Keysight (Agilent/HP) 3551A

For testing the transmission characteristics of telephone and data lines, Frequency range 40 Hz to 40 kHz with up to 1 Hz resolution, Signal level +10 dBm to -60 dBm, Noise voltage measurements with 1 dB resolution. Impedance switchable between 135, 600 and 900 ohms. TRANSMISSION TEST SET