What is Low pass filter: Definition and 50 Discussions

A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design. The filter is sometimes called a high-cut filter, or treble-cut filter in audio applications. A low-pass filter is the complement of a high-pass filter.
In optics, high-pass and low-pass may have different meanings, depending on whether referring to frequency or wavelength of light, since these variables are inversely related. High-pass frequency filters would act as low-pass wavelength filters, and vice versa. For this reason it is a good practice to refer to wavelength filters as short-pass and long-pass to avoid confusion, which would correspond to high-pass and low-pass frequencies.Low-pass filters exist in many different forms, including electronic circuits such as a hiss filter used in audio, anti-aliasing filters for conditioning signals prior to analog-to-digital conversion, digital filters for smoothing sets of data, acoustic barriers, blurring of images, and so on. The moving average operation used in fields such as finance is a particular kind of low-pass filter, and can be analyzed with the same signal processing techniques as are used for other low-pass filters. Low-pass filters provide a smoother form of a signal, removing the short-term fluctuations and leaving the longer-term trend.
Filter designers will often use the low-pass form as a prototype filter. That is, a filter with unity bandwidth and impedance. The desired filter is obtained from the prototype by scaling for the desired bandwidth and impedance and transforming into the desired bandform (that is low-pass, high-pass, band-pass or band-stop).

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  1. A

    Engineering Is it possible for a low pass filter to have such a transfer function?

    Hello! Consider this filter,and that I have to find the transfer function U2/U1 with the norm Ω= ωRC (also double fractions are not allowed) Now I can see that that the resistor and capacitor left as well as right are parallel to each other. So simplifying that ## Z_1 = \frac{R}{jωRC +1}...
  2. R

    Design a linear phase low pass filter

    Design a linear phase low pass filter whose specification is:1) Maximum tolerance in the passband equal to 0.01% (linear) in the passband;2) Cutoff frequency at ω_c = 0.3π and transition band at 0.05π;3) Minimum reduction of the 0.95 rejection band.
  3. Arman777

    Comp Sci Low Pass Filter, plotting a graph

    I wrote this code, but I am not sure its ture or not from numpy import arange from math import floor from pylab import plot, show Vout = 0 #inital condition t_i = 0 #initial time t_f = 10 #final time N = 100000 h = (t_f - t_i) / N RC_values = [0.01, 0.1, 1] def f(Vout, t): if floor(2 *...
  4. W

    Suitable Opamp for this Low Pass Filter

    Homework Statement [/B] Select (with justification) from the op-amps listed in TABLE A those which are NOT suitable for use as the active component in the filter of (a) above. Homework EquationsThe Attempt at a Solution Currently The only Op amp I see that is not suitable is TLV2721 due to...
  5. H

    Second order low pass filter with unity gain

    Homework Statement I need to design a second order filter with unity gain. Homework Equations None - not taking component values into account for now. The Attempt at a Solution Is this a correct method of creating a 2nd order low pass filter with unity gain?
  6. E

    How to graph amplitude vs. frequency for a Low-Pass Filter?

    Homework Statement For my lab class my professor wants us to graph the theoretical amplitude of the output from a low-pass filter vs. frequency. I have the resistance and capacitance that I chose for the RC circuit and a V(in). Is there an equation that I should be able to use to relate these...
  7. J

    Low Pass Filters and Laplace Transform

    Homework Statement Given that r(t) = L^-1 (Inverse laplace) *H(S) and by making the link between the time-domain and frequency-domain responses of a network, explain in detail why the ideal “brick-wall” lowpass filter is not realisable in practice. [/B]Homework EquationsThe Attempt at...
  8. E

    Low pass filter (uniform/gauss distribution)

    Homework Statement [/B] I have got data x, which is formed like uniform distribution. After using discrete low pass filter I got output data u, which is Gauss distribution. What is explanation, why using that filter, we from uniform distribution can get Gauss distribution? Homework...
  9. C

    Filter shape from poles and zeros

    Hey guys, I am learning about the filter shapes deduced from the poles and zero plots, however a lot of sources are describing it differently with no logical explanation. I want to know what exactly the poles and zeros do between the range Ω = 0 and Ω = π. One source i read explains if the pole...
  10. T

    Engineering How to tell if the circuit is a High or low pass filter?

