sinewave trouble

[Kevin Weddle]

With a series RC circuit, you must be wary of the time constant, or you get major distortion. But you can't easily see the distortion with a sinewave. My 8038 has a triangle output, and it's buffered by an opamp, so it's not affected by the RC. I can see the time constant distortion with a triangle. Can anbody explain why my findings would be false?

 

[audioguru2]

Kevin, you forgot to attach a schematic so I am just guessing.

If you feed a triangle wave through a low value capacitor into a low resistance (a highpass filter), the high frequency harmonics are passed and the low frequency fundamental frequency is attenuated. It won't be a triangle waveform anymore.

If you feed a low distortion sinewave the same as above then its fundamental amplitude will be attenuated and it still looks like a sine-wave.

 

[Kevin Weddle]

Very good. But I can manually ramp a voltage up and down like a triangle, and there will zero harmonics, and a corner which would have very weak power down to zero hertz.

 

[audioguru2]

Very good. But I can manually ramp a voltage up and down like a triangle, and there will zero harmonics, and a corner which would have very weak power down to zero hertz.

No. All triangle-waves have harmonics. Only a pure sine-wave has no harmonics.View attachment 40258

 

[Kevin Weddle]

Okay, I made the high pass RC, the capacitor the input. I kept the capacitor at .1 uF and the resistor 100 ohms. I used the outlet power stepped down by a transformer. Bingo, the 60Hz sinewave from the electric company was totally distorted by the RC. So your saying I get this totally distorted sinewave, just because I lose harmonics? If I want a good sinewave from the electric company, I should avoid filtering out the harmonics. The fundamental is there, the higher harmonics are there, the subharmonic is 85% there compared to the fundamental. The second subhamonic is 65% there compared to the fundamental. Maybe I shouldn't filter the third subharmonic and lower so much?

 

[Kevin Weddle]

And look at the graph, almost every single harmonic is 100db down from the fundamental. They almost don't even exist.

 

[audioguru2]

Sorry Kevin,
That graph is rediculous with its scale of 300dB. Here is one that you can see the harmonics:

View attachment 40261

 

[Kevin Weddle]

I made a high pass RC followed by a low pass RC. The lowpass was of a higher impedance, so as not to affect the high pass. I set the fundamental around the knee of the filter curve for both stages. Wow, the sinewave from the transformer looked bad, and after the filter the sinewave looked great. Do you suspect the home outlet sinewave looks distorted? I can't see it with my oscilloscope without blowing the fuse, the ground on the probe causes a problem.

 

[audioguru2]

I have looked at the electrical 60Hz sine-wave at the secondary of a transformer with a light load and see all kinds of distortion and interference. Of course a lowpass filter reduces the level of the distortion harmonics and the interference.