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MRF49XA
3.10 Baseband/Data Filtering
The baseband receiver has several programming
options to optimize the communication for a wide range
of applications. The programmable functions are as
follows:
A suitable bandwidth should be used to achieve various
FSK deviation, data rate and crystal tolerance require-
ments. The filter structure is a 7th order, Butterworth
low-pass with 40 dB suppression at twice the bandwidth
frequency. Offset cancellation is done by using a
high-pass filter, with a cutoff frequency below 7 kHz, in
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Baseband Analog Filter
Baseband Digital Filter
Receive Bandwidth
Receive Data Rate
Clock Recovery
order to achieve the best possible frequency response in
baseband and a good flat response in the pass band.
Figure 3-8 shows the full baseband amplifier transfer
function. This optimizes the chip area, cost and channel
separation.
FIGURE 3-8:
FULL BASEBAND AMPLIFIER TRANSFER FUNCTION (BW = 67 kHz)
40
20
0
-20
-40
-60
-80
-100
-120
-140
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
Frequency (Hz)
The receive bandwidth is programmable from 67 kHz to
400 kHz to accommodate various FSK modulation
deviations. If the deviation is known for a given
transmitter, good results are obtained with a bandwidth
of at least twice the transmitter FSK deviation.
Example 3-1 shows the method to calculate the
recommended frequency deviation and BBBW for the
given specifications.
EXAMPLE 3-1:
FREQUENCY DEVIATION AND BBBW CALCULATION
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Data Rate – 9.6 kbps
Crystal Accuracy – 40 ppm
Frequency Band – 915 MHz
f xerror by the Crystal: 40 x (915000/1000000) = 36.6 kHz
Deviation = Data Rate + 2 x f xerror + 10 = 9.6 + 2 x 36.6 + 10 = 92.8 kHz
The closest possible deviation is 90 kHz.
BBBW = Deviation x 2 – 10 kHz = 90 x 2 – 10 = 170 kHz
The closest possible BBBW is 200 kHz.
The FSK modulated deviation for this example is shown in Figure 3-9 .
DS70590C-page 56
Preliminary
? 2009-2011 Microchip Technology Inc.
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