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low pass filter

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tzc:
I am confused some question about low pass filter
1)Why when cut-off frequency increases average voltage remain constant but ripple voltage is keep increasing?
2)Why the ripple voltage is the highest when reach 50% of the duty cycle ?
3)What is the relationship between cut-off frequency and average voltage or ripple voltage?
Thanks

orolo:
1) You should specify which LPF are you talking about. Many filter topologies scale in frequency with no distortion of the transfer function.

2) Correlate duty cycle to harmonic content. See Figures 1 and 2 of http://www.wenzel.com/wp-content/uploads/choose.pdf .

3) What specific filter are you talking about?

LvW:

--- Quote from: tzc on November 11, 2018, 09:38:05 am ---I am confused some question about low pass filter
1)Why when cut-off frequency increases average voltage remain constant but ripple voltage is keep increasing?

--- End quote ---

No, that is not correct. Where did you find such a statement?
Ripple is independent on cut-offf frequency.


--- Quote from: tzc on November 11, 2018, 09:38:05 am ---2)Why the ripple voltage is the highest when reach 50% of the duty cycle ?

--- End quote ---

I do not understand the queston. The so-called "ripple" of the passband is a property of the transferfunction and does not depend on properties of the input signal.


--- Quote from: tzc on November 11, 2018, 09:38:05 am ---3)What is the relationship between cut-off frequency and average voltage or ripple voltage?

--- End quote ---

What do you mean with "average voltage"?


tzc:

--- Quote from: orolo on November 11, 2018, 11:26:03 am ---1) You should specify which LPF are you talking about. Many filter topologies scale in frequency with no distortion of the transfer function.

Sorry for the unclear information I am using RC low pass filter.

2) Correlate duty cycle to harmonic content. See Figures 1 and 2 of http://www.wenzel.com/wp-content/uploads/choose.pdf .

Okay,thanks

3) What specific filter are you talking about?

RC low pass filter.
--- End quote ---

orolo:
You should explain a bit what you're doing, because answering general questions can be very hard. I guess you're passing a variable duty cycle square wave through an RC lowpass filter, with R a potentiometer, and watching the output voltage (with some high impedance meter, hopefully).

If that is right, you need to specify which R and C values you use, and what frequency is your square wave. Many things could happen depending on that. Probably you see another distorted square wave at the ouput (is that what you call average?), maybe with overshoot at the edges (is that what you call ripple?). The cause of that is a mix of suppressing higher armonics with phase shifting the lower ones.

Please specify the conditions of your question. It is hard enough answering, without having to guess what is exactly being asked.

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