GMSK Modem Data Pump
Page 18 of 37
MX909A PRELIMINARY INFORMATION
4.4.3.2 Control Register B4: DARA - Data Rate
This bit operates in both transmit and receive modes, optimizing the modem's internal signal filtering
according to the relevant bit rate.
If the bit rate used is above 10k bits/sec then this bit should be set to '1' , if not, then it should be set to '0'.
4.4.3.3 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
0
0
1
1
0
1
0
1
Hold
Keep current values of amplitude and offset
Track input signal using bit peak averaging
Track input signal using peak detect
Peak Averaging
Peak Detect
Lossy 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: 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 approximately 2000
BIAS
bit times. Further information of the level measurement system is provided in section 5.3.
4.4.3.4 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
Suggested use
0
0
1
1
0
1
0
1
Hold
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: Further information of the clock extraction system is provided in section 5.3.
¤2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480134.005
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