The signal frequency in a CRO is:

Cathode Ray Oscilloscope (CRO)

• A cathode ray oscilloscope (C.R.O.) is an instrument that converts electrical signals to a visual display.
• A CRO can be Used to Measure Voltage, Frequency, and Phase. It is also used to measure electrical quantities like amplitude and time period.
• The main structure of the C.R.O. is a highly evacuated cathode ray tube (C.R.T.) which emits an electron beam known as a cathode ray beam.
• The cathode ray tube consists of three main components: an electron gun, a deflection system, and a fluorescent screen.
• The electron gun is used to produce a narrow beam of electrons. 
• The deflection system allows the electron beam to be deflected from its straight-line path when it leaves the electron gun. After deflection, the speed of electrons is reduced. So, to increase the speed, the signal frequency in a CRO is kept at more than 10 MHz.
• When electrons in the beam strike the screen, the material fluoresces and becomes luminous or glows. This enables a bright spot to appear wherever an electron beam strikes the screen.
• Electrons are particles and they have mass. Since they move at high speed, they have kinetic energy.
• When these high-energy electrons strike the screen, the fluorescent coating on the screen converts the kinetic energy of the electrons into light energy.