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Radio Frequency Interference (RFI) Characteristics

Have you ever wondered why the interference that you receive on the radio communications bands can sometimes disrupt the intended signals?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 receiverís 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 receiverís 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 receiverís 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 receiverís 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.

Results:Non linear effects in the RF or mixer stages producing receiver desensitization, intermodulation and cross modulation.

3.0 Receiver Out of Band Interference

This is defined as undesired signals with frequency components that are significantly removed from the receiverís RF passband.High level signals may produce spurious responses in the receiver if mixed with local oscillator (LO) harmonics to produce a signal falling within the IF passband.The spurious responses result from the mixing of an undesired signal with the receiverís LO.The amplitude of these responses is directly proportional to the level of the undesired signals prior to mixing with the LO.The spurious responses in a receiver usually occur at specific frequencies.Any other out of band signals are attenuated by the IF selectivity.

Results:An undesired response created by the mixing of an undesired signal with the LO.The undesired signals which mix with the LO and are capable of being translated to the IF stages are the spurious response frequencies.These frequencies and their interference power levels are a function of the receiverís susceptibility to these responses.

4.0 Transmitter Fundamental Emissions

The transmitterís fundamental output signal includes characteristics of the power distribution over a range of frequencies around the fundamental frequency.These are determined by the base-band modulation characteristics and are represented by a modulation envelope function.The primary parameter associated with the modulation envelope is the transmitterís nominal bandwidth (3dB).This may be derived from the transmitter modulation characteristics (by Fourier analysis), measured, or from the manufacturerís specifications.The power distribution in the modulation sidebands may be represented by a modulation envelope function showing the variation of power with frequency.

5.0 Transmitter Harmonic Emissions

The main concern with a transmitterís harmonic emissions is the undesired signal outputs which are harmonically related to the fundamental signal rather than to other oscillator circuits.The relative power associated with the harmonic emissions may be modeled using data for the particular transmitter type.However, since harmonic output power can vary considerably from one transmitter to another for the same type and model, it should be represented statistically.Harmonic emission models may be derived from statistical summaries of measured data or from manufacturerís equipment specifications.Transmitter spurious emission models for prediction of frequencies above the fundamental are based on harmonic emission levels.The modulation envelope must be represented for harmonics as was done for the fundamental.

6.0 Transmitter Noise

Transmitter noise includes the output spectrum that is a result of the thermal noise generated in the driver and final amplifier stages as well as the synthesizer noise from lower level stages.This is a broad-band noise; however, it usually does not cover the immediate modulation sidebands.The level may be specified as the power per bandwidth as a function of frequency (dBm/Hz).

7.0 Transmitter Intermodulation

These are the undesired signals that result from the local mixing of a transmitterís output emission with that of another transmitter.The mixing usually occurs in the non-linear circuits of a transmitter whose antenna receives a high level of RF from another transmitter antenna in close proximity.The mixing products are radiated by the transmitterís antenna as possible co-channel or adjacent signal interference signals.


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