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Post subject: IIP3 measurement Posted: Sun Aug 28, 2005 3:53 pm
I want to measure IIP3 of an amplifier in spice ( LTSpice ) , how can I measure it ? Please help me ,
Post subject: IIP3Posted: Sun Aug 28, 2005 9:49 pm
You will have to derive the IIP3 in much the same way as you would measure it in a lab. (The "Distortion" calculation of SPICE is basically worthless). You'll have to do (at least) two "Transient Analyses". That means "at at least two different power levels", and then use the FFT capabilities of LTSpice to measure the amplitudes of the resulting 3rd order IM products. The more analyses you do, the better your eventual curve-fitting exercise will be. Plot the level of the 3rd order IM products as a function of the input power, and estrapolate to the IIP3. This will be a regression analysis if you use more than two points. And, of course, it's usually plotted in dBm vs dBm (which makes it a log-log curve as far as power in Watts is concerned).
You will need an accurate model of your active device(s) - not all available models will give you realistic results.
I hope this helps.