Minimizing Intermodulation Distortions in Power Amplification and Processing of Radio Frequency Signals in Navigation and Communication
Intermodulation frequency components may occur in power amplification cascades and in passive signal processing channels with a continuous spectrum, which are not available in their input signals. Cross-distortion of transmitted information occurs in amplifying the power of several modulated signals in the common frequency band. They manifest themselves in that a strong signal asynchronously suppresses a weak one, and that the total output power has a decreased level depending on the ratio of signal levels in the channels. To overcome possible contradictions in choosing the active element operating mode in regard of the saturation level, a compromise should be reached between increasing the energy efficiency of the active element, the permissible level of intermodulation in the operating frequency band, and the level of interference in the adjacent frequency bands in accordance with the electromagnetic compatibility (EMC) requirements. The methods used for estimating the power level of intermodulation products in amplifying microwave signals, namely, the one based on a two-tone signal, the one involving a multifrequency test; the one based on the relative noise interference level in the working band (noise power rate, NPR), the method of interference in the adjacent frequency band (ACPR), and the distortion phase diagram method for signals with phase manipulation. The methods used to linearize power amplifier cascades, including the use of feed-forward and feedback circuits, and pre-distortion of amplitude and phase characteristics are arranged in a systematic order. The parameters of adaptive pre-distorting linearizing circuits correcting the AM/AM amplitude compression and AM/FM amplitude-phase conversion are given. Passive intermodulation (PIM) effects that are present in the signal transmission path components give rise to unacceptable interference for the receiving channels operating in adjacent frequency bands. Recommendations on choosing the technology for manufacturing passive components minimizing the level of such interference are formulated. The parameters and characteristics of laboratory instruments for analyzing passive intermodulation levels, as well as and portable testers used for locating the sources of intermodulation interference of different orders and for measuring its level according to a standardized method are compared.