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Don't show this message for one day.Please tell me, Why the cutoff frequency is taken for 3dB and...
Asked 6 years ago Karunya UniversityPlease tell me, Why the cutoff frequency is taken for 3dB and not other values like 1 or 2 db?
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University of BaghdadThe -3dB, come from
20 Log (0.707) or 10 Log (0.5).to determine the bandwidth of signal, when decrease the voltage from maximum to 0.707Max or decreasing the power from max to half power.Hochschule BremenI think, all of the given justifications are appropriate - however, we should not forget that this -3dB limit is an arbitrary one. There may be good reasons for choosing this value - on the other hand, some filter approximations with passband ripple (Chebyshev, elliptical) do NOT use this 3dB limit.In some particular cases it can be necessary - even for Butterworth filters - to specify the band edge at the 1dB or 2dB frequency.Thus, the 3dB limit is not a &natural& threshold - it just makes sense for many applications.All Answers (50)General Industrial Controls Pvt Ltd3db is the power level, its the frequency at which the power is at 3db below the maximum value and 3db means in normal unit its half the maximum power so 3db frequency means the frequency at which the power is half the maximum value so its decided the cuttoff frequency.Circuitsville Engineering LLC3dB is equivalent to 0.707 times the peak Voltage/Current value, also known as the half power point.Usually dB is a measure of power, in electrical work power is the square of current times load impedance or the square of voltage divided by load impedance.So half the power would be half of the squared voltage or the voltage divided by the square root of 2 (1/sqrt(2)) or 0.707.Indian Institute of Technology MadrasAs answered by other friends, actually the frequency corresponding to 3dB magnitude is considered as bandwidth of the signal or of the system. Its at this frequency that the power of the signal is reduced to half its max power/ or the attenuation caused by the system to the signal is such that the magnitude reduces to 1/sqrt(2) of the max magnitude of the signal.This is taken as a standard reference frequency beyond which the response is not very important from the system analysis point of view.Circuitsville Engineering LLCThe -3dB point is at the start of the attenuation.Frequencies beyond that are attenuated at a 20 dB per decade of frequency (per pole) beyond the -3dB frequency.(Assuming a Low Pass Filter)Actually -3dB means that half of _that_frequency_ of the signal has power attenuated.Frequencies below -3dB are not attenuated (by much) , frequencies above are more strongly attenuated.University of LiverpoolThis seems a very inefficient way of finding out very basic knowledge.Why not simply look in a textbook?Dr. D.Y. Patil University3dB corresponds to the half of the maximum power.Central Electronics Engineering Research InstituteIn communication
we want to get the
frequency gain 100%.but it is not possible.the o/p of the gain rate will be 70% to 100%. 70%
gain is a minimum acceptable
we have been using the 3db .Stiefvater ConsultantsIt's completely arbitrary. But a 3 dB change in audio volume is noticeable, so it's commonly acceptable.Karnatak University, DharwadParticularly in communication applications it required to have constatnt gain over lager bandwidth. That is in band pass filter bandwidth is the difference of frequencies between 3 dB points. Over this region, gain reasonably remains constant. Beyond 3dB point attenuation is large so there is a chance of loosing more information.It's not really arbitrary. It's because decibels are logarithmic, and the log (base 10) of 3 is about 50% power. So the 3 decibel cutoff is where power drops off by a half.Jawaharlal Nehru Technological University, Hyderabad3 dB implies 1/2 the power and since the power is proportional to the square of voltage, the voltage will be 0,707 of the pass band voltage.University of AlbertaGenerally speaking, a filter's cutoff frequency is not necessarily defined at -3dB.
Such is the case for Butterworth filters, as a direct result of Butterworth's initial formulation, which ends up with a gain value of 0.707 at the cutoff frequency, ex. see .
In contrast, a Chebyshev filter is defined differently, allowing the designer to specify the desired amount of ripple within a given passband or stop-band.
In contrast to the Butterworth and Chebyshev filters, a Bessel filter is defined with respect to phase response, with the design objective of approximating a delay line, with a maximally flat phase response in a given passband, i.e. near constant delay time for all frequencies within that passband.
