LT5518
8
5518f
Baseband Interface
The baseband inputs (BBPI, BBMI), (BBPQ, BBMQ) pres-
ent a differential input impedance of about 2.9kΩ. At each
of the four baseband inputs, a lowpass fi lter using 200Ω
and 1.8pF to ground is incorporated (see Figure 1), which
limits the baseband bandwidth to approximately 250MHz
(–1dB point). The common mode voltage is about 2.06V
and is slightly temperature dependent. At T
A
= –40
o
C, the
common mode voltage is about 2.19V and at T
A
= 85
o
C
it is about 1.92V.
If the I/Q signals are DC-coupled to the LT5518, it is
important that the applied common mode voltage level
of the I and Q inputs is about 2.06V in order to properly
bias the LT5518. Some I/Q test generators allow setting
the common mode voltage independently. In this case, the
common mode voltage of those generators must be set
to 1.03V to match the LT5518 internal bias, because for
DC signals, there is no –6dB source-load voltage division
(see Figure 2).
Figure 2. DC Voltage Levels for a Generator Programmed at
1.03V
DC
for a 50Ω Load and for the LT5518 as a Load
DAC’s differential output current to minimize the LO to RF
feedthrough. Resistors R3A, R3B, R4A and R4B translate
the DAC’s output common mode level from about 0.5V
DC
to the LT5518’s input at about 2.06V
DC
. For these resis-
tors, 1% accuracy is recommended. For different ambi-
ent temperatures, the LT5518 input common mode level
varies with a temperature coeffi cient of about –2.7mV/°C.
The internal common mode feedback loop will correct
these level changes in order to bias the LT5518 at the
correct operating point. Resistors R3 and R4 are chosen
high enough that the LT5518 common mode compliance
current value will not be exceeded at the inputs of the
LT5518 as a result of temperature shifts. Capacitors C4A
and C4B minimize the input signal attenuation caused by
the network R3A, R3B, R4A and R4B. This results in a
gain difference between low frequency and high frequency
baseband signals. The high frequency baseband –3dB
corner point is approximately given by:
f
–3dB
= 1/[2π • C4A • (R3A||R4A||(R
IN, DIFF
/2)]
In this example, f
–3dB
= 58kHz.
This corner point should be set signifi cantly lower than the
minimum baseband signal frequency by choosing large
enough capacitors C4A and C4B. For signal frequencies
signifi cantly lower than f
–3dB
, the gain is reduced by ap-
proximately
G
DC
= 20 • log [R3A||(R
IN, DIFF
/2)]/[R3A||(R
IN, DIFF
/2)
+ R4A]
In this example, G
DC
= –11dB.
Inserting the network of R3A, R3B, R4A, R4B, C4A and
C4B has the following consequences:
• Reduced LO feedthrough adjustment range. LO to RF
feedthrough can be reduced by adjusting the differential
DC offset voltage applied to the I and/or Q inputs. Be-
cause of the DC gain reduction, the range of adjustment
is reduced. The resolution of the offset adjustment is
improved by the same gain reduction factor.
• DC notch for uneven number of channels. The interface
drawn in Figure 3 might not be practical for an uneven
number of channels, since the gain at DC is lower and
will appear in the center of (one of) the channel(s). In
that case, a DC-coupled level shifting circuit is required,
or the LT5528 might be a better solution.
5518 F02
1.5k50Ω
LT5518GENERATOR
2.06V
DC
2.06V
DC
2.06V
DC
+
–
+
–
50Ω
50Ω
GENERATOR
2.06V
DC
1.03V
DC
+
–
The LT5518 should be driven differentially; otherwise, the
even-order distortion products will degrade the overall
linearity severely. Typically, a DAC will be the signal source
for the LT5518. A reconstruction fi lter should be placed
between the DAC output and the LT5518’s baseband
inputs. DC coupling between the DAC outputs and the
LT5518 baseband inputs is recommended. Active level
shifters may be required to adapt the common mode level
of the DAC outputs to the common mode input voltage
of the LT5518. It is also possible to achieve a DC level
shift with passive components, depending on the appli-
cation. For example, if fl at frequency response to DC is
not required, then the interface circuit of Figure 3 may be
used. This fi gure shows a commonly used 0mA – 20mA
DAC output followed by a passive 5th order lowpass
fi lter. The DC-coupled interface allows adjustment of the
APPLICATIO S I FOR ATIO
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