Anyone know of a design for a -simple-tunable- low distortion sine wave oscillator (audio frequency range)

I have been playing with Wein bridge and other oscillator configurations without any joy. I want low distortion foremost but also the ability to easily tune it over an octave or better tunable over a decade.

Wein bridge works fine, but there is no low cost supply of close tracking pots (1%) available to me and that really sucks. Using a 2 x 12 position multipole switch also doesn't work - at the moment I am thinking of 8 - 10 DPDT stitches on an R-2R network which is really ugly and possibly inconvenient to use.

My question is "is there a low distortion oscillator design where the frequency is tunable by changing a single resistor or a source of electronic pots that track within 1 or 2 percent?

TIA

Reply to
David Eather
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ICL8038, if you can find one. I think somebody like Maxim second-sources a sort of replacement chip, but I've heard bad things about Maxim.

How much schtuff do you have to play with? You could use a counter, LUT and DAC, but that's kind of a lot of hardware.

Good Luck! Rich

Reply to
Rich Grise

I think you have to define clearly what you mean by low distortion 8038 is good for

Hasn't there been a thread about this fairly recently.

Regards, Martin Brown

Reply to
Martin Brown

There is a technique involving switched capacitor digital filters. Basically you clock the filter at, say, 64 times your intended fundamental (or whatever the multiple is for that particular filter), and feed it with the fundamental to filter. What you get out is a very clean sine wave at the fundamental at a known amplitude.

Have a search for digital filters and sine wave generation.

Mark.

Reply to
markp

There was a thread on generating low distortion sine waves. I forget who asked, but their requirement for distortion was was about an order less than mine - I would like .001% - less would be better, but I could (happily) live with .005.

My problem is tuning the sucker. I'm going to go back and try a digital pot (MCP42010). The dual pots don't track as accurately as the data sheets imply because the terminals and wipers are at different potentials and the SPI basically means some form of digital control and noise.

Reply to
David Eather

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Reply to
Bill Sloman

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Asking for 10ppm precision sine waves makes things difficult. You are asking for a 17-bit or better D/A converter if you want to do it digitally.

One option is to make a finite impulse response filter for a square wave by feeding your square wave through a long - say 32-bit - shift register, use your 32 outputs to control 32 single pole double throw analog switches, and connect 32 high precison resistors between a summing junction and a positive reference voltage or an - equal - negative reference voltage.

The resistor values follow the sinc function - (sine x)/x - windowed by a raised cosine Hamming window over at least 90 degrees worth of x. I think I stretched it further when I did something similar back in

1978, but I can't remember why.

The lowest value resistor needs to be a 0.01% part, and even the E196 values are too coarse to let you get away with single resistors any place after the first, so you'd have to figure on measuring the actual resistance (ideally including the on resistance of the analog switch) and padding or trimming the nearest E196 value up to the resistance you actually need.

Truncating the length of the finite impulse response filter gives you Gibbs oscillations on the output (whatever they are) and the Hamming window minimises this problem. I found out about it the hard way and had to rebuild my resistor array to get the desired effect.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

I still think the switched digital filter approach is best (see other post), i.e. clock a switched digital filter (multi-pole) with a multiple of the fundamental, say 64 times depending on filter used, and feed the square wave fundamental into the filter input. You'll get a clean sine wave out, and should be independent of frequency.

Mark.

Reply to
markp

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Switched capacitor filters can get you 80dB below the fundamental, but the OP is asking for 100dB.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

You're basically trying to build your own circa 18-bit DAC from discretes. It's a whale easier to just buy a 20 bit DAC, and drive it from a sine lookup table.

John

Reply to
John Larkin

That's not bad. Use a single pot that tunes an oscillator. The oscillator clocks the switched cap filters, and a div-by-64 or some such divider to feed the filter input.

Some cautions:

The filters will alias the square wave input and make small errors. A little analog pre-filtering will help.

Switch-cap filters make output spikes, so you need an analog output filter as well.

Swcap filters are noisy, have rotten PSRR, and have a bit of distortion of their own. They are, in general, kinda junky.

Probably the best way to get low distortion, tunable sines is by feeding data into a good DAC. A DDS chip does all that for you, for a few dollars.

John

Reply to
John Larkin

markp Inscribed thus:

How about a pair of oscillators at, say 500Khz, mix the outputs and tune them apart. Effectively 0Hz to 20, 30, 40Khz or more, in a single range.

--
Best Regards:
                     Baron.
Reply to
Baron

What kind of distortion level are you trying to achieve. I think I get something < -70dB down with a Wein bridge. (It's kinda hard to measure except when I borrow an SRS spectrum analyzer.) We use these dual ganged pots from Bourns. I can look up a part number if that would help. (The Newark number is 62K3333.. the picture on Newarks website is crap.) I don't know how well they match, but I'm not sure it makes that much differernce in the distortion.

There is also a phase shift oscillator that Phil A. turned me on to. This is nice cause you can get both quadratures out, but I could not get the distortion down as low as the Wein bridge.. and it still needs two pots.

But why not DDS?

George H.

Reply to
George Herold

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0.001%! Wow! How do you measure that?
Reply to
George Herold

0.001% is dead easy. I'm working on a system that measures down to 50ppb (yup, ppb) and that's not too much of a big deal.

Generating is a bit harder, as well as guarantying that what you measure isn't an illusion. Some interesting issues...

--
Thanks,
Fred.
Reply to
Fred Bartoli

What's the learning curve on programming them, and what does the development system cost?

Thanks, Rich

Reply to
Rich Grise

Back in the old days one simply used a ganged variable capacitor for the variable element in a Wien bridge. Values of 30-450 pf with accurate tracking, certainly better than 1%, were readily available. Every superhet receiver ( ie virtually every radio ) had one.

Now you need a DDS to do this simple job.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
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Reply to
Adrian Jansen

Rich Grise wrote in news:id6j8g$g3l$ snipped-for-privacy@news.eternal-september.org:

Analog Devices has free software that runs from the (ugh) parrellel port you can use to evaluate most of their DDS IC's

You can get the AD9833BRMZ for under 9 bucks the 75MHz AD9835 is just over $10.

Reply to
Hammy

0.001% is no problem. 0.0001% is tricky.

Do you know of any DDS chip with a good DAC, especially for a few dollars?

VLV

Reply to
Vladimir Vassilevsky

The varibale capacitors in suoerhets don't track. Some in TRF radios did.

--
For the last time:  I am not a mad scientist, I'm just a very ticked off
scientist!!!
Reply to
Michael A. Terrell

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