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Test EMC using novel time-domain methods (3)

Transient EMI signals
      In cases where the signal consists mainly of spikes, bursts and other transient phenomena, the TDEMI system has important advantages over the conventional EMI receiver. This is due to the fact that the oscilloscope used as ADC can be set up to trigger the acquisition only when a transient has occurred. Figure 5 shows the principle of the triggered DAQ of transient signals. A practical example for this scenario involves measurements on an automotive flasher. The triggered DAQ with the TDEMI assembly consists of the lamp unit, cabling and a leadacid battery. The radiated EMI was examined with a broadband antenna from a distance of about 1m, in a way similar to the previous subsection.
     During the switch-off period, the system radiates a series of impulses, as shown in Figure 6. The impulse series is followed by a long time without any emission. The oscilloscope was set up to trigger on the edge of the impulses. In the first measurement, the TDEMI system used 2,500 spectra with 100,000 points each, sampled at 5GS/s. The EMI receiver was set to a step size of 50kHz, an IF filter bandwidth of 120kHz and a dwell time of 50ms per point. Figure 7 shows the amplitude spectrum for the peak detector evaluation.
     In the second experiment, the dwell time was increased to 1s. Due to time constraints, the  measurement was carried out only up to 170MHz (Figure8). Comparing both results, we see that the amplitude- spectrum measured with the TDEMI system is nearly identical to the upper boundary of the amplitude spectrum measured with the conventional EMI receiver in both cases. While with the spectrum from the TDEMI system, only the variance of the measurement decreases somewhat with the longer observation time.
     Hence, the result obtained from the EMI receiver is strongly dependent on the dwell time. This behavior results from the nature of the objective signal. The EMI receiver requires a long dwell time to be chosen to ensure that every measurement at each frequency point observes exactly one transient. The TDEMI system, on the other hand, will automatically react to the transients and stop acquiring data during the times without EMI emission from the test object. Thus, it is possible with the TDEMI system to perform precise measurements even with very short observation times .(over)

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