Using an inductor for low-pass filter

[spasmous2]

I got a quote for a 20kHz low pass filter: $475. So that settles it,
I'm going to make my own. I really just want something basic and
simple so a series inductor circuit (http://www.allaboutcircuits.com/
vol_2/chpt_8/2.html) would seem to be fine. Now I am not sure what
amount of Henrys are required to get a low pass filter of 20 kHz or
thereabouts. Also, what wire should I buy and how many turns are
needed. Sorry I am such a newbie, any help appreciated!

 

[Eeyore]

I got a quote for a 20kHz low pass filter: $475. So that settles it,
I'm going to make my own. I really just want something basic and
simple so a series inductor circuit (http://www.allaboutcircuits.com/
vol_2/chpt_8/2.html) would seem to be fine. Now I am not sure what
amount of Henrys are required to get a low pass filter of 20 kHz or
thereabouts.

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

 

[spasmous2]

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.

 

[spasmous2]

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:http://www.nuhertz.com/download.html
and it should help you design a suitable filter.

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.