In theory, the mixer's IP3 is related to the semiconductor material, the circuit topology, the manufacturing method, frequency range and LO level.
The LO works as a switch to turn the mixer on and off, higher LO level usually has higher IP3 because the mixer switches on/off more perfectly to provide higher linearity, in addition, the higher LO level has less conversion loss as well. This sounds weird since the mixer itself is a non- linear device
The third interception point(IP3) specification from the manufacture is calculated from the measurement of the third Intermodulation products (IM3), refer this equation:
IP3=(3*Pin-Pim3)/(3-1)
Here Pin is input power of RF signal f1 and f2 in dBm(typical level is -20dbm to -10dbm), Pim3 is the power of IM3 in dBm.
When measure the IM3 (two inputs f1, f2 are in same level), be aware that the spectrum analyzer itself has IM3 too, even between the signal generators, the Intermodulation can be found. So you have to think of some way to get rid of them, for example, using filters between the under tested mixer and spectrum analyzer, etc.
Some +7dbm mixer do have higher IP3 than +13dbm ones, such like Mini-circuits ADE-12 (IP3=17dbm, level 7), MBA-9MH (IP3=15dbm, level 13), this is because of using different technical approach.
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