When measuring signal levels at the tap output, signal amplitude is expressed in:

Signal amplitude at the tap output is commonly expressed in dBmV. This unit specifically denotes the power level of a signal relative to one millivolt (mV) and is typically used in RF (radio frequency) and telecommunications contexts.

Using dBmV for measurements allows installers and technicians to communicate signal strength in a standardized way that accounts for voltage levels, which is particularly important in systems like coaxial cable networks where the signal integrity can directly affect performance. By using dBmV, one can easily determine whether the signal levels are adequate for device operation or if adjustments are necessary.

The other units mentioned have different contexts: dB is a logarithmic unit that simply expresses ratios, not specific power levels; dBm refers to power in milliwatts, which doesn't directly account for the voltage level in the same way dBmV does; and Watts is a measure of power, not a unit for expressing voltage levels relative to millivolts. Therefore, dBmV is the most appropriate choice for measuring signal amplitudes at the tap output in the context of broadband installations.