In a message dated 97-04-23, Roger Magnuson writes: << Our experience so far (after two TBR21 approvals) is that when you actually have a design on the table and you start pre-testing it, some very strange requirements pop up. >>
Roger: I too have noticed the problem that you cite regarding the DC V-I test at extremely low loop current. I have no idea what the technical justification is, but I note that this requirement is similar to one that appears in the German requirements. I have also noticed the tightening of the requirements on the loop current limiter in TBR 21, compared to the requirements that appear in the French B1123A requirements. John Dunn alluded to this problem in his original posting. In TBR 21, the loop current must be limited to between 49.6 mA and 60 mA with a 50V/230-ohm feed. In B1123A, the lower limit is much more generous. I have had difficulty staying within the narrow range specified in TBR 21, especially if I allow the DAA to "cook" for a while on this feeding condition. I think that the 49.6 mA lower limit that appears in TBR 21 is technically unjustified. My guess is that the authors did not realize the implications of what they specified. Certainly, if the entire reason for including the current limiter in TBR 21 is because of France, the requirements that appear in TBR 21 should be no more severe than those that appear in B1123A. This is why I am hoping that the next version of TBR 21 is an improved version of the first attempt. Your posting implies that you believe the technical content of the new version is unchanged. Can you confirm this? Also, is it true that the new version is presently out for voting? Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
