SP NF vs PSS PNoise NF

[Alex_IC]

Hello!
I have simulated a LNA in cadence spectreRF with SP and PSS PNoise and the result for NF is very differ. In SP I have NF=3,5dB and in PSS PNoise NF=6 dB with input power of -15 dBm (ICP=-7dBm). Why the NF degrades so much with input power?

Tnx in advance.

 

[khouly]

the SP is a S paramter , which assume samall singal operation

but PSS is a fullu nonlinear , and take the effects of gain compression and the intermodulation , and ur power is high this make the LNA is complelty nonlinear , and it genrate alot of noise due to intermodulation product and this increase th noise figure

try to make the input power is low in PSS and compare the results with SP , like -80 to-70 dbm

khouly

 

[Alex_IC]

Yes, with very small input power the NF is the same. But what is the input power for NF measument?

 

[khouly]

the NF should be measured at low powers ,

khouly

 

[BigBoss]

Why NF is simulated/measured because NF is a figure of merit of the sensitivity of a system and Dynamic Range starts from MDS level.

MDS: Minimum Detectable Signal

Therfore NF is for low level signals...

 

[khouly]

there is no certain power , for NF simulation , but u should apply power near the MDS

khouly

 

[Alex_IC]

So SNR=Pin-174+10lgB-NF, and as Pin increasing the NF also increase and the SNR is near the same, that why we don't worry about NF at high input power, am I right?

 

[Alex_IC]

Please, answer me, does the NF degradation at high input power degrade sensetivity of the reciever or not?

 

[khouly]

in high power the noise figure is not the main factor , but it is the nonlinear behavior , of the system , where many harmoics and intermodulation products will be the main contributer

khouly

 

[vfone]

You already answered to your question.
The input power it will affect indirectly the sensitivity of the receiver. Is not affecting the Noise Figure of the receiver (or affect very little), but it will affect the Signal-to-Noise Ratio.
There is a tricky RF System question: if a 10dB pad attenuator in the front of the receiver it will degrade the sensitivity? The answer is: depends what the input level is.