MAX1471 Maxim Integrated Products, MAX1471 Datasheet - Page 12

• Manufacturer Part Number: MAX1471
• Description: 3V/5V ASK/FSK Superheterodyne Receiver
• Manufacturer: Maxim Integrated Products

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The data filters for the ASK and FSK data are imple-
mented as a 2nd-order lowpass Sallen-Key filter. The
pole locations are set by the combination of two on-
chip resistors and two external capacitors. Adjusting
the value of the external capacitors changes the corner
frequency to optimize for different data rates. The cor-
ner frequency in kHz should be set to approximately
1.5 times the fastest expected Manchester data rate in
kbps from the transmitter. Keeping the corner frequen-
cy near the data rate rejects any noise at higher fre-
quencies, resulting in an increase in receiver sensitivity.
The configuration shown in
Butterworth or Bessel response. The Butterworth filter
offers a very flat amplitude response in the passband
315MHz/434MHz Low-Power, 3V/5V
ASK/FSK Superheterodyne Receiver
Figure 2. Typical Application Circuit
12
______________________________________________________________________________________
RF INPUT
C5
C6
V
DD
C26
Y1
C4
C7
3.0V
R3
C3
V
L1
DD
C14
C15
1
2
3
4
5
6
7
8
Figure 3 can create a
AV
DSA-
DSA+
OPA+
DFA
XTAL2
XTAL1
LNAIN
DD
L3
32
9 10
V
CC
31
Data Filters
11
30
C11
29
V
L2
C9
DD
C10
28
MAX1471
12
27
V
C8
DD
13
and a rolloff rate of 40dB/decade for the two-pole filter.
The Bessel filter has a linear phase response, which
works well for filtering digital data. To calculate the
value of the capacitors, use the following equations,
along with the coefficients in Table 2:
where f
For example, choose a Butterworth filter response with
a corner frequency of 5kHz:
IN GND
26
14
25
C12
Y2
C
15
is the desired 3dB corner frequency.
16
OUT
PDMAXF
PDMINF
DGND
DV
OPF+
OPF+
DSF+
DSF-
C
C
DFF
DD
F1
F2
ASK DATA OUT
SCLK
DIO
CS
FSK DATA OUT
19
18
17
=
24
23
22
21
21
20
=
a
4 100
(
(
V
100
DD
C21
C21
C23
k
k
b
a
)( )( )
)( )( )
π
π
C22
R8
f
f
C
C
C27