Low distortion sine wave VCO's - state of the art?

Yes, by all means, add a sharp low-distortion filter. Problem solved.

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 Thanks,
    - Win

How about a state variable filter osc, with the two "tuning R's" replaced with current in, current out VCAs, say those in the THAT semi catalog, THAT2151 rings a bell

martin

I'm curious. What application demands an analog tuning voltage?

Allan

(snip)

Does an 8038 0r XR-2206 have a low enough phase jitter to produce a .01% pure frequency, even with reasonable filtering?

The kind of oscillator that came to my mind is the sine cosine oscillator made with two integrators in a loop. But there are still two resistors that have to change in tandem to swing the frequency. But the automatic gain control will compensate for some mismatch, if the frequency changes slowly enough. Or is there a variation that allows the frequency to be changed with only one resistor?

Well, combining Win's idea with another, you could do it like the old HP8640A/B does: make an RF vco using a varicap diode for tuning, and run it through a divide-by-a-lot IC. The square wave output has very low second harmonic (theoretically zero), so with only a +/- 10% tuning range, it's straightforward to filter it. If you don't need super low spurious, you can use a Maxim or Linear Technology switched-capacitor filter, likely running the clock from a tap on the divide-by-a-lot IC. Might have to use one following stage of non-switching filtering to get things clean enough. Given varicap diodes that cover something like

50-500pF, the VCO frequency can be fairly (few hundred kHz or less) low so "a-lot" isn't all that big a number.

You could also make an RC phase shift oscillator, where the C's are large varicaps like that, and do it directly. There are probably other configurations of RC sinewave oscillator that could be made to work...possibly a Wien bridge type...using some sort of AGC to keep the wave from clipping in each case. Varicap diodes with large capacitance could get you to very low frequencies; I believe the old HP200CD uses a couple of about 1200pF sections for the low frequencies, way lower than you need. You'd probably want to run varicap diodes in back-to-back series pairs to lower the distortion from the signal varying the varicap bias, and even then might need a clean-up filter on the output.

BTW, Win, if you see this...did you get my recent email request? If not, please drop me an email...

Cheers, Tom

I'd guess, yes, at low frequencies. That's only 100ppm.

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 Thanks,
    - Win

Okay- you're getting close but still antiquated. State of the art is achieved with DDS, no question about that, and there is that nasty problem of spurious frequency content relatively close to the fundamental which is a complicated function of the number theoretic relations between the CLK and fundamental. This is not likely to support

0.01% THD in general. However, there are fundamental to CLK ratios that will support 0.01% THD, so the obvious answer is fix that ratio, and then tune your RF CLK with the varicap. Output filtering can then be fixed and trivial.

You need pretty clean supply rails to achieve that, since the XR-2206 has a typical supply frequency sensitivity of .01% per volt (100ppm per volt), but the worst case is 10 times that (and that spec applys only to an optimum set of conditions). I didn't look up the 8038.

I would worry that any multivibrator oscillator risks more jitter than one based on a a high Q resonance or a continuous time process that depends on at least a whole previous cycle. Multivibrators make irrevocable decisions based on instantaneous voltages.

It may have little 2nd harmonic, but still plenty of the odd harmonics. Better start with a 8038.

Rene

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Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

I just realized that if you replace the two integrator input resistors with operational transconductance amplifiers, you turn this into a VCO (Well, a current controlled oscillator anyway. Adding a couple resistors to the gain control inputs would fix that.) A pair of OTA's from the same pack should track well enough for the AGC to keep the output amplitude quite constant. With low distortion opamps, I think the second integrator (from the variable feedback point ) might well be able to hit the .01% distortion spec.

OK, let's say 10x worse max: this means he'd need 0.1-volt quiet and stability, which is easily and routinely exceeded.

Agreed, but that's why we'll say 100ppm, rather than 10x better.

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 Thanks,
    - Win

In article , Tom Bruhns wrote: [...]

A couple of ideas:

(1)Once you have this VCO, you could use it as the clock on a DDS chip.

