Using an inductor for low-pass filter

Firstly you need to define aload impedance (for a passive filter).

Youi can make an active filter for about a dollar in parts though.

Graham

You need to tell us more about your application.

Are you trying to filter low level signals, or substantial power levels?

What frequencies are you trying to reject, and which to pass?

What will your load be? Will it be constant or will it vary?

The more you tell us, the better.

The signal is between 0 and 2V and is outputted from an RF amplifier from a BNC jack labelled "Scope", which is a design feature specifically for showing the waveforms on an oscilloscope. It's always scaled between 0 and 2V so it's constant in that respect. I presume it's low power.

The waveforms vary quite smoothly - I can't give an exact frequency cutoff but somewhere around 20-30 kHz should be adequate. However superimposed on the signal is a burst-like noise which I want to smooth away, otherwise the trace on the scope screen is filled with scattered junk. I just want to reduce the scatter so it looks nice.

I wish my 'scope had some digital filtering capability but sadly it doesn't.

Since you're wanting to filter only low power signals, you can use small ferrite cored inductors such as can be bought from Digikey, for example.

Go download "Filter Free" at:

Thanks - that's a useful program. I made a 1-order Butterworth low pass filter with 20kHz passband @ 50ohm load. It's just a 400uH inductor in series! The transition region is kind of broad but that's OK, I'll buy a few inductors from 200 - 1000uH and see what looks nicest. Appreciate your help, The Phantom.