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Phase Shift Oscillator

LvW

Apr 12, 2014
604
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Apr 12, 2014
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Interesting way to do it indeed. Thanks.
But how did we know that 1.732 identical to SQRT(3) ?
Look at the following derivation (like the one for coupled RC sections in post#2):

1.) For three decoupled RC sections in series we have H(jw)=1/(1+jwRC)^3
2.) This can be expanded easily (polynominal of 3rd degree) and becaue we want H(jw)=real we set Im(denominator)=0
3.) This gives 3*w*RC - [w(RC)]^3=0
4.) Hence: w=SQRT(3)/RC
5.) At this frequency, the Re(denominator)=-1/8. Thus, we need a gain stage with a gain "-8".
 

Laplace

Apr 4, 2010
1,252
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Apr 4, 2010
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1,252
I wonder what does making each stage have an impeadance of 10 times more than the last section do? This quite common on cascaded filters without having buffers.

Phase-Shift_N.png
Interesting question, so rather than just 10 times I analyzed this circuit with a parametric factor N. The following graph shows the phase shift for a range of N (N=1 to N=12). The greatest effect occurs for the smaller values of N.
Phase-Shift_Nx.png
 

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