Whereas a Butterworth filter's order can be increased for greater stop-band attenuation while keeping a constant -3dB cutoff frequency, a Bessel filter's order can be increased, also for greater stop-band attenuation, but while maintaining a constant group delay within some passband (ex. see ), but resulting in a varying -3dB cutoff frequency.
Since different filter designs aim at different objectives, it can be mis-leading to compare them on the basis of just a -3dB point.
It is also worth noting, for example, that for a first order low-pass filter, there is a useful symmetry in the Bode phase response relative to the -3dB frequency, which is clearer to see in the Nyquist plot, and even clearer when compared to a first order high-pass filter with the same -3dB frequency.University of LiverpoolMr Kumar has already given you the answer. It is simply a convention.Central Electronics Engineering Research InstituteIn communication we wnat 100% frequency of the output.but unfortunately some of the noises will be added during the
we can't able to
get 100%.the minimum acceptable frequency is 70.7%.so we have been using 3db.
gain=20 log(V0/Vin)
vo/vin=0.707
and then we get the answer.University of BaghdadThe -3dB, come from
20 Log (0.707) or 10 Log (0.5).to determine the bandwidth of signal, when decrease the voltage from maximum to 0.707Max or decreasing the power from max to half power.National Polytechnic School of AlgiersThe best response is given by Hussein AlamariManipal University JaipurDear,We can see the -3 dB point in a frequency response, frequency response is the graph b/w Magnitude in dB {that is 20log(Vo/Vin) or 10log(Po/Pin)} and frequency, plotted at semi-logarithmic scale. In case of low pass filter, design using an Op-Amp and RC circuit and relation b/w gain(Vo/Vin) and input signal frequency (f) is like that:Vo/Vin = Af * (1 / (1+j(/2 pi *f
here Af is gain of Non inverting
OP- AMP configuration.At f = 1/ 2 pi*RC = cut off frequency , the value of frequency dependent gain (Vo/Vin) is |V0/Vin|=|
Af/(1 / (1 + j ) ) | = Af /sqrt(2) = 0.707 Af and 20log |V0/Vin| =20 log ( 0.707 Af) = 20 log(0.707) + 20log(Af)
= -3 + 20 log(Af)
{in dB}That’s why we can easily say that when difference b/w the 20log |V0/Vin| and 20log(Af) is -3, the frequency is known as cut off frequency, that can be easily varied using R and C (Resistance and capacitance ).
In another word, to find out cut off frequency you can directly use –3db gain point at frequency response.
I think it help you to understand the -3 dB point. And Why the cutoff frequency is taken for 3dB and not other values like 1 or 2 db.Manipal University JaipurWell, one more interested thing about it when you linearised the frequency response, then you can see that the extreme point for line is also find at - 3 db, that is clear in attached file. After linearization the plot become piece wise linear.
Please see in attached file that is frequency response of low pass filter.<div class="nova-v-message-item__file-size" data-reactid=" KBUniversidad Politécnica de MadridBesides previous answers, quite useful from an Engineering viewpoint, you might want to know a more “Physical reason” for your question. If this is the case, think about a simple capacitor (a Resistance-Capacitance parallel circuit, capacitors without a shunting resistance R do not exist at temperatures over T=0 K) and put some electrical energy on it, thus a voltage V at time t=0. If you measure the time evolution of this voltage you will find that it decays exponentially with time. This means that the voltage reduction at each instant of time is proportional to the voltage existing at such instant. To say it bluntly: the voltage V has a lifetime that is the time constant Tau=RC (in seconds) of this parallel circuit whose stored energy is defined by its voltage.The inverse of this lifetime gives an angular frequency (in rad/s) that is w=1/(RC), which in turn defines a frequency fc=w/(2Pi). The meaning of this fc is that the synthesis of a sinusoidal voltage of this frequency in the capacitor requires the same amplitude A of current through C (thus in quadrature with the synthesized voltage) than the