Reduction of amplitude modulation interference in digital transmission

The plain-old-telephone-system (POTS) is now increasingly used to carry high-speed data such as for Internet purpose. However, problem can arise if the telephone network is in close proximity to an AM radio transmitter, transmitting high power signal at a frequency, which overlaps the bandwidth of the digital transmission. The external electromagnetic field can induce currents and voltages in the telephone network, which might results in network failure. This paper presents a study on how the electromagnetic waves entered the system, investigated how the system is affected by it and how the effects can be reduced. A GTEM Cell was used to generate a 576 kHz electric field intensity varying from 1V/m to 15 V/m with 80% Amplitude Modulation. The electric field was imposed on a section of a cable and the Bit Error Rate (BER) is noted using Data Tools 5000. It was deduced that the relationship between electric field and BER is given by the expression BER = 2 x 10(-5)e(0.399 vertical bar(E) over bar vertical bar) + 2.48 x 10(-5). Shielding technique was employed using four conducted materials (soft steel, hard steel, aluminum and copper). Measured results showed that copper (a, = 5.87 x 10(7) S/m, mu(r) =1) reduce up to 70% of the BER.