EuroWire July 2024

Technical Article

Single-ended testing of coupling attenuation with passive DUT termination at far end By Harshang Pandya, Dr Zhu Xing and Arvind Patel, AEM Singapore Pte Ltd; Bernhard Mund, bda connectivity GmbH; and Dave Hess, Cord Data

Introduction At an earlier IWCS conference, we reported measurement results of differential parameters using single-ended testing. In this paper, we look at single-ended testing of unbalance parameters like TCL_near and TCTL_far using a similar approach. The ability to measure unbalance parameters using single-ended measurements is very useful, for example, in measurement of single-pair cabling, which is primarily characterised by unbalance attenuation instead of the crosstalk loss that is normally used to characterise four-pair cabling. Single-ended testing has another important use in coupling attenuation measurement using the triaxial method, wherein the triaxial test fixture allows access to only one end of the cable assembly under test [1][2] . Single-ended measurement concept Figure 1 shows a test system with single active device and remote terminator. In single-ended set-up for RF parameter measurement of cables, it is important to consider how the remote (far) end of the DUT is handled.

Figure 2: Single-ended TCL measurement

Insertion loss Single-ended measurement of parameters where typically an active test device would be used at both ends of the DUT is more challenging. Insertion loss measurement, for example, was done using an automated two-step approach. First, the passive terminator was set in reflection mode, and a measurement of return loss was made. Next, the passive termination was set in matched impedance termination mode and return loss measurement was made. The difference between these two measurements represents twice the insertion loss, as the signal would travel back and forth on the DUT.

In this work, a passive termination device that can act as matched-DMCM impedance (100 ohm differential and 25 ohm common mode) or a reflector (open connection) based on DC control excitation from the near end was used. This enabled measuring parameters with the far end terminated or reflected without having to manually change the termination during a measurement.

Here, ports A, B, C and D are the four differential ports. A prefix L indicates near end and an R indicates far end.

Figure 1: Single-ended measurement system

Measuring transverse conversion loss For measuring transverse conversion loss (TCL) from the near end, the passive far-end terminator was set in matched termination mode. Figure 2 shows TCL measurements for a DUT that was 2m shielded (STP) cord.

Figure 3: Measurement of insertion loss using a combination of return loss in reflection mode and return loss in termination mode

42

www.read-eurowire.com

July 2024