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Function Generator Sinewave distortion?

Started by Bill Bowden March 6, 2015
I have an old function generator that has 6 pots to adjust the shape of the 
sinewave for minimal distortion  I don't have a distortion analyser so I was 
thinking of using a DIY sinewave oscillator  through a filter to arrive at a 
good reference and then compare the waveforms on a scope in the add mode and 
adjust the pots so they cancel out indicating minimum distortion. Problem 
is, I can't figure out how to synchronize the 2 waveforms so they both are 
at exactly the same frequency, 180 out of phase and they cross zero at the 
same time.

How do I set this up so I can calibrate the function generator for minimum 
distortion?


. 



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"Bill Bowden" <bperryb@bowdenshobbycircuits.info> wrote in message 
news:mddmot$2off$1@adenine.netfront.net...
>I have an old function generator that has 6 pots to adjust the shape of the >sinewave for minimal distortion I don't have a distortion analyser so I >was thinking of using a DIY sinewave oscillator through a filter to arrive >at a good reference and then compare the waveforms on a scope in the add >mode and adjust the pots so they cancel out indicating minimum distortion. >Problem is, I can't figure out how to synchronize the 2 waveforms so they >both are at exactly the same frequency, 180 out of phase and they cross >zero at the same time. > > How do I set this up so I can calibrate the function generator for minimum > distortion? >
Can't help; with that part, but what would hapen if you just built a very sharp filter to filter out the sine wave ? Then looked at what ever was left.
 Bill Bowden wrote:

> I have an old function generator that has 6 pots to adjust the shape of the > sinewave for minimal distortion I don't have a distortion analyser so I was > thinking of using a DIY sinewave oscillator through a filter to arrive at a > good reference and then compare the waveforms on a scope in the add mode and > adjust the pots so they cancel out indicating minimum distortion. Problem > is, I can't figure out how to synchronize the 2 waveforms so they both are > at exactly the same frequency, 180 out of phase and they cross zero at the > same time.
** That is the main problem with that dopey scheme. Ideas: 1. Use twin-T notch filter to remove the fundamental so you can see all the harmonics on a scope. Piece of cake to make one. 2. Use the FFT function on a DSO to look at the harmonics. 3. Use both above together. 4. Set the generator to about 500Hz and use your ears - it is usually easy enough to put the speaker in spot that almost nulls the fundamental where you are sitting making the harmonics more obvious. .... Phil
On Fri, 6 Mar 2015, Phil Allison wrote:

> Bill Bowden wrote: > >> I have an old function generator that has 6 pots to adjust the shape of the >> sinewave for minimal distortion I don't have a distortion analyser so I was >> thinking of using a DIY sinewave oscillator through a filter to arrive at a >> good reference and then compare the waveforms on a scope in the add mode and >> adjust the pots so they cancel out indicating minimum distortion. Problem >> is, I can't figure out how to synchronize the 2 waveforms so they both are >> at exactly the same frequency, 180 out of phase and they cross zero at the >> same time. > > ** That is the main problem with that dopey scheme. > > Ideas: > > 1. Use twin-T notch filter to remove the fundamental so you can see all > the harmonics on a scope. Piece of cake to make one. >
Isn't that how a distortion analyzer is made? I dont' recall needing an oscillator. Michael
> 2. Use the FFT function on a DSO to look at the harmonics. > > 3. Use both above together. > > 4. Set the generator to about 500Hz and use your ears - it is usually easy enough to put the speaker in spot that almost nulls the fundamental where you are sitting making the harmonics more obvious. > > > > .... Phil > > > >
On Fri, 6 Mar 2015, Bill Bowden wrote:

> I have an old function generator that has 6 pots to adjust the shape of the > sinewave for minimal distortion I don't have a distortion analyser so I was > thinking of using a DIY sinewave oscillator through a filter to arrive at a > good reference and then compare the waveforms on a scope in the add mode and > adjust the pots so they cancel out indicating minimum distortion. Problem > is, I can't figure out how to synchronize the 2 waveforms so they both are > at exactly the same frequency, 180 out of phase and they cross zero at the > same time. > > How do I set this up so I can calibrate the function generator for minimum > distortion? >
If there's a bunch of pots, likely they are generating a triangle wave (easy to do) and then using a set of diodes to "sculpt" it into a sinewave. At the best of times, it will not be a perfect sinewave. HOw much fussing depends on how much you need the sinewave, and even then, I think there will be more distortion than if you set out with an oscillator that generated a sinewave directly. Function generators are for general purpose, an oscillator with a wide range of frequency and different output waveforms. But if you need a good sinewave, you look elsewhere. Maybe stuff about the 8038 would give some ideas about adjusting the sinewave. It generated a triangle and then used diode-resistor shaping network, but there was little external adjustment of the waveform. But the old articles might have given some hints on getting a better sinewave. As someone pointed out, if you need a good sinewave and it's a relatively fixed frequency, you could just pass it through a low pass filter, likely clear up things relatively well. Michael
On Fri, 6 Mar 2015 18:11:40 -0800, "Bill Bowden"
<bperryb@bowdenshobbycircuits.info> wrote:

