What kind of distortion is an asymmetrical sine wave?

"John - KD5YI"

If the positive half is kinda rounded off at the top while the negative side is a sine shape - that is typical of second harmonic distortion in a simple, one device amplifying stage with no NFB.

The spectrum contains mainly 2nd harmonic and some 4th.

If a sine wave looks rounded top and bottom ( or has a smooth kink near each zero crossing ) - that is characteristic of 3rd harmonic and higher odd harmonics.

..... Phil

Excellent!

Thanks Phil, that's the description I was looking for. Unfortunately, the circuit is no longer functional, but that waveform and what it represented (distortion-wise) was bothering me.

Cheers, John

Although I'm really not that young, I seem to be having a Junior Moment.

I had a circuit which produced a sine wave which had a peak positive voltage of, say, 1V. The peak negative voltage was a tad greater, say

-1.1V. But, the average was zero over a full cycle. I have no way to measure the distortion nor the spectrum.

Can someone give me a qualitative idea of what picture that wave might produce on a SA and what its harmonic distribution might look like?

Accuracy not required. Just an idea so I can get a "feeling" for that sort of thing.

Thanks, John

--------------- Measure about 16 amplitude points on the waveform from the same point on adjacent cycles (one exact cycle) and then find some fast Fourier transform software and plug in the values.

As posted by others a flat bottom waveform is indicative of some 2nd harmonic but other harmonics are involved to keep the top of the waveform looking rounded.

One fallacy that people will tell you is that Nth harmonics will make the wave look like this and that. This only applies when the harmonic is in phase with the fundamental at zero crossings. Third harmonics can flatten the peak of a sine wave or increase the amplitude. This can be simulated in Excel with it's charting abilities. We needed this to analyze some waveforms on a job and a co-worker produced one with variable harmonics and variable phase lag in about half an hour. It was really cool to play with the figures and see what happens in different condition of noise.

mike

I meant to tell you.. You nailed the circuit! It was a single stage amplifier where I was trying to get as much gain as possible so feedback was minuscule.

Thanks again.

John

I meant to tell you.. You nailed the circuit! It was a single stage amplifier where I was trying to get as much gain as possible so feedback was minuscule.

Thanks again.

John

Hey, mike -

Thanks for your reply. As I told Phil, the circuit is no longer functional. Sorry I did not put that into my OP. But, it had been bothering me for some time now. I think Phil nailed it.

Sounds cool. I miss having someone to bounce ideas around with.

Cheers, John

My apologies for the multiple post. Thunderbird went berserk on me and I had to close and restart it.

Sorry.

John

Hey, mike -

Thanks for your reply. As I told Phil, the circuit is no longer functional. Sorry I did not put that into my OP. But, it had been bothering me for some time now. I think Phil nailed it.

Sounds cool. I miss having someone to bounce ideas around with.

Cheers, John

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Soooo, you used to be a bouncer, then?

mike

Haw! Well, not in the usual sense of the word.

John

My apologies for the multiple post. Thunderbird went berserk on me and I had to close and restart it.

Sorry.

John

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I always figured that is your NNTP not sending the "confirm" response. Your news readers will continually try, again, until it gets that back to indicate success.

mike

Aha! Thanks for the explanation. It was probably as you say.

John

Um, if you have half wave assymetry, that HAS to be caused by even harmonics. The second is the most likely.

If the frequency is not too high then you could always download Daqarta which will do basic scope functions and spectrum analysis on a PC using the sound card for two input channels. ISTR it is good to about 50kHz. It will easily show which harmonics are present in audio signals, but the most likely cause is second harmonic if the waveform is assymetric and smooth.

It will run for a month or so on evaluation and work as a signal generator forever. It is nicely done with only the odd quirk. No subsitute for a real scope in electronics but useful for public demos when you want a projectable realtime spectrum analyser for voice or music.

Regards, Martin Brown

1. Slightly flattened top (+ side).
2. "Peaky" bottom (- side).
3. Perfect sine with DC offset ruled out by criteria.

Thanks, Martin. Good idea.

Cheers, John

Gotcha. Thanks, Robert.

Cheers, John