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[SOLVED] About the parasitic inductance of capacitor

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greatmarx

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when I use the capacitor of 33pF,my colleagues tell me that 33pF capacitor can filter the microwave of 900MHz.Who can tell me the reason?
I think that maybe cause by the parasitic inductance.
But I don't know the relation between the value of capacitor and the value of parasitic inductance.

My English IS NOT Very Well.
 

keith1200rs

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I am not sure what you mean by "filter 900MHz". A 33pF capacitor should have a self resonant frequency (SRF) above 900MHz so will work as a capacitor at that frequency. Larger capacitors such as 1nF will have a SRF below 900MHz so will behave more like an inductor. There is some useful software you can download here which will allow you to see the impedance characteristics of various capacitors.

KEMET Software

Keith.
 

greatmarx

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I use the package of 0402,and I use the bypass cap in front of mic ,after the cap I place a inductance of 100nH.
This can decrease the Interference of GSM900.
Thank you for your answer.
But I don't know why.
 

keith1200rs

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The inductance adds a series impedance at 900MHz, the capacitance is low impedance. If you put inductance in series and capacitance in parallel it helps to short out the high frequencies.

Keith.
 

greatmarx

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Thank for your your answer.
I have known the reason.But I don't know that Is the 33pF is the best value of the cap.
After using the software you given,I see that the impedance of 33pF around 900Mhz is the lowest.

Yang
 

keith1200rs

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Yes, for 0402 you are about at the SRF so the capacitor is behaving like a series tuned circuit.

Keith.
 

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