# phase shift oscillators

Discussion in 'General Electronics Chat' started by leoed, Feb 26, 2009.

1. ### leoed Thread Starter New Member

Feb 26, 2009
2
0
hey guys,
i am new at this site. can you please help me, i am currently studying the concept of phase shift oscillators. because we are required to make a phase shift oscillator having a variable frequency. i am having a problem on trying to vary the frequency of oscillation of the circuit. from the book electronic devices by thomas floyd it says that the frequency of a simple RC phase shift oscillator is f=1/(2*pi*sqrt6*RC), it just simply shows that if i would vary the value of either the capacitor nor the resistor it will give me a change in frequency. but it also says that the three cascaded RC circuit must have equal in value R1=R2=R3=R, C1=C2=C3=C to compensate the 180 degree phase shift. does it mean that if i have three potentiometers for my RC circuit they must have equal in value as i vary each, for if i will just vary a single resistor it would not sustain the 60° phase shift needed.i am confused, is there any alternative that i can do to vary the frequency.

2. ### Audioguru New Member

Dec 20, 2007
9,411
896
I have a variable frequency phase-shift oscillator. The frequency-adjusting pot has three sections joined to one shaft.

Three matched resistors can be pulsed at a high frequency and the duty-cycle of the pulses (pulse-width-modulation) determines their average value and the frequency of the oscillator.

3. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292
Yes, you will need to twiddle all three resistors. Three-gang pots for doing just this used to be in plenitiful supply. Probably not any longer. You can also use transistors as variable resistors in this application, with just a little added distortion.

eric

4. ### leoed Thread Starter New Member

Feb 26, 2009
2
0
can you please explain to me how the transistors work as a variable resistor for this case.