The intersection point of the fundamental power output and third order spurious
response, when plotted on a log-log scale with output power as ordinate and
input power as abscissa. IP3 is used as a way of estimating the distortion
added to a system by a component.
Intercept point may be referred to the output (common for power amplifiers) or
to the input (used for mixers and LNAs). IP3 can be calculated algebraically
from a measurement of 2-tone third-order intermodulation distortion products
(IM3). For a distortion level IM3 in dBc corresponding to an output of Pout in
dBm resulting from an input power of Pin in dBm,
IP3 out = Pout + IM3 / 2
IP3 in = Pin + IM3 / 2 = IP3 out - Gain
This approach is only valid if the measurement is made in the linear region. It
may yield a slightly different answer from a graphical construction based on
power and distortion measurements at various drive levels, in that it assumes a
perfect 1:1 slope for the Pout-Pin transfer curve and a perfect 3:1 slope for the
IM3-Pout (or IM3-Pin for input IP3) transfer curve. Observed slopes are often
slightly different from these values.
A good rule of thumb is that the output IP3 is 10 dB above the P1dB point of a
component. The range observed is between 8 dB and 20 dB above the P1dB
In regard to mixers, this parameter is highly dependent on the LO and RF
frequency, the LO drive level, and the impedance characteristics of all
terminations at the operating frequency.
See also Third Order Intermodulation Products, Intercept Point