GMSK Packet Data Modem
CMX909B
Control Register B3, B2: LEVRES - Level Measurement Response Time
These two bits have no effect in transmit mode.
In receive mode, they set the ‘normal’ response time of the Rx signal amplitude and dc offset
measuring circuits. This setting will be temporarily overridden by the automatic sequencing of an
AQLEV command.
B3 B2
Setting
Hold
Peak Averaging
Peak Detect
Lossy Peak Detect
Action
0
0
1
1
0
1
0
1
Keep current values of amplitude and offset
Track input signal using bit peak averaging
Track input signal using peak detect
Track input signal using lossy peak detection
For Mobitex systems, and most general purpose applications using the modem, these bits should
normally be set to ‘Peak Averaging’ except when the µC detects a receive signal fade, when ‘Hold’
should be selected.
The ‘Lossy Peak Detect’ setting is intended for systems where the µC cannot detect signal fades or
the start of a received message, as it allows the modem to respond quickly to fresh messages and
recover rapidly after a fade without µC intervention - although at the cost of reduced Bit Error Rate
versus Signal to Noise performance.
Note that, since the measured levels are stored on the external capacitors C6 and C7, they will
decay gradually towards V
when the ‘Hold’ setting is chosen, the discharge time-constant being
BIAS
approximately 2000 bit times. More details of the level measurement system are given in section
1.6.3.
Control Register B1, B0: PLLBW
These two bits have no effect in transmit mode.
In receive mode, they set the ‘normal’ bandwidth of the Rx clock extraction Phase Locked Loop
circuit. This setting will be temporarily overridden by the automatic sequencing of an AQBC
command.
B1 B0
PLL Bandwidth
Hold
Suggested use
0
0
1
1
0
1
0
1
Signal fades
Narrow
Medium
Wide
± 20ppm or better Xtals
Wide tolerance Xtals or long preamble acquisition
Quick acquisition
The ‘hold’ setting is intended for use during signal fades, otherwise the minimum bandwidth
consistent with the transmit and receive modem bit rate tolerances should be chosen.
The wide and medium bandwidth settings are intended for systems where the µC cannot detect
signal fades or the start of a received message, as they allow the modem to respond rapidly to fresh
messages and recover rapidly after a fade without µC intervention - although at the cost of reduced
Bit Error Rate versus Signal to Noise performance.
Note: More details of the clock extraction system are given in section 1.6.3.
ã 2001 Consumer Microcircuits Limited
27
D/909B/1
CMLMICRO [ CML MICROCIRCUITS ]
