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[SOLVED] Design of rc phase shift oscillator..??

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rsashwinkumar

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i need to design an rc phase shift osc. at a freq of 1khz (say)
the frequency of osc. for an opamp based osc., is 1/(2piRC*sqrt(6) if we use 3 rc networks.

Now i choose R and C accordingly. Now since v use 3 RC lead networks, each must be designed to produce a phase shift of 60 degrees.

but for the chosen values of R and C the phase contribution from each network turns out to be 90-arctan(1/sqrt(6)) which does not give 60 degrees.

plz let me know where i have gone wrong....
 

I don`t completely understand the meaning of your inquiry - nevertheless, it is clear that 3 identical CR elements together with an inverting amplifier (-180deg) can oscillate only if the phase contribution of each section is -60 deg. This condition determines the frequency.
 

the formula derived for the frequency of oscillations is fo=1/(2*pi*R*C*sqrt(6)) for 3 identical RC lead networks connected in cascade.
now, the condition needed is phase contribution for each RC n/w at resonant freq must be 60 degrees. But for a lead network phase = 90 -arctan(2*pi*R*C*fo)
since each 2*pi*R*C*fo=1/sqrt(6)
we get phase = 70 degrees... this is my query... instead of 60 deg i end up with 70 deg., where have i gone wrong?? plz kindly help me...
 

But for a lead network phase = 90 -arctan(2*pi*R*C*fo)

Please, can you explain the above formula? What do you mean with "lead network"? Just a single C-R stage?
 

I suppose you forgot that each C-R section is loaded by the next stage - thus, you may not calculate for a simple CR element only.
 
yeah!!! i just thought since they were 3 cascaded stages, i could just add their phases up... thanks a lot :)
 

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