The HP Models 400E and 400EL are versatile ac voltmeters and dB meters. Both models can be used as ac to dc converters or wideband amplifiers. The Model 400E is primarily intended for voltage measurements, whereas the Model 400EL is primarily a dB meter. However, both meters indicate both volts and dB. The 400E has a linear ac scale with a logarithmic dB scale underneath, and the 400EL has a linear dB scale with a logarithmic ac scale underneath. Since the difference in scales is the only difference between the two instruments, this manual will use the term 400E/EL in reference to both instruments.