resistors should be well matched
(0.1% tolerance is recommended).
The purpose of the inductors (L1 and L2) is to reduce the
loading effects of R1 and R2. The impedances of L1 and L2
should be at least several times greater than the IF input
impedance at the desired IF frequency. The self-resonant
frequency of the inductors should also be at least several
times the IF frequency. Note that the DC resistances of
L1 and L2 will affect the DC current and may need to be
accounted for in the selection of R1 and R2.
L1 and L2 should connect to the signal lines as close to
the package as possible. This location will be at the lowest
impedance point, which will minimize the sensitivity of the
performance to the loading of the shunt L-R branches.
Capacitors C1 and C2 are used to cancel out the parasitic
series inductance of the IF transformer. They also provide
DC isolation between the IF ports to prevent unwanted inter
-
actions that
can affect the LO to RF leakage performance.
The differential input resistance to the mixer is approxi
-
ma
tely 10Ω, as indicated in Table 1. The package and
external inductances (TL1 and TL2) are used along with
C3
V
CC
50mA
570mV
570mV
50mA
2k
2k
IF
+
LT5579
C9
C1
IF
INPUT
T1
4:1
C2
L1
L2
R1
R2
5579 F02
3
IF
–
4
TL1
TL2
Figure 2. IF Input with External Matching
