MRF89XA datasheet(63/140 Pages) MICROCHIP

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Preliminary

DS70000622D-page 63

MRF89XA

The POLCFV<3:0> bits (PFCREG<7:4>) set fo, which

is the center frequency of the polyphase filter when

activated. The fo should always be chosen to be equal

to the low Intermediate Frequency of the receiver (IF2).

Because low IF frequency of the OOK receiver denoted

by IF2 can always be replaced by fo for any calculations

or monitoring purposes.

The following setting is recommended:

fo = 100 kHz

POLCFV<3:0> = 0011.

The

value

stored

BUTFILV<3:0>

bits

(FILCREG<3:0>) determines fc, the filter cut-off

frequency. Therefore, fc should be set according to

Equation 3-12.

EQUATION 3-12:

Again, fc, as a function of the BUTFILV<3:0> bits, is

described in Section 3.4.6, Channel Filters Setting in

OOK Mode.

3.4.5

CHANNEL FILTERS SETTING IN

FSK MODE

The fc, the 3 dB cut-off frequency of the Butterworth fil-

ter used in FSK reception, is programmed through the

BUTFILV<3:0> bits (FILCREG<3:0>). However, the

entire receiver chain influences this cut-off frequency.

The channel select and resultant filter bandwidths are

illustrated in Figure 3-7.

Table 4-2 suggests filter settings in FSK mode along

with the corresponding passive filter bandwidth and the

accepted tolerance on the crystal reference.

FIGURE 3-7:

ACTUAL BW OF BUTTERWORTH FILTER

2 fc fo

–

 BW

99%,ook

LOdrifts



Where,

fc is the cut-off frequency

fo is the center frequency

Butterworth Filter BW, FSK

100

150

200

250

300

350

400

450

Val BUTFILV<3:0> [d]

Actual

Theoretical

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