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Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268
Hi, I built a RC oscillator from a Schmitt trigger following the datasheet. It involves one resistor going from output to input with a capacitor from input to ground. The datasheet says the frequency should be 1/(1.7*R*C) I'm using 1200ohm and 10nF which should equal 49khz but when measuring from output to ground it reads 80khz on the DMM which is just RC, without 1.7.
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268
Connection of the output split to another gate with the other leg going to two gates in series to make two terminal a AC signal shows a duty cycle of 55.3% which might be accurate goes one leg has a extra inverter. I don't want to blame my tools if its operator error. Having a series capacitor and bridge recifer reads 11.89DcV from the two terminal. Which I think should be accurate.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
The frequency of Schmitt trigger oscillator depends on the input hysteresis voltage (and symmetry) as fractions of logic output voltage swing. A frequency formula that doesn't include that parameter is too simple.
That might not explain the whole amount of your discrepancy. What Schmitt input threshold voltages are implicit in the formula you are using? You can measure the Schmitt input threshold voltages yourself.
Consider getting some R and C values for much slower oscillator, where you can visually measure the rate of a blinking LED. Or an oscillator where the frequency is audible (with suitable transducer) and can be checked with online references or musical-instrument-tuning apps.
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268
The graph shows T+ as 10V and T- as 5V with a 15V supply, the foumla got shortened to that above, but otherwise the 1.7 value was ln or some log thing.
Probably should have posted this in the last replied.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615
If you look at the datasheet - I'm looking at the fairchild datasheet btw - you can see the hysteresis is not well controlled. Datasheet quotes a range of 2 to 7.2 volts for a 10V supply! So your frequency equation will be just as wildly inaccurate as the hysteresis.
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Schmitt trigger oscillator frequency
