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Selecting inductor for bias-tee application - RF Cafe Forums

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cajoyce
Post subject: Selecting inductor for bias-tee application Posted: Mon Jun 26, 2006 12:03 pm

Lieutenant

Joined: Fri Jun 23, 2006 11:12 am
Posts: 2
I need to implement a bias-tee in a 50 Ohm, 900 MHz circuit to extract a logic signal superimposed on the RF signal. As a coupling capacitor is already present in the existing circuit, I only need to add an inductor to tap the DC off the composite signal. Two questions:

1. Should the inductor value be set to render the DC tap high impedance at 900 MHz, say 500 Ohms or greater?

2. What are the guidelines for considering the inductor's Self Resonant Frequency? I gather that this is the frequency beyond which the inductor no longer behaves as an inductor due to parasitic capacitance? In this 900 MHz application what would be the minimum safe value for Self Resonant Frequency?

_________________
Thanks...

...Christopher


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IR
Post subject: Posted: Mon Jun 26, 2006 3:56 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi cajoyce,

The answers are below your questions:




Quote:
Should the inductor value be set to render the DC tap high impedance at 900 MHz, say 500 Ohms or greater?



Yes.
Quote:
2. What are the guidelines for considering the inductor's Self Resonant Frequency? I gather that this is the frequency beyond which the inductor no longer behaves as an inductor due to parasitic capacitance? In this 900 MHz application what would be the minimum safe value for Self Resonant Frequency?




Yes, the Self-Resonance frequency is the frequency at which the parasitic capacitance form a resonance circuit with the inductor. For 900MHz frequency I would choose 1.5GHz and higher as SRF.

Good luck!

_________________
Best regards,

- IR


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cajoyce
Post subject: Posted: Mon Jun 26, 2006 6:40 pm

Lieutenant

Joined: Fri Jun 23, 2006 11:12 am
Posts: 2
IR,

Thanks very much for your prompt and informative response.

...Christopher


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nubbage
Post subject: selecting inductor for bias-Tee applicationPosted: Tue Jun 27, 2006 2:52 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
The reactance of the inductor at the operating frequency should be at least ten times the circuit impedance level.

Care also needs to be taken with choice of bypass capacitor since this can also have resonant frequencies due to internal inductance.

I have seen articles that give guidelines on parameters, published in trade mags like RF & Microwaves, MWJ etc


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Jeanalmira
Post subject: Posted: Tue Jun 27, 2006 10:58 pm

General


Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi :

the following application note might be useful information on high power device biasing consideration.

http://www.eudyna.com/e/products_e/appl ... cation.pdf

I hope it helps

Cheers,
Jean







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