In a message dated 97-02-17, Duane Marcroft writes: << Joe:
I read your article on U.K. Requirements in the Compliance Engineering 1996 Reference Guide. The article indicates the RT reference network is a 275 ohm resistor in series with a parallel network consisting of a 780 ohm resistor and 310 nF capacitor. A different network is shown in ETS 300 001:March 1996, Chapter 4 [4.1.2 (GB) 1]. It shows a 370 ohm resistor in series with a parallel network consisting of a 620 ohm resistor and 310 nF capacitor. Do you or any other TREG'R know which is correct? >> Duane: Mel Pedersen's answer pretty much sums up the situation. The values that now appear in NTR 3 are 270 ohms in series with the parallel combination of 750 ohms and 150 nF. This matches the values used in TBR 21. In NTR 3, there is no explicit option to use 600 ohms instead. However, as I pointed out in my article, a pure 600 ohm impedance will actually meet the NTR 3 requirement, since only 8 dB return loss is required. It's close at 4000 Hz (about 9 dB), but some minor tweaking will give you good margin for the NTR 3 requirement and still allow you to meet 14 dB referenced to 600 ohms with the same termination. Even though you can still use BS 6305 and NET 4 to get approval in the UK, I don't know why anyone would want to do this. NTR 3 is much easier to meet. About the only thing to watch out for in NTR 3 is that DTMF levels are measured using the complex reference as the load, instead of 600 ohms. If your source impedance is 600 ohms, the pre-emphasis between the low group and high group tones deteriorates in the upper right quadrant of the dial matrix (DTMF "3" and "A"), due to the capacitive element in the load impedance. Programming the preemphasis to a fixed value of about 3 dB is usually sufficient to overcome this problem. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
