Joe, Thanks for the quick response and suggestions.
I'm not one to sit on my hands. So, I went ahead and used NTR 3. I calculated and measured the impedance of the NTR 3 network and discovered it was very flat from 240 hz (bottom end of V.34 carrier) to 3600 Hz (upper edge of V.34 carrier). i.e. 1010 ohms at 240 Hz and 985 ohms at 3600 Hz. It drops to 970 ohms at 4 Khz. As consequence I designed the DAA for 800 ohm input and 600 ohm output. The NTR 3 network gives me worst case return loss of: 18.8 dB @ 240 Hz and 17.9 dB @ 4000 Hz if I design for an 800 ohm input impedance. Even though this modem is running on 3.3.volts I can easily make transmit levels of -10 dB. NTR 3 network has little effect on achieving decent transhybrid loss. It's nothing like the effect the Austel complex impedance network has on THL. The Austel network causes V.34 28.8 bps connect rate (in my designs) to drop from 82% t0 78%. Adding the circuitry in the DAA to give enough pre-emphasis for decent return loss causes too much signal loss. This means transmit signal drive has to be higher to get -10 dB at the line. The higher drive level makes it difficult (even for the DSP) to get decent THL. Thank you for sharing your insights. Regards, Duane
