inductor as RF choke

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STOIKOV

Joined: 18 Nov 2005
Posts: 295
Helped: 6

31 May 2006 0:15 inductor as RF choke

Commercially available inductors can be used as RF chokes ?

flatulent

Joined: 19 Jul 2002
Posts: 4877
Helped: 324
Location: Middle Earth

31 May 2006 0:47 Re: inductor as RF choke

You have to watch out for the DC current rating. When filtering DC you need to look for the series resistance which will cause heating and voltage drop. You also have to look for the reduction of the inductance (when there is some core material other than air or an insulating support) from being biased of the center of the BH curve by the DC current.

boy

Joined: 30 Sep 2002
Posts: 563
Helped: 39

31 May 2006 4:39 inductor as RF choke

One important thing is self resonace frequency (SRF). It has to be less than the operating frequency otherwise your inductor will become a capacitor.

DDavid

Joined: 06 Apr 2006
Posts: 232
Helped: 22

04 Jun 2006 15:25 Re: inductor as RF choke

Hi,

My friend I think you are wrong when you want to use with the inductor as RFChoke you must take inductor with SRF bigger then operating freq.
But if I am wrong please correct me!!!

David

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biff44

Joined: 24 Dec 2004
Posts: 1835
Helped: 244
Location: New England, USA

04 Jun 2006 20:13 Re: inductor as RF choke

Yes, we have all used inductors as "RF chokes" on things like bias lines, etc.

The inductor would have to be used in a frequency range below its SRF, otherwise at resonance it would have no "filtering effect".

However, I do think it would be confusing to use the terms "inductor" and "RF Choke" interchangeably. When I want an "RF choke", I want something that is a LOSSY inductor (ie has part inductance and part resistance in its series impedance). The lossy nature is what allows you to form wideband rf blocking without fear of resonance with shunt bias capacitors! If you look at a typical "ferrite bead" data sheet, you will see that is has considerable resistive loss.

If you used high Q inductors as an "RF Chokes" with impunity, you could get in trouble in some cases with a parasitic bandpass filter formed in your bias network--such as when you have two series RF Chokes with a DC bias line length of around λ/2 between them.