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Design >> RF Design >> simulating P1dB of lna for capacitive loads in LNA:defining port
https://designers-guide.org/forum/YaBB.pl?num=1321371471

Message started by paddu on Nov 15^th, 2011, 7:37am

Title: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by paddu on Nov 15^th, 2011, 7:37am

how to simulate the P1dB of LNA for capacitive loads in cadence? How to define the output port if it is capacitive load? your help is greatly appreciated

Title: Re: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by rfidea on Nov 15^th, 2011, 7:58am

If your LNA is not supposed to deliver power, as if you only has a capacitive load, there is no point in discussing P1dB. But you can discuss and simulate Vout1dB, the 1db compression point but in voltage. Just increase your input signal in dB steps and watch where your output has dropped 1dB from the small signal linear line.

Title: Re: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by paddu on Nov 15^th, 2011, 8:29am

Hi rfidea,
Thanks for your reply. What is the best way to characterize the lna standalone interms of IIP3, when LNA is driving MIXER(as mixer input impedance is capacitive only)

-suresh

Title: Re: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by loose-electron on Nov 15^th, 2011, 5:31pm

inject two tones in a PSS analysis, look at the spectral results and measure dB down of the spurs

Title: Re: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by cct_learner on Nov 15^th, 2011, 10:14pm

I am assuming that your LNA is connected to an external source with a particular impedance, usually 50 ohm. In that case, the input will be a PORT component and input is specified in dBm. Assuming your signal is still well below compression point, you can calculate IIP3 by

IIP3 = Pin + IM3/2

Title: Re: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by aaron_do on Nov 15^th, 2011, 11:06pm

He is asking how to characterize IIP3 for capacitive loads, not 50 ohm...

IIP3 is input referred. Therefore, the input is a power level. You can take the input power which causes the IM3 (voltage) to equal the fundamental voltage. If you want an output referred value, you need to look at a voltage level.

regards,
Aaron

Title: Re: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by cct_learner on Nov 16^th, 2011, 9:23am

When you are talking about IIP3 of an LNA which is fed by a source with certain impedance, the load impedance does not matter as far as the LNA is loaded by that load impedance. The IIP3 is simply

IIP3 = Pin + IM3/2

where IM3 is the magnitude of the IM3 components with respect to the fundamental in dB.

To find IM3, just run PSS or Harmonic analysis and plot the 20log10(Vout) and find the dB difference between the IM and the fundamental component. Pin is just the input power that produces the IM3 measured.

Title: Re: simulating P1dB of lna for capacitive loads in LNA:defining port
Post by aaron_do on Nov 16^th, 2011, 4:54pm

Hi cct_learner.

sorry for the mistake, I misread your original post,

Quote:

I am assuming that your LNA is connected to an external source

for some reason I read it as connected to an external load.

regards,
Aaron