mrf89xa Microchip Technology Inc., mrf89xa Datasheet - Page 66

• Manufacturer Part Number: mrf89xa
• Description: Ultra-low Power, Integrated Ism Band Sub-ghz Transceiver
• Manufacturer: Microchip Technology Inc.

3.4.4
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
receiver and hence its sensitivity. Each filter comprises
a passive and an active section.
3.4.4.1
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.
EQUATION 3-8:
FIGURE 3-6:
FSK mode: The 99% energy bandwidth of an FSK
modulated signal is approximated, as shown in
Equation 3-9.
EQUATION 3-9:
The BUTFILV<3:0> bits from FILCREG set the cut-off
frequency (f
FSK lobes are centered on the virtual “DC” frequency.
DS70622B-page 66
MRF89XA
3 f
•
c ButterFilter
CHANNEL FILTERS
c
-f C
) of the filter. In a zero-IF configuration, the
BW
Passive Filter
99%,fsk
≤
BW
ACTIVE CHANNEL FILTER DESCRIPTION
=
passive,filter
2
•
Polyphase filter for OOK (POLFILEN = 1)
-f o
f
dev
Low-pass filter for FSK (POLFILEN = 0)
≤
-f C
+
4 f
•
BR
------ -
2
c
ButterFilter
Preliminary
0
0
3.4.4.2
The “fine” channel selection is performed by an active,
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 varies between the two demodulation modes.
The choice of f
signal falls in the filter bandwidth, anticipating the Local
Oscillator
temperature and aging of the device as shown in
:Equation 3-10
EQUATION 3-10:
Figure 3-11 illustrates an accurate overview of the filter
bandwidth vs. setting.
the polyphase filter
Canceled side of
f C
frequency
Active Filter
2 f
•
c
should be such that the modulated
c
>
BW
© 2010 Microchip Technology Inc.
99%,fsk
drift
+
over
LO
drifts
the
f requency
f requency
operating