The CRO ( cathode ray oscilloscope), often referred to as a “scope”, is the most powerful tool available for measuring electrical quantities associated with electronic circuitry. It is such an important instrument that a thorough understanding of its operations is absolutely essential for any electrical engineer. Although we will be using a Digital Storage Oscilloscope (DSO), it is instructive to cover the basics of the Cathode Ray tube technology since there are many still in industry. The display on the CRO screen is created by an electron beam exciting a phosphor coating on the inside face of the CRT (cathode ray tube).Fig.3.1(a) shows the major parts of a Cathode Ray Tube (CRT).
The electrons generated by the cathode are accelerated and focused into an electron beam. When the beam strikes the fluorescent screen, it emits a tiny spot of visible light. The position of the electron beam (hence the spot on the screen) generated by the electron gun is controlled by the vertical and horizontal deflection plates. The spot on the screen changes its position depending on the voltages applied to the vertical and horizontal deflection plates. Continuously changing signals on vertical and horizontal deflection plates causes the beam to trace out a path on the screen. Even though only one spot is highlighted on the screen by the electron beam at any given moment, the persistence of the phosphor makes the path appear to be continuous. The path must be retraced frequently in order to render a steady display.
Resistance is an expression of degree to which electron flow will be impeded through a conductor. The unit is the Ohm. In simple circuits resistance determines the relation between voltage and current. At the extremes a short piece of wire will have a resistance of nearly zero Ohms. While an air gap (for example an open switch) has very large resistance. Intuitively a couple of relationships will hold in conductor, a voltage difference between the two ends will cause a current to flow. How much current will be determined by how much resistance the conductor offers. If there’s less resistance more current will flow. In fact, given a power source of high enough density, if you half the resistance, you will double the current. If you double the resistance, you will half the current.
A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the rated breakdown voltage or "Zener voltage". The device was named for Clarence Zener, who discovered this electrical property.