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Design >> RF Design >> iip3 simulation vs. calculation
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Message started by YYou on Mar 3^rd, 2015, 1:13pm

Title: iip3 simulation vs. calculation
Post by YYou on Mar 3^rd, 2015, 1:13pm

Hello,

I am simulating a chain with mixer + PA driver. I simulated voltage gain and IIP3 for mixer and PA driver, respectively.
Then I connected them together, and simulated the whole chain. The IIP3 is about 5 dB lower than that calculated using cascaded IIP3 equation.
I tried to figure out why, but no luck.
Does someone have any suggestions ? Did I miss sth. ?

BTW, the cascaded IIP3 calculation should be correct. I used them before to do sth. similar. Proved good.

Thank you.

Title: Re: iip3 simulation vs. calculation
Post by aaron_do on Mar 4^th, 2015, 7:08am

Hi,

just FYI, the cascaded IIP3 equation is a worst case calculation. It only works when the phases of the IM3 and fundamental components line up nicely. This is obviously not the source of your problem however as your simulated IIP3 is worse than what was calculated. I think you need to provide more information. For example, are you sure it isn't a loading issue? Does the curve look well behaved? How did you simulate individual IIP3? In order to be accurate, you need to use the correct source/load impedances for the individual blocks.

You could do an ideal simulation where you cascade the blocks, and then probe the internal node in between the blocks and check the IIP3 of the mixer. For the PA driver, you could inject the input signal at the internal input node while the system is still cascaded.

regards,
Aaron

Title: Re: iip3 simulation vs. calculation
Post by totowo on Mar 12^th, 2015, 12:45am

There may be two reason:
1. the load impedance (PA as load of mixer) is not constant but a function of frequency.
IM3 production depends on not only the impedance at fundamental frequency, but also impedance at f1-f2, f1+f2, 2f1, 2f2.

2. Besides 3rd nonlinearity, 5th nonlinearity could also generate IM3. PA always shows a strong nonlinearity.

Simply caculation ignore these effects.