Determining Federal Permissive Change Thresholds for Transceiver Software Updates
Software updates altering RF output power, occupied bandwidth, duty cycle, or band limits exceed Class I limits and demand Class II or Class III filings.
EIRP Drift
Meaning
Transmitter performance variations during continuous operation can result in changes to the calculated or measured radiated power in a specific direction. The occurrence of eirp drift refers to the fluctuation of the equivalent isotropically radiated power from its nominal calibrated level over time or across different environmental conditions. This variation is typically caused by temperature changes in the power amplifier or impedance mismatches in the antenna system.
Thermal Influence
Power amplifiers generate heat during transmission, which changes the gain of the active semiconductor devices. As the temperature rises, the amplifier gain often decreases, leading to a reduction in the output power of the transmitter. This thermal effect must be compensated for in the design of the radio module.
Compensation Method
Calibration routines stored in the device firmware adjust the transmitter gain dynamically based on readings from internal temperature sensors. These routines apply correction factors to the digital-to-analog converter to maintain a stable output. This adjustment prevents the radiated power from exceeding regulatory limits or falling below the level required for a stable wireless link.
Measurement Protocol
Test laboratories evaluate the stability of the output power during extended transmission periods under extreme temperature conditions. The measurements are performed in an anechoic chamber to isolate the device from external reflections. The maximum drift allowed is defined by the relevant standards.