LT5527
9
5527fa
APPLICATIONS INFORMATION
Introduction
The LT5527 consists of a high linearity double-balanced
mixer, RF buffer amplifi er, high speed limiting LO buffer
amplifi er and bias/enable circuits. The RF and LO inputs
are both single ended. The IF output is differential. Low
side or high side LO injection can be used.
Two evaluation circuits are available. The standard evalua-
tion circuit, shown in Figure 1, incorporates transformer-
based IF matching and is intended for applications that
require the lowest LO-IF leakage levels and the widest
IF bandwidth. The second evaluation circuit, shown in
Figure 2, replaces the IF transformer with a discrete IF
balun for reduced solution cost and size. The discrete
IF balun delivers comparable noise fi gure and linearity,
higher conversion gain, but degraded LO-IF leakage and
reduced IF bandwidth.
RF Input Port
The mixer’s RF input, shown in Figure 3, consists of an
integrated transformer and a high linearity differential
amplifi er. The primary terminals of the transformer are
connected to the RF input pin (Pin 3) and ground. The
secondary side of the transformer is internally connected
to the amplifi er’s differential inputs.
One terminal of the transformer’s primary is internally
grounded. If the RF source has DC voltage present, then
a coupling capacitor must be used in series with the RF
input pin.
The RF input is internally matched from 1.7GHz to 3GHz,
requiring no external components over this frequency
range. The input return loss, shown in Figure 4a, is typi-
cally 10dB at the band edges. The input match at the lower
band edge can be optimized with a series 2.7pF capacitor
at Pin 3, which improves the 1.7GHz return loss to greater
than 20dB. Likewise, the 2.7GHz match can be improved
to greater than 30dB with a series 1.5nH inductor. A series
1.5nH/2.7pF network will simultaneously optimize the lower
and upper band edges and expand the RF input bandwidth
to 1.1GHz-3.3GHz. Measured RF input return losses for
these three cases are also plotted in Figure 4a.
Alternatively, the input match can be shifted down, as low
as 400MHz or up to 3700MHz, by adding a shunt capacitor
(C5) to the RF input. A 450MHz input match is realized with
C5 = 12pF, located 4.5mm away from Pin 3 on the evaluation
board’s 50Ω input transmission line. A 900MHz input match
requires C5 = 3.9pF, located at 1.3mm. A 3500MHz input
match is realized with C5 = 0.5pF, located at 4.5mm. This
Figure 3. RF Input Schematic
Figure 4. RF Input Return Loss With
and Without External Matching
(4b) Series Shunt Matching
(4a) Series Reactance Matching
RF
IN
Z
O
= 50Ω
L = L (mm)
C5
RF
5527 F03
EXTERNAL MATCHING
FOR LOW BAND OR
HIGH BAND ONLY
TO
MIXER
3
FREQUENCY (GHz)
0.2
–30
RF PORT RETURN LOSS (dB)
–25
–20
–15
–10
1.2 2.2
3.2
4.2
5527 F04a
–5
0
0.7 1.7
2.7
3.7
SERIES 1.5nH
SERIES 2.7pF
NO EXTERNAL
MATCHING
SERIES 1.5nH
SERIES 2.7pF
RF FREQUENCY (GHz)
0.2
–30
RF PORT RETURN LOSS (dB)
–25
–20
–15
–10
1.2 2.2
3.2
4.2
5527 F04b
–5
0
0.7 1.7
2.7
3.7
NO EXTERNAL
MATCHING
900MHz
C5 = 3.9pF
L = 1.3mm
450MHz
C5 = 12pF
L = 4.5mm
3.5GHz
C5 = 0.5pF
L = 4.5mm
