AIS Baseband IC with/without RF Synthesiser
CMX7032/CMX7042
7.9 RF Synthesiser (CMX7032 only)
The CMX7032 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. Then, a
single C-BUS command will change over from the transmit to the receive division ratios, or vice versa,
enabling a fast turnaround.
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
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.
Note: 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
CMX7032 is kept as short as possible. The loop filter components should be placed close to the VCO.
Both RF synthesisers utilise on-chip phase locked loops (PLLs) of the same design, together with external
VCOs and loop filters. The VCOs need to have good phase noise performance although it is likely that the
high division ratios used will result in the dominant noise source being the reference oscillator. The phase
detectors are of the phase-frequency type with a high impedance charge pump output requiring just
passive components in the loop filter. Lock detect functions are built in to each synthesiser and the status
reported via C-BUS. A transition to out-of-lock can be detected and communicated via a C-BUS interrupt
to the host µC. This can be important in ensuring that the transmitter cannot transmit in the event of a fault
condition arising.
2012 CML Microsystems Plc
44
D/7032/42_FI1.2/13
CMLMICRO [ CML MICROCIRCUITS ]
