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.
** That is the main problem with that dopey scheme.
Ideas:
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.
Use the FFT function on a DSO to look at the harmonics.
Use both above together.
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.
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.
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.
" and then using a set of diodes to "sculpt" it into a
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.
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.
That's OK. It is his. He did it. No problem.
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.
** 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%.
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
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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.
Thanks for the plug! The OP can probably do his calibration for free, before the 30-session / 30-day trial expires. (I say "probably", because I don't know if there is a good strategy for adjusting a multiple-diode wave shaper; I seem to recall reading a *design* article back in the Bad Old Days, though.)
Anyway, after the trial period, Daqarta's signal generator keeps working, forever. (Plus a whole bunch of other stuff, but not external signal *inputs*.) So even if he can't get his old signal generator working he'll have a replacement that not only produces lower sine distortion, but has a lot of exotic modulation options. 2 channels are standard, with up to 4 independent streams per channel. (Sneak preview: Daqarta v8.00 will allow 8 independent output channels.)
One shortcoming of sound cards, however, is that they are all AC-coupled so you can't get decent results below a few Hz without circuit modification. (Some high-end cards seem to claim DC coupling, but it's an "audiophile lie": They just mean that there are no capacitors in the signal path. They use a servo circuit instead, but still have an AC-coupled high-pass response.)
Regarding sine-from-triangle generators, I think the average function generator is around 1% (-40 dB). The "overdriven differential pair with feedback" approach can do that easily, and as Phil Allison notes it is easy to adjust by ear. You have only 2 adjustments, and once you are close it comes down to trading between 2nd harmonic and 3rd harmonic, both pretty audible if your fundamental is in the 100-500 Hz range. I have seen one (ancient) article where by careful design and oven control of the differential pair, they got down to -60 dB.
Best regards,
Bob Masta DAQARTA v7.60 Data AcQuisition And Real-Time Analysis
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Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!
Yes, they use 15 resistors and 10 diodes. There are 3 voltage dividers of 5 resistors each and wierd values. The center divider has the triangle input which is already calibrated for symetry and offset. 5 diodes on one side are used for half the waveform and the other 5 diodes for the other half. There are 4 pots to set the DC voltage at each end of the 2 dividers. And a couple more pots for other minor adjustments. I think this thing does less than
Why the op-amp? This generator puts out 20 volts p-p open circuit, or 10 volts p-p into 50 ohms. Why not just feed the generator into a passive twin-T notch filter and look at the other end of the filter?
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