Intermodulation
Understanding Intermodulation
Intermodulation is the most important distortion mechanism in multi-signal RF systems. When two or more signals pass through a nonlinear device, new signals appear at frequency combinations that can fall in-band and interfere with desired signals.
Intermodulation Products
- Second-order (IM2): f1+f2 and f1-f2. Usually out of band. Suppressed by balanced topologies.
- Third-order (IM3): 2f1-f2 and 2f2-f1. Close to the desired signals. Cannot be filtered. Most critical.
- Fifth-order (IM5): 3f1-2f2, etc. Weaker but still relevant for high-dynamic-range systems.
IP3 (Third-Order Intercept)
IM3 (dBc) = 2 x (P_in - IIP3)
At IIP3 = +20 dBm and P_in = 0 dBm:
IM3 = 2 x (0 - 20) = -40 dBc
Cascaded IP3:
1/IP3_total = 1/IP3_1 + G1/IP3_2 + G1*G2/IP3_3
Frequently Asked Questions
What is intermodulation?
Intermodulation generates unwanted signals at sums and differences of input frequencies due to nonlinearity. Third-order IM3 products (2f1-f2, 2f2-f1) are most critical because they fall close to the desired signals and cannot be filtered.
What is IP3?
IP3 (Third-Order Intercept Point) is the hypothetical input power where IM3 products would equal the fundamental. Higher IP3 = better linearity. IIP3 is referenced to input; OIP3 to output. OIP3 = IIP3 + gain.
How does IP3 cascade in a receiver chain?
Each stage contributes to the system IP3. The last stage (highest-level signals) typically dominates. 1/IP3_total = sum(Gi/IP3_i) where Gi is the gain preceding stage i. Attenuators and mixers with high IP3 help preserve dynamic range.