>I have an old function generator that has 6 pots to adjust the shape of the >sinewave for minimal distortion I don't have a distortion analyser so I was >thinking of using a DIY sinewave oscillator through a filter to arrive at a >good reference and then compare the waveforms on a scope in the add mode and >adjust the pots so they cancel out indicating minimum distortion. Problem >is, I can't figure out how to synchronize the 2 waveforms so they both are >at exactly the same frequency, 180 out of phase and they cross zero at the >same time. > >How do I set this up so I can calibrate the function generator for minimum >distortion? >
Since you are writing this on a PC, you already have the instrument of your dreams. Connect the "sinus" to the sound card, run a free FFT software and - bingo! w.
>"If there's a bunch of pots, likely they are generating a triangle wave=20
(easy to do)" Are you thinking Wavetek ? I lik those. They claim to do 0.5 % harmonic dis= tortion. I think tha tis some damn good engineeriong myself. It is not just= diodes, it is also using the knee of the beta curve of amost common bas st= ages. I have a Wavetek 111, and I thihnk for being back in the 1960s, the t= hing is fantastic. for what they diud at the time I tink they were like Tek= tronix. Not as complex but just as anal about certain things.=20 " and then using a set of diodes to "sculpt" it into a=20
>sinewave. At the best of times, it will not be a perfect sinewave"
The Wavetek 1XX series used three stages. Emitter fed by the triangle throu= gh the diodes, each biased a bit differently. Totally push pull, positive a= nd negative sides matched to a "T". Those pots adjusted the parts of a sine= wave that the circuit acted upon, and it acted upon it. Wavetek found a wa= y to make it a bit better. It hadd negative current feedback and brought th= e low current hfe of the transistors into play. It has it just coming into = conductance, and nulling it. there is a set of precision resistors that if = not right, you would NEVER be able to align it down to 0.5 % harmonic disto= rtion. Ever.=20 My buddy has a 103 that has a distortyd sine output. I found it to have a D= C offset From the triangle output to the sine convertor. This is consistent= with the fact that his "sine" is compressed at the top and ponty at the bo= ttom. I temporarily fixed it after gettijng aggeavated working on it by put= ting a blocking capacitor between the triangle generator and the sine conve= rtor. It worked fine but he decided to be anal and get it workin right so h= e took it apart and now we have a basketful. He frigged someting up and now= it won't even oscillate.=20 That's OK. It is his. He did it. No problem.=20 Anyway, I was never impressed by generators tha tneed AGC and all that to s= tay flat response. And thought I REALLY respect GOOD analog design, when it= comes to this, digital is probably good enough now. Although I will not se= l my old Wavetek, thsat doesn't mean I don't want something else.=20 See how we are ?
>"Isn't that how a distortion analyzer is made? I dont' recall needing an >oscillator. "
Some have it included, others do not. And some that do have it included give you the option of using an external oscillator.
Michael Black wrote:

Phil Allison wrote:

 >
> > 1. Use twin-T notch filter to remove the fundamental so you can see all > > the harmonics on a scope. Piece of cake to make one. > > > Isn't that how a distortion analyzer is made? I dont' recall needing an > oscillator. >
** THD analysers use one or more notch filters to remove the fundamental frequency and then measure what is left. Most allow that to be read a percentage of the original signal and be viewed on a scope. Most are purely manual while more expensive models adjust the notch frequency automatically. My home brew, Wien Bridge notch filter job has three fixed frequencies ( 66, 998 and 5280 Hz) plus two 10 turn WW pots to trim the notch frequency and depth. With a little patience, it can resolve down to 0.002%. ... Phil
On Sat, 07 Mar 2015 00:01:00 -0700, Helmut Wabnig --- -.dotat>  
<"<hwabnig"@.-> wrote:

> On Fri, 6 Mar 2015 18:11:40 -0800, "Bill Bowden" > <bperryb@bowdenshobbycircuits.info> wrote: > >> I have an old function generator that has 6 pots to adjust the shape of >> the >> sinewave for minimal distortion I don't have a distortion analyser so >> I was >> thinking of using a DIY sinewave oscillator through a filter to arrive >> at a >> good reference and then compare the waveforms on a scope in the add >> mode and >> adjust the pots so they cancel out indicating minimum distortion. >> Problem >> is, I can't figure out how to synchronize the 2 waveforms so they both >> are >> at exactly the same frequency, 180 out of phase and they cross zero at >> the >> same time. >> >> How do I set this up so I can calibrate the function generator for >> minimum >> distortion? >> > > Since you are writing this on a PC, > you already have the instrument of your dreams. > > Connect the "sinus" to the sound card, > run a free FFT software and - bingo! > > w.
Hear, hear! The dynamic range is more than 120dB and most function generators using a triangular wave will be around 60dB from memory. Either use your soundcard and run the software yourself, or get FFT programs. I was impressed with the simplicity of the GUI done by Bob: Bob Masta DAQARTA v7.60 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card! Notice he now has an LCR meter built-in! shows you Resr of caps.