amplitude of current through R (thus in-phase with the synthesized voltage). This means that the aforesaid synthesis of sinusoidal voltage in this Two-Terminal Device (2TD) takes an amount of reactive power linked with C that is equal to the active power linked with R.Therefore, the total power entering the above 2TD (no matter if it is a capacitor or a resistor, both share the same R-C circuit) is 50% reactive power responsible for Fluctuation of energy in the 2TD and 50% active power responsible for Dissipation (actually Conversion into heat in this example) in the 2TD. Scientists speaking about electrical noise should handle in this way the Fluctuation-Dissipation Theorem derived from Callen & Welton pioneering work (1951)….. Anyway, this gives you the physical reason why you observe 50% power at fc if you measure voltage on the C of the low-pass, first order R-C filter or if you measure voltage on the R of the high-pass, first order R-C filter both driven by an input voltage generator Vg.To say it bluntly: when you are measuring any of the above two filters driven by the input generator Vg, you actually are measuring the voltage on BOTH (R and C) or in our Engineering language: you are measuring the voltage on the Complex Admittance formed by R and C in parallel, driven by a current generator of value (Vg/R) Amps. The latter sentence simply requires a Thevenin-Norton transform, “very familiar” to us, the Electrical Engineers (EE), but not “well known” as most EE use to believe. Perhaps, this is enough for you, but if you feel curiousity about electrical noise and electrical conduction, you may find interesting this open access paper:A Fluctuation-Dissipation Model for Electrical Noise.José-Ignacio Izpura, Javier MaloCircuits and Systems. 01/-120.NeuralynxPlease see Hussein ALameri&#x27;s answer. While all these posts are good and interesting, I think your question was more basic. Its simply the 1/2 power bandwidth.Cochin University of Science and TechnologyMr. Hussein ALameri and Arivu Che has put the things more simply and in an abstract way... As an example, suppose we are hearing a sound (a musical show), the changes in the sound can be detected by our ear when its loudness has reduced by .707 times the peak value... that is when it is below .707 the peak value, its difference is felt more and is more obvious.Auckland University of TechnologyIt is the intersection of two asymptotes to the magnitude response - the 0dB one and one at -20dB/decade. Look at any book on control engineering under Bode Plots and see the maths.National Polytechnic School of AlgiersToo many time is spent for this simple question. Very interesting has been given. I suggest to to close our discussion.Doordarshan Kendra Shillong, Indiahuman ear can not distinguish maximum and 3dB frequency level. 3 dB is maximum permitted level. another reason :importance of 3 dB is , at this point the power level of a device is reduce to half value within a pass band.ResearchGateJust summarizing a bit from former messages:The -3dB comes from the logarithmic value of sqrt(0,5) (0,707), as pointed by Hussein. This is done, because when talking about signals and filters, you are usually interested on the value of the power.As you may know, P=V^2/R or I^2·R (where P stands for power, V for voltage, I for current and R for resistance)Hence, when the value for the voltage (or current) reaches 0,707·Vmax (0,707·Imax), the power will be 0,5·Pmax [since sqrt(0,5)^2 = 0,5].The points where a filter attenuates the power of a signal to the half of its maximum are considered the frequency limits. Any signal with a frequency smaller than the lower limit and bigger than the upper limit will be attenuated, at least, by 2.And since those frequency limits are taken as the borders for the bandwidth of a filter, the cutoff frequencies result to be those where the power is 3dB lower than on the maximum. (Remember that P(dB) = 10 log(P))And why 0,5? Well I think that 0,5·Pmax is a reasonable cutoff value, isn&#x27;t it? ;)Lam Research Corporation-3db fall corresponds to the magnitude of root(2)*maximum value. And the reason we take root(2) factor is, for a sinusoidal