MLX71122_16 Datasheet, PDF (33/59 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71122
300 to 930MHz FSK/FM/ASK Receiver
Datasheet
3.4. Data Path
The data path contains all circuitry that is used to process the baseband signal. The MLX71122 comprises a
second order Sallen-Key lowpass filter, two peak detectors and an output comparator as digital signal
output.
3.4.1. Data Filter (DF)
The receive part of the MLX71122 contains a 2^nd order Sallen-Key low-pass filter that can be configured by
connecting two external capacitors C8 and C9 to the IC (see sec. 6). This data filter removes high frequency
components and noise from the demodulated signal that may otherwise lower the signal to noise ratio at
the comparator input. The filter bandwidth has to be adjusted to the maximum data rate. A good choice
for the -3dB bandwidth is 85% of the data rate for NRZ codes and 170% for bi-phase or Manchester codes.
A characteristic between Bessel and Butterworth is best used in the data filter.
Since the internal resistors of the filter are both 200k and the overall gain is set to unity we obtain the
following table for the capacitor values:
Coding
NRZ Code
Bi-Phase Code
C8
1.47  C9
1.47  C9
C9
640 pF
data rate / kbps
320 pF
data rate / kbps
C9 should be rounded (down preferred) to the closest E-series value. C8 should be calculated from the
rounded value before rounding it (down preferred) to the closest E-series value.
Example: base band signal 4kbps, NRZ coding
C9  ^640pF  160pF
4 [kbp s]
in E-series  C9  150pF
(24)
C8  1.47 150pF  220.5pF
in E-series  C8  220pF
(25)
REVISION 014 – AUGUST, 2016
390 10 71122 01
Page 33 of 59