Radiated Spurious Emissions Mitigation in Multi Radio System Design
Radiated spurious emissions in multi-radio designs demand aggressive board-level isolation and predictive intermodulation filtering to secure market approvals.
Intermodulation
Meaning
Signal distortion occurs when two or more distinct frequencies pass through a non-linear component or transmission medium. The phenomenon of intermodulation generates new, unwanted frequencies that correspond to the sum and difference of the original inputs and their harmonics. These generated signals can fall directly into the operating band of adjacent wireless receivers and cause severe reception issues.
This electrical behavior ceases to be a design obstacle when signal paths operate strictly within their linear regions and input power levels remain low enough to prevent amplifier saturation.
Spectral Behavior
Third-order products are particularly troublesome because they appear closest to the original carrier frequencies. When signals at two adjacent frequencies are amplified, the resulting third-order terms cannot be removed by simple filtering. The power of these third-order products increases at three times the rate of the fundamental signal power.
Circuit Impact
Low-noise amplifiers and mixer stages in transceivers are highly susceptible to performance degradation from these unwanted mixing products. A receiver facing strong out-of-band signals can experience desensitisation as the mixing products mask weaker, desired signals. This issue is quantified by the third-order intercept point, which defines the theoretical limit where the distorted products equal the fundamental signal power.
Mitigation Method
Addressing this degradation requires high-linearity components and proper impedance matching. Loose connectors or oxidized metal contacts in the RF path act as non-linear junctions that generate passive intermodulation. Rigorous cleaning, correct torque application and high-quality materials ensure that these distortion products remain below the noise floor.