Suppose I have an unknown system and I would like to know its linearity.
I am confused by the proper measurement of the IIP3. The gain is defined as "total output power" divided by "total input power".
By sweeping a single input tone and obtaining a the power of the single output tone, I know that the gain is about 24.
But to create an IIP3 plot, one uses TWO tones and measures the power of ONE output tone and ONE IM3 (e.g., https://www.maximintegrated.com/en/app-notes/index.mvp/id/5429).
Option 1: Use one output bin and subtract 6dB from the total input power (since it consists of two tones)
- The gain is correct
- The IIP3 is worse than I would expect
- The input power is the DUT is actually wrong ... in reality it is twice as powerful (since I have two tones but only consider one).
Option 2: Add the power of both output signal bins and both output IM3 bins (in linear domain) - use total input power:
- The gain is off by 3dB
- The IIP3 comes closer to the region I would hope for
- I do not like that because the amplifier is frequency dependent and in general, the two tones (as well as the two IM products) can be (slightly) different.
Option 3: Use one output bin and one IM3 output bin and total input power
- The gain is off by 6dB (This is expected because I am considering only half the power at the output than I have at the input)
- The IIP3 is closer where I could hope it to be
Option 4: Use total input power. For output power, use sum (in linear units) from both signal bins ... for the IM power, only use one bin
- The gain is again off by 3dB
- The IIP3 is where I would hope :-)
- It does not sound right to take the sum of the output fundamentals but only consider one output IM3.
In any case, none of these 4 options looks "correct" to me.