FX919B Datasheet, PDF (23/47 Pages) CML Microcircuits – Excellent Radio Fading Performance

4-Level FSK Modem Data Pump
FX919B
Control Register B5, B4: FSTOL - Frame Sync Tolerance
These two bits have no effect in transmit mode. In receive mode, they define the maximum number of
mismatches which will be allowed during a search for the Frame Sync pattern:
B5 B4
00
01
10
11
Mismatches allowed
0
2
4
6
Note: A single 'mismatch' is defined as the difference between two adjacent symbol levels, thus if the symbol
'+1' were expected, then received symbol values of '+3' and '-1' would count as 1 mismatch, a received symbol
value of '-3' would count as 2 mismatches. A setting of '4 mismatches' is recommended for normal use.
Control Register B3, B2: LEVRES - Level Measurement Modes
These two bits have no effect in transmit mode. In receive mode they set the 'normal' operating mode of the
received signal amplitude and dc offset measuring circuits (the automatic sequencing of an AQLEV command
may temporarily override the 'normal' setting).
B3 B2 Mode
0 0 Hold
0 1 Level Track
1 0 Lossy Peak Detect
1 1 Slow Peak Detect
In normal use the LEVRES bits should be set to '0 1' (Level Track), the other modes are intended for special
purposes, for device testing, or are invoked automatically during an AQLEV sequence.
In ‘Slow Peak Detect’ modes the positive and negative excursions of the received signal (after filtering) are
measured by peak rectifiers driving the DOC1 and DOC2 capacitors to establish the amplitude of the signal and
any dc offset wrt VBIAS. This mode provides good overall performance, particularly when acquiring level
information at the start of a received message, but does not work well with certain long sequences of repeated
data byte values. It is also susceptible to large amplitude noise spikes such as can be generated during deep
fades.
The ‘Lossy Peak Detect’ mode is similar to ‘Slow Peak Detect’ but the capacitor discharge time constant is
much shorter, so this mode is not suitable for normal data reception and is only used within part of the
automatic AQLEV acquisition sequence.
In ‘Level Track’ mode the DOC capacitor voltages are slowly adjusted by the FX919B in such a way as to
minimise the average errors seen in the received signal. This mode provides the best overall performance,
being much more immune to large amplitude noise spikes than ‘Slow Peak Detect’ and being much less
sensitive to long sequences of repeated data byte values. It does, however, depend on the measured levels
and timing being approximately correct. If either of these is significantly wrong then the correction algorithm
used by the ‘Level Track’ mode can actually drive the voltages on the DOC capacitors away from their optimum
levels. For this reason the automatic AQLEV acquisition sequence (see 1.6.3) forces the level measuring
circuits into ‘Slow Peak Detect’ mode until a Frame Sync pattern has been found.
© 1997 Consumer Microcircuits Limited
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D/919B/1