how to select the PLL parameters

Discussion in 'Analog & Mixed-Signal Design' started by anstxfw, Sep 14, 2016.

  1. anstxfw

    Thread Starter New Member

    Sep 14, 2016
    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 the parameters of R3/R4,and C18/R5/R6 of the filter.How to calculate?
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  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
  3. anstxfw

    Thread Starter New Member

    Sep 14, 2016
    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.
  4. anstxfw

    Thread Starter New Member

    Sep 14, 2016
    who can help me ?
  5. Sensacell


    Jun 19, 2012
    So you are trying to make a water level sensor with a PLL driven from a fixed oscillator block?

    There are much better, simpler ways to detect changes in capacitance, especially using an MCU.