Rene, The DAA calculations I use take into consideration the effect of the transformer (and CO shunt) on a set of 16 values of loop impedance (Rline+Xline), over which the local transmitter echo (side tone) is to be minimized. These values were selected from scatter plots of data from AT&T 1977 and 1983/84 Loop surveys using the following rationale. Most of the data was taken at 1 , 3 and 3.6 KHz. In the past we considered the 1 KHz data to be most important, but the advent of V.34 caused us to soften that concept and place a little more consideration at the high end (3.6 KHz) of the channel. In the beginning the 1 KHz was favored more (10 of the 16 values. The 1 KHz data also favored non-loaded loops, which are both a majority (80%) and statisically more tightly grouped than loaded loops. All chosen points are capacitive and that 600=j0omega is not included. This the reason it does not work well with 600 ohms reflected impedance.
There are a number of engineers out there that doubt the old AT&T local loop surveys are still relevant. But, my experience says they are and these hybrids keep proving it. To insure we could produce some semblance of balance in the hybrid we did a least squares fit of the 16 points. This provided a curve we could used in one leg of the hybrid bridge to in sure balance over a large percentage of real loop lengths. The circuit in that leg is actually a simple RC network. The calculations have a limitation and that is: New calculations must be made every time a different transformer is used. In the beginning I considered this to be pure drudgery. Now I don't mind. I hope this gives you some idea how this hybrid functions John Bingham is no longer at Telebit. He is with Amati. Amities, Duane
