MRF49XA Datasheet, PDF (56/102 Pages) Microchip Technology – ISM Band Sub-GHz RF Transceiver

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:
• Baseband Analog Filter
• Baseband Digital Filter
• Receive Bandwidth
• Receive Data Rate
• Clock Recovery
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
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
Frequency (Hz)
1.0E+05
1.0E+06
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 trans-
mitter, good results are obtained with a bandwidth of at
least twice the transmitter FSK deviation.
Example 3-1 shows the method to calculate the recom-
mended frequency deviation and BBBW for the given
specifications.
EXAMPLE 3-1: FREQUENCY DEVIATION AND BBBW CALCULATION
• Data Rate – 9.6 kbps
• Crystal Accuracy – 40 ppm
• Frequency Band – 915 MHz
• fxerror by the Crystal: 40 x (915000/1000000) = 36.6 kHz
Deviation = Data Rate + 2 x fxerror + 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.
DS70590B-page 54
Preliminary
© 2009 Microchip Technology Inc.