MLX71121_15 Datasheet, PDF (12/29 Pages) Melexis Microelectronic Systems – FSK/OOK Receiver

MLX71121
300 to 930MHz
FSK/OOK Receiver
1.16 Data Output and Noise Cancellation Filter
The data output pin DTAO delivers the demodulated data signal which can be further processed by a noise
cancellation filter (NCF). The NCF can be disabled if pin CINT is connected to ground. In this case the
multiplexer (MUX) connects the receiver output DTAO directly to the data slicer output.
Fig. 9: Data output and noise filter
data slicer
output
MUX
DTAO
NCF
CINT
noise cancellation filter
CF3
The noise cancellation filter can suppress random pulses in the data output which are shorter than t_min.
CF3  15 10^-6  t_min
 ^{15 106}
R_NRZ

7.5 10^6
R_RZ
The NCF can also operate as a muting circuit. So if the RF input signal is below sensitivity level (or if no RF
signal is applied) then the data output will go to a constant DC level (either HIGH or LOW). This can be
achieved by setting the bandwidth of the preceding data filter (sec 1.13) about 10 times higher than the
bandwidth of the NCF. Further the data filter cutoff frequency must be higher than the data rate, so the noise
pulses are shorter than the shortest data pulse. Otherwise, the NCF will not be able to distinguish between
noise and data pulses.
Having the NCF activated is a good means for reducing the computing power of the microcontroller that fol-
lows the receiver IC for further data processing.
In contrast to a conventional muting (or squelch) circuit, this topology does not need the RSSI signal for level
indication. The filtering process is done by means of an analogue integrator. The cut-off frequency of the
NCF is set by the external capacitor connected to pin CINT. This capacitor C_F3 should be set according to
the maximum data rate. Below table provides some recommendations..
During receiver start-up a sequencer checks if pin CINT is connected to a capacitor or to ground. The maxi-
mum value of C_F3 should not exceed 12nF. This defines the lowest data rate that can be processed if the
noise cancellation filter is activated.
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
C_F3[nF]
12
6.8
4.7
3.3
2.2
1.5
1.2
39010 71121
Rev. 012
Page 12 of 29
Data Sheet
Mar/15