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Old 30th June 2008, 10:48 PM   #1
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Default Wien Bridge, I HATE YOU

Dear Friends,

For many times i'm searching for a good Function Generator to use in place of the good design with ICL8038, MAX038 and XR2206 because them are IMPOSSIBLE to found in Brazil.

I make the Wien Bridge with a OPA228, OPA227, OPA134, NE5534. I used styroflex capacitors, and a Wave Switch.

The capacitors are 1uf, 0.1uf, 0.01uf, 0.001uf, I tested for the potenciometer a 47k wich I have good results. The waveform choose when be good and when don't oscillate.
I put the 10kohm potentiometer and the waveform was good in these 4 scales. But... It choose when oscillate or when don't oscillate. ( The waveform wasn't so good )

Does anybody have a opinion about this? I need a Function Generator!

A digital switch is a good choice to pulll off the waveswitch ( I don't know the name of this switch in English, but it is a rotative switch )?

Best Regards,
Felipe Navarro
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Old 30th June 2008, 11:00 PM   #2
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What are you using to control positive feedback in the oscillator? Pots don't work! Traditionally light bulbs were used because of their positive temperature coefficient. Finding the right one might be an adventure, but seems a bit easier than using a precision rectifier and jfet as I once did. (Bulb worked better.) You might want to use one bulb for the higher frequency ranges and add another for the LF ranges or select one with a longer time constant in order to reduce LF distortion.

Look at the old HP201 oscillator, not to mention the old Thorens TD-125 (MKII) electronics as examples of bulb regulated Weinbridge oscillators.

A well implemented Weinbridge will blow the doors off of any function generator in terms of thd.

Note that the chips you listed weren't that great either. I built a generator around the XR-2206 and despite great attention to detail it never worked well. (I was an eng tech at the time I designed and built it.) Incidentally I don't think any of those types are commonly available now, IIRC most of the listed types were in production over 20yrs ago and have not stood the test of time that well..
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Old 30th June 2008, 11:08 PM   #3
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What functions, amplitude and repetition rate do you require?

You can output functions from a soundcard.

You can find some free software for tones, e.g TTG, or you can make a .wav file with any waveshape you want with a sample editor or a bit of programming. You might even be able to create the values and export it from Excel.

w

Obviously the rise time is limited and you will only get a really good square wave at fairly low frequencies, but this is true to an extent with any FG.
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Old 30th June 2008, 11:19 PM   #4
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I found this circuit a while ago, maybe it helps.

Sine Wave Generator with bulbs (Sine-lightenment)
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Old 30th June 2008, 11:20 PM   #5
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The Wein bridge is a thing of beauty but hard to get right.

I assume your computer has a sound-card? Why not just use a software function generator. Google Visual Analyzer, Praxis or search your favorite freeware site.

waki- you type faster than me!
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Old 30th June 2008, 11:20 PM   #6
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There was a project of a state variable oscillator with stabilisation by diodes which was described by Ian Hickman in Electronics World. Disto lmuch less than 0.1%, simple and interesting.

About 25 years ago, Elektor made a full project with double sided PCB using an XR2206. It got rid of the spikes on the sines at high frequencies quite well. Useful for applications where stabilisation of the output voltage is more important than distorsion (about 0.5 %), I still use it today.
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Old 1st July 2008, 12:03 AM   #7
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Quote:
Originally posted by Ecclesiastes
"The race is not always to the swift, nor the battle to the strong."
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Nice to meet you Iain
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Old 1st July 2008, 12:08 AM   #8
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Quote:
Originally posted by forr
There was a project of a state variable oscillator with stabilisation by diodes which was described by Ian Hickman in Electronics World. Disto lmuch less than 0.1%, simple and interesting.

About 25 years ago, Elektor made a full project with double sided PCB using an XR2206. It got rid of the spikes on the sines at high frequencies quite well. Useful for applications where stabilisation of the output voltage is more important than distorsion (about 0.5 %), I still use it today.

I'm using the stabilisation by two 1n4148 diodes. And a negative feedback of gain is 3.

I need from 10hz to 100khz to test amplifiers. Sine and Square waves.

I tested mine soundcard a two weeks ago. It's horrible the square and sine wave at 10khz.

Best Regards,
Felipe Navarro
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Old 1st July 2008, 12:14 AM   #9
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This Wein Bridge oscillator is getting me crazy! It works when it wants to.

Best Regards,
Felipe
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Old 1st July 2008, 12:21 AM   #10
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The schematic
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