Oscilloscopes
Early radar displays used adapted oscilloscopes with various inputs. An oscilloscope generally receives three input "channels" of varying (or oscillating) voltage as input and displays this information on a cathode ray tube. The oscilloscope amplifies the input voltages and sends them into a deflection magnet. the "intensity" channel, which controls the brightness of the spot on the screen. A bias voltage source for each of the three channels allows the operator to set a zero point. Varying the voltages sent into the input channels makes the cathode beam to move as a spot on the display.
In a radar display, the radar receiver provides one of three input channels to the oscilloscope. Early displays generally sent this information to either X channel or Y channel to displace the spot on the screen to indicate a return. More modern radars typically used a rotating or otherwise moving antenna to cover a greater area of the sky, and in these cases, electronics, slaved to the mechanical motion of the antenna, typically moved the X and Y channels.
Read more about this topic: Radar Display