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Design >> RF Design >> Solution for inductor behaving like a Capacitor at desired frequency and layout
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Message started by sweetchoto on May 24^th, 2013, 1:29pm

Title: Solution for inductor behaving like a Capacitor at desired frequency and layout
Post by sweetchoto on May 24^th, 2013, 1:29pm

Hi,

I am designing a LNA for 4GHz band. I am having a problem with inductor of lets say 5nH value which i am using as DC short. With ideal inductor the LNA circuit gives accurate results as i expect. But when i replace ideal inductor with real inductor i get in to problem. To analyze the inductor, i simulated it separately. Plotted its impedance vs Frequency graph and it clearly shows me that at frequency of interest inductor is actually behaving more like a capacitor rather than inductor. Inductor resonant frequency is around 10 GHz but around 4 GHz it gets a small dip and than it starts rising again (Like an inductor impedance should with increase in frequency) till 10 GHz and then falls again. Also, its Q factor is really low at 4 GHz.

1-If any one can suggest how to make inductor behave like an inductor at my desired 4 GHz frequncy and improve its Q value ?

2- My second question is related to layout design rules. If anyone can point me to any post/document where i can read about rules and best practices about how to place components from RFin to RFout to make sure that parasitics does not effect the circuit RF performance a lot. It would be really nice.

Really looking forward to some good feedback from the folks here.

/SC

Title: Re: Solution for inductor behaving like a Capacitor at desired frequency and layout
Post by raja.cedt on May 25^th, 2013, 12:18am

Hello,
You should post your inductor layout as well as frequency response, otherwise it's difficult to explain. Looks like you have large area, so better reduce turn width, decrease turn spacing, increase the diameter(means leave higher space between opp current carrying turns).
Use this online cal to get rough estimation.
http://www-smirc.stanford.edu/spiralCalc.html

Use this pap http://smirc.stanford.edu/papers/JSSC99OCT-mohan.pdf

Hope this helps,
Raj.

Title: Re: Solution for inductor behaving like a Capacitor at desired frequency and layout
Post by sweetchoto on May 25^th, 2013, 6:24am

Hi,

Thanks a lot. I will try the things you have mentioned next week. If i still couldnt solve it. I will post the inductor layout along with its response. Thanks a lot for the help. Paper looks really helpful.

/SC