Designing Regional RF Front End Components for Multi SKU Hardware Deployment
Standardizing RF front-end pad layouts allows single-PCB hardware deployments across distinct regional sub-GHz band plans without board respins.
Spurious Domain
Meaning
Frequency range lies outside the transmitter’s allocated bandwidth where unintended signal emissions must be kept below strict regulatory limits. Emissions falling within the spurious domain are caused by non-linearities in the power amplifier, mixer products, or digital clock noise. These unwanted signals can cause interference with adjacent radio services if they are not suppressed.
The boundary of this domain is defined by international standards and typically begins at several times the channel bandwidth offset from the carrier frequency.
Emission Limit
Regulatory bodies enforce strict limits on the power level of any signal detected within this outer frequency band. Failing to meet these requirements during compliance audits will prevent the device from being sold. Designers must analyze the output spectrum during the development phase to ensure that the spurious domain emissions remain below the specified limits.
This is especially critical for multi-mode devices that transmit on several bands, as the risk of intermodulation is much higher.
Filter Requirement
Bandpass filters are placed between the power amplifier and the antenna to attenuate these out-of-band signals. The filter must provide high rejection at the spurious domain frequencies while maintaining low insertion loss at the desired transmission frequency. This requires a carefully designed RF front end with high-quality components.
Measurement Boundary
Compliance testing defines the starting point of this region based on the channel bandwidth and the center frequency of the transmitter. In most wireless standards, the transition from the out-of-band domain occurs at a standard offset of two hundred and fifty percent of the channel bandwidth. This boundary ensures that the test results are repeatable across different test laboratories.