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# Can 0.5dB S11 cause oscillation in an LNA design?

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##### Junior Member level 1
i am desingin an LNA at 500 MHz .. S11 is under zero for all frequencies except at 1.3 GHz it's 0.5 dB . Is there any risk of oscillation for this small value of S11 ???

s11 device

Hello,

You said you designed means simulated or fabricated a prototype!!!

The S11 can't be positive their may be some problem with the device model
parameter or file...

Yes, it is unstable (oscillations), plot the K or B1 stability factor plots or stability circle measurements on smith chart....
See more here **broken link removed**

Or verify the matching network & the device models...!
By the way what is your operating frequency range...!!!

---manju---

Points: 2
how to use awr to simulate lna

i am simulating my LNA on ads , my operating frequency is from 300MHz to 800 MHz . 10x

Re: LNA S11

Hello,

I haven't used ADS, but their is a measurement...(I use AWR Microwave Office)
may be go through the tutorials or the following pdf...

In ADS there may be an icon for stability in S-Simulation, if it is not their you can write an equation to extract it from S-parameters
k = {1- |S11|**2 - |S22|**2 + |S11*S22 - S12*S21| **2} / {2*|S12*S21|}

See here...

---manju---

Points: 2
Re: LNA S11

Hi

In ADS u have stabfact in simulation tools, use that for checking the stability

Points: 2
Re: LNA S11

I was under the impression that when S11 (Input reflection coefficient )exceedes a magnitude of 1 then you possibly have an Oscillator.

Re: LNA S11

Its the stability factor which u should consider...
k>1 or mu>1 is the condition for stability ( No oscillations)

Points: 2
Re: LNA S11

i agree with u.
|Г|>1,then the system is unstable

darcyrandall2004 said:
I was under the impression that when S11 (Input reflection coefficient )exceedes a magnitude of 1 then you possibly have an Oscillator.