# how to select the PLL parameters

#### anstxfw

Sep 15, 2016
4
The voltage waveform of the PLL circuit VCOin is shown(The PLL can lock).I need to get the stable voltage.If I change C18 from 0.1uF to 10uF, PLL cann't lock phase,the VCOin is always 5V,How to select C18/R5/R6 of the filter.How to calculate?

#### Attachments

• pll.jpg
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#### anstxfw

Sep 15, 2016
4
First,I illustrate the application of this circuit,as shown in Figure 1.It is called capacitive liquid level

detection.When the needle touches the liquid level,the capacitance value will become larger.So using phase locked loop

to detect the change of capacitance.My schematic diagram is shown in Figure 2.Network named signal is connected to the

needle.Normally,the voltage of TP1 is 2.5v,if the neddle touches the liquid level,the voltage of TP1 will change,as

shown in Figure 3.However, When the needles are filled with ionized water, the voltage will become unstable,as shown

in Figure 4.The frequency of the TP1 point voltage is 50Hz.It will affect the judgement of the waveform.Now I have two

solutions,one is change the low pass filter parameter of PLL,so it can filter out the interference of 50hz, but to

consider the timeliness of change.Another is to add a notch filter.where to put the notch filter,I want to put it in

the output of OP U4,so the signal is passed through the notch filter before it is passed to MCU.
Now my question is that which will be better,and how to make it.If you have some advice,I will appreciate it very

much.Thank you very much.

Sep 15, 2016
4
the diagram

#### Attachments

• figure.jpg
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#### anstxfw

Sep 15, 2016
4
who can help me ?

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