Intermodulation
Understanding Intermodulation Distortion
Intermodulation is the primary distortion mechanism in multi-signal RF environments. Unlike harmonics (which fall at integer multiples of a single frequency), IM products fall at sum and difference combinations of multiple input frequencies. Third-order products are particularly dangerous because they land near the input frequencies.
IM Product Frequencies
- Second-order: f1+f2, f1-f2, 2f1, 2f2. Usually far from the signal band and easily filtered.
- Third-order: 2f1-f2, 2f2-f1, 2f1+f2, 2f2+f1. The difference products (2f1-f2, 2f2-f1) fall closest to f1 and f2 and are the hardest to remove.
- Fifth-order: 3f1-2f2, 3f2-2f1. Even closer to the fundamentals than third-order.
IM3 and Two-Tone Testing
Two signals of equal power are input. The third-order IM products are measured relative to the fundamental tones. IM3 level increases at 3x the rate of the fundamental (3 dB per 1 dB of input increase). This rate of increase defines the IP3 intercept point.
Example: f1 = 10.0 GHz, f2 = 10.1 GHz
IM3: 9.9 GHz (2x10.0-10.1) and 10.2 GHz (2x10.1-10.0)
IM3 level vs input power:
IM3 (dBc) = 2 x (P_in - IIP3) for small signals
IIP3 from measurement:
IIP3 = P_in + deltaP/2
where deltaP = P_fund - P_IM3 (dB)
Frequently Asked Questions
What is intermodulation distortion?
Intermodulation occurs when two or more signals mix in a nonlinear device, creating new signals at mathematical combinations of the original frequencies. Third-order products (2f1-f2, 2f2-f1) are most problematic because they fall close to the desired signals.
Why are IM3 products the most important?
Third-order IM products fall close to the original signal frequencies. For example, two signals at 10.00 and 10.01 GHz produce IM3 at 9.99 and 10.02 GHz, right next to the original signals. These cannot be removed by filtering and directly interfere with desired signals.
How do you reduce intermodulation?
Use more linear amplifiers (higher IP3), operate amplifiers well below compression, use balanced amplifier configurations (which cancel even-order products), and minimize the number of nonlinear components in the signal path. Digital pre-distortion can also reduce IM in transmitters.