What is Filter: Definition and 604 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. 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...
  2. 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...
  3. I

    Surge protector for EMI protection in place of EMI filter

    can i use surge protector for EMI protection inplace of EMI filter?what the difference between them?
  4. N

    MATLAB Creating Noise / Filtering With Matlab FFT for Tap Sound Masking

    I am trying to create noise that has similar spectral properties of a recorded tapping sound (so that I can mask the tapping sound). I was thinking that if I generate noise, and then multiply the fft of the noise by the fft of the tap sound, it would sort of "filter" the noise to the desired...
  5. S

    Can an IIR filter be applied by convoluting a signal with its impulse response?

    Hi all.. I'm a new to filters and I need some help understanding filters (I'm in mechanical engineering) So far, I've found out that FIR filters use convolution and has a finite impulse response. On the other hand, IIR filters are recursive and have an infinite impulse response. But...
  6. H

    DSP - IIR filter, anlog to digital

    hi, I'm studying for a test, and found this question in one of the previous exams but there is no solution and I was not able to solve it myself. It would be very helpful if anybody could help =] Homework Statement h(t) is an analog IIR filter, with the poles λ_1 = -1, λ_2 = -1+i, λ_3 =...
  7. Z

    Choosing the correct type of filter

    hi i bought a machine which determines led colours by measuring intensity for a product. the product has three leds:green,red,orange. the equipment works well for the green and red leds but fails most of the time with the orange leds. i looked up wavelength values for the colours and i...
  8. 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...
  9. V

    A filter with a narrower-than-Fourier Transform passband?

    If I have a signal that I'd like to filter very precisely (say, a few Hz bandwidth), I feel like the "best" one can possibly do is to take a time sample long enough to get a discrete transform frequency resolution of the same width, erase all but the frequency component of interest, and then...
  10. J

    Transforming a low pass filer to a high pass filter

    Hi, I have trouble understanding how to achieve this transformation Suppose for example that we have a low pass filter which impulse response is the following h(n) = 1 for 0<=n<=2 where n is an integer In the frequency domain we would have the following frequency response: H(e^{j\omega}) =...
  11. E

    Finding the frequency response of an IIR filter

    Homework Statement IIR filter with poles at .7±j0.2 and zeros at 0.2±j0.7 Calculate H(f) and the Differance Equation when the DC gain |H(f=0)|=4 The Attempt at a Solution I know to find H(f) I can evaluate H(z) along the unit circle. so H(z) evaluated at z=exp(j2pifT) However...
  12. H

    MATLAB Wiener filter - MATLAB implementation

    Hello, I'm currently working on a project in which I am required to implement an optimum 11-tap linear equalizer (Wiener filter) to remove additive noise from a QPSK signal over a multipath channel. The channel impulse response h{k} is known. My understanding of the implementation...
  13. R

    Filter DC offset - apply High-pass filter (in time-domain or frequency domain?)

    Hello all, I want to filter DC offset for a signal by applying a high-pass filter to it (I've been told that to filter the DC offset this is what I should do - I'm not sure why exactly!) My question is; do I do that in the time-domain or frequency-domain after applying FFT? And I would...
  14. W

    RC constant of capacitor in RC circuit as a filter

    We know that for RC circuit as filter or in multiplier we use capacitors having larger RC constant than the period of input signal.but I am confused here: As long as i know the charging and discharging period of a capacitor are equal wether it is fully charged or not...(tell me i am right or...
  15. M

    Is Including Pressure in a Kalman Filter for Wind Forecast Causing High Error?

    Hi, I am now using multiple parameters kalman filter to remove the systematic error of the output from the numerical weather prediction model. when applying kalman filter to the wind forecast, there is high error if pressure is included to be the predictors. I would like to know if there any...
  16. A

    Filter Practice Problem for P.E. Exam, HELP

    Hey everyone, I'm trying to get prepped from my EE Professional Engineering exam and there is a question that I cannot solve in the prep-guide. I'm hoping that someone might be able to help because it can be solved in MatLab (as noted in the book). Here is the problem description: You are an...
  17. R

    Designing a lowpass Butterworth filter

    Hi all, I am trying to design a lowpass filter using the following conditions Passband edge f = 10 Khz (Ωp) passband stop f = 30 Khz (Ωs) Minimum stopband attenuation = 50db (1/A) Maximum passband attenuation = 0.4db( 1/\sqrt{1+ε2} ) and am having quite a lot of trouble with it. I have tried...
  18. Andy Resnick

