Phase Lock Loop Design

Discussion in 'The Projects Forum' started by robertlee, Mar 8, 2011.

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  1. robertlee

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    Mar 8, 2011
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    I need to design a Phase Lock Loop for my Project.
    The input signal is a Resonant freq that will shift from 10Mhz @ 26 degrees Celsius downwards as temperature decreases @ 4khz per degrees Celsius
    Output signal is required to be in digital value.

    input freq varies linearly so i suppose a 2nd order LPF will suffice.
    however i am not sure on how to design the VCO LPF MULTIPLIER circuit using inductors, resistors, capacitors e.t.c
    can anyone help?
     
    Last edited: Mar 16, 2011
  2. Ron H

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    What is the purpose of the PLL?
     
  3. robertlee

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    My main project is to design a mems temperature sensor making use the the resonant freq shifting property of the mems chip to output the temperature.
    MEMS sensor will output temperature by means of its shifting resonant frequency with reference to temperature.
    therefore i need to design a PLL to get the output
     
  4. Ron H

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    I don't see why you need a PLL to read the resonant frequency.
     
  5. robertlee

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    this is because eventually i need a digital value as output for the temperature sensor
     
  6. robertlee

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    i have decided to use a 3rd order butterworth LPF filter and Colpitts Oscillator for the PLL.
    Gain for oscillator is designed to be 1.
    Input and Output impedance set to 50 Ohms.
    However i do not know what other design considerations do i have to take into account.
    I have read up on various Colpitts oscillator designs and they come in different schematics with BJT or MOSFET transistors. I am confused on how to design the circuit
     
  7. Ron H

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    Are you planning to replicate the resonant frequency with the PLL? If so, why don't you just pass the signal from the resonant circuit through a comparator? That will give you a digital version of the analog signal.
    Am I missing something?
     
  8. robertlee

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    Hi,
    I am not replicating the resonant frequency.
    My aim is to detect the shifting resonant frequency and convert it to a digital value.
     
  9. Ron H

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    I don't see how a PLL will help you do that. Can you explain? Perhaps a block diagram would help. It sounds to me like you need a frequency counter. You don't need a PLL for that.
     
  10. robertlee

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    Hi Ron, I have consulted my prof and yes a PLL does work.
     
  11. hgmjr

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    So 155KHz represents one end of temperature range while 160KHz represents the other extreme of the temperature range. So you are looking to use the PLL to demodulate the shifting frequency from the MEMS sensor. Is that more or less what you are attempting to do?

    hgmjr
     
  12. robertlee

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    Mar 8, 2011
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    YES
    Fo for my case @ 26 degrees Celsius is roughly 10Mhz and as the temperature decreases, the Fo will decrease by 4kHz per degrees Celsius
     
  13. mjhilger

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    Feb 28, 2011
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    I'm confused about your design criteria. You have stated throughout this thread that the freq's of interest are 155 - 160 KHz. Then in the above you indicate 10 Mhz as Fo.
    Trying to read between the lines, I think you have a mems sensor who's resonate frequency changes with temp and @ 26 Celsius this is at approx 10 Mhz. The sensors resonate freq will decrease by 4 KHz for each 1 degree C.
    So, are you already mixing with a local osc to achieve the shift down to 155 KHz range? Or is the operating temp environment for the sensor such that the area of interest is 155/160 KHz. If you are going to need a digital response across a broad range, then hgmjr has already suggested the best solution - a counter. If the temp is such that the only frequencies that can exist are the 155 or 160 KHz, then you will use a PLL with a local VCO locked to the resonate freq. You would then compare the VCO control voltage and provide either a 1 or 0 (digital from a comparator using a threshold voltage level to adjust the compare [ I would also use hystersis]). I think hgmjr was trying to point you in that direction, but we are not clear about your needs.

    Perhaps if you could provide all inputs and outputs that are necessary (and resolution of the signals as well). The more information you provide, the easier it will be to point you in the right direction.
     
  14. robertlee

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    hi sorry for the misunderstanding, i will not be using the PLL anymore though it works.
    I have updated my first post nevertheless.
    My Prof told me to design a low power Temperature sensor using MEMS Resonant freq and PLL is gg to take up a low power...
    Therefore I started another thread.

    THREAD CLOSE thnx
     
  15. hgmjr

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