How to make Oscillator Variable

[yasir_ali]

hey! Guys I've designed a phase-shift oscillator. now i want to make it variable frequency oscillator.? what should i do.? what i think is to use a single variable resistor instead of three fixed resistors, but i don't know is there any type of variable resistor available. look at the picture.

 

[audioguru]

Your simple oscillator does not have an amplitude stabilizer so its output clips.

My phase-shift oscillator uses a triple-gang variable resistor for tuning.
It has low-pass RC filters instead of the high-pass ones you use so the amplifier stage that clips is filtered 3 times by the RC filters for very low distortion and a fixed output level.

 

[yasir_ali]

i don't know why i can't your reply here coz my subscription shows that i've received reply but it's not showing it.what should i do.?

 

[yasir_ali]

i have changed three fixed resistors with a triple ganged variable resistor. now i don't know how to stablize the gain of the oscillator.

 

[audioguru]

You need to learn about the phase-shift oscillator including how to stabilize its output level with a light-bulb, a Fet transistor or diodes.

 

[MrAl]

Hello again,


Here are a couple of root locus plots for your original circuit. The first plot shows a more continuous graph but it's harder to pick out the right value of K where the plot crosses the imaginary axis, so the second plot was done with K stepping by 5k for each dot, starting at 10k. The values labeled on the plot are in k ohms.
Looking at the second plot which also has values of K labeled, we can see that the locus crosses the imaginary axis near the point where K is approximately equal to 26.5k. We can then read off the jw frequency from the imaginary axis, divide by 2*pi, and get the frequency, which comes out in the neighborhood of 1000Hz. Higher resolution would get us closer to the exact value, but this plot was done assuming a perfect op amp anyway so the real life value of K required for oscillation would be a bit higher. Still, it's interesting to look at the plot and see what information we can get from it.

 

[Transistorman]

what are you using it for ?

 

[MrAl]

Hi,

I am not using it, just analyzing it.