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MRF89XA
3.4.4
CHANNEL FILTERS
EQUATION 3-8:
The second mixer stages are followed by the channel
select filters. The channel select filters have a strong
influence on the noise bandwidth and selectivity of the
3 ? f c
ButterFilter
≤ BW passive,filter ≤ 4 ? f c
ButterFilter
receiver and hence its sensitivity. Each filter comprises
a passive and an active section.
3.4.4.2
Active Filter
3.4.4.1
Passive Filter
The “fine” channel selection is performed by an active,
Each channel select filter features a passive second-
order RC filter, with a bandwidth programmable
through the PASFILV<3:0> bits (FILCREG<7:4). As the
wider of the two filters, its effect on the sensitivity is
negligible, but its bandwidth must be set up to optimize
blocking immunity. The value entered into this register
sets the single side bandwidth of this filter. For optimum
performance it should be set to three to four times the
cut-off frequency (fc) of the active Butterworth (or
Polyphase) filter described in Section 3.4.4.2, Active
Filter , and as shown in Equation 3-8 .
third-order, Butterworth filter, which acts as a low-pass
filter for the zero-IF configuration (FSK), or a complex
Polyphase filter for the low-IF (OOK) configuration. The
POLFILEN bit (SYNCREG<7>) enables or disables the
Polyphase filter.
Figure 3-6 illustrates the required bandwidth of this
filter that varies between the two demodulation modes.
FIGURE 3-6:
ACTIVE CHANNEL FILTER DESCRIPTION
Low-pass filter for FSK (POLFILEN = 0)
- f c
0
f c
frequency
Polyphase filter for OOK (POLFILEN = 1)
Canceled side of
the polyphase filter
- f c
- f c
0
frequency
FSK mode : The 99% energy bandwidth of an FSK
modulated signal is approximated, as shown in
Equation 3-9 .
EQUATION 3-9:
EQUATION 3-10:
2 ? f c > BW 99%,fsk + LO drifts
= 2 ? f dev + -------
BW 99%,fsk
BR
2
Figure 3-11 illustrates an accurate overview of the filter
bandwidth vs. setting.
OOK mode : The 99% energy bandwidth of an OOK
modulated signal is approximated, as shown in
BW 99%,ook = ------- = 2 ? BR
The BUTFILV<3:0> bits from FILCREG set co the cut-
off frequency ( f c ) of the filter. In a zero-IF configuration,
the FSK lobes are centered on the virtual “DC”
frequency. The choice of co should be such that the
modulated signal falls in the filter bandwidth,
anticipating the Local Oscillator frequency drift over the
operating temperature and aging of the device as
shown in: Equation 3-10
Equation 3-11 .
EQUATION 3-11:
2
t bit
DS70622C-page 62
Preliminary
? 2010–2011 Microchip Technology Inc.
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