electronics equation question

Discussion in 'General Electronics Chat' started by scoundrel421, Oct 8, 2018.

  1. scoundrel421

    Thread Starter New Member

    Oct 8, 2018
    I know this equation (f=1/3.3RC) is an Oscillator Frequency equation; just wondering where the '3.3' came/derived from. NOTHING I have studied thus far used that figure as a Constant...any ideas?
  2. OBW0549

    Distinguished Member

    Mar 2, 2015
    Don't feel bad; I've been "doing electronics" since the late 1950's and I've never seen it either.

    My guess: it's probably specific to a particular integrated circuit oscillator, and not applicable to anything else.
  3. scoundrel421

    Thread Starter New Member

    Oct 8, 2018
    Most likely, that is the case. I ran into the equation in a practical exercise; it posed no problems in solving it, but I just didn't understand where the figure used as the constant came from or was derived from. The practical exercise was building and testing an oscillator for a logic probe, and that formula was for calculating the frequency of the oscillator with particular variables, i.e., the resistance (R) and capacitance (C) values
  4. MrChips


    Oct 2, 2009
    As a general rule of thumb, we know that tau = RC.
    For a first order filter 2πf = 1/RC,
    or f = 1/6.3RC

    If you check the application notes of various timing circuits you will find variations on the xRC theme.

    The period can be xRC where x can be anything from 1 to 6 depending on the switching thresholds of the oscillatory circuit.

    The devil is in the details of the circuit.
  5. BR-549

    AAC Fanatic!

    Sep 22, 2013
    Something about a phase shift resonant frequency struck me when I saw it.
  6. scoundrel421

    Thread Starter New Member

    Oct 8, 2018
    I thought of Phase differences, too; but there wasn't anything explained/mentioned about what '3.3' represented, other than a constant of some sort...and I kinda guessed at that, too, with no real way of checking if I was right...all I know is that I had no problem doing the math and making the equation work, and having it make sense to me
  7. danadak

    Well-Known Member

    Mar 10, 2018
    Not sure why they got this result but -


    Maybe its a composite factor derived from phase shift and Tpd of inverter
    and non symmetric inverter thresholds, but then at low freq Tpd must be

    Regards, Dana.
    Last edited: Oct 8, 2018
  8. MisterBill2

    Well-Known Member

    Jan 23, 2018
    For a number of devices there are special formulas for the time constant with that particular device. two that come to mind right away are the CD4047 one-shot IC and the 555 timer IC. In addition, there are some circuits where a specific RC time constant formula applies because of the trigger voltages in that specific circuit.
    So the formula you present would only apply to a very specific instance.
  9. crutschow


    Mar 14, 2008
    It's a phase-shift oscillator.
    The frequency of oscillation is where the phase-shift of each RC LPF is 180°/3 = 60°, (for a total feedback phase of 360° ) and that occurs at a frequency of 1/3.3RC.

    Note that the bottom of the capacitors must all go to ground.
    Tonyr1084 and AnalogKid like this.
  10. noweare


    Jun 30, 2017
    Here is a great video from youtube on RC phase shift oscillator. Your impressive Crutschow !
    Last edited: Oct 9, 2018
    Ramussons likes this.
  11. ebp

    Well-Known Member

    Feb 8, 2018
    60 degree phase shift occurs at 1/3.63RC Hz

    The circuit at #7 is not a conventional buffered phase shift oscillator. It is a string of discrete time delays using (usually) CMOS inverters and can be built with any odd number of stages. The waveforms will depend on the time delay of each stage and the number of stages. An unbuffered CMOS inverter will typically switch at half of Vcc, but the gain isn't especially high and the output transitions won't be fast, so with very small RCs the outputs may spend more time slewing than sitting at a rail. Once the RC gets sufficiently great, each output will become more square and swing rail to rail. If the number of stages built that way is large enough, peak to peak voltage on each capacitor will get close to being rail to rail, which of course means the delay of the stage is longer because the cap voltage has to swing farther to cross the switching threshold. Schmitt trigger inverters can be used and the delay times will be quite different for the same RC values. A Schmitt based circuit will reliably oscillate with a single stage, a simple inverter probably won't. The RC values don't need to be the same for each stage.
    Last edited: Oct 10, 2018
  12. scoundrel421

    Thread Starter New Member

    Oct 8, 2018
    THAT is exactly what I was thinking it may have been; I was just having some trouble explaining it to myself...it now makes Perfect Sense; now, Life Can Go On, and the world can start turning again *lol*...Thank You for the explanation