Understanding Patch Cord Testing
The patch cord adapter jacks are hand-selected based on how unbiased the jack behaves when it is mated with plugs from an acceptable performance range. For the mated NEXT coupling between the pairs 3,6 and 4,5 the difference between the worst margins produced by mating the best (low) and worst (high) plugs in the acceptable range must be less than 2 dB according to IEC 61935-2. This concept is further illustrated in the following three hypothetical curves (Figure 1 through Figure 3).
Figure 1 shows a jack that produces a same worst margin value for both low and high plugs. This jack is considered to be truly unbiased and “centered”. Other plugs between the low and high plugs produce higher worst margin values than the low and high plugs. This jack is very rare in the field.
Figure 2 shows a jack that favors high plugs. This jack produces more margins for high plugs; however, the high plug worst margin must not exceed more than 2 dB from the low plug worst margin in order for the jack to be considered for patch cord testing. This particular jack in Figure 2 is not considered to be perfectly centered, but still acceptable for patch cord testing when referencing the standards.
- Unlike the jack in Figure 2, Figure 3 shows a jack that favors low plugs. This jack will produce more margins for low plugs; however, the low plug worst margin must not exceed more than 2 dB from the high plug worst margin in order for the jack to be considered for patch cord testing. This particular jack in Figure 3 is not considered to be perfectly centered, but still acceptable for patch cord testing when referencing the standards.
It is not uncommon to see a difference of 2 dB in margin reported between different adapters. This reflects the reality of interoperability of all the plugs and jacks in a typical system.
Additional measurement uncertainty
Your RJ45 plug may exhibit movement after being inserted into the RJ45 jack/connector. Where the RJ45 sits inside the jack/connector will have an impact on your reported NEXT margin. How much depends on whose RJ45 plug you are using. In the following experiment, the mated NEXT of the DTX-PC6AS was measured using a DTX-AXTERM. The DTX-AXTERM was first pushed in as far as it would go and a NEXT measurement made. It was then pulled back as far as it would go and the NEXT measurement repeated. The results are displayed below:
Establish with the operators as to whether the cords should be tested witht he RJ45 plugs fully inserted or pulled back.
Using the same adapters above, a Category 6A patch cord was certified with the plugs pushed all the way in and then with the plugs pulled back as far as they would go.