Data Sheet

4-Level FSK Modem Data Pump Page 40 of 47 MX919B PRELIMINARY INFORMATION

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5.6 Received Signal Quality Monitor

In applications where the modem has to monitor a long transmission containing a number of concatenated

Frames, it is recommended that the controlling software include a function which regularly checks that the

modem is still receiving a good data signal and triggers a re-acquisition and possibly changes to another

channel if a problem is encountered. This strategy has been shown to improve the system's overall

performance in situation where fading, large noise bursts, severe co-channel interference, or loss of the

received signal for long periods are likely to occur.

Such a function can be simply implemented by regularly reading the Data Quality Register, which gives a

measure of the overall quality of the received signal, as well as the current effectiveness of the modem's clock

extraction and level measurement systems. Experience has shown that if two consecutive DQ readings are

both less than 50 then it is worth instructing the MX919B to re-acquire the received signal levels and timing

once it has been established that the received carrier level is satisfactory. Re-acquisition should follow the

procedure given in Section 5.3.

The intervals between Data Quality readings is not critical, but should be a minimum of 64 symbol times

except for the first reading made after triggering the AQSC and AQLEV automatic acquisition sequences,

which should be delayed for about 250 symbol times.

A suitable algorithm is shown in Figure 27.

Note: Times are symbol times.

Reset timer.

Set µC variable 'LAST_DQ' to 99

Read DQ register into

µC variable 'THIS_DQ'

Copy 'THIS_DQ' to 'LAST DQ'.

Reset Timer.

Ye s

AQSC/AQLEV

task issued

Re Acquire

Timer > 250 ?

Timer > 64 ?

'THIS_DQ' < 50 ?

'LAST_DQ' < 50 ?

Figure 27: Received Signal Quality Monitor Flowchart