opamp sine wave oscillator

Does the definition of "nicely" include "producing a low distortion sine wave"?

What a bunch of guys. What's the THD of those ones? More than a bit of clipping, I bet.

That's usually cad sulphide (CdS), cad selenide (CdSe), or an alloy of the two. They're very sensitive, adequately slow for lots of things, but unfortunately their photoresponse depends on their previous history...would you believe 5:1 change in resistance for CdSe based on how recently it's been in room light? For oscillators that tends to turn into lots and lots of amplitude drift unless you're careful.

And you need to linearize it really carefully---Jim Williams's chapter on ALC oscillators is a classic. (It's in one of his books, but I don't have it handy.) Residual nonlinearity in the JFET (even with the 1:1 voltage divider between gate and drain) turns out to be one of the big problems, iirc.

Cheers,

Phil Hobbs

An analog multiplier is the obvious choice.

John

If the "Q" of the effective "tank" is high enough, all you need to do is limit energy input.

Not the best example in the world, since it has ~1% distortion...

But I have some AGC'd versions around here someplace that do several orders of magnitude better.

I'll post when I find where I filed them away ;-)

...Jim Thompson

We also discussed it well in our book. WRT an exact factor of two, 1:1 ratio, that can be viewed as advantage, having a distortion- adjustment point. Our Wien-bridge oscillator design (for AoE 3rd ed) uses 1M and 976k+50k pot for distortion trimming, 1:1 +/-2.5%, and along with a gain trimpot (to minimize the JFET's work), achieves 2ppm distortion. We used an inexpensive Fairchild 2n5458 JFET.

ell

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Your amplitude stabilization consists of setting the loop gain to just a little bigger than unity and hitting the rails.

Changes in the rail voltages will show up as both amplitude changes and frequency changes in the output.

Sounds like a very real step backwards from the Wien bridge with a light bulb (or whatever).

Tim.

Well, now you brought it upon yourself: When is that new edition going to hit the store shelves?

Not in this case unless we are speaking of very high values of "low".

If you use the inverting pin of the op-amp as the output, the 3rd harmonic is down by about 20dB.

Mine doesn't clip at all when run on normal supply voltages. It makes a sawtoothy waveform that doesn't get tot he rails.

The ones I was speaking of are in a black housing. The no-light resistance depends on what happened last week but the full light resistance only really depends on the last minute or so.

You just tell the user to leave it on for a week before use. A nonlinear feedback gets it settled more quickly. For large errors, you turn the LED to full power or off.

Yes, it adds distortion for even fairly modest swings. "deleveraging" the JFET with capacitors and not resistors like:

! === C1 ! !------+ Control-->! ! !-- === C2 ! ! GND GND

helps.

It divides down the amplitude the JFET sees and C2 also tends to short out the harmonics.

That's not even my main concern. I can wait. What I'm worried about is: How is the community going to get their hands on what's finished of AoE3 when Win dies of old age 20 or 30 years from now?

robert

Actually, we've ramped up our work pace, and plan on introducing H&H AoE 3rd-ed early in 2010. We also committed to this in an edit of the Wikipedia "Art of Electronics" entry, so now it must be true. :-)

It is such a wide ranging book, that (unlike most textbooks) also tries to describe the actual current state of the art. By the time it is finished it is ten years later and out of date - it's a moving target, and you will always be trying to catch up without a huge, sustained effort.

But I guess you know that :)

You should have broken the thing up into multiple volumes, we'll all be dead by 2010...

That is awesome. IIRC Williams's sinewave generator of four opamps claimed the THD of 3 ppm. How do you measure the distortion levels that low? The design of the notch filter for the measurement should be as challenging as the design of the oscillator itself.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

Wait, what year is it now? I meant to say 2009.

It's easy. You build a couple hundred of them, sum their outputs, and THEN measure the distortion.

Bob

So when good old analog TV goes kaputt and we finally have free time in the evening, voila, a new AoE appears? Great!

Pre-orders, with a penalty clause? ;-)

Interesting. We didn't choose to do that, and instead placed the JFET directly in the DC signal pathway. Hmm, would you say we could have further reduced our distortion below the 2ppm = 0.0002% we observed?