EMRFD Message Archive 5525

Message Date From Subject
5525 2010-12-14 10:15:14 Vincenzo Calandra Phase shift oscillator
Hi all,

at page 7.31 (fig. 7.25) of my EMRFD there is the schematic of a "simple
audio generator".

Which it is the correct formula to determine the frequency? 1/2*Pi*R*C?

Many thanks for the reply.
73 de IZ5GVP - Vincenzo

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5526 2010-12-15 03:56:45 Tim Re: Phase shift oscillator
Theoretically the oscillation frequency is when the phase shift through each of the individual RC hi-passes is 45 degrees.

For 4 stages of cascaded RC hi -pass with C=0.0027 uF and R=47K, that's close to 1kHz.

Things are made a little complicated when the output of each individual hi-pass is loaded by another hi-pass.

If you look at the circuit in fig 7.25 this is "worked around" by using 3 R's of 47K, and the 4th R is effectively 75K in parallel with 39K.

So the "right R" to use in the phase shift calculati