# Circuit analysis help.

#### ecjohnny

Joined Jul 16, 2005
142
In a factory floor environment, a microphone is used in an intercom system. The frequency range of interest is up to 3000Hz. The voice signal at microphone output is 100mV.
There is a high frequency vibrator in the factory floor, which produces interfering sine wave noise at 10kHz. This interferes with the voice signal and produces 40mV peak sine wave at microphone output. Microphone output is fed to preamplifier. The arrangement is shown in fig.1.

Voice - up to 3kHz, 100mV peak
Vibrator noise - at 10kHz, 40mV peak
Capacitor is 100 microfarad

Q1 Suggest a suitable subsystem(filter) that can reduce interfering noise to 26dB below signal at preamplifier input without attenuating voice signal by more than 3dB. We would like to have flat frequency response in the lower frequencies.

Q2 Design the sub system, do a simulation on Circuitmaker(any other software).

hey guys... need help in the above problem. Actually i really dont know where to start. This is a thinking problem so my teacher gave a fast briefing.
Anyway, i only know that its a Butterworth filter(low-pass filter) that is going to be used. And finding some values for SNR ,order,frequency scaling,impedance scale factor..etc...so any guidance/ advise would be appreciated.

#### beenthere

Joined Apr 20, 2004
15,819
Read up on low pass filtering. Take a guess where to put the corner (where the filter starts to attenuate the signal frequencies). Then look at the magnitudes of the voice and noise signals to get an idea of the reduction of the noise signal amplitude so it's 26 dB less than the voice.

#### ecjohnny

Joined Jul 16, 2005
142
Read up on low pass filtering. Take a guess where to put the corner (where the filter starts to attenuate the signal frequencies). Then look at the magnitudes of the voice and noise signals to get an idea of the reduction of the noise signal amplitude so it's 26 dB less than the voice.
i know the basic of low pass filter. But this question is asking me to design a practical filter. Any links(read up) to this? and my book suggest me to find fc(frequenct cut-off) first. i am thinking of fc = 3khz?? Maybe you guys can give me hints and i will try my best to work it out. i will keep posting back. Thanks for the help.

#### mrmeval

Joined Jun 30, 2006
833

#### ecjohnny

Joined Jul 16, 2005
142
10khz notch filter.
http://en.wikipedia.org/wiki/Notch_filter

Can you sample the offending noise without the audio? If so there are filtering methods for that as well but I'm blanking on what it is.

Thanks. But my teacher told me the question uses low-pass filter and is a Butterworth type filter and that notch filter is a bandstop filter.

Can you sample the offending noise without the audio? actually i am not sure what you mean but
my teacher flashes the solution up on board for a few minutes to let us have a overlook on how to format is like. i only know it requires lots of mathematics/formulas and thinking involve to calculate the resistor and stuff.

Oh and i forgot to mention that the capacitor is given as 100mirco farad.

Help me please while i search on my book and online for solutions.

#### electroperk

Joined Jun 30, 2007
8
in the case you want a flat passband response then butterworth filter is a good choice..
and in case of 1st order filter i think that any filter will give the same response.. the response rolls off at −6 dB per octave (−20 dB per decade).. so if you use 3khz... it may not give -26db attenuation for 10khz noise..
so better use 2 order filter..the response rolls off at −12 dB per octave.
choosing 3khz as your cut off would easily provide you -26db attenuation for 10khz noise..

#### electroperk

Joined Jun 30, 2007
8
use fc=1/2*pi*Rs*c
as you know c and fc you may find Rs from here..

attaching a picture of second order filter.. this may help you abit..
i tried my level best.hope it may help you..

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#### ecjohnny

Joined Jul 16, 2005
142
in the case you want a flat passband response then butterworth filter is a good choice..
and in case of 1st order filter i think that any filter will give the same response.. the response rolls off at −6 dB per octave (−20 dB per decade).. so if you use 3khz... it may not give -26db attenuation for 10khz noise..
so better use 2 order filter..the response rolls off at −12 dB per octave.
choosing 3khz as your cut off would easily provide you -26db attenuation for 10khz noise..
thanks it helps alot. Thanks for the picture too. Can you explain a little or any read up on how you get the order 2 filter part below?
" so better use 2 order filter..the response rolls off at −12 dB per octave.choosing 3khz as your cut off would easily provide you -26db attenuation for 10khz noise"
and also order 2 is the answer which my teacher told me,but i will have to do a prove on why order2(n = 2) is used.

so am i right about fc = 3khz for my question? if thats the case, i will go work out Rs soon.

#### electroperk

Joined Jun 30, 2007
8
its practically proven circuit and a standard one.. -12db per octave means that whenever you double the frequency i.e cut-off frequency.. the attenuation will increase by 12db..so having 3khz cutoff..and for cut off you get 3db attenuation..so now doubling the frequency you will get a attenuation of 15db at 6khz.. then again doubling the frequency you will get an attenuation of 27db at 12khz(approx 10khz,your noise frequency).. and that's it.. you wanted not more than 3db attenuation of informatin..and above 26db for noise..and theoretically you are getting it.. go for it practically also..

#### electroperk

Joined Jun 30, 2007
8
and you can observe this by drawing a db Vs frequency graph.. you may also consult a book by Ramesh Gaekawad..an indian author

#### ecjohnny

Joined Jul 16, 2005
142
can anyone suggest any other possible solutions to this problem?? Any other method or ways other than using the butterworth filter??

#### JoeJester

Joined Apr 26, 2005
4,390
You could visit http://sound.westhost.com/p-list.htm. Look at the mid range filter used in project 09.
You could use the buffer and mid-range filter before the amplifier in your PA system.