Why is my Waveform of a simulated Multivibrator wrong?

[johannes23]

He there!

I'm trying to simulate a multivibrator using to NAND-element, a capacity and a resistor. The waveform of the signal grabbed at the nod between resistor and capacity is correct, still those between the two NANDs and the resistor as well as the one between second nand and capacity are discrete.

What am I doing wrong? Something as simple as a multivibrator should be easily possible to simulate.

[chaitu2]

Hi please don't forget attach your multisim file to get some help.

If you post your circuit you will definitely get the solution with good explanation of the mistake.

...........chaitanya 

[johannes23]

Thank you, so may I found a solution if any of you sees that. 

[chaitu2]

Hi johannes......

              The only mistake in your circuit is that you have connected one input of the first NAND gate to the triggering source (used for oscilloscope triggering). If your intention is that we need to trigger the nand gate to start oscillations you should give a single negative pulse not a continuously varying square wave.

one input of the first nand gate should be high or same as that of the other input in order to work as a inverter(NOT).  (1 1=0(1'), 0 0=1(0'), 1 0=1(0') )

this is the reason that your getting response when the clock(trigger) is high.

I have attached the file which uses a push button switch to do give a negative pulse trigger. push and release that button to start the correct operation. 

other wise you should give initial condition voltage to the capacitor to start the oscillations. I hope you understood.

don't forget to give kudos if you like the answer. 

.......chaitanya 

Message Edited by chaitu2 on 11-11-2009 03:38 AM

[johannes23]

Thank you chaitu2!

But my actual problem is, that the waveforms on Channel B and D should also show the exponential charging of the capacity (what they in reality do). Instead of that I get a full rectangular pulse.

[chaitu2 · Accepted solution]

Dear johannes,

           channel B and D are showing the out put voltages of the first and second NAND gate output voltages. Those are standard TTL output voltages they will be 0 or 5 volts(discrete levels). If they output voltage as the channel c does where do you take the astable multi vibrator out put? multivibrator in astable mode itself is a square wave generator, is n't it?

           perhaps you are confused with currents and voltages....... The current wave forms at these points show the exponential charging of the currents. To see current waveforms connect the current probe as shown in the attachment.    

    Regards

A. chaitanya

[johannes23]

Thank you!

All I have to do now is figure out why I made that mistake!

Greetings, Johannes