AX.25 Modem
CMX7031/CMX7041
Notes:
1. X1 can be a crystal or an external clock generator; this will depend on the application. The tracks
between the crystal and the device pins should be as short as possible to achieve maximum stability
and best start up performance. By default, a 19.2MHz oscillator is assumed, other values could be
used if the various internal clock dividers are set to appropriate values.
2. R5 should be selected to provide the desired dc gain of the discriminator input, as follows:
GAIN
= 100k / R5
DISC
The gain should be such that the resultant output at the DISCFB pin is within the discriminator input
signal range specified in 7.12.2.
3. R7 should be selected to provide the desired dc gain (assuming C13 is not present) of the alternative
input as follows:
GAIN = 100k / R7
ALT
The gain should be such that the resultant output at the ALTFB pin is within the alternative input
signal range specified in 7.12.
4. R9 should be selected to provide the desired dc gain (assuming C15 is not present) of the
microphone input as follows:
GAIN = 100k / R9
MIC
The gain should be such that the resultant output at the MICFB pin is within the microphone input
signal range specified in 7.12.1. For optimum performance with low signal microphones, an additional
external gain stage may be required.
5. C13 and C15 should be selected to maintain the lower frequency roll-off of the discriminator inputs as
follows:
C13 1.0µF GAIN
ALT
C15 30nF GAIN
MIC
For 9600bps GMSK operation the DISCN input should be DC coupled.
6. ALTN and ALTFB connections allow the user to have a second discriminator or microphone input.
Component connections and values are as for the respective DISC and MIC networks. If this input is
not required, the ALT pin should be connected to AVss.
7. C5 (AUDIO/MONITOR out) should be increased to 1.0µF if frequencies below 300Hz need to be
used on this pin. Used for test/debug in this Function Image™.
8. A single 10µF electrolytic capacitor may be fitted in place of C4 and C24, providing the two VDEC
pins are connected together on the pcb with an adequate width power supply trace.
2013 CML Microsystems Plc
Page 12
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CMLMICRO [ CML MICROCIRCUITS ]
