Wien Bridge Tunable Oscillator 100Hz - 100kHz
Wien-Bridge Oscillator (100 Hz - 100 kHz)
Overview
This project is a prototype for analog sinewave oscillator based on Wien Bridge circuit topology. Building sinewave oscillators was a dream since my early electronics hobbyist years.
The objective is to make analog, low harmonic (THD < 0.2%), frequency tunable, amplitude adjustable oscillator. This prototype is a part of a larger future project to make a multi-function-generator.
The oscillator has a mechanical selector (4P3T) to select the decade. Three decades are available:
100 Hz – 1kHz, 1 kH – 10 kHz, 10 kHz – 100 kHz
This is done by changing the two capacitors of the Wien-Bridge network. In each decade, a dual potentiometer (10k) is used to tune the frequency by changing the two resistors of the Wien-Bridge synchronously.
Automatic gain control is used to stabilize the amplitude at a fixed 1V while keeping the harmonics at low levels. The resulting fixed amplitude sinewave from this circuit can then be fed to adjustable gain amplifier to produce the waveform at the desired amplitude.
Using a single controller over 3 decades of frequency could not achieve the objectives because the transfer function of the plant changes when frequency changes. Hence, The frequency spectrum was divided into 5 bands. Each band covers approximately 3 folds (Band 1 is 100 Hz to 300 Hz, Band 2 is 300 Hz to 1 kHz, etc). In each band, the controller parameters (k, wz, and wp) are adjusted to meet the design objectives.
it is found that bands 5 and 6 could be combined in one band spanning the entire decade (10k to 100k).
The slides show the following:
• Breadboard prototype circuit.
• Circuit schematic built using schematic.com.
• Summary of important results.
• Simulation built using LTspice to test envelop function that was derived mathematically (For the mathematical derivation, see last slide.)
• Some plots produced using Matlab for the root locus, bode plot, and the step respond of the controller for first two bands.
• Oscilloscope results showing waveform, FFT, and step response for sample bands.
• Scans of the 14 scratch papers showing derivations and progress of project for reference. They also show the parameters of controller chosen for each band.
Overview
This project is a prototype for analog sinewave oscillator based on Wien Bridge circuit topology. Building sinewave oscillators was a dream since my early electronics hobbyist years.
The objective is to make analog, low harmonic (THD < 0.2%), frequency tunable, amplitude adjustable oscillator. This prototype is a part of a larger future project to make a multi-function-generator.
The oscillator has a mechanical selector (4P3T) to select the decade. Three decades are available:
100 Hz – 1kHz, 1 kH – 10 kHz, 10 kHz – 100 kHz
This is done by changing the two capacitors of the Wien-Bridge network. In each decade, a dual potentiometer (10k) is used to tune the frequency by changing the two resistors of the Wien-Bridge synchronously.
Automatic gain control is used to stabilize the amplitude at a fixed 1V while keeping the harmonics at low levels. The resulting fixed amplitude sinewave from this circuit can then be fed to adjustable gain amplifier to produce the waveform at the desired amplitude.
Using a single controller over 3 decades of frequency could not achieve the objectives because the transfer function of the plant changes when frequency changes. Hence, The frequency spectrum was divided into 5 bands. Each band covers approximately 3 folds (Band 1 is 100 Hz to 300 Hz, Band 2 is 300 Hz to 1 kHz, etc). In each band, the controller parameters (k, wz, and wp) are adjusted to meet the design objectives.
it is found that bands 5 and 6 could be combined in one band spanning the entire decade (10k to 100k).
The slides show the following:
• Breadboard prototype circuit.
• Circuit schematic built using schematic.com.
• Summary of important results.
• Simulation built using LTspice to test envelop function that was derived mathematically (For the mathematical derivation, see last slide.)
• Some plots produced using Matlab for the root locus, bode plot, and the step respond of the controller for first two bands.
• Oscilloscope results showing waveform, FFT, and step response for sample bands.
• Scans of the 14 scratch papers showing derivations and progress of project for reference. They also show the parameters of controller chosen for each band.
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