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decoupling and coupling.. - RF Cafe Forums

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ngks79

Post subject: decoupling and coupling.. Posted: Fri Apr 21, 2006 3:50 am

Lieutenant

Joined: Sun Apr 09, 2006 8:48 pm

Posts: 2

Location: Penang

good day everybody,

Can anybody explanain definition of coupling and decoupling? what is their's purpose? how is their connection in RF matching circuit?

Thanks

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IR

Post subject: Posted: Fri Apr 21, 2006 8:33 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello ngks79,

AC or RF coupling is done with a series capacitor which its purpose is to deliver only the alternating or high frequency signal and block the DC in order not to vary the operating point of the active device (Usually transistor or amplifier). These series capacitors are often connected between a matching network and the input or output of the device. Their capacitance should be calculated in a way that their reactance is low in the lowest frequency of operation (not more than few ohms).

Decoupling means suppressing high frequency signals, this is usually done in supply voltages by connecting capacitors between the supply and GND rails. The capacitors short high-frequency signals (Interferers and noise) to GND.

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Best regards,

- IR

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Kirt Blattenberger

Post subject: Posted: Fri Apr 21, 2006 9:11 am

Site Admin

Joined: Sun Aug 03, 2003 2:02 pm

Posts: 308

Location: Erie, PA

Greetings:

A similar question was asked of me the other day at work on a related topic, and that was: What is the difference between a decoupling capacitor and a bypass capacitor? The questioner had been told by his college professor that a decoupling capacitor was used in series with an RF signal to block DC, and that a bypass capacitor was used on DC supply lines to shunt AC to ground.

IR's response is in line with my own usage of the term "coupling capacitor" for one used to pass RF and block DC. The terms "decoupling capacitor" and "bypass capacitor" are used interchangeably almost universally in technical literature and on all of the online electronics dictionaries (and app notes) I checked. Both terms describe a capacitor used to conduct AC to ground, or at least away from the signal path.

…just another data point.

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