AK1590 Datasheet, PDF (13/36 Pages) Asahi Kasei Microsystems – 1GHz Delta-Sigma Fractional-N Frequency Synthesizer

[AK1590]
3. Charge Pump and Loop Filter
AK1590 has two charge pumps; Charge Pump 1 for normal operation and Charge Pump 2 for Fast Lockup mode.
The internal timer is used to switch these two charge pumps to achieve a Fast Lock PLL.
The loop filter is external and connected to [CP], [SWIN] and [CPZ] pins. [CPZ] pin should be connected to R2 and C2 ,
which are intermediate nodes, even if the Fast Lockup is not used. Therefore, R2 must be connected to [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 operation.
Phase Detector
up
down
Timer
Loop Filter
CP
R3
C1 _R2'
R2
C3
VCO
SWIN
C2
CPZ
Fig. 5 Loop Filter Schematic
MS1478-E-01
13
2013/2