I need some help with understanding the deviation in results of spice simulation vs. theoretical. If we look at the most basic phase shift oscillator as found in wikipedia http://en.wikipedia.org/wiki/Phase-shift_oscillator, we always want the feedback resistance to be 29 times the resistance R1. I have looked at the derivation of this relationship but I am still not quite sure how we are getting this. Is this relationship applicable for any RC phase shift oscillator? And in order to find the frequency of oscillation I am using the formula provided in the wikipedia link I have provided. My question is for 10KHz frequency of oscillation with R = 6.5 and C = 0.1microF, my oscillator starts oscillating and when I take the fourier transform, I should get a peak at 10KHz frequency provided that my feedback resistance Rf = 29R = 188.5 in my case.
In my simulation, I get the peak at 8KHz but if I reduce the Rf to 134.7, it shifts to the correct position 10KHz. When I asked my professor, he told that this might be due to the input impedance and capacitor values of the opamp. I am not sure what that means. Can anyone explain this deviation in my results?
In addition to this, when I try to do slightly more complicated model similar to the basic model, my simulation doesn't even work... gives me a straight line and no matter how much I played with the Rf resistance, I couldn't even get the oscillations to occur in spice!
I am just researching on this phase shift oscillator and I am very new to this concept and I have read other posts based on this topic. I will really appreciate it if someone can help me on this.
Thanks
In my simulation, I get the peak at 8KHz but if I reduce the Rf to 134.7, it shifts to the correct position 10KHz. When I asked my professor, he told that this might be due to the input impedance and capacitor values of the opamp. I am not sure what that means. Can anyone explain this deviation in my results?
In addition to this, when I try to do slightly more complicated model similar to the basic model, my simulation doesn't even work... gives me a straight line and no matter how much I played with the Rf resistance, I couldn't even get the oscillations to occur in spice!
I am just researching on this phase shift oscillator and I am very new to this concept and I have read other posts based on this topic. I will really appreciate it if someone can help me on this.
Thanks