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How to build a oscillator by using a Sallen Key filter.

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jdocez

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I m trying to build an oscillator with a Sallen Key filter by using simulator LT spice. I grounded the input of the filter. C = C1 = C2 = 1uF; R = R1 = R2 = 2k. Non-inverting amplifier gain = 3. ( From the Transfer Function of Low Pass filter, I calculate the K value which is the Non-inverting amplifier gain when the one of the pole from Transfer Function is on the origin of the s-plane) I think this system can become marginally stable. But from the result that i got, its output is OV.

Any advices and any suggestions. TQ

Notes
V(n002) = Vout
 

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Of course, you can use a S&K filöter as a core of an oscillator!
However, you need a filter that has a phase shift of -180 deg (together with an inverting amplifier) or of 0 deg (together with a non-inv. amplifier) - in both cases, for a single frequency only!
In the first case, you need a 3rd order lowpass or highpass and in the 2nd case you need a bandpass.
These requirements follow from the oscillation condition (Barkhausen).
Based on a 2nd-order lowpass it will not be possible to realize such an oscillator.
 
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    jdocez

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Two faults:
1. A marginal stable won't start oscillation, at best keep an existing AC excitation. You need a small gain plus above the oscillation condition.

The given circuit will oscillate for G >3.

2. Other than real circuit, SPICE oscillator simulation need a means to stoke up oscillation. Most simple, use a DC excitation and skip initial transient solution.

The example shows how the circuit keeps an initial excitation for G=3.

RC.PNG
 
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    jdocez

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TQ LvW and FvM for helping me.
I managed to get the circuit to oscillate by set the K/G >3.
However, I got a saturated sinusoidal wave. I try to increase the Vsat+ and Vsat- to 30 V and -30V respectively but still get a saturated sinusoidal wave.
After that, I added two diodes which are 1N4148 to regulate the wave but the sinusoidal wave is weird or it came out weird wave that similar withe sinusoidal wave.

Question- How to get the perfect sinusoidal wave without saturated with this circuit?
That is the circuit and the result.

Link: https://imgur.com/rUD4M68

The Green wave which is the Sinusoidal wave with 159hz and a 600ms Amplitude to compare the red wave which is the circuit's output
 

The distortion is present by design of the circuit. There are different ways to overcome it.
- use a magnitude control circuit with linear gain variation, e.g. a FET as controlled resistor
- use low-pass instead of a band-pass feedback network and get a low distortion signal at the low pass output
- reduce the excess gain by bringing the circuit gain nearer to 3 (this will be probably thwarted in a real circuit by capacitor tolerances)
- make the 1N4148 gain reduction more smooth by adding a series resistor to the diodes

There are previous discussions about low distortion oscillators.
 

Just an other way to get sine wave is to add low-pass filter stages to the output to remove harmonics from the distorted signal.
 

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