LT5511
11
5511i
The LT5511 consists of a double-balanced mixer driven by
a high-performance, differential, limiting LO buffer. The
mixer has been optimized for high linearity and high signal
level operation. The LT5511 is intended for applications
with LO frequencies of 0.4GHz to 2.7GHz and IF input
frequencies from 10MHz to 300MHz, but can be used at
other frequencies with excellent results. The LT5511 can
be used in applications using either a low side or high side
LO.
LO Input Port
The LO buffer on the LT5511 consists of differential high
speed amplifiers and limiters that are designed to drive the
mixer quad to achieve high linearity and performance at
high IF input signal levels. The LO
+
and LO
–
pins are the
differential inputs to the LO buffer. Though the LO signal
can be applied differentially, the LO buffer performs well
with only one input driven, thus eliminating the need for a
balun. In this case, a capacitor should be connected
between the unused LO input pin and ground. The LO pins
are biased internally to about 1.4V, and thus must be DC
isolated from the external LO signal source.
The LO input should be matched to 50Ω. The impedance
match can be accomplished through the use of a reactive
impedance matching network. However, for lower LO
frequencies (below about 1.5GHz), an easier approach is
to use a shunt 62Ω resistor to resistively match the port.
(The resistor must be DC isolated from the LO input pin).
This method is broadband and requires LO power levels of
only –10dBm. At higher frequencies, a better match can be
realized with reactive components. Transmission lines
and parasitics should be considered when designing the
matching circuits. Typical S-parameter data for the LO
input is included in Table 1 to facilitate the design of the
matching network.
IF Input Port
The IF
+
and IF
–
pins are the differential inputs to the mixer.
These inputs drive the emitters of the switching transis-
tors, and thus have a low impedance. The DC current
through these transistors is set by external resistors from
each IF pin to ground. The typical internal voltage on the
emitters is 1.2V; thus, the current through each IF pin is
approximately:
I
IF
= 1.2/R
IF
R
IF
is the value of the external resistors to ground. Best
performance is obtained when the IF inputs are perfectly
balanced and 0.1% tolerance resistors are recommended
here. The LT5511 has been characterized with 75Ω
resistors on each of the IF inputs.
The IF signal to the mixer must be differential. To realize
this, an RF balun transformer or lumped element balun can
be used. The RF transformer is recommended, as it is
easier to realize broadband operation, and also does not
have the component sensitivity issues of a lumped ele-
ment balun.
The differential input impedance of the IF input is approxi-
mately 12.5Ω; therefore, a 4:1 impedance transformation
is required to match to 50Ω. Selecting a transformer with
this impedance ratio will reduce the amount of additional
components required, as the full impedance transforma-
tion is realized by the transformer. DC-isolating trans-
formers or transmission-line transformers can be used,
as could lumped element transformation networks. Be-
cause the IF ports are internally biased, they must be DC
isolated from the IF source. Additionally, IF
+
and IF
–
must
be DC isolated from each other in order to maintain good
LO suppression.
APPLICATIO S I FOR ATIO
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