MRF49XA-ISLASHT datasheet(56/102 Pages) MICROCHIP

56 / 102 page

MRF49XA

DS70590C-page 56

Preliminary

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

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

Frequency (Hz)

• 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.

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