    Bayer Filter Effects: Image Comparisons

    I had mentioned that I dropped the idea of posting a lens testing lab I am trying to develop for class here because the technical details got out of control, but I didn't post any images showing why I came to that conclusion. Here are 4 images of the central region of a Richardson test slide...
  19. 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...
  20. D

    Fabry Perot interferometer used as a filter

    Hi all! I'm studying Fabry Perot optical interferometer (with monochromatic light without taking time into account so far) and as its first application we studied it can be used as a filter, letting pass only certain frequencies \nu= \frac{mc}{2n_fd\cos(\theta_t)},\,\,m\in\mathcal{N} where c...
  21. Y

    Prism Design to Filter Out Infrared Light

    Is it possible to filter out infrared light using a prism, and if so, how? What is the math involved to do this? I want to come up with some effective, durable ways to filter out IR light for solar panels, to keep their temperature lower to make them more efficient.
  22. A

    Help writing a FIR Filter in Verilog

    Hi, Homework Statement Design and build a co-processor that interfaces to the Leon that implements an FIR filter. Use memory mapped registers on the AMBA APB bus device 12 to communicate with the co-processor. Your C program (and Modelsim Testbench) will transmit the 16 bit filter coefficients...
  23. N

    Active Filter Output: Help Solving Confusion

    hey I put some filter what will be the out put of this? is it depend on y3 or depend on y4 and y2? im very confusing in here... can anybody help me?
  24. A

    How can I filter out non-prime numbers in a list using Standard ML?

    Hi, I'm using SML and I'm trying to make a program/function that finds all the prime numbers in an Int list of numbers. what I'm trying to do is make a function that removes any (and all) elements x of the int list where x mod p = 0, and where p is the first prime number (2). then i want to...
  25. N

    Blackbody radiation with frequency filter

    Let's say there's a perfect blackbody inside a box (at room temperature) composed of material such that only x-rays are able to enter and leave. If the absorption and emission of the x-rays weren't the same, the box would heat up above room temperature, and this would contradict thermodynamics...
  26. J

    What Is the Velocity of a Charged Particle in a Crossed Magnetic Field?

    Homework Statement In the figure (see attached) what is the velocity of the charged particle to the nearest tenth of a m/s that will allow it to travel undeflected through the crossed magnetic field if E = 6852 N/C and B = 3.6 T? Homework Equations F = E*q + q*V*B The Attempt at a...
  27. S

    Optimal Filter Coefficients: Correlation versus Least Squares

    I found a claim in a paper (BSSA, Vol 81, No. 6: "A Waveform Correlation Method for Identifying Quarry Explosions", By D.B. Harris) concerning finding filter coefficients. The statement is given without proof. I cannot locate a reference or theorem for the following, and have not been able thus...
  28. R

    Understanding Sine Wave Production with an LC Filter and Inverter PWM Waveform

    Hi, Could someone please explain how EXACTLY a sine wave is produced using an LC filter from a PWM waveform generated by an inverter. Thanks
  29. S

    Is a polarizer a filter or a converter?

    Does a polarize simply filter photons or does it change the polarization of photons (as well)? let's consider only one photon being sent, at a time, via the polarizerI think the answer might be that: for photons polarized at say 45 degree to the horizontal (or vertical) plane of the polarizer...
  30. D

    Need help in Filter Housing Design

    Need help in Filter Housing Design...! I have to design the filter housing using the data provided below. Can anybody help me ,how to calculate the shell diameter and length of the shell of filter housing.which all formula are taken into consideration into filter Housing design ?? Flow...
  31. K

    How to make a simple water filter?

    I was wondering if any of you have a very simple way to make a water filter using buckets. I'm going to Africa next september and will be visitng a secondary school for a week and just thought it would be nice to teach the kids how to make a water filter.
  32. M

    Prediction covariance matrix with Kalman filter

    Hello all. I have set up a model using the Kalman filter to estimate automobile prices. I'm having difficulty in figuring out how to formulate a prediction covariance matrix based on the model, i.e. given a set y_{new} = y_1, \ldots, y_N of N cars, finding the covariance matrix based on the...
  33. R

