opamp sine wave oscillator

[...]

The down side is that the frequency of operation is more sensitve to component values. BTW

That's because the phase shift depends on the control voltage. If you used two resistors instead, you could get the same effect, minus the couple of dB worth of harmonic rolloff, and no significant phase shift.

Cheers,

Phil Hobbs

I'd imagine that most capacitors add distortion, too.

John

Our 2ppm may well be coming from the caps.

Touru Kuroda in Japan published a low distortion oscillator with JFET and opamp. He achieved 0.000064% distortion with a 2SK30A (a jelly bean JFET in Japan, very cheap there) withtwo NE5532 and a TL071. So 5 opamps not 2 like Win's design. Kuroda used 1:1 resistor divider instead of capacitors like this. I wonder this idea could reduce the distortion in Kuroda's circuit, too. Hmmm, interesting. Oh, the outputfrequency of the actual assembly was 995Hz and the output was 7.04Vrms with 0.000064% distortion.

By the way, the oscillator was published in a book titled "First hands on Transistor Circuit Design", a book intended for a beginner. A sine wave oscillator with -124dBc distortion (0.000064%) for a beginner?? Well, you judge. The book contained a full PCB pattern so a lot of people in Japan made this oscillator. I don't know how many of us could measure the actual distortion of the oscillator, though.

Satoru

speaking of distortion measurements, how does that work, anyway? Feed a "zero-distortion" signal into the DUT, subtract the output from the input, and what's left is the distortion? I'd image gain and phase adjustment finicky, to say the least. Or some synchronous demodulation technique -- the same thing in the frequency domain, essentially an ultra-narrow bandpass. What's not inside the delta transfer function has got to be distortion.

So how is it done?

robert

Don't know but I've stressed some philips leaded 500V-100pF NP0 at 1kV while measuring the cap deviation which stayed under the bridge resolution, ie under 0.001% (not discernible from the normal last digit flip).

Using these at normal IC voltages would certainly ensure sub ppm cap induced distortion.

At this level, lots of weird things can happen from class B operation and 'hidden' components (PCB, parasitics) interaction.

--
Thanks,
Fred.

I still wonder if sending e.g. 100 Hz and 5 kHz at the same time to a speaker wouldn't give a 100Hz FM modulation of the 5 kHz tone (doppler effect caused by the cone movement). But maybe the microphone has an inverse effect? But then, what about a pressure sensor as microphone (assuming no diafragm movement)?

Arie de Muijnck

First, the fundamental should be attenuated by a notch or lowpass filter so the distortion of the instrument itself would not affect the result. After that we can measure the harmonics by the synchronous detector or some other technique. The bandwidth of the detector determines the noise floor, and it is determined by the short term frequency stability of the oscillator, which is not so good for the Wein bridge.

For the claimed distortion levels below -120dB, the notch should attenuate the fundamental by at least 20dB or so, and the nonlinear artifacts of the filter itself should stay below -130dB. I don't think that is attainable with the active filter. With the passives, the huge L and C should be used to keep the distortion at minimum.

Nobody yet provided an attestation of their claims. When it comes to the THD levels below 0.001%, I have some doubts.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Down at 2ppm, I would start worrying about resistor linearity, also.

--
Regards,

John Popelish

** No point in testing a multi-way speaker ( ie the vast majority ever sold) for IM distortion.

The famous Quad ESL63 electrostatic speaker has THD in the order of 0.03% at

97dB SPL.

...... Phil

"Arie de Muynck"

** Cone displacement modulates the *phase* of the high frequency tone - but it ain't audible.

Any resulting Doppler effect is undetectable since the sidebands are submerged beneath similar ones produced by the IM distortion of the driver when operated with large cone excursions.

** Nope.

..... Phil

Yes, I knew the cause. The few dB were worth the frequency error.

How about ppb instead of ppm:

Linear Technology Magazine, Feb 94 pp 26-28 - LTMag_V04N1_Feb94.pdf at

"Using Super Op Amps to Push Technological Frontiers: an Ultra-Pure Oscillator

An Ultra-Low-Distortion, 10kHz Sine-Wave Source for Calibration of

16-Bit or Higher Analog-to-Digital Converters

... The measurement of the harmonic distortion of this oscillator defies all of our resources, but appears to be well into the parts-per-billion range."

And only 11 Op amps :-).

ISTR another LT article on measuring the distortion of this oscillator using a 120? dB notch filter also designed with the "Super Op Amp" idea, but can't find it now.

Some references...

Newsgroups: alt.binaries.schematics.electronic Subject: Re: opamp sine wave oscillator (SED) Message-ID:

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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It will certainly produce the FM however the Doppler effect is not the biggest of the problems. The velocity of the cone is only at the order of meters per second. Other nonlinearities are much higher.

If the membrane moves with the same velocity as the cone. This is not realistic.

The DSP can be successfully applied to compensate all sorts of distortion in a speaker. However it is much cheaper to make a reasonably good speaker then to make a good speaker from a bad one by means of the DSP.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

How can that be measured? Do you believe the number?

John

"John Larkin"

** Huh ?

No idea how THD can be measured ?

** No faith required, it ain't religion.

....... Phil

No idea of how to measure THD in a sound wave to this sort of resolution. Are microphones this good? If so, how do people measure

*them*?

A lot of audio stuff seems to be. It sure ain't science. Do you believe that you can buy cables that are six-nines pure copper?

John