BU2373FV_11 Datasheet, PDF (13/19 Pages) Rohm – High Performance VCOs for Image Sampling

BU2373FV,BU2374FV
Technical Note
(Functioning of PLL System)
In order to configure the stable PLL system, the following section describes the functional principle, open loop
characteristics, and closed loop characteristics by block shown in the Block Diagram below.
PLL System Block Diagram
FIN-A ^θi
Phase-comparator
LPF
Kp
F(s)
FIN-B ^θo
VCO
Kv/s
VCO_OUT
1/N
Divider
Fig.58
① Phase Comparator
The phase comparator shows the characteristics shown in figure below. Assuming that the Gain is Kp,
Kp=(VOHVOL)/4(V/rad)
② VCO (Voltage Controlled Oscillator)
The VCO shows the characteristics shown in figure below.
Assuming that the Gain is Kv, Kv=2   (fmax-fmin)/(Vmax-Vmin)(rad/s/V)
VOH
frequency
　 ^(MHz)
fm ax
-2π
0
2π
VOL
Fig.59 Phase Comparator Characteristics
fm in
0
Vm in
Vm ax
Fig.60 VCO Characteristics
VCO_IN (V)
③ LPF (Lag-Lead Filter)
Calculate the Gain of the lag-lead filter. It is recommended to use the filter having the pattern shown below.
R2
PDOUT
VCOIN
R1
C1
C2
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Fig.61
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2011.08 - Rev.B