Testing And Analysis Of FM Broadcast Third-order Cross-modulation Interference

FM broadcast is an important means of covering radio programs effectively, In recentyears, with the rapid development of the national economy and wireless mobilecommunications industry，FM broadcast makes the space electromagnetic environmentincreasingly complex.FM broadcast is one kind of devices which used a wirelesstransmitter to transmit broadcast.In reception, FM broadcast is more susceptible to allsorts of interference, including generated the interference on the aviation system andwireless communication systems, in which the third-order cross-modulation interferenceis particularly serious.Third-order intermodulation is the amplitude modulation of signals containing twodifferent frequencies in a system with nonlinearities. The intermodulation between eachfrequency component will form additional signals at frequencies that are not just atharmonic frequencies (integer multiples) of either, but also at the sum and differencefrequencies of the original frequencies and at multiples of those sum and differencefrequencies. In order to measuring the existence of spurious signals, first, we cantheoretically determined the third-order intermodulation interference signal frequencyband which was probably generated by the two signals of transmitter, then through thereceiving antenna and test equipment in the band, we can confirm and validate thecorrectness of our method. Finally, according to the actual situation through effectivemethod reduces or completely remove the interference caused by spurious signals, finallywe can create a clean and efficient environment for signal transmission system.This paper will take the living example from the the Shilu business district whichinterfered by the third-order cross-modulation interference from the104.8MHz FM radio Suzhou Radio and Television Traffic Economic channel, and theoretically calculates thetwo-tone third-order intermodulation distortion of the nonlinear system and gives thereasonable way of testing third-order intermodulation distortion and the elimination ofthird-order intermodulation distortion, which also means under the shutdown state of104.8MHz, testing the possible frequency by using the third-order intermodulationinterference, with a large number of field test data, we confirm the theoretical inferences,moreover, we analyze and determine the effective way of eliminating the third-orderintermodulation distortion, and finally we will display the result of drawing the spectrum.Actually, this analysis method can also be applied to analyze third-order intermodulationof the other nonlinear systems.