phase change

Discussion in 'General Electronics Chat' started by shreyas_bhat, Feb 26, 2005.

  1. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
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    Hi ppl
    if in a Twin T oscillator, the capacitor changes the freuquency of oscillation changes. Would there be a change in the phase shift. Could i use the phase change in the output voltage to detect a change in the capacitance ?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    You are correct that the overall phase shift will change as the capacitance changes in a twin-T oscillator. However, as the frequency out of the oscillator must track the frequency change, it will not reflect any of the phase shift. You would need a reference frequency to track such a change, unless the change was of short duration and occurred very rapidly. You simply have to have a reference point to measure phase shift.
     
  3. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
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    Hi beenthere
    thanks for sharing the info. i'd appreciate if you could exactly tell me how to go about it (how to choose a reference signal etc.) ?


     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    That is an interesting question. With a constant signal, it is easy to see if another signal has shifted its phase relationship. An oscillator changes its frequency with the phase shift, so I am not certain that it could serve as its own reference.

    If you have a dual beam oscilloscope, set it up so the top trace displays the oscillator output. Use it for the trigger. Put the second probe at the feedback point and see if you can detect phase shift from the output, and then see if you can see it change as you vary the capacitance.

    Many years ago, I used a Gersch complex ratio bridge to measure phase relationships. I have no idea what instruments might be available to day to do the same thing.
     
  5. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
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    could I use a PLL to do the same. If yes, how ?

    Shreyas
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    I'm not able to answer your last question for certain. As far as I know, the phase shift causing the oscillation is constant. Adjusting the value of a capacitor will cause a transient phase shift, but as soon as the new frequency stabilizes, the phase shift will be effectively nulled out. Tracking the frequency change with a pll will give you an indication of the phase shift - if you can get at the error voltage signal. You would need to be able to record this transient, as the error will go to zero as the oscillator steadies on the new frequency. Changing capacitance as a step function rather than as an adjustment will make the brief error voltage easier to see.

    I'm mostly guessing here. I've used twin-T's as filters and oscillators, but have never been concerned about actual phase shift. I was just happy if I got my result. Technicians are like that.

    Good luck.
     
  7. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
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  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    You might as well try to eliminate your sensors as a possible source of instability. Try replacing them with fixed capacitors of about the same value. Use polyester or mylar film types to make them as stable as possible. Then see it the oscillator settles to a constant frequency. Also a good time to look for long-term frequency drift that might point to other components.

    Assuming that fixes your drift problem, you will need to find out what is causing the disturbance in the sensors. Do they need more shielding? Are they microphonic and need better mounting?

    If the oscillator still has poor frequency stability, try changing your op amp for one with better specs. Use film resistors rather than carbon. This can be tedious, but you have to find the source of the problem.
     
  9. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
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  10. shreyas_bhat

    Thread Starter Active Member

    Jul 26, 2004
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    Thanks for the step by step approach on troubleshooting. Really appreciate ur help. Shall try ur suggestions and get back in case of need.

    Thanks
    Shreyas
     
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