wave, root(2) is R.M.S value. By which it means that the power consumed by load for the sinusoidal source will be equivalent to the DC constant power source when the DC source&#x27;s magnitude should be R.M.S value of sine wave.NXP SemiconductorsI often find there is a simpler way to think about it. The input impedance of a parallel RC circuit is (w=omega, the angular frequency)Z=R/(1+j*w*R*C)and it is true, as Moir notes, that the cutoff frequency fc is where the two asymptotes |R| (w-&0) and |1/(j*w*C)| (1-&inf) meet on a Bode plot. However, also at that point the real and imaginary parts of the denominator are equal. The magnitude of the denominator is then sqrt(2) of its value at low frequency, so it is clear where that factor comes from (or the factor 2 when squared for power).The advantage of thinking about it this way is that you can determine fc from low frequency simulation and do not have to have any a priori knowledge of fc to know what range of frequencies to simulate over to properly bracket it. If you run a small-signal ac simulation with voltage stimulus V=1+j*0 at a frequency f=1, so w=2*Pi, then the real part of the current I is 1/R and the imaginary part is 2*Pi*C, thenfc=Re(I)/Im(I).You cannot easily measure at this low frequency, and if there is any nonlinearity then a 1V signal will introduce distortion, but in a simulator an ac analysis is done on the circuit linearized at its dc operating point and you can easily simulate at any frequency, so it works like a charm.Hochschule BremenI think, all of the given justifications are appropriate - however, we should not forget that this -3dB limit is an arbitrary one. There may be good reasons for choosing this value - on the other hand, some filter approximations with passband ripple (Chebyshev, elliptical) do NOT use this 3dB limit.In some particular cases it can be necessary - even for Butterworth filters - to specify the band edge at the 1dB or 2dB frequency.Thus, the 3dB limit is not a &natural& threshold - it just makes sense for many applications.Vignana Bharathi Institute of TechnologyThe 3dB point corresponds to half Power Point ie. Half the Power Supplied which is the Maximum Power that can be supplied to the Load according to Maximum Power Transfer Theorem.Dr.P.S.Ain Shams UniversityCOMMENT!No answer is voted up by Cyril Robinson, the man who asked this question. Till now there is no response from him on all the above answers. IS he still not satisfied with these answers. Excuse me,t he most
important vote is that of the asking person!From my point of view the voting system is not a sufficient measure for answers evaluation. Some better answers collected less votes!Hochschule BremenYes - you are right.A small &thank you& would suffice.Ain Shams UniversityThank you Prof. Lutz. Avery respectful man.Vignana Bharathi Institute of TechnologyThank you Prof. Lutz for providing info on the relevance of 3 dB cutoff points regarding some other filters
like Chebishev,
Butterworth Filters etc. Dr.P.S.Bhabha Atomic Research CentreAn international definition for bandwidth, while u can defined yours say 6dB or 10dB. Depends on the manufacturer.Else what so ever other members say is correct. and dont forget it is for power in a signal with respect to a known power.Karunya UniversityThank you.. very kind of you for the answer.. i didnt turn up to research gate so long.. Sorry for replying late sir.Universiti Malaysia PerlisCutoff frequency is the frequency either above or below which the power output of a circuit, such as a line, amplifier, or electronic filter has fallen to a given proportion of the power in the passband. Most frequently this proportion is one half the passband power, also referred to as the 3 dB point since a fall of 3 dB corresponds approximately to half power. As a voltage ratio this is a fall to sqrt {1/2} is approximately 0.707 of the passband voltageCardiac Science CorporationRemember that in a normal expression of the frequency response of a simple RC/RL circuit a normal term in the response expansion is of the form A/(1+tau*s) so 1/tau radians/sec is naturally the -3dB point, the half power frequency, related to that term.
