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Gain measurement using S21 and transient simulations using Cadence Spectre (Read 3333 times)

Sharath Raju

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Gain measurement using S21 and transient simulations using Cadence Spectre
Dec 31^st, 2012, 6:27pm

Hi,

I am trying to measure the gain of a low noise amplifier designed using MOS technology in Cadence. The LNA is a made of an NMOS in common source configuration operating at 10 GHz (narrow-band). S-parameter simulations reveals the gain S21 to be 10.6 dB or 3.4 V/V.

I wanted to verify the gain value using transient simulations and replaced the input and output ports using "vsin in series with 50 ohm resistance" and a 50 ohm load resistance respectively. I applied a 1 mV amplitude, 10 GHz sine-wave at the input and see a 4.61 mV amplitude sine wave at the output, suggesting a gain of 4.61 V/V

Why is there a discrepancy between S-parameter and transient simulation. ? Am I missing something ?

Attached are the testbenches of the S-parameter and transient simulations.

Thanks,
Sharath

Archive_002.zip

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Sharath Raju

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Re: Gain measurement using S21 and transient simulations using Cadence Spectre
Reply #1 - Jan 3^rd, 2013, 1:21pm

Update: I solved the problem!

First off, I learned that there is more to the measurement of gain than simply measuring the S21. The following link talks about three different gains: transducer power gain, operating power gain and available power gain in the context of LNAs.
http://www.ek.isy.liu.se/courses/tsek03/SpectreRF_LNA533AN.pdf

I followed the above link and re-measured the gain. In the process, I discovered that I had made a few changes to the circuit in between measuring the S21 and transient gain! With the version inconsistency removed, I see an agreement between the values of operating power gain and transient gain �:)

Note: For those who are looking at the numbers in detail, my design does not have output impedance matching. Therefore the available power gain is negative �:o

Gain_LNA_SS.png

« Last Edit: Jan 4^th, 2013, 2:17am by Sharath Raju »

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rfidea Senior Member Offline Posts: 159 Europe	Re: Gain measurement using S21 and transient simulations using Cadence Spectre Reply #2 - Jan 4^th, 2013, 2:15am Thank you for updating the thread! I read it from the beginning and thought the results was strange. What can I say, been there, done that (-:
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