A spectral mask, also referred to as a transmit spectrum mask or emission mask, specifies the maximum permissible signal power levels at various frequency offsets from the centre frequency of a transmitted RF signal (e.g., Wi-Fi). It ensures that the transmitted signal remains confined within its allocated channel bandwidth and minimizes out-of-band emissions, thereby preventing interference with adjacent channels or systems.
Mask Margin is the difference between the spectral mask limit (defined by IEEE or regulatory bodies) and the measured power of the DUT at a specific frequency offset.
The purpose is to ensure that the transmitted signal does not leak power into adjacent channels or exceed spectral emission limits. Regulatory standards (like IEEE 802.11 or FCC) define a spectral mask that limits how much RF energy can be present at frequencies outside the main channel.
A higher mask margin = better (more compliant).
- If the limit is -30 dBr at ±20 MHz offset, and the DUT emits -35 dBr:
Mask Margin = 5 dB (DUT is 5 dB below the limit) → PASS - If the DUT emits -28 dBr instead:
Mask Margin = -2 dB → FAIL - If Mask Margin > 0 dB, the transmitter passes the emission requirements with some margin.
- If Mask Margin < 0 dB, it means the emission exceeds the allowed mask (a fail).
A generic diagram for a permitted transmitted spectrum mask at different frequency offset:
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