Digital comb-generator for validating sub-30 MHz radiated emission test system using double-gapped shielded loop antenna

Recently, the needs for the emission measurement of electronic devices are increasing. Especially, in association with the development of RFID system, electromagnetic cooker, and the wireless power transmission for automobile, the radiated emission measurement in the frequency range from 9 kHz to 30 MHz become very important. Comb-generators as a reference signal source are very useful equipment for validating the radiated emission test system. Conventional comb-generators periodically generate pulse waveforms with a certain period, T. As a result, comb-like spectrum with the frequency-spacing of 1/T are generated from the conventional comb-generators. The period of pulse generation and the output level of comb-generators have inverse relationship because longer period decrease the waveform energy per second. In addition, the frequency spectrum of the conventional comb-generator and the pulse waveform are Fourier transformation pairs. Therefore, the fine control of the spectrum of the comb-generator is quite difficult. Additionally, if the frequency-spacing of the generator is set to 1 kHz, the frequency spectrum is too dense around 30 MHz and the output level of the comb-generator are too low due to the inverse relationship of the period and the output level. On the other hand, if the frequency-spacing of the generator is set to 1 MHz, the spectrum of less than 1 MHz cannot be generated. To solve these problems, a digital comb-generator using FPGA-based arbitrary waveform generator and double-gapped shielded loop antenna is proposed in this paper. This digital comb-generator can cover the frequency range from 150 kHz to 30 MHz, which correspond to CISPR Band B. The proposed comb-generator can also generate almost equally-spaced comb-like spectrum in the logarithmic frequency scale. For emission test in the frequency bellow 30 MHz, the log-scale frequency scale is frequently used. Therefore, the log-scale comb-generator is very useful for full-band measurement in the frequency range from 9 kHz to 30 MHz. Moreover, by the use of double-gapped shielded loop antenna, the proposed comb-generator can generate both magnetic-field and electric-field by controlling the phase of waveform. The generated magnetic field strength from the proposed comb generator is about 44 dBuA/m to 54 dBuA/m. Additionally, the standard deviation in 3-hour measurement of the output spectrum is less than 0.05 dB in the frequency less than 30 MHz. Therefore, the output spectrum of the proposed comb-generator is quite stable. From these results, the proposed comb-generator can be used as a reference signal source for validating sub-30 MHz emission test systems.