Meaning
Unwanted radio frequency emissions arise when a strong interfering signal mixes with a modulated transmitter signal in a non-linear active component. These unwanted outputs are referred to as cross modulation spurious emissions when they fall within adjacent channel allocations. The phenomenon occurs in multi carrier environments or when a powerful external transmitter operates close to a co-located transmitter.
Mixing Process
Active semiconductor devices such as power amplifiers or mixers generate these spurious products when driven into their non-linear regions by multiple high power signals. The non-linear behavior of the device acts as a frequency mixer, combining the modulation of the desired carrier with the amplitude variations of the interferer. This creates sidebands that copy the modulation profile of both signals across a broader frequency band.
In multi radio IoT gateways, this mixing can compromise both the local cellular uplink and the co-located satellite receiver module simultaneously.
Bandwidth Impact
Receiver desensitization occurs when these spurious signals land directly on the receive channel of an adjacent radio system. The resulting noise floor elevation reduces the effective operating range of the affected receiver. Filtering at the antenna port offers limited protection because these emissions often sit too close to the operating frequency to be attenuated without also degrading the primary signal.
Mitigation Strategy
Linearity optimization in the power amplifier stage remains the primary method to control these spurious outputs. Designers use digital pre-distortion to linearize the amplifier response and minimize the generation of unwanted mixing products. Correct configuration of the bias voltage also ensures the amplifier operates within its linear range even under peak power conditions.
