Can we improve this low distortion sine oscillator ?

[gaetan8888]

Hello

Here is a low distortion sine oscillator, it is simple and with the schematic suggested LT opamp it have .0025% distortion at 1khz.

For myself I use a NE5534A opamp.

Is there a way to lower the distortions of this oscillator ?

Using modern fast opamp, LM4562, etc.., may do a bit better but can it become unstable ?

For myself I use batteries with a regulator.

With this oscillator I need to keep the output lower than arround .5 volts, to keep distortions very low, maby I should use an output buffer.

Thank

Bye

Gaetan

[GK]

Follow it with an active or passive filter.
A multiple feedback bandpass filter would make a simple and effective active option:

http://sound.westhost.com/project63.htm


The center frequency can be trimmed (to the oscillator frequency) just by varying one component (R2 in the schematic on that page).

[gaetan8888]

Hello

I did use a passive high pass filter, it done a bit better.

Thank

Bye

Gaetan

[gaetan8888]

Hello

I will try this multiple feedback bandpass filter.

Btw, is there a limit of slew rate and ft for any opamp to be used in my oscillator circuit ?

Thank

Bye

Gaetan

[GK]

Originally posted by gaetan8888
is there a limit of slew rate and ft for any opamp to be used in my oscillator circuit ?

Not much at 1kHz. Any "audio" opamp will be fine. For active filters you generally want an opamp with a bandwidth at least 10 times the fc or f3.

[infinia]

Geez can't you up date that 70 yr old HP design. Use a LED instead of a non green incandescent perhaps

[Elvee]

I'd say the main distortion source in this circuit is the regulator bulb.
A 40mA filament has a pretty low time constant, and the frequency of 1KHz is low, meaning there are probably interactions at this level.
I'd try to "dilute" the resistance of the bulb with series and parallel resistors, preferably adjustable, in order to reduce the output level; this too will contribute to lower distortion.

[Bonsai]

Anothe r option is to build the osclillator section of Bob Cordells THD analyzer - if you go to his web page you can find a reprint of the original article (.pdf).

I've been toying with the idea of re-doing this but modernizing it a bit . . . . .big project and so little time.

[infinia]

Distortion is related to the ratio of the bulb heating time constant to osc freq. or tau.
The distortion should get lower as the osc. frequency is increased. Or a bigger bulb filliment is used.

Consider using a Bandpass filter buffer perhaps if you just need a narrow tuning range.

430 ohm = 2 times bulb resistance at equalibrium

[rayfutrell]

Look at Linear Technology app note AN43, page 30. They have methods of lowering the distortion of these oscillators.

Regards,
Ray