# anyone help me about low pass filter....

#### jack_sparrow

Joined Aug 17, 2006
17
can anyone help me how to test a low pass filter and a power amplifier?
i design a 5th order butterworth low pass filter which has a cut-off of 8KHz and a power amplifier...i do have an oscilloscope but i don't exactly knew how to test the filter circuit and what should be the settings...anyone help how to test these circuit....

#### hgmjr

Joined Jan 28, 2005
9,027
Ideally what you need is a function generator capable of outputing a relatively distortion-free sine-wave whose frequency range is at least 4 or 5 octaves above your filter's corner frequency of 8KHz. That would be around 128KHz to 256Khz. The low end frequency should be at least 100 Hertz or so just to be able to see what the response of the filter looked like near the low end of the frequency spectrum.

hgmjr

#### jack_sparrow

Joined Aug 17, 2006
17
...i also have a function generator....how about the amplitude?does it vary?
also the voltage peak-to-peak?and the time/div..?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

Amplitude and p-to-p voltage are closely related.

Be less concerned with the settings on the oscilloscope than confirming the filter action. It's low-pass, so frequencies into the filter should not be affected below the corner frequency. You should have a response curve that will predict the filter's action, so a meter is adequate test equipment.

Set up your function generator for a sine output at some level, and at some frequency well below the corner. Watch the voltage out of the filter as you approach the corner frequency.It should stay up until you get close to 8 kHz. The corner freq is defined by the -3 dB point where the amplitude of the signal is attenuated by that amount. The filter response plot will predict the signal level at higher frequencies. Use your meter to see if the filter and the response plot match.

#### jack_sparrow

Joined Aug 17, 2006
17
thanks.....about my filter circuit...i used a 5th order butterworth filter implementing a sallen & key topology...it's all about calculation...right?as long as you know the formula...my question is that does it matter if i use any types of capacitor?example like my case...there are no available ceramic cap. in our town
only mylar and alectrolytic...i dont have a choice but to use a mylar...then the value of cap. will change so as the resistors...but still the calculation are the same...so, is it ok?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

Since your siganl is probably AC in nature, use the mylar capacitors. They are non-polar. The accuracy tplerance of the components - resistors as well as capcaitors, will affect the accuracy of the filter.

#### jack_sparrow

Joined Aug 17, 2006
17
hi,

just for confirmation...

about testing my filter circuit,,...i set up the input freq of the scope at some level below the corner freq and then adjusted up to the corner freq w/c is about 8khz,..the voltage doesn't drop...and still i continued adjusting the freq
higher and then the voltage drop down and turned 0V...is my filter good?

#### hgmjr

Joined Jan 28, 2005
9,027
That is pretty much it.

A 5th-order Butterworth filter should drop off rapidly as you adjust the sinewave frequency of your function generator once you pass the -3dB corner frequency of 8KHz. If all of your poles have been proper placed, then the output from your filter should drop off at a rate of -30 dB per octave.

Would you be willing to post a schematic of your filter in the AAC forum?

hgmjr