low pass filter  - Page 1
Forum section: electronics / beginners

author: tzc
author_rank: Newbie
timestamp: November 11, 2018, 09:38:05 am
content: I am confused some question about low pass filter1)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

author: orolo
author_rank: Frequent Contributor
timestamp: November 11, 2018, 11:26:03 am
content: 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 ofhttp://www.wenzel.com/wp-content/uploads/choose.pdf.3) What specific filter are you talking about?

author: LvW
author_rank: Frequent Contributor
timestamp: November 11, 2018, 11:41:28 am
content: Quote from: tzc on November 11, 2018, 09:38:05 amNo, 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 amI 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 amWhat do you mean with "average voltage"?

author: tzc
author_rank: Newbie
timestamp: November 11, 2018, 02:08:02 pm
content: Quote from: orolo on November 11, 2018, 11:26:03 am

author: orolo
author_rank: Frequent Contributor
timestamp: November 11, 2018, 02:24:10 pm
content: 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.

author: tzc
author_rank: Newbie
timestamp: November 11, 2018, 02:26:49 pm
content: Quote from: LvW on November 11, 2018, 11:41:28 am

author: orolo
author_rank: Frequent Contributor
timestamp: November 11, 2018, 03:19:11 pm
content: Ok, now I think I understand.1) The DC component of your wave is the 0th harmonic, which will pass unmolested, as long as your filter is lowpass. So your average DC component will be the same independent of cutoff frequency. What you call ripple are the higher harmonics (1kHz and upper), which will become the more attenuated the lower your cutoff frequency is.2) Look at Wenzel's Figure 2. The fundamental 1kHz harmonic is maximum for 50% duty cycle. That is the lowest "ripple" (non DC) frequency, so the most unaltered by the filter. It will dominate in the output.3) Roghly speaking, the lower the cutoff and duty cycle, the lower the ripple, since you have lesser dominant terms (lower duty cycle) and all harmonics get more attenuated (lower cutoff). Your RC filter generally should reduce ripple voltage at a rate of about 3dB per octave of cutoff reduction below 1kHz.For a 100% rigorous answer, you need the transfer function for your RC filter, generally Vout = Vin / (1 + jwRC), where w is the angular frequency and j the imaginary unit. Apply that frequency-dependent operator to the coefficients of the Fourier series of your variable duty-cycle wave (Wenzel, Fig. 1) and then compute the RMS norm of the result.

author: LvW
author_rank: Frequent Contributor
timestamp: November 11, 2018, 03:39:19 pm
content: Now I have realized that my first reply did not really was an answer to your problem.However, the reason was the uncomplete description of your problem.So - you did not mention that it was your task to filter a squarewave.You spoke about low pass filtering and "ripple".Perhaps you don`t know that some lowpass filters have a "passband ripple" - and that was the background of the misunderstanding.More than that, when you are asked "what kind of filter" - it is by far not enough to answer "RC filter".There are many, many different RC filter alternatives - at least the degree of the filter schould be given.Finally: A good and complete answer requires at least a good and clear problem description.

author: bson
author_rank: Unknown
timestamp: November 11, 2018, 10:09:27 pm
content: Quote from: tzc on November 11, 2018, 09:38:05 amBecause you're passing through more noise?