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AS3932

8.5 Channel Selector / Demodulator / Data Slicer

When at least one of three gain channel enters initial wake-up state the channel selector makes a decision which gain channel to connect to the

envelope detector. If only one channel is in wake-up state the selection is obvious. If more than one channel enters wake-up state in 256µs

following the first active channel the channel with highest RSSI value is selected. The output signal (amplified LF carrier) of selected channel is

connected to the input of the demodulator.

The performance of the demodulator can be optimized according to bit rate and preamble length as described in Table 16 and Table 17.

If the bit rate gets higher the time constant in the envelop detector must be set to a smaller value, this means that higher noise is injected

because of the wider band. The next table is a rough indication of how the envelop detector looks like for different bit rates. By using proper data

slicer settings it is possible to improve the noise immunity paying the penalty of a longer preamble. In fact if the data slicer has a bigger time

constant it is possible to reject more noise, but every time a transmission occurs, the data slicer need time to settle. This settling time will

influence the length of the preamble. Table 17 gives a correlation between data slicer setup and minimum required preamble length.

Note:

These times are minimum required, but it is recommended to prolong the preamble.

The comparator of the data slicer can work only with positive or with symmetrical threshold (R3<6>). In addition the threshold can be 20 or 40 mV

(R3<7>)

In case the length of the preamble is an issue the data slicer can also work with an absolute threshold (R1<7>). In this case the bits R3<2:0>

would not influence the performance. It is even possible to reduce the absolute threshold in case the environment is not particularly noisy

(R2<7>).

Table 16. Bit Setup for the Envelop Detector for Different Symbol Rates

R3<2>

R3<1>

R3<0>

Symbol rate [Manchester symbols/s]

0

0

0

4096

0

0

1

2184

0

1

0

1490

01

1

1130

1

0

0

910

1

0

1

762

1

1

0

655

1

1

1

512

Table 17. Bit Setup for the Data Slicer for Different Preamble Length

R3<5>

R3<4>

R3<3>

Minimum preamble length [ms]

00

0

0.8

00

1

1.15

01

0

1.55

01

1

1.9

10

0

2.3

10

1

2.65

11

0

3

11

1

3.5