CMX983 Datasheet, PDF (49/93 Pages) CML Microcircuits – Programmable Channel Filter

Analogue Front End (AFE) for Digital Radio
CMX983
12 Fractional-N Frequency Synthesizers
PLL1_CFG
PLL1_STATUS
PLL1
lock status
PLL1_CON
PLL1_LOCKDET
PLL1_BLEED
PLL1_RDIV
MCLK
Ref divider
(÷1 … ÷128)
Lock
detector
Phase
detector
Bleed
current
Charge
pump
Multi-modulus divider
(÷32 … ÷2047)
dither
PRBS
CLK
Fastlock
timer
Σ-Δ mod
16/24 bit
Divider
update
Fastlock
control
PLL1_FDIV1
PLL1_FDIV0
PLL1_IDIV
PLL1_FLCK
CP1VDD
CP1
C1 R1
RF1P
RF1N
VCO
R3
C2
R2 C3
FLCK1
Ground capacitor
on RF1N if VCO
output is single-
ended
Figure 21 Fractional-N Frequency Synthesizer
The CMX983 has two identical, independently programmable, 2.1 GHz fractional-N frequency synthesizers; a
block diagram of one of the synthesizers is shown in Figure 21. Note that a single ended VCO signal can be
used; if so, it should be ac-coupled to RF1P and the capacitor connected to RF1N should be grounded.
The synthesizers use a sigma-delta modulation technique that allows use of a high reference frequency, thus
providing rapid frequency switching and low phase noise performance. The 24-bit fractional divider resolution
provides an ultra-fine step size for narrowband applications, and can be used to compensate for crystal
oscillator frequency drift or Doppler shift.
A fast locking mechanism is provided that increases the transition rate when changing to a new operating
frequency. This is done by temporarily modifying the loop filter characteristics and charge pump gain
whenever the main divider settings are updated, allowing the responsiveness of the closed loop system to be
increased without compromising the loop stability. The fast lock mode automatically turns off after a
predetermined delay, thus reverting the PLL to its standard, low noise mode of operation.
Each synthesizer has a programmable lock detector circuit that indicates when the loop is in lock. The lock
detectors can be configured for analogue or digital operation, and no external components are required.
 2015 CML Microsystems Plc
49
D/983/6