In a message dated 10/17/2002, Alan Shaw writes:
> So, with regard to European CTR21, is the complex Reference Z equal to the > telephone line impedance or does represent the complex conjugate so as to > provide a power match when I design the TE's input impedance to provide a > good return loss in the measuring bridge? > Hi Allen: Yes, the complex reference Z in CTR 21 is intended to represent the impedance of a "typical" phone line as seen by the terminal device. It is not the complex conjugate of the phone line impedance. The goal in specifying return loss is to minimize reflections. So, if you want to achieve good return loss with respect to the reference Z, your interface should present an impedance that is similar to the reference Z. It sounds like you are trying to understand what it takes to meet the CTR 21 return loss requirement. You might be interested to know that even a pure 600 ohm impedance will meet the requirement, although some shunt capacitance will improve the compliance margin at 4000 Hz. Some purists may insist that better matching is needed, but I am not convinced. The fact is that the actual impedance seen looking into a phone line varies over a tremendous range. I have scatter plots of measurements on a large number of actual lines, plotted on the complex plane. It looks like someone threw food at the wall. When studying these plots it is possible to conclude that 600 ohms is sort of in the middle of the mess. It is also possible to conclude that a complex network such as the one called out in TBR 21 represents a closer approximation (which makes sense, since phone cables have some distributed capacitance). Even though the phone networks in various countries are fairly similar, different engineers have come up with different fixed-compromise representations for the reference impedance. That's why some countries use a 600 ohm reference and some have created their own complex reference. So, that is all the TBR 21 network is: somebody's idea of a fixed compromise approximation of the "center" of the range of possible impedances experienced in the real world. My point here is that for most applications, it is not productive to strive for extraordinary matching to whatever reference impedance is provided by a particular standard. If the end product will be used on an arbitrary PSTN line, the actual matching achieved on a given line is likely to be rather poor. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
