I have a question about the Attenuation Selection on the Display. I can select: 0dB, -20dB, -10dB, -20dB, -30dB, -40dB, and -50dB. First, the two -20dB positions are not the same. Second, the first -20dB position seems to be the ideal setting. Third none of these positions behave like putting an external attenuator in the series with the input......... N6AS, Phil