Reference Voltage Ouptut Impedance A test process that sources two sinusoidal AC Voltage signals at different voltage levels keeping the frequency constant. The ReferenceVoltage signal is sourced and the voltage is measured by the UUT and saved as the Reference value. Then the voltage level is changed and the Test value is measured on the UUT. The two measured values are compared, the results can be expressed as (e.g.) as a voltage ratio, dB or percent of change. These two sinusoidal signals are often used when calibrating linearity of a device.