T&L Publications

Hi I am a electronics technician that came across equation confusion from various books for the three network phase shift oscillator circuit.One book"hints and kinks by QST"says f=(tan 60 deg)/2prc=1.73/6,28 rc=
0.28/rc. Another book "electricity,electronics and electromagnetic principles" by Robert Boylestad says: fo=1/(2xpi x square root of 6 x rc) =1/(2xpi x tan 180deg xrc);and the last book "Introduction To electronic Circuit Design" by Richard Spencer says : f=(square root of 6 /rc)/(2 x pi).
Which equation will give the correct answer? I think it's Boylestads book but when I rearrange the equation to solve for c I get :
c = 25330/fo^2 6r . I would really like to know the correct answer. I will build the circuit as in the Hints And Kinks Book and make measurements to see if their correct or not. Tim Rasch [email protected]

Hi Tim,

Could you post the circuit somewhere? That's the only way we can
tell what circuit you are dealing with.

Mr Al,I can't send the circuit from protolab or spice program .I will try to describe the network : it's a three resistor network with 10k in series and three caps .1uf going to ground from each resistor node and a output resistor from the resistor string ,10k going to -input of ua741. there is a adjustable resistor 1meg going from - input to output pin 6 of the ua741 op amp. Thats the complete oscillator circuit except for the +- vcc of 9v.
I think I have found the correct equation on the internet:F= 1/(2xpi rc )x 2.4494897 So the QST book is wrong !!! :they calculated the frequency based on 60 degrees instead of 180 degrees of the total phase shift network. Tim Rasch [email protected]

Timothy Rasch wrote:Mr Al,I can't send the circuit from protolab or spice program .I will try to describe the network : it's a three resistor network with 10k in series and three caps .1uf going to ground from each resistor node and a output resistor from the resistor string ,10k going to -input of ua741. there is a adjustable resistor 1meg going from - input to output pin 6 of the ua741 op amp. Thats the complete oscillator circuit except for the +- vcc of 9v.
I think I have found the correct equation on the internet:F= 1/(2xpi rc )x 2.4494897 So the QST book is wrong !!! :they calculated the frequency based on 60 degrees instead of 180 degrees of the total phase shift network. Tim Rasch [email protected]

Tim, I'm lazy. I simulated the RC network and got 2.44955 as the constant. probably close enough to the equation you found for government work.
This assumes that the op amp has infinite bandwidth - not a very good assumption, especially with a 741, but at this frequency (~390Hz), probably OK.