An oscilloscope is an electronic test instrument that graphically displays varying signal voltages The standard operating mode displays voltage on the vertical (Y) axis versus time on the horizontal (X) axis The time base is generated internally, sweeping the electron beam horizontally at a constant rate The input voltage signal deflects the beam vertically This allows visualization of the waveform's amplitude, frequency, period, and shape over time While current or other quantities can sometimes be displayed indirectly (e g , by measuring voltage across a known resistor for current), the fundamental display is Voltage versus Time