    Hilbert transform filter phase

    We know that hilbert transform is a linear filter whose frequency response is given as -j*sgn(f), where f is the baseband frequency. Hence magnitude response of this filter is 1 and phase response is -pi/2 for f > 0 and pi/2 for f < 0. Hence phase response curve is like a staircase function (...
  34. J

    Bounds of a band pass filter for image processing

    Hi all, I'm working on some image analysis as a part of my research, specifically trying to match images. The method I am using transforms the image into the frequency domain and then applies a band pass (or mesa) filter to eliminate noise and the dc component of the image. I have never...
  35. M

    Piezoelectric filter

    Homework Statement Consider a quartz crystal and a PZT ceramic filter both designed with resonant frequency equal to 1 MHz. What is the bandwith of each? Given Young's modulus (Y) and density (ρ) for each, and that the filter is a disk with electrodes and is oscillating radially, what is the...
  36. S

    Recurrence Relation for Alpha beta filter

    The alpha beta filter equation is given by this Xni = AXn + BXn-1 Xn-1 is subscript i want to figure the solution to this recurrence relation, but is it a recurrence relation? i am wondering weather alpha beta filters grow faster than exponential smoothing filters.- i said it is a 1st order...
  37. H

    How to Determine Correct Values for Q and R in Designing a Kalman Filter?

    Hey, I designed a kalman filter for my project. But how can I determine the correct values for Q and R? Thx!
  38. P

    Electrical - What type of filter is this (Low Pass?)

    The title says it all, what type of filter is this? I thought it was a "low pass" filter but the image on Wikipedia is different. Thanks
  39. L

    Using Kalman Filter to Estimate Motion of Object Along Line Segment

    I want to use a Kalman filter to estimate the motion of an object. However, the catch is, the measurements I have only tell me that the object is somewhere along a particular line segment. Typically Kalman filters require normally distributed measurements. I'm trying to work out how best to...
  40. D

    How Do Inductors Function in Bandpass Filters Without Capacitors?

    Can I get some help with this? I don't even know where to begin.
  41. C

    Calculating the cutoff frequency on analog filter

    for Butterworth filter -> Ha(s).Ha(-s) where s=jΩ I got (Ω^4)/1+Ω^6 hence this not Butterworth filter, and I managed to find all the poles, where all the poles lie on the left hand side so this filter is stable.. My problem is, I'm not quite sure how to calculate the cutoff frequency, for...
  42. N

    A question on passive low-pass filter

    Homework Statement This is a simple passive low-pass filter stated in the Wikipedia. I am wondering why we need the resistor to allow it operates as a low pass filter. Can we just ignore the resistor(by shorting it)? What will the consequence be? Thank you for reading. Homework...
  43. M

    MATLAB Implementing Particle Filter with Matlab

    I'm now implementing different kinds of Particle Filters using Matlab. One is the Regularized Particle Filter(RPF) and my code for 1 time-step filtering is as follows: function [xcap,wcap]=RPF(x,z,k,N_thre,Q) N=size(x,1); %no. of particles m=size(x,2); %dimension of state space...
  44. P

    Capacitor input filter for voltage regulater

    This question has been boggling my mind. The capacitor input(C2) filter in this schematic helps me filter the output from the rectifier. With combination of Rchoke (is just a resistor and not an inductor) of 2.4ohm, it limits my peak current and filters the output voltage from the rectifier...
  45. 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
  46. C

    Inertial Navigation Integration FIlter

    Hello, I am trying to create a filter that can integrate inertial measurements with GPS measurements. My inertial sensors are a yaw axis gyro, an odometer, and possibly a 3axis accelerometer. Could you please help me with some guidance? I have read papers, articles and books on the subject but...
  47. C

    Band pass filter and twin-T notch filter

    Hi, I've encountered some problem during solved the band pass filter and twin-T notch filter. I don't really understand what the question exactly meant...hope can get some idea in here..thx 1.) A signal conditioning system uses a frequency variation from 6 kHz to 60 kHz to carry...
  48. 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...
  49. P

    Capacitor-input or choke-input filter?

    Capacitor-input or choke-input filter for power supplies? Whats the difference and when to use them?
  50. D

    Phase constant for high pass filter

    Homework Statement If the input voltage is given by Vin = Vin peak cos ωt, then the output voltage is Vout = VH*cos(ωt - δ) where VH = Vin peak/(1 + (ωRC)^(−2)) (Assume that the output is connected to a load that draws only an insignificant amount of current.) Find an expression...
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