CMX838 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX838' PDF : 71 Pages View PDF
FRS/PMR446/GMRS Family Radio Processor
CMX838
4.3 RF Synthesizer
This section describes the implemented core functions of an Integer-N frequency Synthesizer. This includes
modules for the RF 32/33 prescaler, programmable divider, phase detector, lock indicator, reference counter
and charge pump. The Block diagram for the module is shown in Figure 20.
REFIN
-RFIN
+RFIN
divide 32/33
12 bit programmable
reference counter
programmable divider
phase
detector
lock
detect
charge
pump
S
ISET
CPOUT
SVDD SVSS
Figure 20: RF Synthesizer block diagram.
4.3.1 Operating Range and Specifications
The RF synthesizer is capable of supporting narrowband (6.25kHz < channel BW <25kHz) applications in the
RF range from 100MHz to 500MHz. In other words, there are no blind channels over this range.
4.3.2 Main Divider
An input buffer amplifies and limits the RF signal from the VCO to a level that drives the dual modulus
prescaler. The main RF divider is implemented using the dual-modulus 32/33 prescaler in conjunction with a
programmable counter. This counter is realized using two programmable counters (A & M Counters).
The M-counter uses a 12-bit programming word and the A-counter uses a 5-bit word, see Figure 21.
DATA WORD
17
RF IN
divide 32/33
5
12
programmable 5 bit
MODULUS
A-counter
programmable 12 bit
M-counter
TO PHASE
DETECTOR
RESET
Figure 21: Block diagram of main programmable divider.
The forward division ratio, N, can be expressed as:
N = (32M + A)
Where A and M represent the programmed data words.
4.3.3 Phase Detector & Charge Pump
A Phase/Frequency detector is implemented where steps have been taken to remove the dead-band normally
associated with this type of detector and charge pump arrangement.
An external resistor, RSET, sets ICHP, the nominal charge pump current. The current through this resistor is set
by a 1.26V on-chip reference at the ISET pin where, ISET = 1.26 ÷ RSET. The magnitude of the charge pump
current is either 40*ISET or 80*ISET depending upon the state of the IHL bit programmed through the C-BUS
serial interface, see Section 5.1.2.8 for programming details.
IHL = 0, ICHP = 40*ISET
IHL = 1, ICHP = 80*ISET
The value of RSET can vary between about 50kΩ and 250kΩ. This gives a charge pump current range of
0.2mA to 2.0mA.
2003 CML Microsystems Plc
26
D/838/8
Share Link: 
