# Phase shift oscillators....

Discussion in 'General Electronics Chat' started by digitalenigma, Apr 24, 2008.

1. ### digitalenigma Thread Starter Member

Apr 11, 2008
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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

Nov 17, 2003
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Dave

3. ### Audioguru New Member

Dec 20, 2007
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The value of your feedback resistor is so low that it shorts the low current from the output of the weak lousy old opamp.

Are we going to have every kid in your class asking the same questions here???

4. ### Dave Retired Moderator

Nov 17, 2003
6,960
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That was an old thread - January 2008. Check your PMs I think you replied to the same question twice because I moved the original post (the first post in this thread) whilst you were writing a reply.

Dave

5. ### digitalenigma Thread Starter Member

Apr 11, 2008
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I tried the phase shift opamp on the TI link but I am not able to make the circuit oscillate at a stable frequency ....it does oscillate but not totally stable. Any ideas? I am sorry I am new at this.

6. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Which phase shift oscillator at TI? Can you repost the link?

7. ### digitalenigma Thread Starter Member

Apr 11, 2008
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1)Now that I understand the working of phase shift oscillators, I need to understand its application at present times. I know phase shift oscillators has audio frequency of oscillation and hence can be used in audio equipments like guitar amplifiers etc. Can anyone tell me more details about the application of phase shift oscillators and its reliability?

2)I also had another question... I was told that the the stability of frequency and phase noise reduction can be achieved using MEMs RF resonator ....can anyone elaborate on that? Like is this resonator a kind of phase shift oscillator too?

3)Is quadrature oscillator a type of phase shift oscillator? Can phase shift oscillator have LC sections instead of RC sections or maybe both?

I will be very very very grateful if someone could respond to the above questions fast.

Thanks

~D

8. ### digitalenigma Thread Starter Member

Apr 11, 2008
11
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how do you start a new thread? It may sound stupid but some how I couldn't figure it out !

9. ### hgmjr Moderator

Jan 28, 2005
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Look for the heading "Forum Tools" at the top of each of the forums and then click on it to obtain the drop down menu. In the menu you will see "Start a new thread". Click on it and you will open up the form you need to start a new thread.

Alternately, you can click on the button "NEW THREAD" at the top and left-side of the forum that you have selected to place you thread in.

hgmjr

10. ### digitalenigma Thread Starter Member

Apr 11, 2008
11
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< In response to creating new thread>

Ok got it. thanx
~D