Multisim and Ultiboard

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relaxation oscillator does not simulate


I read some previoius posts about this problem,  where Multisim has some difficulty simulating mixed-mode.  ( I don't know how to include the threads here).   I tried the different possible solutions but cannot get the circuit to oscillate if R = 1.5k. However, at 1.4k or lower, it works.  Tried inital conditions,  tried setting simulation to use 'Gear' but no success..  Any suggestions are welcome.

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Message 1 of 4



I have manage to reproduce your situation.


Look, what is happening here is you are using  the digital component in analog mode by connection its  in and out with resistor.


that situation  is heavily depending on value of that resistor and properties of inverter and barely predictable.. 


so IMHO - there is no problem with MS. 


I am surprised the MS takes it at first place. good for them.


BTW, why 1.4k does work for you and 1.5 k does not?


and another BTW - in real word that resistor might be completely different and  depends on what family of 74xxx do you use.


with hope it helps








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Message 2 of 4

You asked why does 1.4k ohm resistor simulate ok, while the 1.5k ohm does not.  Actually that was my question.   In my circuit I'm actually using 10k ohm resistor,  but I found that it did not simulate properly ( no oscillation ).  So I played with the simulation to see what did work. 


 I think you have the right idea as to why this strange behaviour; mixing of digital and analog simulations is difficult for SPICE..

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Message 3 of 4

hello, thanks for response.


what I meant was - it does not make a big reference what value is working for simulation, in real word it gonna be different anyway.


so my advice here is - to use for simulation value that makes MS happy and make sure your project works.

then when you switch to real hw  - you have to find the fittest value for your specific situation.


with hope it helps





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