Radio Frequency Interference
Interference probability is based on the potential power transfer densities involved due to the proximity of equipment and antenna systems; the various transfer mechanisms, and equipment performance. The electromagnetic transfer mechanisms may vary depending on modes of operation, propagation conditions, and other variables. The propagation paths that exist for signal transfer from the transmitters to a receiver within the RF environment of a radio communications band can be numerous. Antenna-to-antenna coupling parameters may vary depending on antenna gain, directivity, beam width, side lobes, polarization, separation, propagation conditions of the path (path loss), etc. The receiver characteristics which influence performance include noise, dynamic range, sensitivity, selectivity (RF, IF), desensitization, adjacent signal susceptibility, intermodulation, cross modulation and spurious response susceptibility. Once a particular type of interference is determined to be likely, any analysis should be limited to its most predominate effects. The following types of interference are applicable to Radio Frequency (RF) communications equipment.
1.0 Receiver Co-Channel Interference
This is defined as undesired signals with frequency components that fall within the receiver’s RF passband and are translated into the Intermediate Frequency (IF) passband via the mixer stage. The interfering signal frequency is equal to the sum of the receiver’s tuned frequency and one half of the narrowest IF bandwidth. These signals are amplified and detected through the same process as the desired signals; therefore, a receiver is very susceptible to these emissions even at lower levels.
Results: Receiver desensitization, signal masking, distortion.
2.0 Receiver Adjacent Signal Interference
This is defined as undesired signals with frequency components which fall within or near the receiver’s RF passband and are translated outside of the IF passband via the mixer stage. These signals must be of sufficient amplitude to produce non-linear effects within the receiver’s RF amplifier or mixer stages. Some of the resulting non-linear response signals may be converted to the IF passband frequency via the mixer stage where they are amplified and detected through the same process as the desired signals. These become similar to co-channel interference signals at this point. The undesired emissions which are translated outside of the IF passband may still pass through the remaining receiver stages, if at high enough levels to survive the out-of-passband attenuation. They may then be processed by the detector. The predominant response for this case is desensitization.