4-Level FSK Modem Data Pump
Page 43 of 50
MX929B PRELIMINARY INFORMATION
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 situations 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 MX929B 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 interval 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.
AQSC/AQLEV
task issued
Reset timer.
Set µC variable 'LAST_DQ' to 99
No
Note: Times are symbol times.
Timer > 250 ?
Read DQ register into
µC variable 'THIS_DQ'
No
No
'THIS_DQ' < 50 ?
Yes
'LAST_DQ' < 50 ?
Yes
Copy 'THIS_DQ' to 'LAST DQ'.
Reset Timer.
Yes
No
Re Acquire
Timer > 64 ?
Figure 27: Received Signal Quality Monitor Flowchart
©2001 MX-COM, INC.
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Doc. # 20480171.003
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