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Oscillator

Discussion in 'Circuit Simulation & PCB Design' started by desan2012, May 12, 2017.

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  1. desan2012

    desan2012 New Member

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    I have attached an oscillator circuit simulated in LTspiceIV.

    Interested in knowing what kind of oscillator this is and its theory of operation.

    Appreciate your explanations and comments.
     

    Attached Files:

    • PWM.asc
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  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    It's a conventional inverting Schmitt-trigger oscillator. R1,2,3 set the bias point for the non-inverting input of op-amp U1. When the inverting input is below the bias point, U1 output is high, which means the bias point is raised above half the supply voltage and defines an upper threshold. U1 output charges C1 via R5 until the inverting input voltage goes above the threshold. This causes U1 output to go low, so R1,2,3 now set a lower threshold and C1 discharges into U1 output, via R4 and R5 in parallel, towards the lower threshold. When that is reached, U1 output goes high again and the cycle repeats.
     
  3. atferrari

    atferrari Well-Known Member

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    Next time please post a common image file to show the circuit and, why not, the .asc file. Many here do not have LTSpice installed but could be interested in taking a look.
     
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  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Back in the day, we called this circuit an astable multivibrator. It could just as easily be built with a 555 chip except a 555 will not run at 1.8V.

    U1 is a high performance, low-voltage, rail-to-rail in/out opamp which you dont need for such a low frequency oscillator.

    Get rid of the 10second time-step in the .TRAN directive.
     
  6. desan2012

    desan2012 New Member

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    Many thanks everyone.

    Will endeavour to attach a common image file in the future. A .jpeg image is now attached.

    What exactly does the diode D1 do?

    The circuit is intended to be used for an indoor photovoltaic energy harvesting system; hence the low voltage.
     

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  7. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    It makes the charge path into C1 different than the discharge path.

    Here are two simulations, one with R4=750KΩ, and the other with it not in the circuit (R4=1GΩ). Note that with R4 open, the output is a square wave. With R4=750K, the duty cycle changes (as does the period).
    pwm.png
     

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