FX919B Datasheet, PDF (40/47 Pages) CML Microcircuits – Excellent Radio Fading Performance

4-Level FSK Modem Data Pump
FX919B
1.6.6 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 includes 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 FX919B to re-acquire the received signal levels and timing
once it has been established that the received carrier level is satisfactory. This re-acquisition should follow the
normal procedure given in section 1.6.3.
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 illustrated below.
Figure 22 Received Signal Quality Monitor Flowchart
© 1997 Consumer Microcircuits Limited
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