RC-LOW PASS FILTER

© Aditya Narayan Nanda

LOW PASS RC FILTERS:

WHAT IS THIS??

FILTERS are used to change the frequency content of a audio ... that means u can clip off some of the frequency( in some range) and pass others .. wow!! sounds great.....

understand any thing??? i know your ans is NO!!!

so lets see this practically::

first download VLC MEDIA player and open the song : given up by Likin park

i choose this one because this band is my fav and songs have almost every frequencies ... cool na??

OK now play the song and go to the equalizer control and then change the frequency bars as shown...

(CLICK HERE TO GET A MAGNIFIED VIEW)

have you noticed any change in sound?? probably your answer is YES..

its just like your pre-loaded FULL BASS mode.  

look at bar setting what you have done?? u have just allowed the low frequencies and blocked the higher ones that mean you have clipped off high frequencies and allowed lower one.. that means you have made a LOW PASS FILTER...

<<notice you can still hear the song but some frequencies are clear>>

HOW TO MAKE LOW PASS FILTERS??

u need resistor, capacitor, DC source , jumper wires and bread board

Example Circuit:

this works so because the voltage of capacitor can't instantaneously change. and you have a resistor in series which slows down the charging process: OK THAT MEANS IF RATE OF CHANGE IS HIGH then OUTPUT VOLTAGE IS NOT GOING TO REPRODUCE IT IMMEDIATELY .. BUT WHEN YOU HAVE LOW FREQUENCY THEN RATE OF CHANGE IS SLOW SO YOU GET THE EQUIVALENT OUTPUT.

IN PRACTICAL CIRCUITS YOU HAVE TO CHOOSE A HIGH RESISTANCE AND LOW CAPACITANCE TO BUILD A LOW PASS FILTER

cut-off frequency formula:

so our example filter has a Fcutoff = 1591.54 Hz that means any frequency greater then this is not going to sound your ear buds!!!

how to choose r c values for your own filter?

FIRST use the speaker data sheet and look for the max current... then look at your source DC voltage... the choose a resistor to limit the current .....then use your cut off formula to calculate the capacitor value.. very easy.... 

SOME SIMULATIONS :

below i have given 3 examples. R1 and C1 are filter elements and R2 is  a load resistance and stands for head phone or speaker etc.

in all cases R1=10K, C1=10n 

so
Fcutoff= 1591.54 Hz
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1:WHEN INPUT FREQUENCY IS LOWER THAN CUT OFF FREQUENCY:

Fcutoff= 1591.54 Hz

Applied Frequency= 50 Hz

Here applied frequency(in green color) is very low so output frequency ( in red color) is same as input...

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2:WHEN INPUT FREQUENCY IS HIGHER THAN CUT OFF FREQUENCY:

Fcutoff= 1591.54 Hz

Applied Frequency= 2000 Hz

Here applied frequency(in blue color) is slightly high so output frequency ( in green color) is clipped off and we get very low amplitude output...

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3:WHEN INPUT FREQUENCY IS MUCH HIGHER THAN CUT OFF FREQUENCY:

Fcutoff= 1591.54 Hz

Applied Frequency= 200000 Hz

Here applied frequency(in green color) is very very high so output frequency ( in red color) is totally clipped off... so we get a nearly zero output

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NEXT TUTORIAL:

 RC HIGH PASS FILTERS then REAL AND IDEAL FILTERS