Can we improve this low distortion sine oscillator ?

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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
 

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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.
 
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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.
 
Wein bridge oscillator

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

Upon closer inspection you find that the lamp is not intended to produce light.

Here is the link for AN43. Skip to page 29. Appendix C discusses the early days of HP. Appendix describes distortion measurements. By all means, see the last page.

AN43
 
with extra op amps you can rearrange the Wein Bridge use only inverting gain stages which have lower distortion from eliminating common mode V at the input - which is generally worse with fet input amps

see already mentioned LT AN-43 pg32 fig 47 for common mode supression and better gain control

there's also Eagar"s "super op amp" oscillator using composite opamp gain stages - claims ppb distortion

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1031,C1060,P1029,D4156
(pg 67)
 
jcx said:
with extra op amps you can rearrange the Wein Bridge use only inverting gain stages which have lower distortion from eliminating common mode V at the input - which is generally worse with fet input amps

see already mentioned LT AN-43 pg32 fig 47 for common mode supression and better gain control

there's also Eagar"s "super op amp" oscillator using composite opamp gain stages - claims ppb distortion

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1031,C1060,P1029,D4156
(pg 67)

I always wanted to play with that circuit. It has a distortion trim doesn't it, which seems a little funny in a way? Jim Williams sent me his collection of papers on Wein bridge oscillators after I mentioned in passing that one without distortion actually can't oscillate. There is a wonderful tech note from HP (1960's) on this.

EDIT - I should correct that to "as the distortion goes to zero the amplitude settling time goes to infinity"
 

GK

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scott wurcer said:


I always wanted to play with that circuit.


Me too. Way back when I was a first year apprentice my boss let me souvenir this from the parts drawer as part of my education expenses.
It’s a never-soldered RA53 glass encapsulated thermistor that is now unobtanium but was once the defacto standard bouncing feedback stabilisation component in budget commercially manufactured test bench audio oscillators.

I originally worked out a discrete fully-symmetrical opamp circuit to make a Wein bridge oscillator with it, but then moved onto other things and never got around to it. I’ve been meaning to etch a board for the design for nostalgia for a while already.

Also, I think I can pretty much guarantee that for the circuit presented in this threads opening post at 1kHz the distortion introduced by the light bulbs thermal time constant would be on the negligible side.
I’ve got published oscillator curves for the RA53 and THD doesn’t begin to pull out of the noise until <100Hz or so (if my memory serves me correctly).

The thermistor blob of the RA53 is absolutely tiny and the thermal time constant is far from great. This is one of the reasons it was the preferred device for oscillator stabilisation – the boing-boing would settle quickly after readjusting the frequency.

Cheers,
Glen
 

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G.Kleinschmidt said:



Me too. Way back when I was a first year apprentice my boss let me souvenir this from the parts drawer as part of my education expenses.
It’s a never-soldered RA53 glass encapsulated thermistor that is now unobtanium but was once the defacto standard bouncing feedback stabilisation component in budget commercially manufactured test bench audio oscillators.


I used a Tellabs precision PTAT resistor (the one they made for compensating log amps) to make a .1ppm oscillator once.
 
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