ISO 21806-13:2021 pdf download – Road vehicles — Media Oriented Systems Transport (MOST) — Part 13: 50-Mbit/s balanced media physical layer conformance test plan.
8.4.2 Test procedure general The evaluation of cable attenuation shall follow the principle as specified in EN 50289-1-8. This is performed with a network analyser, using a 4-port arrangement. 8.4.3 Example set-up test procedure A test fixture is being used to connect the differential cable system to the single ended measurement equipment. For details see Annex C. The status of Annex C is informative. When acquiring a complete set of the mixed mode scatter parameters (S-parameters) for an interconnect under test, extract the magnitude of the transfer characteristic from differential port 1 to port 2 and vice versa (differential insertion loss: SDD21 and SDD12) in dB-scale. NOTE The cable attenuation varies with environmental conditions (e.g. temperature) and also depends on production process, properties of used materials and stability of geometric properties. It also depends on the way the interconnect under test is being arranged for the test. The cable under test shall be placed on an arrangement with metal plane (GND-reference) at the bottom and an isolation layer (thickness 10 mm, ε r ≤ 1,4) on top. The cable shall be placed on top of the isolation layer, the cable shall be laid out with a minimum distance of 30 mm between the cable portions (either meander shaped on a flat plane or cable assembled on a conductive drum with a minimum distance to each winding). GND of test fixture shall be connected to the metal plane. 8.4.4 Test procedure for data acquisition Figure 6 shows the example measurement of attenuation of an electrical interconnect.
8.4.5 Impact of attenuation on the data signal The attenuation characteristic of electrical interconnect follows a function of frequency. Therefore, the spectrum of a data signal being fed into such channel shall be attenuated in a non-uniform manner. Attenuation affects high frequencies more than low frequencies. In consequence, transition times decrease. Shorter pulses of the signal might not achieve full amplitude swing anymore. The effect is called intersymbol interference. The graph in Figure 7 gives an example: the SP2 signal starts with a nearly uniform amplitude on all pulses. A small intersymbol interference effect is already visible, which is caused by AFE band-filtering. The SP3 signal shows the resulting signal shape after having passed a typical electrical interconnect. Additional intersymbol interference is visible.