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Calculate the oscillation frequency of oscillator circuit with inverter IC

S.Satoshi
Guru 21675 points

Hi,

My customer has question about how to calculate oscillation frequency of attached circuit that is oscillator circuit by using inverter IC. 

  • Do you have any document that include formula to calculate output frequency?
  • Do you have other idea of oscillator circuit by using logic IC?

Best Regards,

Sonoki

  • 0
    Guru 279850 points

    This image was stolen from Fairchild's application note AN-118: CMOS Oscillators.

    In there, it is labeled "Less Than Perfect Oscillator"; the text explains:

    A popular oscillator is shown in Figure 5a. The only undesirable feature of this oscillator is that it may not oscillate. This is readily demonstrated by letting the value of C go to zero. The network then degenerates into Figure 5b, which obviously will not oscillate. This illustrates that there is some value of C1 that will not force the network to oscillate. The real difference between this two gate oscillator and the three gate oscillator is that the former must be forced to oscillate by the capacitor while the three gate network will always oscillate willingly and is simply slowed down by the capacitor. The three gate network will always oscillate, regardless of the value of C1 but the two gate oscillator will not oscillate when C1 is small.

    If this circuit is actually used, and actually happens to oscillate, its frequency is about 1 / (2.2 × C × R2).

    Consider using any of the better circuits from that appnote.
    The oscillator using a single Schmitt-trigger inverter would have a frequency of about 0.8 / (R × C).