Determining Permissive Change Classification for Encapsulated Radio Modules
Modifications to encapsulated radio modules require Class II permissive change filings whenever conducted power increases or radiated emissions degrade.
Harmonic Spurious Emissions
Meaning
Radio-frequency transmitters generate energy at specific intended frequencies, but they also produce unwanted signals at multiples of those frequencies. The harmonic spurious emissions are the radiated or conducted signals occurring at integer multiples of the fundamental carrier frequency. These emissions must be suppressed to prevent interference with other communication services.
They are a primary metric checked during regulatory approval.
RF Degradation
Non-linearities in power amplifiers and mixers are the primary source of these unwanted signals. The harmonic spurious emissions can distort the transmitted signal and leak into adjacent frequency bands, causing communication errors for nearby receivers. This leakage reduces the overall efficiency of the transmitter by wasting energy on frequencies that cannot be used.
Filter Application
Suppressing these unwanted frequencies requires inserting specialized component blocks in the transmit chain. Low-pass or band-pass filters are placed between the power amplifier and the antenna to attenuate the harmonic spurious emissions while allowing the primary signal to pass with minimal loss. This filter block must be designed with high attenuation at the second and third harmonics, which are typically the strongest spurious signals generated by the non-linear amplifier.
The layout of the filter on the board is critical, as any parasitic capacitance can bypass the filter elements entirely.
Compliance Test
Regulatory bodies such as the Federal Communications Commission enforce strict limits on the power levels of these out-of-band signals. Lab technicians measure the harmonic spurious emissions using a spectrum analyzer while the transmitter operates at maximum power. This measurement confirms that the device will not cause harmful interference when deployed in the field.