How do I ensure my RF device meets the out of band emission requirements of its regulatory class?

Out-of-band (OOB) emissions are unwanted emissions immediately outside the necessary bandwidth of an RF transmission, while spurious emissions are all other unwanted emissions farther from the carrier. Every regulatory framework (FCC, ETSI, ITU) specifies limits on both. FCC general spurious emission limits: for intentional radiators, spurious emissions below 1 GHz must not exceed -43 dBm (20 μW). Spurious emissions above 1 GHz must not exceed -46 to -53 dBm depending on the Part. Many specific Parts have additional, tighter requirements. Ensuring compliance requires: (1) Output filtering: low-pass or bandpass filters after the final amplifier to suppress harmonics. Second and third harmonics of the fundamental are typically the strongest spurious emissions, requiring 40-60 dB of filtering. For a 900 MHz transmitter: the second harmonic at 1800 MHz and third harmonic at 2700 MHz must be suppressed to below -43 dBm. If the transmitter output power is +30 dBm (1W), the filter must provide at least 73 dB rejection at the second harmonic. (2) PCB shielding: inadequate shielding allows the PA to radiate harmonics directly from the PCB, bypassing output filters. A metal can over the PA section provides 30-50 dB of additional isolation. (3) Power supply decoupling: switching regulator harmonics can couple into the RF chain, creating spurious emissions at frequencies unrelated to the RF carrier. (4) LO leakage: in superheterodyne transmitters, LO feedthrough creates spurious outputs at the LO frequency.