The frequency/half power relationship is intrinsic to the physics and very useful in recognizing one critical design parameter in the usual run of circuits.L.J.Institute of Engineering & Technology, Ahmedabad -3db is half power frequency point...National Polytechnic School of Algiers A level of 3 dB corresponds to half of the signal power ( that is to say the half of the signal power is lost) .In many applications: detection , extraction, transmission and other signal processing operations ... to a level lower (less than) - 3 dB is difficult or impossible to receive the information signal.This signal is often corrupted by noise : making them unreliable communications. Reliable communications is a necessary condition (high priority).Hochschule Bremen&....the voltage of the input will drop dramatically.&Dramatically? What does this mean? For my opinion, the rate of change depends on the order of the system only.V?B-Technical University of OstravaFor the approximation of the filters we use also other borders (Chebyshev, Bessel).APPLIED SIGNALS INTELLIGENCE, INC.Different cutoff values are used for different practical or theoretical purposes.Convenient for theoretical analysis is 1/2 power, which is really 3.01... dB but we round to 3.Also convenient for theoretical analysis is 1/2 voltage, which is really 6.02... dB but we round to 6.Another practical reason we sometimes choose 6 dB is because the noise bandwidth of many analog filters is at about their 6 dB down points. For example, MIL-STD-461G uses 6 dB for this reason, and gives an example that for a Gaussian shaped bandpass filter, the noise bandwidth is at the 6.8dB down points.Yet another reason we use 6 dB is because the loss versus frequency of real filters, with strays and box resonances etc. can have hooks and glitches make the loss-vs-frequency slope relatively flat, or not beyond the ripple, and therefore not easy to precisely measure, at the -3dB point. The slope is steeper at -6dB and therefore easier to precisely measure from a practical, hands-on, point of view.In fact, the FCC&#x27;s regulation of UWB says 20% bandwidth at the -10dB points. In this case, 10dB was specifically chosen for exactly this reason. At -10dB, the slope of the skirts of the transmitted spectrum are clearly established and steep enough to be easily, precisely, and repeatably measured.Hope that helps.The University of Tennessee Health Science CenterLutz,Very well generalized.In my filter projects, my PassBand shapesare frequently Non-Gaussian
( see attached file )and do not compare on equal-ground with other accepted measurements.
So, for me, -3dB is not a good comparison.
Normally, in Audio Filters,
for my Non-Gaussian pass-band shapesI use the slope of best fit from -3 dB to -12 dB.In my later project circuits I extend this to -3 dB to -60 dBor even -3 dB down to -100 dB.In the attached Magnitude Plot, marked in -dB Volts.you can see that the light-blue and dark-blue traceshave the same -3 dB measurement. But ... Obviously ... not the same Band-Width all the way down !In radio operations, we measure R.F. power band-width in dB.In radio operations, I find PSK-31 techs using the audio -3dB .In radio operations, I find the F.C.C. using -28 dB.Band-Width is a Pragmatic Measurement.<div class="nova-v-message-item__file-size" data-reactid=" KBIndian Institute of Technology Roparhello As I understand: The relation between power and db is expressed as
1db=10log(Pout/Pin) and so when Pout/2 = Pin ( half power loss) then 10log(1/2)=-3db.JIET Group of InstitutionsWe cannot take other than 3 dB point for cut off frequency. because on this point power delivered to the o/p is = half of P. w.r.t maximum power transfer theorem. We know that due to proper impedance matching, power delivered to the load is max.,
if zo=zin*.And in communication system, we also know that for proper reconstruction of signal (s/n), s is required to be atleast 50% or more than 50%.For any filter Fc is the point at which power is just half or the signal strength is 50%, is called cutoff frequency.Can you help by adding an answer?Answer
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Related PublicationsABSTRACT: The computer software encodes the control algorithms and the logical commands for the interfacing of the I/O and for the emergency and other routines that the system is expected to perform.Chapter · Jan 2001 ABSTRACT: The use of multiple antennas at both transmitter and receiver to formmultiple input and multiple output (MIMO) channels currently hold the potential to drastically improve wireless links spectrum efficiency or robustness, increasing capacity and the ability to increase the transmission speed in future wireless communications system as well as radar. For broadband communications, OFDM turns a frequency selective channel into a set of parallel flat channels, which significantly reduces the receiver complexity.Problems in multipath fading and interference are severe on road condition and environment also so dynamic. Existing wireless system may be utilized single frequency, singleantenna and pulse for multicarrier transmissionand reception. Problems of such system are that in case offailure the total system will become non-operational A distributed system in terms of multicarrier, multiantenna and coded pulse can provide a more suitable communication and sensor give rise toHybrid (SS-OFDM-MIMO) technology is the ultimate solution. The hybrid approach has been essentially been developed for 60GHz frequency provided we have the necessary bandwidth.The performance analysis for the MIMO-OFDM system has been carried out based on MATLAB simulation. The experimental results have been verified using the simulation, the results of simulation have been verified with the various works being carried out in this area and the results conferred to be correct. Full-text · Article · Feb 2013 +1 more author...Article · Jul 2017
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