Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
C4FM RX Modes:
The C4FM RX Mode encodings are specific to receiving C4FM. When writing a new C4FM RX Mode after the
signal type has been detected, in addition to setting the task code bits, the applicable receive mode, C4FM
should be enabled and the other receive modes disabled.
Bit 3
0
Bit 2
0
Bit 1
0
DEFAULT TRACKING MODES.
X
X
X
X
0
1
Stochastic Gradient symbol clock tracking.
Fixed symbol clock.
0
0
1
1
0
1
0
1
X
X
X
X
Slow symbol level tracking.
Fast symbol level tracking.
Partial Response symbol level tracking.
Fixed symbol levels.
Symbol Clock Extraction Modes:
1. Stochastic Gradient Clock Tracking mode is able to extract the symbol clock in the presence of relative
clock drift between the transmitter and the receiver in excess of 40ppm. This is the default clock
tracking mode following a reset.
2. Fixed Symbol Clock mode fixes the symbol sample point relative to the last tracked symbol sample
point, using only the local receiver clock. This mode can be selected to override the clock tracking mode
in circumstances where excessive noise could cause the tracking to shift to an inappropriate sampling
phase.
When a C4FM signal is detected, by recognising the frame sync pattern, the optimum symbol sample timing is
extracted from the received FS signal. This is irrespective of the selected clock tracking mode.
Symbol Level Tracking Modes:
1. Slow Symbol Level Tracking mode adjusts the symbol level thresholds to minimise the average symbol
sample level errors (ie. the deviation of the sampled level from the optimum level). This is the default
level tracking mode following a reset.
2. Fast Symbol Level Tracking mode adjusts the symbol level thresholds in the same way as Slow Symbol
Level Tracking, but applies a larger level adjustment at each level change period.
3. Partial Response Symbol Level Tracking mode filters the received signal (after the sin(x)/x filtering) and
tracks the peaks, from which the symbol levels are derived. The peak values decay slowly whilst the PR
filtered signal is within these peak levels. This mode is particularly effective when receiving a noisy
signal, but will not work well with certain long sequences of repeated symbol patterns.
4. Fixed Symbol Levels mode sets the symbol levels and thresholds to the last tracked symbol level. This
mode can be selected to override the other level tracking modes in circumstances where excessive
noise could cause the tracking to shift to inappropriate sample levels.
When a C4FM signal is detected, by recognising the frame sync pattern, the optimum symbol sample levels are
extracted from the received FS signal. This is irrespective of the selected level tracking mode.
ã 2001 Consumer Microcircuits Limited
56
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