    Homework Statement Draw the circuit diagram of a second-order series High pass filter RLC filter. Given: R = 50 ohm Q_s = .5 f_0 = 30 MHz determine L and C Homework Equations f_0 = 1/(2*pi*sqrt(LC)) Q_s = 2*pi*f_0*L/R = 1/(2*pi*f_0*R)The Attempt at a Solution I know how to find L and C, given...
  11. R

    Engineering Where is the bandpass filter in this circuit?

    Homework Statement I am doing a project on the Elenco AM/FM superheterodyne radio kit. I have been stuck for ages trying to figure out how the two common emitter amplifiers are tuned to filter out anything that is not around 455 kHz. I tried to find a bandpass filter, or a high-pass and a...
  12. J

    Low pass filter: cutoff and peak output

    Homework Statement (a) If R1 = 4.7kΩ, what value for C1 will give a cutoff frequency of 50kHz? (b) If the input voltage is described by the equation Vin = 3cos(2π(4000)t), what will the peak output voltage be?[/B] Homework Equations (a) fc = 1/(2πRC) (b) Vout/Vin = 1/√(1 + (2πfRC)2) The...
  13. M

    Building a High Power Low Pass Filter for a TIG Welder

    A friend of mine has a TIG welder, which uses a high frequency signal to make the start of the weld much smoother. He's noticed that the welder seems to be tripping his GFIs (works fine on non-GFI circuits). So, that got me thinking, is it possible to build a low pass filter for something like...
  14. E

    On deriving response function in simple low pass filter

    Homework Statement I have a problem where the circuit is as follows: (pic attached I hope) but if you can't see it it's just a power source (AC), resistor and inductor with 2 terminals across the inductor (from were you measure the voltage). I want to derive the response function, and I am...
  15. B

    How Do You Calculate the Signal Gain of a Low Pass Filter?

    Homework Statement The schematic shows a low-pass filter identical to the one you built in studio. This filter uses a 60 k resistor and a 250 pF capacitor, and the input signal is at 30 kHz. (1 pF=10−12 F)...
  16. mishima

    Optical Low Pass Filter, how does it hit CCD?

    I'm confused about optical low pass filters (found on CCD imaging devices). As I understand it is a double layer of birefringent material which in effect splits light into 4: (2*25%) green, 25% red, and 25% blue (for a standard bayer matrix). The part I'm not sure about is if all 4 fall on the...
  17. applestrudle

    Help analysing this circuit - how does it act like a low pass filter?

    In my simulation it did act like a low pass filter so it definitely is one! <a href="http://tinypic.com?ref=657vdh" target="_blank"><img src="http://i59.tinypic.com/657vdh.jpg" border="0" alt="Image and video hosting by TinyPic"></a> please tell me if the following is correct...
  18. B

    The design of a low pass filter

    I am trying to design a 24V dc to 230V ac inverter operating at 50Hz. So far, by using multisim to simulate the circuit I have been able to achieve these parameters but my output is essentially a square wave. I am using a variation of a circuit that I have found on line, and I am now trying...
  19. B

    How Do You Calculate Capacitance for a Low Pass Filter to Attenuate 60Hz Noise?

    A resistor and capacitor are in series. The signal is at 3 herts and a passive filter must be used to remove a 60hz noise with -10db attenuation. The resistor is 100 Ohms. HOw do you find the capacitance, I've been tryin it for an hour. Usually you would find it with the corner frequency but...
  20. P

    Low pass filter for 60Hz rejection

    Hi All, I am looking to design LPF with Inductor and Capacitor that can reject 60Hz frequency. I tried on google about the same but did not get an idea. Basically, I am trying to do is this, Incoming mains (AC Signal) has 60Hz frequency and I wanted to make it like a DC (close to DC) which...
  21. S

    Low Pass Filter Exam: Help Needed with Part C

    Exams are coming up and I've been doing some revision from past papers. Having issues with working out part c of the attached problem. I've already calculated part a and b, possibly correct. Basically I simplified resistances and capacitance. And to have a constant frequency I let Z equal the...
  22. T

    Low pass filter (arctan domain)

    Homework Statement when I'm trying to calculate and show the graphic of the phase characteristic i don't understand why the domain range of the arctan function is [0,90] : Homework Equations The Attempt at a Solution shouldn't be the domain of arctan function between...
  23. S

    Implementation low pass filter on array

    hi guys: if i have signal(YT) consist of two frequencies(30 and 300 Hz),as the following: t=0:1/1000:1; Y30=sin(2*pi*t*30); Y300=sin(2*pi*t*300); YT=Y30+Y300; i want remove the frequency of 300 Hz from(YT) by using low pass filter so how can i do that(by using MATLAB command)?
  24. P

