HP 400GL AC Voltmeter. The range is in 8 steps from .1mV to 1000V full scale, -80 to +60dB.
The 400 GL has the most sensitive ranges of 0.1 mV (100 µV) full scale. The “L” indicates the presence or emphasis of the log scale on the display. This meters also has amplified AC V output terminals and can be used as preamps for a digital scope, less sensitive AC voltmeter, or a chart recorder with an AC voltage range. They also have front panel switch selectable 100 kHz filters. Note that the hp 400 series are all “average responding” AC voltmeters. Average responding means that a meter is calibrated for AC sine wave waveforms in volts RMS (root mean squared). For noise measurements the reading will be low by a factor of 1.13, thus hp 400 noise values will read low and should be multiplied by a correction factor of 1.13.