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cowell

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== how to improve S11 in broadband circuit design?
May 21^st, 2006, 7:40pm

I'm designing a 40MHz~1GHz splitter in CMOS process.
To lower noise, i have to increase the size of the input CMOS transistors and DRAIN current.
However , the S11 of the circuit becomes very bad when the frequency is as high as near to 1GHz because of the shunt input Cgs.
To improve the S11 in high frequency, i have made a try to put a series small inductance before the GATE of the input transistors.
It seems not so effective than i had expected.
Does anybody have any hints on solving the problem like this?

By the way, is there anyboy here who have good experiences on designing broadband splitter to get
good distoration in large-signal condition and good noise simultaneously?

best regards
cowellen@gmail.com

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schehrazi Community Member Offline Posts: 45 University of CA, LA	Re: == how to improve S11 in broadband circuit des Reply #1 - May 25^th, 2006, 2:22pm You may like to use a ladder filter type matching circuit at the input. Embed the parasitic capacitances in the ladder filter.
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cowell Junior Member Offline Posts: 18 World	Re: == how to improve S11 in broadband circuit des Reply #2 - May 25^th, 2006, 6:36pm thanks a lot for your good suggestion. But i wonder is it possible to solve this problem for 40MHz~1GHz band? Besides this , i won't be at the cost of size and NF of all circuit because of the large amounts of inductances or other outside devices. :'( Any enlightening words will be appreciated.
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schehrazi

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Posts: 45
University of CA, LA

Re: == how to improve S11 in broadband circuit des
Reply #3 - May 25^th, 2006, 6:54pm

Hi,

The cut off frequency of this circuit is 20GHz and if I have understood your question correctly, such a circuit should help you solve your problem. You might need to optimize it though. In order to keep the NF low, the inductor should be optimized for lowest series resistance possible cause its noise adds directly to the input signal. I hope this helps.

Sorry, I don't know how to attach a picture to this but I write the netlist.

Vin 1 0 sine ...
Rs 1 2 50
C1 2 0 1p
L2 2 3 5n
C3 3 0 1p
RL 3 0 50

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cowell Junior Member Offline Posts: 18 World	Re: == how to improve S11 in broadband circuit des Reply #4 - Jun 2^nd, 2006, 3:26am Hey, schehrazi, Thank u so much, it did work to some extent!! ;D Before this, i had thought i can't solve the problem which resulted from my missing of C1, :-[
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