Datasheet

MRF49XA
DS70590C-page 56 Preliminary © 2009-2011 Microchip Technology Inc.
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)
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
-140
-120
-100
-80
-60
-40
-20
0
20
40
1.0E+02 1 .0E+03 1 .0E+04 1.0E+05 1.0E+06
Frequency (Hz)
Out put Powe r Leve l (dB)
• 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.