
TQ5135
Data Sheet
10
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TQ5135
G
GND
RF In
GND
LNA
Bias
MXR
In
I
I
GND
LO
In
G
L
V
DD
V
D
V
D
Vdd
Vdd
IFout
Minimize These
Lengths
Figure 2. Critical signal Paths
Mxer – Filter Interaction:
Before attempting a new TQ5135 application, it is important to
understand the nonlinear interaction between the image filter
and the mxer. The device IP3 is a strong function of this
interaction. For this reason it is helpful to consider the filter
and mxer as one nonlinear block.
Figure 3 shows a much simplified block diagramof the LNA,
image filter, and mxer. The RF signal is amplified by the
LNA, passes through the image filter, and is converted down
to the IF where it is amplified by the IF output FET. The
quiescent current in the IF amplifier is set by the GIC network.
Both the filter and the mxer termnate the RF signal with
50ohms.
However, the situation is much different with the LO signal. At
the LO frequency the image filter looks like a short circuit.
Some LO energy leaks out of the mxer input, bounces back
off of the image filter and returns back into the mxer with
some phase or delay. The delayed LO signal mxes with the
normal LO to create a DC offset which is fed into the IF
amplifier and changes the quiescent current. Depending on
the phase of the reflected LO, the IF stage current may be
higher or lower.
The DC offset also affects the passive mxer FET to some
degree as well. It has been found empirically that varying the
delay between the filter and mxer can have positive or
negative consequences on IP3, CG, and NF. It is for this
reason that an LC network is useful between the SAW and
mxer input, even though the mxer input can have an
adequate match at the RF frequency without any external
components.
25-100 ohms at RF-
short circuit at LO
band pass
LO Leakage
LO Leakage
( f )
Mixer
LO
IF + DC
Offset
RF in
Mixer Portion
of TQ5135
+ LO) = DC Offset
(LO Leakage
( f )
at Mixer IF Output
Idd + Idd Offset
IF Output
FET
to GIC
Mxer in
IF Output
12
9
8
7
LNA Portion
of TQ5135
LNA Out
2
14
Figure 3. Non-linear filter-Mxer Interaction