How to calculate the IM3?

1. Suppose PA OIP3=+40dBm
2. At PA output port, one CW is +26dBm.
3. At PA output port, another CW is +14dBm.
4. How to calculate the IM3 of the two CWs at the PA output?

Thank you very much.

The following equations can be used if you are in the small signal region of your amplifier. Note though that once you get close to the P1dB, these relations are not valid and you'll need a more complicated model or actual test data to get IM3.

at 2*f.1 - f.2
IM3 = 2*P.1 + P.2 - 2*IIP3

at 2*f.2 - f.1
IM3 = 2*P.2 + P.1 - 2*IIP3

There are some assumptions to define OIP3.Namely, OIP3 is an "extrapolation point" so that amplifier will never reach to this output power.
-Two carriers will have same amplitude that is at least 10dB below P1dB
-Carriers are as close as possible to observe IM3 products

OIP3=Single Carrier Level+Delta/2
Delta=Single Carrier Level-IM3 Level which is greater

So you can do the math to find IM3 level

But I warn you again... This equation is valid only with the assumptions that we have done...

If you already know the OIP3 and use unequal input levels the equation for IM3 @ (2*f1-f2) is:

IM3 @ (2*f1-f2) [dBm] = P1[dBm] ? 2*[OIP3[dBm] ? P2[dBm]]

As was stated, your amplifier have to be way below compression.

Hi, pstuckey and Bigboss and vfone,
Thank you very much.