+ Post New Thread
Results 1 to 10 of 10

25th March 2012, 17:20 #1
 Join Date
 Jan 2012
 Location
 Coimbatore
 Posts
 9
 Helped
 0 / 0
 Points
 351
 Level
 4
Cutoff frequencies from Frequency response curve
While calculating the cutoff frequencies using the Frequency response curve, why do we draw a line 3dB below the curve to mark the Cutoff frequencies? Why is it 3dB and not something else?

25th March 2012, 17:20

25th March 2012, 17:50 #2
 Join Date
 Dec 2010
 Location
 USA, midwest
 Posts
 960
 Helped
 308 / 308
 Points
 5,915
 Level
 18
Re: Cutoff frequencies from Frequency response curve
A 3 dB reduction in power is equal to 1/2 of the original value. I don't know the historical origin, but marking the point where signals are reduced by 1/2 or more, seems like a good point of reference.

25th March 2012, 17:54 #3

25th March 2012, 17:54

25th March 2012, 18:48 #4
 Join Date
 Mar 2012
 Posts
 308
 Helped
 56 / 56
 Points
 2,232
 Level
 10
Re: Cutoff frequencies from Frequency response curve
Dineshkumar005,
While calculating the cutoff frequencies using the Frequency response curve, why do we draw a line 3dB below the curve to mark the Cutoff frequencies? Why is it 3dB and not something else? .
RatchLast edited by Ratch; 25th March 2012 at 19:33. Reason: Changed 10*log(1/√2) to 20*log(1/√2)
1 members found this post helpful.

25th March 2012, 18:48

25th March 2012, 19:03 #5
 Join Date
 Jul 2002
 Location
 South Africa
 Posts
 1,014
 Helped
 192 / 192
 Points
 7,067
 Level
 20
Re: Cutoff frequencies from Frequency response curve
The 3dB point indicates 1/2 the power of the signal. (Not the voltage)

25th March 2012, 19:32 #6
 Join Date
 Mar 2012
 Posts
 308
 Helped
 56 / 56
 Points
 2,232
 Level
 10
Re: Cutoff frequencies from Frequency response curve
klystron,
The 3dB point indicates 1/2 the power of the signal. (Not the voltage) .
Ratch

25th March 2012, 20:12 #7
 Join Date
 Mar 2012
 Posts
 1
 Helped
 0 / 0
 Points
 133
 Level
 1
Re: Cutoff frequencies from Frequency response curve
i am doing project on frequency resonse analyser card..... and i dont know any thing about these Can anyone hel me in this?

26th March 2012, 02:11 #8
 Join Date
 Dec 2010
 Location
 USA, midwest
 Posts
 960
 Helped
 308 / 308
 Points
 5,915
 Level
 18

26th March 2012, 02:11

26th March 2012, 02:59 #9
 Join Date
 Jan 2012
 Location
 Indonesia
 Posts
 20
 Helped
 1 / 1
 Points
 837
 Level
 6

26th March 2012, 03:16 #10
 Join Date
 Dec 2010
 Location
 USA, midwest
 Posts
 960
 Helped
 308 / 308
 Points
 5,915
 Level
 18
Re: Cutoff frequencies from Frequency response curve
Decibels is a way to quantify the measure of something to a known reference level. You can measure decibels (dB's) of any magnitudebased quantity... voltage, current, power, resistance, etc. Most often people will talk about the 3 dB point of a filter, which is the point at which the output signal is attenuated by 1/2.
To calculate dB's of voltage, current, resistance, etc... you use 20*log(x)
To calculate dB's of power, you use 10*log(X).
When someone says a filter has 20 dB of rejection at 100 MHz away from the center frequency, they are saying that a signal at that particular frequency will go in at some power X, and come out of the filter at a power level of 0.01*X. 20 dB = 10*log(?), so ? = 0.01.
For more information, start reading up here: http://en.wikipedia.org/wiki/Decibel
2 members found this post helpful.
+ Post New Thread
Please login