    Low Pass Filter in Software question

    Hi Everyone, I read an article by Barry L. Dorr about how to implement a Low Pass Filter in software, however, the equation that he gave confused me as I didn't see any sort of convergence to the value desired. So here is the equation: y(n) = (1 - 2^(-k)) * y(n-1) + x(n) x is the input, y...
  25. K

    Active Low Pass Filter Design: 1kHz, Gain 20, 22kΩ Input Impedance

    Homework Statement Design this circuit to give a cut off (-3dB) frequency of 1kHz, a low frequency gain of 20, and an input impedance equal to 22kΩ Homework Equations The Attempt at a Solution So, I started off with Vout/Vin = [-R2/R1]/[1 + sCR2. I also know that the dc gain...
  26. F

    Designing an LPF with a 17kHz Cut-off Frequency: Question and Guidance

    Hello, I have been asked to: - design an LPF with a cut off frequency of 17kHz and a roll-off of 40dB Decade. I have attached a picture of the circuit diagram I came up with. Can someone please confirm if I have this correct ? The second part of the question was: - prove both time domain...
  27. F

    Why Might a Low-Pass Filter Cause Distortion in a 600Hz Waveform?

    Homework Statement We have a waveform with peak to peak amplitude 2V and a frequency of 600Hz. If this signal is to pass through a low-pass filter with a passband of 3.5kHz, explain why some distortion may occur to the waveform when shown on an oscilloscope. The Attempt at a Solution...
  28. H

    Low Pass Filter Design: Choosing L & C for Cut-off Frequency of 50Hz

    Hello, I am designing LC low pass filter for single phase sinusoidal pwm inverter, where the cut off frequency is f = 50 Hz. And i have to chosse L, C such that: f = 1/2π√(LC) but there is many combinations of L and c that result in this cut off frequency. So my question is, how to choose...
  29. G

    High pass filter and low pass filter question?

    You have a series circuit consisting of a ac power supply, a 1150.0 ohm resistor and a 144.0 nF capacitor. If the circuit is configured as a low pass filter, what frequency will cause the gain to be 0.25? Formula Gain = Vout / Vin = R / [R^2 + (1/ωC)^2]^1/2Solution Plugging in R=1150 ohms...
  30. T

    Engineering Low pass filter circuit questions

    Homework Statement Why does a low pass active filter give a constant gain and no filtering when the capacitor is removed? What type of circuit does this become? Support your answer with equations. also Does the amplitude eventually roll off as the frequency was increased toward to a...
  31. J

    Transforming a low pass filter to a band pass and low pass filter

    Homework Statement My problem is I want to know how to transform a low pass filter into a band pass and high pass filter. Suppose that we are given the impulse response of the system which is h(n) The Attempt at a Solution In order to transform this system into a band pass filter, I need to...
  32. cepheid

    What is the purpose of the second inductor in a T-section LC low pass filter?

    I have a dumb question about "T-section" filters like the one shown on this page: http://www.radio-electronics.com/info/rf-technology-design/rf-filters/simple-lc-lowpass-filter-design.php If the output it taken across the two rightmost terminals, then what is the point of the second...
  33. A

    Ferrite bead as low pass filter search

    Hi i'm looking for a ferrite bead that will block 1-20 Mhz frequancies and pass all lower ones is it possible ? can someone to advice me for one specific? Arye
  34. F

    First Order Active Low Pass Filter

    http://img39.imageshack.us/img39/2991/captureucm.jpg Attempt to solve: omega = 1/R2C1 ; omega = 2pi*f so C1 = 1/R2*2pi*f , where f = 1000Hz so C1 = 0.53 nF (nanofarads) then Ao = -R2/R1, so absolute value is 15 gain V/V? in terms of dB its approx 23.52dB using 20 log X...
  35. D

    AC Power RC series circuit low pass filter, find frequency given gain.

