Meaning
Electronic performance metrics describe the ability of a receiver to reject interference caused by the transfer of modulation from an unwanted signal to a desired carrier. This specific cross modulation immunity level defines the signal strength of an interferer that results in a measurable degradation of the intended communication link. The test involves applying a modulated interfering signal at a frequency offset while monitoring the output for spurious modulation.
Interference Threshold
Receiver circuits must maintain a high degree of linearity to prevent non-linear components from mixing the envelopes of two distinct signals. When cross modulation immunity is low, the audio or data information from a loud neighbor channel appears on the quiet desired channel. This effect remains independent of the carrier power if the non-linearity occurs in the early stages of the front end.
Evaluation Protocol
Measurement of this property requires two signal generators and a spectrum analyzer to observe the depth of the transferred modulation. Technicians increase the power of the undesired signal until the modulation on the desired carrier reaches a predefined percentage, typically one percent or three percent. Robust shielding and filtering at the input stage prevent the unwanted energy from reaching the first active mixer or amplifier.
Mitigation Strategy
Improving the third order intercept point of the amplifier usually increases the resistance to this type of interference. Filtering out of band signals before they reach active components ensures that the receiver remains focused on the assigned frequency range.
