Digital PMR Radio Processor
CMX7131/CMX7141
6.11 RF Synthesiser (CMX7131 only)
The CMX7131 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 Synthesiser Data (CMX7131 only) - $B2 write
RF Synthesiser Control (CMX7131 only) - $B3 write
RF Synthesiser Status (CMX7131 only) - $B4 8-bit read
External RF components are needed to complete the synthesiser circuit. A typical schematic for a 446MHz
synthesiser (3.125kHz comparison frequency) is shown in Figure 24.
RFVDD
CPVDD
RF
Output
Reference Frequency
VCO
R34 C34
RFnP
R33
R32
Dividers
and
Phase
Detector
+
-
Charge
Pump
CPnOUT
RFnN
C35
C33
C31
C32
ISETn
R31
RFVSS
CMX7131
RFVSS
RFVSS
Note: n = 1 or 2 for Synthesiser 1 or 2
Figure 24 Example RF Synthesiser Components
R31 0
C31 22nF
C32 470nF
C33 10nF
C34 1nF
C35 1nF
R32 5.6k
R33 10k
R34 100
Resistors 5%, capacitors and inductors 20% unless otherwise stated.
Note: R31 is chosen within the range 0 to 30k and selects the nominal charge pump current.
It is recommended that C34 and C35 be kept close to the VCO and that the stub from the VCO to
the CMX7131 be kept as short as possible. The loop filter components should be placed close to
the VCO.
2014 CML Microsystems Plc
Page 55
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