    1. You have a series circuit consisting of a ac power supply, a 600 ohm resistor and a 73 nF capacitor. If the circuit is configured as a low pass filter, what frequency will cause the gain to be 0.25? 2. If the circuit is configured as a high pass filter, what frequency will cause the gain...
  36. C

    Transfer function for a low pass filter

    Homework Statement The input signal of a first-order lowpass filter with a transfer function given by eqn 1 below and a half power frequency of 200 Hz is eqn 2 below. Find an expression for V_outHomework Equations eqn1: H(f) = 1/(1+j*(f/f_B) where f is the frequency and f_B is the half-power...
  37. N

    Low Pass Filter and Fourier Transforms

    What is the general Fourier Transform for a low *** filter if the lowpass filter also has a bandwidth of W Hz, and fc >>W,? I think it would this: ∂(fc) + ∂(f-fc) + ∂(f-fc)
  38. C

    Why isn't this circuit behaving like a low pass filter?

    I'm trying to design a low pass filter from a transfer function (with one pole and one zero), and according to my textbook, the circuit which represents such a transfer function is of the following kind: DC gain is 16. Yet I get complete attenuation for low frequencies? Could my component...
  39. B

    Ideal Low Pass Filter: Why Can't It Be Real?

    Hey guys, I have some problem understanding why an ideal low pass filter cannot be implemented in reality. I do not understand the reason given in wiki " because the sinc function's support region extends to all past and future times. The filter would therefore need to have infinite delay, or...
  40. J

    MATLAB Low Pass Filter using Matlab for Data Filtering

    Hi, I need to filter my data using a low pass filter. I would like to do this using Matlab. The sampling rate of my data is 200 Hz, and I would like a cut-off frequency of 30Hz. Someone can help me, please? jo
  41. U

    How to filter noise from an inductive pickup. low pass filter.

    i am making a tachometer which will count pulses from an inductive pikup connected to an HT lead on an engine. i am having great difficulty when measuring my signal from the pick up as there is a huge amount of noise. can anyone raccomend the best way to clean this up? someone mentioned an...
  42. somasimple

    Comparing Two Circuits: Can We Say Low Pass Filter?

    Hi all, Here is a picture of two circuits. The one at the left is stimulated with a current source and the other with a voltage source. They give the same results V(1) = V(2). Is it possible to say that the primer is a low pass filter?
  43. L

    Low pass filter or envelope detector in Excel

    I am playing with some accelerometer data and need to filter out the noisy output. How do I implement a Low pass filter in excel? I tried avg, rolling avg. They all smooth out the data but not what I want (averaged accn is finite when it is actually zero!) If I can create an envelope...
  44. E

    ECG Signal Noise Reduction with Low-Pass Filter Implementation in Excel

    Homework Statement I have data for an ECG and I want to filter it. Homework Equations The problem tells me to run the signal through a low pass filter. I tried a single pass double pole filter (essentially the average of three averages). If we have data points A, B, C, D, E, and F...
  45. P

    Deriving Expression for Vout/Vi in Low Pass Filter

    Q: Derive an expression for the Vout/Vi, the ratio of the output and source voltage amplitudes, as a function of the angular frequencie W of the source. The L-R-C circuits looks like this.. source(ac)----------capacitor-----inductor----resistor----(back to start) Vout is taken...
  46. B

    MATLAB Design Low Pass Filter in Matlab: Help Needed!

    can anybody help me design a low pass filter in matlab? its a simple task but i still have trouble filtering a wav file taken by recording using the sound card. i can't seem to get it right since the length of the input is variable... can anyone show me the right way to make a filter? thanks! i...
  47. X

    Understand how a low pass filter

    Hi, I'm new into this subject. I'm trying to understand how a low pass filter (where the frequency f is from 0 Hz to f_u ) for a speacker is working. The fuction for the low pass filter is \mid H \mid = U_out / U_in, that is \mid H \mid = R / \sqrt{R^2 + ( \omega L )^2} where...
  48. B

    Low Pass Filter: Clues to Cutoff Frequency of 100kHz

    im sorry if this is lame for you guys but i posted a thread for some info on farads and the info was good but i still cannot find the answer to this question any clues for a low pass filter from the following list choose the two components which would produce a cut off frequency...
  49. B

    Help Needed: Low Pass Filter Cutoff Frequency Closest to 100 kHz

    hi there if anyone could help i would be very grateful for a low pass filter from the following list choose the two components which would produce a cut off frequency closest to 100 khz resistor capacitor 1.6 k ohm 1 nf 33 k ohm 330 nf 150 k ohm 10 uf...
  50. B

    Low Pass Filter Help: Values for Capacitor Confusion

    hi there i would be very gratefull if anyone could help. im trying to work out some values for a low pass filter. the problem is the values for the capacitor I am not sure exactly what amount nf or uf are, for example 1 nf or 10 uf what is the exact number any help would be appreciated