Help with Active Low Pass Filter

[ade77]

I am trying to get the magnitude response of the simple active low pass filter . I have attached the diagram.

I use the AC analysis , under the expression I typed 20*log(Vo/Vin); my Vo and Vin were of course changed to my nodes.

However, I am not geting the right waveform. The formula above is what we were asked to use to get the waveform in decibel.

[lacy]

Heres something to try. Instead of your expression just add the 2 nodes. This gives a cutoff frequency of approx 1Khz according to what I am seeing in the graph.

The thing is that on the first page of set up you are already selecting the output to be in decibels and I believe Multisim is doing the calculations for you. By entering your expression into the output varibles it is confusing the simulators calculation and this is why the waveform is incorrect. This is my theory and you can try my suggestion and see if that corrects the situation and that the waveform is what you are expecting. It looks correct to me after the changes I made in the analysis setup.

I hope this helps

[ade77]

Thanks Lacy, yes like you pointed out the formula I use is in decibel. So what I just did is to use the expression Vo/Vin, then pick decibel as the output of the vertical axis. It worked fine. Although the cutoff freq based on my calculations is suppose to be 2.3kHz,  I don't know why the cut off freq seems to be around 1kHz in the simulation graph.

Thank you.

[lacy]

I used your sweep settings in my test. You might want to try to tighten the bandwidth on the sweep and see if the accuracy of the graph is increased. What we may be seeing is a broad range on the sweep and it may not be as accurate to our naked eye when interpreting the graph.

This is just a theory for you to try. I hope that is what we are seeing and that it helps.

[lacy]

I revisited this today because it was so late last night that I didn't have a chance to look at this like I wanted to. Tightening the bandwidth did little or nothing. What I did find is that the 1Khz marker is deceiving to the eye and is not the true cutoff frequency. What I measured was approx. 2.3Khz as the cutoff frequency.

What you have to do is turn on the cursors and move one of them to -3 decibels (half power point) on the "y"  and then read the frequency off of "X". You have one cursor for X1 and Y1 and one cursor for X2 and Y2.

I hope I explained this properly and that you can understand what I am doing here. I also hope this helps.