Pi filter with an LF351N Oscillator

Discussion in 'General Electronics Chat' started by Neyolight, Jan 25, 2012.

  1. Neyolight

    Thread Starter Member

    Dec 13, 2011
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    Hi All

    I have a "pi filter" with connects to the output an oscillator. The oscillator is connected to a 12 V DC power supply.

    I dont really know whats happening in the circuit - kindly help :confused:

    The circuit is supposed to oscillator at a certain frequency, but im only reading 0s :mad:

    Regards Neyolight
     
    Last edited: Feb 2, 2012
  2. Audioguru

    New Member

    Dec 20, 2007
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    We also don't know what is happening in your circuit because you forgot to post its schematic with parts values.
     
  3. Neyolight

    Thread Starter Member

    Dec 13, 2011
    54
    3
    Oooops hehe true that :D! Will have to draw it out in paint as i dont have a scanner here !

    Schematic coming soon.....
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    By the way, the LF351 is a single JFET-input operational amplifier, not an oscillator.
    It is a very close cousin to the TL071.

    Neither of these opamps can see within 3v of the negative rail, or within 1.5v of the positive rail.

    I am almost willing to bet that you are attempting to operate your circuit using a single-rail supply, and it simply is not enough to allow it to function - either that, or you are trying to operate the circuit on < 6v.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    You did not specify values for R, R1, R2, C, C1, nor C2.

    Do you know any of the physical characteristics of the inductor? Like how many turns of wire, average diameter, length, does it have a core, if so what type/dimensions/part number?
     
  6. Neyolight

    Thread Starter Member

    Dec 13, 2011
    54
    3
    Yup, and an op-amp can be made to oscillate as a certain frequency ? This is what im actually trying to achieve-getting that circuit to oscillate at its resonant frequency! Is it possible to do so?

    Once I have the circuit oscillating, I will change the inducatance value gradually and will see its effect on frequency! :rolleyes:
     
  7. Neyolight

    Thread Starter Member

    Dec 13, 2011
    54
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    yes, I can get some information about the wire used to build that inductance. Its copper and has about 30 turns!
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    An opamp cannot drive your pi phase shifter because the capacitor at the output of the opamp is a dead short to the opamp. Add a 100 ohm to 1k ohm resistor between the output of the opamp and the first capacitor of the pi phase shifter.

    I think the gain of the opamp must be a little more than 1 to make up for losses.
     
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  9. Audioguru

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    Dec 20, 2007
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    Points A and B are joined by a winding of wire that has an extremely low resistance.
    The output pin of the opamp (Point A) is at half the supply voltage (about 6V) and since the input pin of the opamp has an extremely low DC current then Point B also has a DC voltage the same as point A.

    Of course there is almost no DC voltage dropped across the coil of wire that has an extremely low DC current in it.

    Please show us the parts values and opamp part number that you tried.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    R and R are 1k ohms and since the opamp has Jfet inputs it has no input current and the comparator has almost none so the resistors can be 100k ohms for less battery current wasted.

    The battery needs a 47uF to 100uF capacitor and a 0.1uF capacitor across it. Pin 3 of the opamp needs 0.1uF to ground, not 1000uF.

    I don't know what frequency you want but C1 is 150nF which is a reactance of only 107 ohms at 10kHz which the opamp cannot drive.

    The opamp has a signal gain of 1 which is too low because there are losses so the circuit will not oscillate.
     
  11. Neyolight

    Thread Starter Member

    Dec 13, 2011
    54
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    Allright , so I will try build this curcuit on a breadboard today and will see if it works:

    I will :
    1) Add 0.1uF capacitor from pin 3 to ground ( remove 1000 u F)

    2) What else do I need to change?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    You need to change everything I talked about.
    The opamp cannot drive the dead short of the 150nF capacitor C1.

    A breadboard has lots of stray capacitance between rows of contacts, intermittent contacts and lots of wires that pickup interference. Use a compact SOLDERED stripboard or pcb design instead.

    Why don't you make an oscillator that works like a Wien Bridge or a Phase Shift oscillator?
     
  13. Neyolight

    Thread Starter Member

    Dec 13, 2011
    54
    3
    Why is that ?

    I looked at phase shift and wein bridge oscillator- they seem to work better with RC combo than LC !

    This combo that I presented is Colpitts Oscillator I belive?
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    I found your oscillator circuit with an online calculator of its frequency.
    What frequency do you want?
     
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  15. Neyolight

    Thread Starter Member

    Dec 13, 2011
    54
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    I have an unknown inductor, so taking a guess at frequency i want to see would be 10 Khz. Any frequency below 65 Khz is fine as i cannot capture frequency above 65 Khz in my code.

    So ya, the link you presented cleary shows how the resonant frequency relates to L and C. For 10 Khz, and C1= 150nF and C2= 68nF , the inductor value should be 5.4 mH .

    Thats all sorted, what i dont get is what should be the value for the other components in the circuits eg- feed back resistor, R , R1 , C and so on ?
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    Two resistors set the gain of the opamp. Try a few different amounts of gain.
     
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