IP3
Understanding IP3
IP3 is the universal metric for characterizing the linearity of RF amplifiers, mixers, and receivers. When two or more signals at different frequencies pass through a nonlinear device, they interact to create intermodulation (IM) products at new frequencies. The third-order products (at 2f1-f2 and 2f2-f1) are the most problematic because they fall close to the original signals and cannot be easily filtered out.
Input IP3 vs Output IP3
- IIP3 (Input IP3): Referenced to the input power level. Used for receivers and cascaded systems. IIP3 = OIP3 - Gain.
- OIP3 (Output IP3): Referenced to the output power level. Typically specified for amplifiers. OIP3 = IIP3 + Gain.
IP3 in System Design
In a cascaded system, the total IP3 is degraded by each stage. High-gain stages before low-IP3 stages (like mixers) amplify the two-tone signal, driving the mixer harder. The cascade IIP3 formula accounts for the gain of preceding stages.
IM3 frequencies: 2f1-f2 and 2f2-f1
IP3 from two-tone measurement:
IIP3 = P_in + (P_fund - P_IM3) / 2
OIP3 = P_out + (P_fund - P_IM3) / 2
Typical relationship:
OIP3 = P1dB + 10 to 12 dB
Cascade IIP3:
1/IIP3_total = 1/IIP3_1 + G1/IIP3_2 + G1*G2/IIP3_3 + ...
Frequently Asked Questions
What is IP3?
IP3 is a measure of amplifier or mixer linearity. It represents the theoretical power level where third-order intermodulation products would equal the fundamental signal. Higher IP3 means better linearity and greater immunity to intermodulation distortion in multi-signal environments.
What is the difference between IP3 and P1dB?
P1dB is the actual power where gain drops by 1 dB (compression). IP3 is a theoretical extrapolation used to predict intermodulation distortion. IP3 is typically 10-12 dB higher than P1dB. P1dB characterizes single-signal behavior; IP3 characterizes multi-signal behavior.
Why does IP3 matter for receivers?
Receivers must handle multiple signals simultaneously. If a strong adjacent-channel signal intermodulates with another signal in the front end, the resulting IM3 product can fall on the desired channel and cause interference. Higher receiver IIP3 means better rejection of this interference.