CISPR 22 conducted common mode emissions: a discussion on the effect of Balun quality on the calibration of impedance stabilization networks (ISNs)

CISPR 22 (1997) defines the limits of conducted common mode emissions (disturbances) permitted from information technology equipment (ITE) over the frequency range of 150 kHz to 30 MHz. CISPR 22´s preferred test method for measuring the common mode (CM) conducted voltage emissions on unshielded twisted pair (UTP) telecommunications cables specifies the use of an impedance stabilization network (ISN). CISPR 22 requires the ISNs to have specific levels of longitudinal conversion loss (LCL), which is a measure of the balance of the ISN. To measure the LCL of an ISN (i.e. calibrate the ISN), a balanced to unbalanced transformer (Balun) is used in conjunction with a network analyzer. Every Balun has its own inherent balance on the balanced output port. This inherent balance is critical to accurately measuring the highest level of ISN LCL (-80 dB) specified by CISPR 22 (1997). If the balance of the Balun is worse than the balance of the ISN, the measured LCL of the ISN can have significant errors that are not obvious to the operator. Any connector added to the Balun, and/or any adaptor installed between the Balun connector and the ISN input, will also have its own inherent balance that can affect the measured balance of the ISN. This paper presents the following information: How the balance of an ISN is measured (i.e. calibrated). An example of measured balance of a commercial off-the-shelf Balun is compared to the manufacturer´s published balance specification. Demonstration of the effect on the measured balance by the addition of a connector and an adaptor to the Balun.