Rene, I apologize for taking so long to respond. I've got too many projects to work on at the moment.
I do not like the local loop simulators supplied by TAS or PTT. In my opinion they are a poor representation of real world local loops. They are sourced at 600 ohms and should be sourced at 900 ohms. I don't use them unless a client insists they must be used. In my opinion TSB-37A testing makes poor and good modems look good. My performance figures come from testing with real local loops. I have been designing modems since 1969. My first modem was a discrete Bell 202. I was employee #13 at Vadic (we later sold it to Racal). While there I was in the development engineering department for 10 years and advanced development department for 5 years. During those years I was privileged to work for and with John Bingham (the inventor of 1200 bps full duplex modems). During that time I learned a lot about full duplex modem from FDX1200 bps to 9600 bps. During that period I developed my knowledge of DAA's, return loss, 2W to 4W hybrids, the effect real local loops have on modem performance and how to squeeze all the modem performance possible out of a modem. When I stated I've been asked to diagnose poor performing modems and come up with a way to fix them I meant just that. In almost all these cases the original DAA was changed so the modem would pass approval in a specific country. This was done sans ever testing the modem performance after the changes were made. The changes to the DAA were made for one reason and one reason only, to insure the modem would pass approvals. In my opinion it was done sans any regard or thought on how the changes might effect modem performance. What me and Joe were trying to say was a return loss of 6 dB is adequate for a modem DAA. 6 dB is not adequate for terminal equipment which is used for voice conversations. For obvious reasons. We were talking about two different things. One is the requirement for stringent return loss on a modem DAA . And second I was saying the addition of circuitry within the DAA to meet stringent return loss requirements can effect the performance of the modem. This is very true if the return loss circuitry added to the DAA over compensates for return loss and causes an inordinate transmit signal of loss within the DAA. To overcome the loss the transmit signal must be increased compensate for the loss. This will generally cause poor performance. I design my own external 2W to 4W hybrid that takes into account the variable length of the local loop. This hybrid will generally have a THL of at least 20 dB on about 96% of all local loops and as much as 30 dB or more is not uncommon. It does not work well when the reflected impedance is 600 ohms. I have to provide an option strap for that. I won't try to describe how it functions. This is not the forum, time or place to do so. Who is the manufacturer of the modem with Rockwell chips that has good performance? I worked for Rockwell 7 years and designed several hundred DAA's for customers. The one's I designed are not the passive hybrid DAA's used in the Rockwell reference designs. The Rockwell passive designs were designed by someone else. Mine always have an active 2W to 4W hybrid. When Joe Randolph and I were speaking about return loss we were referring to PTT return loss speed to used a complex impedance as a reference. I n the U.S. Part 68 does not have a return loss requirement. The only U.S. return loss specification I know is a recommendation in RS-496. Which states: "It is DESIRABLE the return loss conform to the tables below: Frequency Minimum Return Loss 260 - 500 Hz 7.0 dB (SRL Lo) 560 - 1960 Hz 11.0 dB (ERL) 2200 - 3400 HZ 14.0 dB (SRL Hi) The reference impedance is 600 ohms in series with a 2.16 uF capacitor. In my DAA 's designed to source 600 ohms the return loss is generally between 30 and 45 dB. I am unable to get that much if the DAA is required to used a complex impedance as a reference. If I do the modem performance suffers. Amities, Duane
