Intermodulation Distortion
Understanding IMD
IMD is the most important distortion mechanism in multi-signal RF systems. Any two signals passing through a nonlinear device generate intermodulation products at frequencies mf1 +/- nf2. The third-order products (m+n=3) are the most problematic because they fall closest to the desired signals.
IMD Products
- 2nd order: f1+f2, f1-f2, 2f1, 2f2. Usually far from signal band, easily filtered.
- 3rd order: 2f1-f2, 2f2-f1. Fall near the input signals. Cannot be filtered. Most problematic.
- 5th order: 3f1-2f2, 3f2-2f1. Even closer to signals but lower level.
IMD Level vs Input Power
Third-order IMD products increase 3 dB for every 1 dB increase in input signal level. This 3:1 slope allows extrapolation to the theoretical third-order intercept point (IP3).
IM3 grows at 3 dB / 1 dB input (3:1 slope)
IM5 grows at 5 dB / 1 dB input (5:1 slope)
IMD from IP3:
IM3 (dBc) = 2(P_in - IIP3)
Example: IIP3 = +20 dBm, P_in = -10 dBm
IM3 = 2(-10 - 20) = -60 dBc
Frequently Asked Questions
What is IMD?
IMD is the creation of unwanted frequencies when two or more signals pass through a nonlinear device. Third-order IMD products (2f1-f2, 2f2-f1) fall near the desired signals and cannot be filtered, making them the most problematic form of distortion.
How is IMD measured?
Two equal-amplitude signals (two-tone test) are applied to the device. The IMD products are measured relative to the desired signals in dBc. IM3 is typically 30-70 dBc below the carriers depending on the device linearity and signal level.
What is the relationship between IMD and IP3?
IMD level (dBc) = 2 x (Pin - IIP3). Lower IMD (more negative dBc) indicates better linearity. Doubling the input power increases IM3 by 6 dBc (worsening). IP3 is the extrapolated point where IM3 would equal the carrier.