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# LNA noise figure problem , plz help ........

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#### bcdeepak

##### Member level 5
lna noise match

hi

i am designing a dual band LNA , S parameters are fine and their dips are occuring at the required frequencies but NOISE FIGURE dip is not occuring at the required frequency but NF value at the wanted freq is fine( 2.4 dB )

what can be the problem ?? what should i do ??

deepak

what is more important noise figure or s11

What is dip ?

u mean there is a dip in the S11 , this dip means that the S11 at this freq shows good matching

be aware that the S11 doesn't indicate the NF , so the S11 dip doesn't make NF dip

khouly

hi khouly ,

at 2.4GHz i am getting a dip in S11 and S22 curves, so there is impedance matching at 2.4GHz .

but NF dip is occuring at 1.6dB , that means NF is minimum at 1.6 , but not minimum at 2.4

can it be like that , i mean ,my doubt is wherever impedance matching occurs , is it not guranteed that NF can be minimum at the same freq ???

Usually, Noise match is not located at the same frequency of inout math, please reference the Book writen by Thomas Lee.

You should find some method to keep the impedence match at the minimum NF area

You are right. The optimum source impedance for minimum NF may not match the input impedance of a LNA. You have to trade-off RL for NF. Because you are designing a LAN, NF should be more important. Keep the RL within the spec and the lower NF you can get the better. Thanks.

bcdeepak said:
hi khouly ,

at 2.4GHz i am getting a dip in S11 and S22 curves, so there is impedance matching at 2.4GHz .

but NF dip is occuring at 1.6dB , that means NF is minimum at 1.6 , but not minimum at 2.4

can it be like that , i mean ,my doubt is wherever impedance matching occurs , is it not guranteed that NF can be minimum at the same freq ???

there is a difference between the impedance match and noise match , so may be the soure impedance is noise optimum at 1.6

khouly

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