The Study Of Suppressing Co-frequency Ambient From The Measured Signal

The electric communications develop very fast at present. Much more devices of electric and communication become parts of people life, so people keep more attention to the effect in health by the electromagnetic radiations from the devices. The sensitivity of electronic devices become higher, and these devices just work in a low lever of ambient. The technology of electromagnetic compatibility testing is very important in the electromagnetic compatibility subject. That isn't precision measuring the radiated emissions in the present of co-frequency ambient signals. There is no ambient signal inside of the RF anechoic chamber. However, for some larger electrical system, it can't do EMC testing in anechoic chamber by the reason of the size, such as High-Voltage Transmission Lines. There is no appropriate approach of the method of measuring radiated emission in the present of strong co-frequency ambient signals domestic.In this paper, a system based on Digital Signal Processing for measuring electromagnetic precise will be presented. The system removals the co-frequency ambient utilized that the signal uncorrelated with noises. The system comprises two receivers, the primary one operative to receive the co-frequency ambient signals and the radiated emission from the electronic device, and the reference one operative receive primarily the correlated ambient signals. The signals will be down converted and then digitized. Because the ambient signals of two channels are correlated, and the measured signal is uncorrelated with the ambient, the system removal the same frequency noises from the radiated emission utilized the adaptive interference cancellation based on the least squares adaptive algorithms for FIR transversal filter. The simulation results indicate that the system suppresses the co-frequency ambient signal.In fact, two channels would add some channel-noises, and the reference channel can receiver little measured signal, called crosstalk. The effect of low level channel-noises is little, but that of crosstalk has a theorem called "Power Reciprocal" leading to higher input SNR, lower output SNR. At present, some related improvements to the canceller had been made domestic and foreign for the applications for some trades in domestic and foreign, but these improvements can't be apply to this measurement of electromagnetic radiation. An improved method is presented based on that the ambient signal is AM signal with known carrier frequency in this paper. This method result for a good output SNR when the input SNR is in the range of OdB to 20dB.We deride a formula for estimation of input SNR based on the characteristics that the difference of the measured signal power is larger between two channels, so that we can choose which structure should be used to suppressing the co-frequency according to the input SNR.