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Spurious-free dynamic range (SFDR) is two-thirds the
difference between the 2-tone, third-order intercept point (IP3) and the minimum discernible signal (MDS). Referring back
to Figure 1, it can be seen that the 3rd-order product line has a 3:1 slope while
the fundamental output line has a 1:1 slope. Therefore, the 3rd-order products are
always three times as far from the IP3 as the fundamental output. If you use the
SFDR as the power of the output tones (dBm) and plug it into the equation for IP3
calculation from the Intercept Points (IP3 & IP2) section of the RF Cafe web
site, the power of the 3rd-order products will be right at the MDS power level.
Figure 1
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