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Originally Posted by G0HZU_JMR
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Since the calculation of IP3 involves the difference between the fundamental and the average of the key troublemaking anti-social neighbours; (2f1-f2)-LO, (2f2-f1)-LO (or upconversion LO-(2f1-f2), LO-(2f2-F1) ), the contribution of IM products of the first-stage mixer of the spectrum analyzer is subtracted out.
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I'm not sure what you mean here, but in the simplistic case of the DUT being an RF amplifier, then the analyser IMD levels need to be about 20dB lower than the IMD levels of the amplifier if the aim is to keep the measurement uncertainty of the amplifier IMD below about +/-1dB. Otherwise, the analyser IMD tones can sum or cancel with the IMD tones of the DUT and cause uncertainty exceeding +/- 1dB.
If the input TOI of the analyser is the same as the output TOI of the DUT, then the DUT IMD and analyser IMD tones will be at the same level, and they generally sum together in the analyser and the analyser will then display the IMD levels about 6dB higher than the true IMD level of the DUT. The tones can also cancel (in theory) and the analyser might show much lower distortion than the true result. That's why the analyser IMD levels ideally need to be about 20dB lower than the IMD levels of the DUT. |
Sorry I was confused and wrong about the subtraction bit. To improve the DANL in IP3 measurement, it is necessary to switch on the pre-amp of the spectrum analyzer, so the displayed IMD readings also include the IMD of the spectrum analyzer's pre-amp. I have been struggling with the idea why not the spectrum analyzer's pre-amp and its mixer IMD do not mess up with the true IMD level of the DUT.
The IP3 is a fictitious figure of merit that linearizes something is really non-linear. At times, I get confused when I try to understand things underneath the surface. The definition of OIP3 is simple but i feel it is deceptive, the more i think about it, the more I realize it is complicated.
Constructive and destructive IMD interference is a completely new concept to me.
Earlier I got confused by these sentences from the link below:
The spectrum analyzer also produces internally generated, third-order distortion products at exactly the same frequencies as those generated by the DUT. These distortion products are relative to the mixer level, rather than the output level of the DUT. This distinction enables the measurement of a DUT with a significantly higher TOI than that of the spectrum analyzer.
https://www.mwrf.com/technologies/te...i-measurements