Howdy,

Does anyone out there have a useful collection of logic gate RC oscillators? I'm aware of AN-88/118 from NSC/Fairchild, and have scrounged the web and found a few circuits, but am looking for more information.

In particular, although the following circuits all look quite similar, and yet different enough to make me ask, how would one calculate the output freq of say, the second and third? And given these all have an even number of gates, which the aforementioned app notes frown upon as they may not auto-start, it seems curious that there are so many oscillators out there like this.

The second circuit appears to use the two diodes and Rs for marking the high and low periods. Is this kind of stuff documented somewhere?

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Go Trojans!

 

Don't know what you mean by documented. The designs are in print.

Here's another that uses the diode trick to control duty cycle.


John

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Folks,

There must be someone out there that can show me how to calculate the osc freq of the above circuits (second and third), and/or provide a bit of explanation in determining the duty cycle?

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Go Trojans!

 

the circuit posted by jpanhalt is quite easy and actually gives a clue for the others: T=RC. That goes for both the high level and the low level so the period is P=2*RC.

You can actually us a diode and a small resistor in the feedback path will allow you to tie a resistor to the rail to act like a 555, or even let you sellect multiple delays in a sequencer.

 

You won't find any accurate formulas because the switching points of the CMOS gates are highly variable, so your mileage and frequency will vary.

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Inside every little problem, is a big problem trying to get out.

 

The period is a function of the two switching threshold voltages of the 74HCT14, the two resistors, and the diode voltage drops. And the capacitor, of course. In general, T≠RC.

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Ron

 

True, my bad. I forgot to mention my usual schmidt reccomendation.

 

Hi,


Here are two of the oscillators and their formulas.
They both use the 74HC14 ic chip.

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