# How to measure cutoff frequency of a low pass audio filter?

I've built a low pass filter for use in a synthesizer and would like to measure the cutoff frequency compared with various different control voltages to profile its characteristic.

How can I measure cutoff frequency?

• I think you will need a signal generator too. Jul 24, 2015 at 8:45
• At first you must decide at which frequency you want to define the end of the passband for your particular application. It is not always the 3dB frequency. And this definition gives you - automatically - the parameter/value to be verified (amplitude or phase).
– LvW
Jul 24, 2015 at 9:24
• As it's audio, you can use a PC soundcard as the sig gen.
– user16324
Jul 24, 2015 at 10:26

As Andy mentions in a comment, you need a signal source that you can tune to make a frequency that you measure the filter with.

Luckily you say it is an Audio filter, so you can use an audio source.

Find a program that you can use in the freeware (or sample-ware) world to make signals around the cut-off frequency.

Use your scope to measure the signal going into the filter and going out. The scope can also probably even do some maths (since it's a digital one) and show "Input" - "Output" for you to easily then calculate the factor.

Since you designed the filter you should probably know about the log() scale of Decibels, but since I'm in a decent-ish mood today I'll continue to explain that the "cut-off" frequency is the 3dB point for many filter designs.

The 3dB point is where the power is reduced by a factor of two, but that means for the voltage that it should be a factor of sqrt(2) =~ 1.41 lower. Half the voltage output would be 6dB.

This website has a nice table to use if you don't feel like maths.

Many digital scopes can also (or have a purchasable upgrade for it) calculate the dB change in voltage. All you'd need to do is tweak the sine-wave output by the sound card until it says "-3dB". These digital toys have really taken the art out of it :-).

In fact, if it has "infinite persistence" you can make your soundcard sweep the frequency over 10seconds and then set the scope to a timebase that gets 10 seconds for a full screen and you can get a nice 2-line frequency plot and just use a ruler to instantly find a 3dB, 6dB or 20dB point.

Since you are interested in measuring an audio filter you can use a sound card as signal generator and spectrum analysator.

Search the web with keywords "sound card signal generator spectrum analysator" for appropriate software.
See e.g. here.