Characterization of common-mode choke for automotive ethernet networks enabling 100 Mbit/s

In this article, a surface mounted device (SMD) common-mode choke (CMC) required for unshielded-twisted-pair automotive communication systems (like 100Base-T1) is characterized in detail regarding its high-frequency filtering characteristics. Firstly, the S-parameters and mixed-mode S-parameters of the mentioned choke were measured using a 4-port vector network analyzer in a frequency range of 1 MHz to 1 GHz. In addition, the same parameters of the choke were measured with common-mode termination in the same way as before. Furthermore the common-mode (CM) and differential-mode (DM) impedance of the choke was measured using an impedance analyzer which covers a frequency range up to 120 MHz. Next, a SPICE equivalent circuit model was extracted from impedance measurements of the common-mode choke, both in CM and DM. Thus, the proposed model incorporates all parasitic effects including packaging, mounting and soldering related effects respectively. In order to characterize the CMC, a set of wideband PCB boards with 3-port and 4-port configurations were designed and fabricated to function as a test fixture for the CMC measurements. In order to remove test fixture effects de-embedding method was applied.