MLX71122_16 datasheet(33/59 Pages) MELEXIS

33 / 59 page

MLX71122

300 to 930MHz FSK/FM/ASK Receiver

Datasheet

REVISION 014 – AUGUST, 2016

Page 33 of 59

390 10 71122 01

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

1.47 

kbps

rate

data

640

Bi-Phase Code

1.47 

kbps

rate

data

320

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

160pF

[kbps]

640pF



in E-series

0pF

C9 



(24)

220.5pF

150pF

1.47







in E-series

0pF

C8 



(25)

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