Preliminary
8-80
RF2449
Rev A4 001016
8
Application Schematic
1
2
3
4
5
6
7
8
9
10
11
12
24
23
22
21
20
19
18
17
16
15
14
13
33 nF
30 nH
6.8 nH
100 pF
6.8 nH
3.0 pF
22 k
Ω
33 k
Ω
1.5 nH 33 nF
22 nH
ENABLE
IP SET
IF SEL
LNA GAIN
MIX GAIN
*
100 pF
*NOTE:
Microstrip Inductor, Z
O = 50 Ω, L = 100 mils
Suggested component values for 85.38 MHz IF and
R
OUT = 1 kΩ:
L1 = 470 nH
C1 = 10 pF
R = 7.5 k
Ω
RF IN
510
Ω
1 nF
V
CC
RF SAW Filter
56
Ω
LO IN
1 nF
R
L1
C1
L2
V
CC
C1
C2
L
Filter
IF2+
IF2-
100 pF
R
L1
C1
L2
1 nF
VCC
C1
C2
L
Filter
IF+
IF-
Output Interface Network
L1, C1 and R form a current combiner which performs
a differential to single-ended conversion at the IF fre-
quency and sets the output impedance. In most cases,
the resonance frequency is independent of R and can
be set according to the following equation:
Where CEQ is the equivalent stray capacitance and
capacitance looking into pins 16 and 17. An average
value to use for CEQ is 2.5pF.
R may then be used to set the output impedance
according to the following equation:
where ROUT is the desired output impedance and RP is
the parasitic equivalent parallel resistance of L1.
C1 should be chosen as high as possible, while main-
taining an RP of L1 that allows for the desired ROUT.
L2 and C2 serve dual purposes. L2 serves as an out-
put bias choke, and C2 serves as a series DC block.
In addition, L2 and C2 may be chosen to form an
impedance matching network if the input impedance of
the IF filter is not equal to ROUT. Otherwise, L2 is cho-
sen to be large and C2 is chosen to be large if a DC
path to ground is present in the IF filter, or omitted if the
filter is DC blocked.
f
IF
1
2
π
L1
2
------ C1
C
EQ
+
()
-----------------------------------------------
=
R
1
4 R
OUT
⋅
---------------------
1
R
P
------
1
–
–
=
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