MLX71120 Datasheet, PDF (12/30 Pages) Melexis Microelectronic Systems – 300 to 930MHz FSK/FM/ASK Receiver

MLX71120
300 to 930MHz
FSK/FM/ASK Receiver
1.15 Data Output and Noise Cancellation Filter
The data output pin DTAO delivers the demodulated data signal that can be further processed by a noise
cancellation filter (NCF). The NCF can be disabled if pin CINT is connected to VEE. In this case the multi-
plexer (MUX) connects the receiver output DTAO directly to the data slicer output.
data slicer
output
MUX
DTAO
NCF
CINT
Fig. 8: Data output and noise filter
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
PRELIMINARY ^{The NCF can also operate as a muting circuit for RF input signals that are below sensitivity level if the band-}
width of the preceding data filter is selected much higher than the bandwidth of the NCF. This would be the
case if no RF signal is present.
In contrast to conventional muting (or squelch) circuits, 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]
R_NRZ [kbit/s]
C_F3[nF]
0.6
1.2
12
1.2
2.4
6.8
1.6
3.2
4.7
2.4
4.8
3.3
3.3
6.6
2.2
4.8
9.6
1.5
6.0
12
1.2
In shutdown mode pin DTAO is set to Hi-Z state.
39010 71120
Rev. 005
Page 12 of 30
Data Sheet
Feb/08