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What capacitor to use to filter out 10GHz signal - RF Cafe Forums

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antennaant

Post subject: What capacitor to use to filter out 10GHz signal Posted: Mon Dec 11, 2006 3:20 pm

Lieutenant

Joined: Mon Dec 11, 2006 3:17 pm

Posts: 2

I have a 10GHz signal riding on all my DC inputs. I am thinking of using a low pass RC filter. But what kind of capacitor to use? Ceramic MLCC?

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fred47

Post subject: 10 GHz bypassPosted: Wed Dec 13, 2006 12:44 am

General

Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104

Hi!

You say that you have "have a 10GHz signal riding on all my DC inputs".

A classic approach is to bypass the DC inputs, and use a resistor, inductor, or lossy ferrite to isolate the DC from the RF.

However, I'd like to caution you that one possible problem with an RC lowpass is the quality of the ground. At 10 GHz (wavelength about 3 cm), it doesn't take much distance to be an appreciable fraction of a wavelength. So, while a small MLC SMT capacitor would probably work, proper functioning would really almost more depend on the available ground path. As a rule of thumb, you should reach a ground plane within 1/10 of a wavelength, or 3mm. (And preferably less!)

Physical layout is increasingly critical as the frequency goes up.

Good Luck!

Fred

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sm7ovk

Post subject: Posted: Thu Jan 18, 2007 2:57 pm

Lieutenant

Joined: Thu Jan 11, 2007 12:59 pm

Posts: 4

Just wanted to mention that 10GHz easily can get outside an ordinary T-filter often used in digital circuits, as mentioned earlier the dimensions of the filters are often larger than 3mm. In a project some years ago I found feedthrough capacitors specified to 10GHz, think it was Murata but there are probably plenty of them out there - just be sure that they really are specified at least to the frequency you use or have problems with!

Regards

Jens

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yendori

Post subject: Posted: Thu Jan 18, 2007 5:52 pm

General

Joined: Thu Sep 25, 2003 1:19 am

Posts: 50

Location: texarcana

Check out Vishay. They have some pretty ok mos caps. I use them up to 10GHz.

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