AK1541_17 Datasheet, PDF (12/37 Pages) Asahi Kasei Microsystems – 20 to 600Mhz Delta-Sigma Fractional-N Frequency Synthesizer

[AK1541]
2. Charge Pump and Loop Filter
The AK1541 has two charge pumps; Charge Pump 1 for normal operation and Charge Pump 2 for Fast Lockup.
The internal timer is used to switch those two charge pumps to achieve a fast lock PLL. The loop filter is external and
connected to [CP], [SWIN] and [CPZ] pins.
The [CPZ] pin should be connected to the R2 and C2 , which are intermediate nodes, even if the Fast Lockup is not used.
Therefore, R2 must be connected to the [CP] pin, while C2 must be connected to the ground.
R2 and R2’ are connected in parallel with internal switch in Fast Lockup. These R2 and R2’ parallel resistance value is
required for calculating loop bandwidth and phase margin in Fast Lockup.
Phase Detector
up
down
Timer
Loop Filter
CP
R3
C1 _R2'
R2
C3
VCO
SWIN
C2
CPZ
Fig. 5 Loop Filter Schematic
MS1043-E-05
12
2013/03