AX.25 Modem
CMX7031/CMX7041
See:
o
AuxDAC Control - $A8 16-bit write
7.9
RF Synthesiser (CMX7031 only)
The CMX7031 includes two Integer-N RF synthesisers, each comprising a divider, phase comparator and
charge pump. The divider has two sets of N and R registers: one set can be used for transmit and the
other for receive. The division ratios can be set up in advance by means of C-BUS registers. A single C-
BUS command will change over from the transmit to the receive division ratios, or vice versa, enabling a
fast turnaround.
See:
o
o
o
RF Channel Data - $B2 write
RF Channel Control - $B3 write
RF Channel Status - $B4 8-bit read
External RF components are needed to complete the synthesiser circuit. A typical schematic for one
synthesiser, with external components, is shown in Figure 16.
Figure 16 Example RF Synthesiser Components for a 512MHz Receiver
R31 0
R32 18k
R33 18k
C31 820pF
C32 8.2nF
C33 680pF
C34 1nF
C35 1nF
Resistors 5%, capacitors and inductors 20% unless otherwise stated.
Notes: R31 is chosen within the range 0 to 30k and selects the nominal charge pump current.
It is recommended that C34 and C35 are kept close to the VCO and that the stub from the VCO to
the CMX7031 is kept as short as possible. The loop filter components should be placed close to
the VCO.
2013 CML Microsystems Plc
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CMLMICRO [ CML MICROCIRCUITS ]
