[Moved]: Testing an oscillator made by me in Orcad 16.5

[eletmoraes]

I got this message after the simulation, as the picture below



which may be the problem or what is missing?

 

[Audioguru]

You made an odd Cmos oscillator. Usually only two inverters are used but you have three. Fairchild shows a Cmos oscillator like yours.
Since you are using 2-inputs Nand gates then only one input should be used (connect the unused input to the positive supply voltage) then it will switch symmetrically.
The value of R1 should be at least 10 times higher than the other resistor.

 

[eletmoraes]

I tried to ride another scheme using the CD4011 as the image and the waveform output.


and the same circuit mounted on multisim 13 already has wave output. Because in multisim presents waveform at the output and Orcad not?

 

[Audioguru]

Your ORCAD simulation software does not know that the CD4011 has noise that it amplifies which causes it to begin oscillation.
Why did you use a CD4011A that is about 40 years old instead of a CD4011B that replaced it? Its datasheet shows an output high when both of its inputs are at half the supply voltage so I do not know why ORCAD shows an output low.

Usually a two-inputs gate as an oscillator uses only one input for an ocillator then the output will be a more symmetrical squarewave. Also usually the value of R2 is 10 times or more the value of R1 (the value of R2 is 100 times the value of R1 in my circuit) so that the frequency is not changed much when the supply voltage changes.

 

[FvM]

There's nothing wrong with the waveform in post #3, as far as is displayed. To see the oscillation, the anaylsis time must be increased to reasonable values (at least several ms).

The other question is what you see in Multisim? Are the RC values the same?

There might be a problem that an oscillator made from buffered standard gates (actually three transistor stages cascaded) can show parasitic ring oscillator effects. If they occur in SPICE simulation would depend on various simulation settings and is hard to predict.

 

[Audioguru]

I forgot to look at the ORCAD 'scope timing that shows a time period of only 1us.
Since the timing resistor is 68k and the timing capacitor is 22nF then the frequency is 374Hz and half of one cycle will take about 1337us.
Here is the formula: