Reference Frequency Output Impedance A test process the sources 2 sinusoidal RF signals at different frequencies keeping the power levels constant between the two frequencies. The FrequencyRef signal is sourced and the voltage is measured by the UUT and saved as the Reference value. Then the signal is changed to the test Frequency signal and the test value is measured on the UUT. The two measured values are compared, the results can be expressed as dB or percent of change. These two sinusoidal signals are often used when calibrating bandwidth or frequency response on an Oscilloscope or similar device.