Modification of transformer coupled permeameter (TC-Perm) for wider bandwidth

Abstract

Power electronics is attracting strong attention as one of the key technologies for realizing the carbon neutral society. A higher switching frequency in a power module is sought for making it more compact. However, inductors hinder it because the loss in the magnetic core rapidly increases. Thus, it is crucial to study and eventually suppress the magnetic loss in MF, HF and VHF bands (300 kHz – 300 MHz).Conventional RF permeameters measure mutual or self inductance of the coil wound around the magnetic sample. This method has the following problems, 1) it requires the sample to be the final toroidal shape, and 2) it cannot apply an external magnetic field. It is desirable to measure the RF permeability of a single particle in the powder stage as a function of magnetic field to gain more insight. For this purpose, we developed the transformer coupled permeameter (TC-Perm), and successfully measured a single Sendust particle used in noise suppression sheet over 10 MHz – 20 GHz1). Then we modified it to further expand the bandwidth.In the initial configuration of TC-Perm, the lower bandwidth limit is set by the vector network analyzer (VNA). We replaced it with another VNA covering 100 kHz – 44 GHz. The S21 parameter of the jig is proportional to frequency at low frequencies because the two CPWs are inductively coupled. This causes the signal level becoming smaller than the noise floor level of the VNA below a few 10 MHz, making it impossible to measure at low frequencies. We modified the TC-Perm as shown in Fig. 1 to avoid this problem. A low noise amplifier (LNA) with noise figure of 2.4 dB is inserted between the jig and VNA below 100 MHz. Above 100 MHz, the LNA is bypassed by two semiconductor RF analog switches. The Permalloy calibration sample used in Ref. 1 was measured using this system under a bias field of 200 mT. The modified TC-Perm showed approximately 40 times smaller noise level at low frequencies than before modification, which allows it to seamlessly measure the permeability over 300 kHz – 44 GHz.Acknowledgement - This work is supported by MIC SCOPE JP195003002.  Block diagram of TC-Perm after modification.  Measurement result of a Py sample with and without LNA.