MLX71120_12 Datasheet, PDF (11/31 Pages) Melexis Microelectronic Systems – FSK/FM/ASK Receiver

MLX71120
300 to 930MHz
FSK/FM/ASK Receiver
1.13 Data Filter
The data filter is formed by the operational amplifier OA1, two internal 100kΩ resistors and two external ca-
pacitors. It is implemented as a 2^nd order Sallen-Key filter. The low pass filter characteristic rejects noise at
higher frequencies and therefore leads to an increased sensitivity.
CF1
DF1
CF2
DF2
data filter
100k
100k
OA1
DF0
Fig. 5: Data filter
The filter’s pole locations can be set by the external capacitors CF1 and CF2. The cut-off frequency f_c has to
be adjusted according to the transmission data rate R. It should be set to approximately 1.5 times the fastest
expected data rate. For a Butterworth filter characteristic, the data filter capacitors can be calculated as fol-
lows.
CF1 =
1
2 ⋅ π ⋅100k ⋅ f_c
CF2 = ^CF1
2
R_RZ [kbit/s]
0.6
1.2
1.6
2.4
3.3
4.8
6.0
R_NRZ [kbit/s]
1.2
2.4
3.2
4.8
6.6
9.6
12
fc [kHz]
0.9
1.8
2.4
3.6
5
7.2
9
CF1 [pF]
2200
1200
1000
680
470
330
220
CF2 [pF]
1000
680
470
330
220
150
100
1.14 Data Slicer
The purpose of the data slicer is to convert the filtered data signal into a digital output. It can therefore be
considered as an analog-to-digital converter. This is done by using the operational amplifier OA2 as a com-
parator that compares the data filter output with a threshold voltage. The threshold voltage can be derived in
two different ways from the data signal.
SLCSEL
0
1
Description
Averaging detection mode
Peak detection mode
39010 71120
Rev. 008
Page 11 of 31
Data Sheet
Jun/12