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Post layout simulation results differ from prelayout simulation- S21 is negative

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sp.bhuvana

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Hi,
Iam a beginner in cadence Virtuoso...
Iam working on LNA, In prelayout simulation, S11 = S22 < -10dB; S21 = 20dB; S12 <-30dB;
In post layout Simulation,
S11 = S22 < -10dB; S21 = -10dB; s12 < -30dB
Iam getting negative gain...
Can anyone tell me what may be the reason for negative gain?? Kindly help me...
 

We cannot judge without information of layout and its' parasitic elements.If especially the frequency is too high, layout parasitic components which have been extracted may play very important degradation role.
 

BigBoss said:
We cannot judge without information of layout and its' parasitic elements.If especially the frequency is too high, layout parasitic components which have been extracted may play very important degradation role.
Click to expand...

Hi,
The Layout of LNA is designed for 5GHz
Herewith i have attached the LNA layout and RCX image for your reference,
Kindly check it..
Pls tell me what should i do???
 

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  • lna-rcx.png
    lna-rcx.png
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Now you need to look at the extracted netlist and carefully reason about the parasitics. Find the aggressor. With that new information, you can go back to your layout and apply changes to overcome parasitics. This is analog layout 101. Iterate, iterate, iterate.
 

How have you extracted the layout parasitics? I also see inductors in your layout.. How have you extracted them?
 

The Interconnections are very thin as I could see..They will bring extra inductances.
Have you ever simulated the whole layout in a EM Simulator ?? This is very important because Extraction Tools cannot predict Inductances accurately.
I recommend you to simulate whole layout in ADS using with RFPRO feature..
 

First thing to check is - whether your inductors were extracted as R parasitics, or defined as p-cells or cells that are blackboxed for extraction.
If it's the former - you have a big problem, because now instead of inductance you have resistance.
 

BigBoss said:
The Interconnections are very thin as I could see..They will bring extra inductances.
Have you ever simulated the whole layout in a EM Simulator ?? This is very important because Extraction Tools cannot predict Inductances accurately.
I recommend you to simulate whole layout in ADS using with RFPRO feature..
Click to expand...

Hi,
As u said, i have increased the thickness of routing lines, my gain increased to positive(s21=7dB(In prelayout simulationS21 = 20dB); but frequency shifted to 7.9GHz)... still working on it...
Thank you...
 

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