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Old 11th August 2008, 06:41 PM   #1
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Default Pulse Width Modulation Circuits

Could a discrete circuit be potentially better then a IC? What is the best IC / solution available? Note: i have a gate driving and level shifting circuit designed. The PWM circuit only drives a small signal fet.

Won't a 1% distorted triangle wave distort the audio output by 1%? A 1% triangle wave at 250khz is hard to get.
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Old 11th August 2008, 09:18 PM   #2
ssanmor is offline ssanmor  Spain
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Hello,
Your statement is true, a distorted triangle wave will distort the audio output, unless you use feedback.
You can get quite a good quality triangle using a video opamp with some decens of v/us slewrate and a hundred MHz bandwidth.
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Old 11th August 2008, 10:01 PM   #3
Pafi is offline Pafi  Hungary
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Quote:
Won't a 1% distorted triangle wave distort the audio output by 1%?
Maybe, or may not. It depends on the type of distortion, and also on the feedback. If the distortion changes one side of the signal in such a way that changes the other side in the opposit direction, then the distortion in audio is much smaller then the one in triangle.

With the attached "triangle" wave...
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File Type: gif pwmquad triangle3.gif (11.8 KB, 818 views)
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Old 11th August 2008, 10:04 PM   #4
Pafi is offline Pafi  Hungary
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...output is distorted only this much. Without feedback!
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File Type: gif pwmquad output3.gif (12.2 KB, 804 views)
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Old 11th August 2008, 10:16 PM   #5
luka is offline luka  Slovenia
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Simple clock generator driving RC will I think produce undistorted triangle wave, only if "noise" comes from some where else will you get distortion
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Old 12th August 2008, 12:04 AM   #6
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If the square wave is near perfect and the capacitor charges and discharges in a linear fashion that would make for a very precise triangle wave.

http://members.aol.com/sbench102/caps.html

Paper/oil, polycarbonate, polypropylene, or polystyrene should work well?
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Old 12th August 2008, 05:57 AM   #7
luka is offline luka  Slovenia
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I don't know which type I got, but that yellow, bigger one is the one, 4n7 with 250k freq., all you need is some resistor to set how big you would want to have that /\/\ wave
Click the image to open in full size.
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Old 12th August 2008, 10:29 AM   #8
Gyula is offline Gyula  Hungary
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Hi Pafi!

Distortion can`t be removed even with feedback. It can only be reduced, depending on the open-loop gain, at the desired frequency.
By the way, don`t you think the feedback with infinite harmonics with their sidebands, what remainded from the square wave can cause harmonic distortion? I say it can, because there's sampling at the triangle comparation. But I'm not sure about it.
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Old 12th August 2008, 10:33 AM   #9
Pafi is offline Pafi  Hungary
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Quote:
Originally posted by Gyula
Hi Pafi!

Distortion can`t be removed even with feedback. It can only be reduced, depending on the open-loop gain, at the desired frequency.
Gyula!

Who said any contradictionary?
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Old 12th August 2008, 02:05 PM   #10
Pafi is offline Pafi  Hungary
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By the way, don`t you think the feedback with infinite harmonics with their sidebands, what remainded from the square wave can cause harmonic distortion?
What do you mean? There are many different cases. The extremely simplest case: quasy DC modulating signal, perfect triangle wave. In this case there is no distortion, it can be verified in both time and frequency domain. But in case of high frequency modulating signal (in practice ftriangle/fmod<=5..10) the mathematical analysis suddenly becomes very difficult. There are different type of PWM methods (main groops: naturally sampled, and periodically sampled PWM) and they have distortion without feedback already, but very different kind of distorton: periodically sampled PWM have harmonic distortion, but naturally sampled PWM have non-harmonic distortion! And we could start to analyse feedback after this, but linear assumptions are unusable if you suspected a nonlinear behaviour ("feedback makes distortion"). Not simply the amount of feedback, but the exact way of feedback determines closed loop distortion. For example: delay in feedback loop is evil, especially with multi-level PWM. This area of researches deserves not just a post, but a big topic. But to take a shortcut: experiments tell that feedback decreases distortion significantly, however sometimes not as much (1/loop gain) as we expect.

Prof. Halász Sándor had an interesting article about PWM distortions in IEEE in the end of 70's, but now I can't find it. There is a list of his publication, but I guess it's not full.
http://www.vet.bme.hu/mts/publ/halasz_sandor.htm
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