AX.25 Modem
CMX7031/CMX7041
Rx
SIGNAL I/P
1200 BAUD
LOGIC '0'
LOGIC '1'
LOGIC '0'
LOGIC '1'
Figure 12 Modulating Waveform for 1200bps AFSK Signals
Figure 13 Modulating Waveform for 9600bps GMSK Signals
Table 5 Data Frequencies for 1200bps
Baud Rate
1200baud
Data
1
0
Frequency
1200Hz
2200Hz
In receive mode, a PLL is used to extract the data from the incoming signal.
7.6.5 9600bps GMSK Modem
The 9600bps GMSK modem directly modulates the RF carrier via the MOD1 and MOD2 signals (two point
modulation). The data stream from the host is first filtered via a Gaussian filter with a Bt of 0.5 in order to
maximise the signal for efficient reception but at the same time meet the channel emission mask so as not
to interfere with adjacent channel users. If the modulator is capable of being dc coupled, then single point
modulation can be employed, otherwise it is necessary to modulate both the VCO and the Reference of
the Tx PLL to maintain the required frequency response and modulation fidelity.
In addition to the NRZI encoding used in 1200bps operation, the GMSK signal is also passed through a
data scrambler to reduce the dc content of the transmitted signal. This is based on a 17-bit maximum
length LFSR scrambler.
Figure 14 9600bps Data Scrambler
7.7
Auxiliary ADC Operation
The inputs to the two auxiliary ADCs can be independently routed to any of the Signal Input pins under
control of the AuxADC, $A7. Conversions will be performed as long as a valid input source is selected, to
stop the ADCs, the input source should be set to “none”. Register $C0, b6, BIAS, must be enabled for
Auxiliary ADC operation.
2013 CML Microsystems Plc
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