Meaning
Unwanted radiations from an electronic device appear at frequencies outside the necessary bandwidth and can interfere with other communication services if not properly filtered. These spurious emissions are a byproduct of the non-linear behavior of amplifiers, oscillators and digital switching circuits. They are distinct from the intentional transmissions on the main carrier frequency and are subject to strict limits set by international regulatory bodies.
Measuring these signals ensures that a product does not disrupt television, radio or emergency networks. The requirements apply to both the conducted power at the antenna port and the radiated field from the enclosure.
Interference Source
High speed digital circuits and switching power supplies are common origins of unwanted electromagnetic noise within a system. When a signal travels along a trace on a printed circuit board, it can act as a miniature antenna and radiate energy into the surrounding environment. If the spurious emissions are not controlled at the source, they can couple into the radio frequency path and be amplified by the transmitter.
This leads to a degradation of the signal quality and an increase in the interference for nearby devices. Designers use decoupling capacitors, ferrite beads and proper grounding techniques to minimize the generation of these unwanted signals. The mechanical fit of the enclosure also plays a role, as gaps in the shielding can allow noise to leak out.
Regulatory Limit
Agencies like the federal communications commission define the maximum allowable levels for any signal that falls outside of the assigned frequency band. These spurious emissions must be kept very low to protect the integrity of the radio spectrum for all users. The limits are typically expressed in decibels relative to the carrier power or as an absolute power level in dBm.
During the certification process, the device is tested across a wide range of frequencies, sometimes up to several gigahertz, to identify any peaks. If a device exceeds the limit, it cannot be sold until the design is modified to reduce the emissions. These rules are non-negotiable and are enforced through market surveillance and laboratory testing.
Suppression Method
Reducing the unwanted radiation requires a combination of good board layout and effective electromagnetic shielding. Filtering the input and output lines of the power supply can prevent noise from being conducted along the cables. Adding a metal shield over the sensitive radio components can block the radiated spurious emissions from leaving the board.
The use of differential signaling for high speed data helps cancel out the electromagnetic fields, which reduces the overall noise level. Engineers also use software techniques, such as spread spectrum clocking, to distribute the energy of a clock signal over a wider frequency range, lowering the peak value of any single emission. Testing the effectiveness of these methods in an anechoic chamber provides the data needed to confirm compliance.
A successful design balances the need for high speed performance with the requirement for a clean electromagnetic profile. This ensures that the device is a good neighbor to other wireless systems in the environment.
