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参数资料
| 型号： | CXA3266Q |
| 元件分类： | PLL合成/DDS/VCOs |
| 英文描述： | PHASE LOCKED LOOP, 0.12 MHz, PQFP48 |
| 封装： | PLASTIC, QFP-48 |
| 文件页数： | 31/62页 |
| 文件大小： | 929K |
| 代理商： | CXA3266Q |
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CXA3266Q
- 37 -
The loop filter F (s) is described below.
The loop filter smoothes the output pulse from the phase comparator and inputs it as the DC component to the
VCO. In addition to this, however, the loop filter also plays an important element in determining the PLL
response characteristics.
Typical examples of loop filters include lag filters, lag-lead filters, active filters, etc. However, the CXA3266Q's
LPF is a current input type active filter as shown below, so the following calculations show an actual example
of deriving the PLL closed loop transmittance when using this type of filter and then using this transmittance to
create a formula for setting the filter constants.
Current input type active filter
Next, substituting (2) into (1) and obtaining the overall closed loop transmittance for the PLL :
(
θo/N)/θr = ((KPD KVCO τ/NC) S + (KPD KVCO/NC))/(S2 + (KPD KVCO τ/NC) S +
(KPD KVCO/NC)) .................................. (3)
= (2
ζωnS + ωn2)/(S2 + 2ζωnS + ωn2).......... (4)
ωn =
....................................... (5)
ζ = 1/2 ωnτ..................................................... (6)
The filter transmittance is as follows.
The Bode diagram for formula (2) is as follows.
VO/A + VO = (R + 1/SC)
F (S) = (1 + SRC)/SC A/(1 + A)
= (1 + S
τ)/SC A/(1 + A)
Therefore ;
τ = RC
Here, assuming A > 1, then ;
F (S) = (1 + S
τ)/SC ......................(2)
Vo
CR
ii
–Vo
–1
–A
log w
–45deg
0
–90
phase
[deg]
log
scale
gain
[dB]
1
τ1
DPD KVCO
NC
-------------------------------------
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