Kevin Weddle Posted December 28, 2006 Report Share Posted December 28, 2006 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? Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 28, 2006 Report Share Posted December 28, 2006 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. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 29, 2006 Author Report Share Posted December 29, 2006 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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 29, 2006 Report Share Posted December 29, 2006 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. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 30, 2006 Author Report Share Posted December 30, 2006 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? Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 30, 2006 Author Report Share Posted December 30, 2006 And look at the graph, almost every single harmonic is 100db down from the fundamental. They almost don't even exist. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 30, 2006 Report Share Posted December 30, 2006 Sorry Kevin,That graph is rediculous with its scale of 300dB. Here is one that you can see the harmonics: Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 31, 2006 Author Report Share Posted December 31, 2006 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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 31, 2006 Report Share Posted December 31, 2006 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. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.