TQ5633 Ver la hoja de datos (PDF) - TriQuint Semiconductor
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TQ5633 Datasheet PDF : 10 Pages
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TUNING:
TQ5633
Data Sheet
IF Amplifier- The IF amplifier output at pin 5 requires a
match down to 50 ohms in addition to a source of DC bias. A
simple matching network that performs well for both functions is
a shunt-L series-C type. See Figure 1. The L3 inductor
provides DC bias to the IF amplifier while functioning as part of
the IF matching network. The C9 capacitor provides a DC block
and functions as the second AC matching component. During
the design phase it is usually fairly easy to empirically determine
these components by attaching a network analyzer to the
50ohm side of the IF network, and vary L3 and C9 until good
return loss at the IF frequency is obtained. Typical values for
L3 will be 180nH to 270nH and typical values of C9 are from
4.7pF to 15pF. Note that unlike a single-ended mixer type
design, a shunt-C element at the output is not required. The
TQ5633 leaks only a small amount of LO energy out of the RF
port, so no additional shunt-C filtering is required.
IF Tank Circuit- As discussed in the introduction, the
parallel LC circuit on pin 3 functions by creating a high IF
impedance at the sources of the IF amplifier, improving common
mode rejection. Once a prototype phone board layout is
finished there remains the task of assigning the values of these
components.
For a parallel circuit Rp = Q x Xp. Thus the higher Q and Xp
(i.e. Lp) are, the better IP2 will be. However, too high of a value
for L2 will make the circuit more prone to parasitic capacitances.
A good compromise would be to follow the evaluation board
example and start with a 3.3pF capacitor for C3. Then using a
network analyzer probe measure the impedance at the pin 3
pad with the TQ5633 absent. Adjust L2 until the network
analyzer measures S11 as close as possible to Γ = 1 at an
angle of 00 (i.e. an open circuit). In practice an |S11| of 7.5k
ohms is attainable with a 0805 size 470nH inductor. Smaller
package 0603 and 0402 inductors may not be obtainable in
such high values so that some compromises will have to be
made if inductor size is an issue. See Figure 2.
RF
Input
Vdd
Mixer
F1
COAXIAL
PROBE
1
REMOVE
TQ5633
FROM
BOARD
2
Ground at pad 2
3
C3
L2
R4
4
C4
MEASURE S11
NETWORK
ANALYZER
8
C5
7
6
5
C9
Vdd
R3
RFA
C6
R1
C7
LO
Input
C2,
Gain
Select
Vdd
L3
R5 IFA
C8
IF
Output
S11
Marker at IF freq
e.g. 110MHz
Figure 2 IF Tank Tuning
Further Improving IP2:
Although the TQ5633 is exceptional in its isolation of the LO
signal from the RF port, there is still a miniscule amount of LO
energy present, typically –40dBc. That energy tends to bounce
off of the image filter and reenter the downconverter where,
depending on its phase, it creates a very small DC offset in the
mixer. The phenomena occurs in the Low Gain mode where it
can create 4 to 5 dBm variation in IP2 depending on how closely
the image filter is located to the TQ5633. Thus for applications
demanding the highest Half-IF spur rejection, higher IP2 can
possibly be obtained by tailoring the length of the transmission
line between the filter and chip. The specific line length will
depend upon board layout and will vary between filter types. If
it appears that the needed length will be long, the line can be
“U” shaped in order to conserve space. Measurements have
indicated that there is some reduction in gain at the optimum
line length.
8
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