Meaning
Regulatory standards defining the maximum allowable power spectral density of a transmitter at specific frequency offsets ensure that wireless devices do not interfere with adjacent communication channels. Achieving spectrum mask compliance is a mandatory step for certifying any radio frequency transmitter for commercial use. The limits are set by international bodies such as the FCC and ETSI.
Measurement Method
Spectrum analyzers measure the output power of the transmitter across a wide span of frequencies around the center channel. The measured power must remain below the defined limit line at all frequency offsets. This test is performed under worst-case modulation patterns and maximum transmit power.
If any portion of the signal exceeds the mask, the device cannot be sold.
Linearity Impact
Amplifier non-linearities create intermodulation products that spread the signal energy into adjacent channels. This spectral regrowth is the primary cause of mask failures in modern digital transmitters. Designers use digital pre-distortion techniques to linearize the amplifier response and reduce these out-of-band emissions.
High-quality power amplifiers are selected to maintain linearity over the full output range.
Filter Design
Bandpass filters are used to suppress any remaining out-of-band energy before the signal reaches the antenna. These filters must have a sharp roll-off to meet the mask requirements while keeping insertion loss to a minimum. Higher-order filters provide better suppression but add cost and board area.
The final filter design must balance suppression with signal attenuation.
