74hc14 relaxation oscillator

[david_c_partridge]

I wanted to simulate a circuit using a standard 74HC14 relaxation oscillator using a resistor and capacitor.

The NXP datasheet (page 17) says that for a 74HC14 the frequency can be determined by the formula F = 1/T = 1/(0.8*RC).

Unfortunately the simulation doesn't seem to give results that match this.   I wanted to set up a relaxation oscillator with a period of about 66mS, and with an 825k resistor, working the formula backards gives a value of about 100nF.

If I use a 100nF capacitor, the actual period for the oscillation that the MultiSim simulation creates is about 157mS, and I need to reduce the capacitor to 43nF to get the period to be 66mS.

I attach an MS10 file showing the problem.

The second relaxation oscillator and 74HC393 are there to actually get the simulation to do anything at all - without these the output of the upper circuit sits a 5V all the time.

[Tien_P]

Hi David,

Attached is a circuit that is oscillating and to make it run I change the simulation method from trapezoidal to gear, this setting is under Simulate>>Interactive Simulating settings>>Analysis Options>>Customize>>Transient.  Unfortunately, getting this circuit to oscillate at the correct frequency is difficult and this is usually true with many self-oscillating circuit in any SPICE simulators.   In Multisim, you can tweak the rise/fall time on the 74HC14 by double click on the component and select the Edit Model button under the "Value" tab this should have some affect to the oscillating frequency.