Meaning
Unwanted radio frequency signals generated by a transmitter that fall within the operating bands of adjacent receivers represent a severe source of local interference. These colocation spurious emissions arise when multiple wireless technologies operate simultaneously within the same compact product or enclosure. The measurement of these signals defines the boundaries where a high power transmitter will desensitize a nearby sensitive receiver, guiding the design of shielding and filtering.
Filter Design
Bandpass filters suppress out of band energy before it can reach the antenna port. These components must offer high attenuation in the receive band of the colocated module while presenting minimal insertion loss to the desired transmit signal. Multi layer ceramic filters are commonly used because they offer a small footprint and high thermal stability.
Isolation Strategy
Physical separation of the radiating elements provides the primary defense against mutual interference in a multi radio system. Placing antennas on opposite sides of the printed circuit board reduces the coupled energy, protecting the frontend circuits from saturation. In compact designs where physical distance is limited, orthogonal polarization of the antennas helps maintain the required isolation.
Designers must evaluate the coupling path at each critical frequency to ensure that the transmitted energy does not degrade the noise figure of the adjacent receiver.
Compliance Testing
Regulatory standards specify the maximum permissible power levels for these unwanted signals in shared environments. Testing requires a conducted setup where the transmitter output is connected directly to a spectrum analyzer through a notch filter. The filter prevents the fundamental frequency from overloading the analyzer frontend during the measurement.
