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Fitted Versus Input Impedance

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Fitted Versus Input Impedance Fitted Versus Input Impedance There has been some confusion in the industry about the impedance and SRL measurement method that Belden uses for its DataTwist® 350 and MediaTwist®. Some people have actually stated that Belden does not use the "accepted method" for measuring impedance. These people are trying to the illusion that Belden is somehow "cheating". The fact is that the method Belden uses is not only accepted, but it is more stringent than the "accepted method" to which some people are referring. This paper will explain the two methods and show how the results compare. The electrical symbol for impedance is the Greek letter Zeta, or "Z". The symbol Zo is the electrical symbol for the characteristic impedance of a transmission line. Impedance is the ac resistance to the flow of electrical current. Impedance is made up of resistive, capacitive, and inductive components. All three of these can vary with the frequency of the transmitted signal. Because of this, impedance can also vary with frequency. In a perfect transmission line, impedance would be constant for all frequencies. However, due to material properties at different frequencies and dimensional variations in the cable construction, the impedance of most twisted pair cables exhibits variations with frequency. By choosing materials with electrical properties that are stable over the applicable frequency range, and by minimizing dimensional variations, it is possible to minimize the impedance variations in a twisted pair. The variations in impedance are centered around a nominal impedance level. This nominal level is typically designed to be 100 ohms for Category 3,4 and 5 cables. The 100 ohm nominal value allows the equipment manufacturers to design the input and output impedance of the electronics to match 100 ohms. This minimizes the amount of signal loss due to impedance mismatches between the cable and electronics. The nominal impedance can be thought of as the average of the impedance across a frequency range. This is the idea behind "Zo-fit". Zo-fit is simply a curve fitting function that converts the varying input impedance trace for a given pair into a smooth "nominal" impedance trace. This fitted curve can still vary with frequency, but the variations are much more gradual than those in the original input impedance curve. See graph 1 below for an example of an input impedance measurement and its corresponding Zo-fit curve. GRAPH 1 - Input and Zo-fit impedance The input impedance is fitted for each pair over a specified frequency range. The fitting applies to each pair individually and is not an average of all the pairs combined. TIA/EIA 568-A states that the fitted impedance must be between 85 and 115 ohms (100 ohms +/- 15%) at all frequencies for all categories. This is to limit the mismatch of impedance between the cable and the electronics. However, this requirement does not limit the actual impedance variation in the cable, which causes signal reflections and thus signal loss within the cable itself. This signal loss is called Structural Return Loss, or SRL. SRL is a measurement of the difference between the input impedance at a given frequency and the nominal impedance at that same frequency. When the input impedance is the same as the nominal impedance, there is no return loss (or SRL becomes a large negative number). As the the two impedances differ, there is more SRL (or SRL becomes a small negative number). Graph 2 shows the SRL calculated from the input impedance and Zo-fit impedance. At 1.2 MHz, the fitted impedance is approximately 90 ohms. At this frequency, the input impedance has its peak difference from this fitted impedance line. Hence the SRL has its maximum at this frequency. (Remember that SRL is the difference between the actual measured impedance and the nominal impedance.) At 1.3 MHz, the fitted and input impedances are equal and, therefore, the SRL is at a minimum. GRAPH 2 - Comparison of impedance and SRL The question is which impedance is nominal, the Zo-fit or 100 ohms. TIA/EIA 568-A contains an SRL requirement that allows for the SRL to be calculated using the Zo-fit impedance as the nominal. This may or may not be 100 ohms at all frequencies, as shown in graph 1. If the Zo-fit for a given pair is equal to 100 ohms at all frequencies, then the Zo-fit method will yield the same SRL results as the method using 100 ohms as the nominal. However, if the Zo-fit varies from 100 ohms, but still remains within the 15 ohm tolerance, then the two methods can yield significantly different SRL results. The method using the Zo-fit impedance will always yield equal or better results than the method using 100 ohms, allowing manufacturers to ship more product. To demonstrate the advantage of using the Zo-fit method for calculating SRL, both methods were used on the pair shown in graph 1. Graph 3 below shows the "improvement" in SRL given by the Zo-fit test method. GRAPH 3 - SRL calculated using 100 ohms and Zo-fit as nominal impedance This example shows how much more difficult it is to meet the same SRL requirement when using 100 ohms to calculate SRL instead of the Zo-fit impedance. Belden DataTwist 350 and MediaTwist use the more stringent SRL measurement method without fitting the impedance. This method guarantees that the product performance will not only meet but exceed the performance specified in TIA/EIA 568A. Remember, the system sees SRL as an unfitted, pure phenomenon, so why disguise it! .
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