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 purposes. However, problems can arise if the telephone network is in close proximity to an AM radio transmitter, transmitting high power signals 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 result in network failure. This paper presents a study on how the electromagnetic waves enter the system, investigate 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 × 10^-5 e^0.399|E~| + 2.48 × 10^-5. A shielding technique was employed using four conducting materials (soft steel, hard steel, aluminum and copper). Measured results showed that copper (σ_r = 5.87 × 10⁷ S/m, μ_r =1) reduced up to 70% of the BER.