Re-Evaluating Radiated Emissions and Exposure for Potted Transmitter Housings
Encapsulating transmitters alters dielectric loading, detuning antennas and shifting RF exposure boundaries, demanding re-evaluation of emissions and SAR compliance.
Gain Suppression
Meaning
Nonlinear effects reduce the amplification factor of an active device when input signals reach high power levels. The occurrence of gain suppression marks the transition from linear operation to saturation. Efficiency drops as energy is converted into heat.
Signal Saturation
Compression points are used to quantify the level at which the output power deviates from the ideal linear curve. In systems where gain suppression is present, the P1dB point indicates a one decibel drop from the small signal gain. This value defines the usable dynamic range of the radio frequency chain.
Signal peaks are clipped which results in bit error rate increases. Harmonic content rises as the device reaches its physical voltage or current limits. Measuring these deviations requires a power sweep across the intended operating frequency.
Performance Boundary
Operating beyond the linear region causes intermodulation products to appear in adjacent frequency bands. Avoiding gain suppression is necessary for maintaining signal quality in modulated waveforms like QAM. Filtering cannot remove the in band noise generated by the saturated stage.
The device must be backed off from its peak power to restore linearity.
Design Constraint
Thermal management becomes more difficult when an amplifier enters the compressed state. Because gain suppression involves a loss of efficiency, more DC power is dissipated as heat within the semiconductor. Active cooling or larger heat sinks are often required to prevent thermal runaway.
The design process balances power output against the distortion limits set by the communication standard.