Low distortion three-phase oscillator

This low distortion oscillator uses the three phases of a three-phase circuit, plus the inverse signals to generate an effective six-phase control signal to control the amplitude of the oscillation. This six-phase control circuit requires little additional filtering, enabling a low distortion control signal to be obtained with less than one cycle stabilisation period. It operates effectively from 10Hz to 30kHz, but the bandwidth limitation of the op-amps generates slight distortion at higher frequencies that might be eliminated with a faster op-amp. Performance figures of the prototype circuit show better than -90dB distortion, nothing being visible on a signal anlyser trace at 1Hz and 1kHz, and only a second harmonic at -85dB at 30kHz.

Project Elements

1hz-distortion-3.jpg (JPG, 1.07 MB)

threephaseoscv5-1.png (PNG, 19.45 KB)

Discussion (3 comments)

EN140174 9 years ago

Where are the output/s?

neonlamp 9 years ago

Hi These are marked on the diagram. Phase 1 (red) is from IC1; phase 3 (blue) from IC5, the inverted output from IC2 and phase 3 (yellow) from IC3. If volume level control is needed, these should be routed to additional IC buffers thorugh a 10k potentiometer. These could be individual or another three-ganged pot. The lowest distortion is from the third stage (yellow).

1 Comment(s)

ClemensValens 11 years ago

Hello neonlamp,

For some reason I always have been attracted to 3-phase oscillators and I think your's is quite nice. Looking at the project picture it seems that you went through the effort to put it into an enclosure. Why? What do you use it for?

The low distortion, was that a design goal or a happy bonus?

Regards,

Clemens

neonlamp 11 years ago

The main reason was to provide a low distortion signal to test audio power amplifiers. I had looked at John Linsley-Hood's phase shift oscillator from the 90's and compared the Radford LDO3 design. THe JLH oscillator seemed to me to be "back to front" i.e. for simplicity, the CR network acts as a high pass filter, thus passing high frequency components but the RC approach filters higher frequencies and should therefore cut harmonics. The Radford LDO used a 90 degree quadrature signal with a sample and hold taken off the 90 degree phase. COnsidering that the main issue is to provide a feedback signal that operates within a cycle to stabilise the amplitude, it was clear to me that a multiphase approach would be able to provide in-cycle feedback, and thus a three-phase circuit was chosen. Adding the inverse gave a 6phase control signal which halved the filter requirements, and the three phase circuit amplitude is nicely damped that a moderate filter in the control amplitude path was sufficient. I was also interested in three-phase for possible brushless motor control and as three-phase demos. I had a future project in mind to possibly develop a contactless drive for a hifi turntable at some point. using the oscillator to drive three power amps driving field coils spaced around an aluminium turntable. But that project has not got past this concept stage yet.

1 Comment(s)

Filmann Faul 11 years ago

Thanks for sharing, the distortion is remarcable. The resolution of the schematic is quite low, could you please provide higer resolution. Could you give examples what you do with this generator?

Tilmann