# How to build a Oscillator using a Sallen Key low pass filter

Status
Not open for further replies.

#### Swsean

##### New Member
Hi. I am a college student studying E&E engineering. I am currently taking Modern Control System Enginnering subject.

Recently, I was given a assignment asking me to build a Oscillator using a Sallen key Low pass filter. For my circuit, I am using a Sallen key low pass filter with R=R1=R2=10K and C=C1=C2=1uF, I also grounded the input of the filter circuit. Using the Sallen Key filter formula, I got t(s)=10000/(s^2 + (300-100k)*s + 10000). From there for my circuit to be marginally stable, so that it can oscillate, I need my value of K=3, so that the poles of the system is located on the imaginary axis of the s-plane but the problem with that is during my ltspice simulation, I cannot get the circuit to oscillate without providing a pulse input into the filter input.

tspice simulation, I cannot get the circuit to oscillate without providing a pulse input into the filter input.
Welcome to ETO. That's a common problem when simulating oscillators.
The simulation doesn't have the intrinsic noise that helps start a real oscillator.
Oscillators depend upon AC positive feedback for oscillation, but if there's not a small value of AC (noise) to start the process it may just sit without oscillating.

Instead of a pulse, you might try using the top and/or bottom Transient simulation options (below) which avoids the initial DC operating point calculation that can generate a quasi-stable state of no oscillation.
The initial power-on transient will then usually give enough of a jolt to start the oscillation. Last edited:
I also grounded the input of the filter circuit.
you need feedback to the input for it to oscillate. the feedback also needs to be positive feedback to make it unstable which starts the oscillation. using the startup option in the simulator starts all voltages at 0, and the "turn-on thump" usually will start an oscillator.

Status
Not open for further replies. 