(2) if you use a moderately long Johnson counter as the last section of the divider, a set of weighted resistors can get the first few odd harmonics down under 1%. This makes the following filter a lot easier to make.

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kensmith@rahul.net   forging knowledge

FWIW, I slapped together an 8038 circuit right out of the data book, and no matter how much I twiddled R12 and R13, I couldn't get rid of that little tittie on the top of the sine output. I didn't have a THD meter (who does? ?:-\\ ), but that little tittie had to have introduced some distortion.

I haven't tried the eXaR chip, but it could be worth a shot.

Otherwise, i'd probably look into the Wein(Wien?)-bridge or a phase-shift oscillator - with CMOS parts these days, you should probably be able to make a very nice sine wave.

Good Luck! Rich

More likely (as elsewhere suggested in this thread, and in AOE as well) I'd use a ring counter so I don't have to worry much until some high harmonic (AOE says IIRC that with a 8-stage ring counter, 15th harmonic is the first assuming perfect resistor balance).

And at that point no need for varicap stuff, the VCO from a HC4046 would be more than good enough.

I'm reluctant to put nonlinear components like varicaps in the oscillator because I'd rather start with low distortion rather than put it in. As you point out enough back-bias and small enough oscillator amplitude might make this not so bad.

My first inclination for a "controllable low-distortion tuning" was some Silonex photocell-coupled-to-LED thingies in a Wien bridge. I had very good success with these in low distortion (0.01%) stuff in the past and while I don't remember the guarantees about matching, the ones I've used happened to be very well matched.

Tim.

Any worthwhile electronics engineer could duplicate the performance of this in the home lab:

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As I recall you can also get a couple of them in the same housing,
manufactured as a matched pair, so that ought to work nicely, as
well.  Or was that what you were using then?

John P raised a useful point re possible jitter sources on the triangle shaping chips. Extreme curiosity forced examination of my cheap function generator, a Xtal based sine synthesiser and a GP Wein bridge oscillator. The Topward function genny is identical in working to the XR and ICL internals yet gave no noticable jitter, even at the 1MHz mark (1kc THD 0.6%). Biggest problem was 50Hz power supply related, carrier sidebands at -50dB. The digital synth offered a cleaner sine but throws out wideband, low level noise at the -80dB level, ( THD

0.1%). Wien bridge, no jitter, clean -90dB noise floor and THD of 0.2%.

(-10dBm refs and not counting slow frequency drift as jitter)

I'd go with the guys. Start with a '8038 etc and filter down to 0.01% john

[Have a pair of 8 year old Silonex devices. Originally used as Wien bridge arms, with a HF pilot tone running through to allow dynamic balancing. They still match within 10%, pretty much the same as when bought!.]

Yes you are spot on. I have a 1980-ish design doing exactly that...... fast frequency slewing, with no bounce. 5x multipliers instead of OTA's though.

2x multipliers in series with the integration R's for the voltage control of frequency, as above. 2x multipliers squaring Sin and Cos, outputs added together to produce a fast-response rectification without the need for a smoothing cap. Vref was subtracted at the same time to produce the error voltage for the multiplier that increased/decreased the oscillation gain.

I got about 0.005% THD in the 1980 prototype, which was at the limit of the 400Hz notch filter I had at that time. That was with 2x low THD multipliers in the signal path (AD634?), using the correct low THD X or Y input for the AC, and Vpk held down to no more than about 7v (4-5Vrms).

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Tony Williams.

BTDT with trying to voltage control a Wien Bridge oscillator. The initial problem to solve is that one of the resistive elements has a CMV on it, but the real problem is moving the frequency without transient amplitude changes/bounces.

The underlying problem there is that rectification of a single phase (to get a low ripple dc for the agc) cannot be made fast enough. I have a vague memory in one of those reader Design Ideas at that time, of someone resorting to a complicated s/hold on Vpk to get a fast response dc for the agc.

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Tony Williams.