LT5560
23
5560f
C
DC
C
O
C
O
R
B
L
O
L
O
L
DC
R
A
5560 F19
APPLICATIO S I FOR ATIO
WUU
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Lumped Element Matching
The applications described so far have employed external
transformers or hybrid baluns to realize single-ended to
differential conversions and, in some cases, impedance
transformations. An alternate approach is to use lumped-
element baluns to realize the input or output matching
networks.
A lumped element balun topology is shown in Figure 19.
The desired component values can be estimated using
the equations below, where R
A
and R
B
are the terminat-
ing resistances on the unbalanced and balanced ports,
respectively. Variable f
C
is the desired center frequency.
(The resistances of the LT5560 input and output can be
found in Tables 2 and 6).
L
RR
f
O
AB
C
=
•
••2 π
C
fRR
O
CAB
=
1
2• • • •π
The computed values are approximate, as they don’t ac-
count for the effects of parasitics of the IC and external
components.
Inductor L
DC
is used to provide a DC path to ground or to
V
CC
depending on whether the circuit is used at the input
or output of the LT5560. In some cases, it is desirable to
make the value of L
DC
as large as practical to minimize
loading on the circuit; however, the value can also be op-
timized to tune the impedance match. The shunt inductor,
L
O
, provides the DC path for the other balanced port.
Capacitor C
DC
may be required for DC blocking but
can often be omitted if DC decoupling is not required.
Figure 19. Lumped Element Balun
In some applications, C
DC
is useful for optimizing the
impedance match.
The circuit shown on page 1 illustrates the use of lumped
element baluns. In this example, the LT5560 is used to
convert a 900MHz input signal down to 140MHz using a
760MHz L
O
signal.
For the 900MHz input, R
A
= 50Ω and R
B
= 28Ω (from
Table 2). The actual values used for C
O
and L
O
are 4.7pF
and 6.8nH, which agree very closely with the calculated
values of 4.7pF and 6.6nH. The 15nH shunt inductor, in
this case, has been used to optimize the impedance match,
while the 100pF cap provides DC decoupling.
At the 140MHz output, the values used for R
A
and R
B
are 50Ω and 1080Ω (from Table 6), respectively, which
result in calculated values of C
O
= 4.9pF and L
O
= 265nH.
These values are very close to the actual values of 4.7pF
and 270nH. A shunt inductor (L
DC
) of 270nH is used here
and the 33pF blocking cap has been used to optimize the
impedance.
