Oscilloscope and Electron Beam

Topics: Oscilloscope, Square wave, Cathode ray tube Pages: 9 (2595 words) Published: September 19, 2013
An oscilloscope is the standard instrument that is used to examine time dependent voltages in a circuit. They are widely used in the medical professional to examine the electrical output of a heart, and in operating rooms. The voltage can be drawn on graph paper with the voltage on the vertical axis, and the time on the horizontal axis. Adjusting the scope will allow you to analyze part of one cycle or many cycles of the waveform. The objective of this lab is to become familiar with 2 types of oscilloscopes, the model 2530B made by BK Precision, and the Data Studio scope.

The main part of an analogue scope is a cathode ray tube (CRT), which is just a bean of electrons. Electrons are emitted from a hot cathode an accelerated by an electrode, then focused into a thin beam by electrostatic lenses. The “tube,” which is the glass vacuum envelope, is necessary so that the electrons will not become scattered by air molecules and the cathode will not burn up.

The electron beam is then passed through 2 pairs of deflection plates, where one pair of deflection plates deflects the electron beam in the vertical direction, while a voltage across the other pair deflects the electron in a horizontal direction. The electron beam passes through the deflection plates, and then strikes a phosphor material that covers the inside surface of a flat portion of the vacuum tube. Where the electron beam strikes the phosphor and some of this light passes to the outside of the tube is where the light is emitted and can be observed. When the electron beam is suddenly turned off, the light from phosphor does not immediately stop but decays in less than a second.

The electron beam points out of the flat part of the tube called the screen. When the beam is moved around the screen, it creates a pattern of light on the phosphor often called a trace. As the phosphor decays quickly, the trace must be constantly refreshed by the electron beam so the image can continuously be observed. It is refreshed by the electron beam executing the same path on the screen.

The most simple way a scope is operated is to apply the voltage you want to examine to the vertical deflection plates, and this will allow the trace to proportional to the input voltage. A linear ramp voltage created by the scope is applied to the horizontal deflection plates. This ramp voltage sweeps the electron beam horizontally across the screen at a uniform rate.

The trigger voltage, Vt, is the value of the voltage that tells the electrons beams to start the beginning of the curve. The x-axis is a time scale of the curve and is equal to how fast the electron beam is swept across the phosphor. This is determined by the “time-base” of the scope – if the voltage has a very high frequency the time base needs to sweep the electron beam across the phosphor quickly. A good scope can display voltages with frequency in the MHz range.

A periodic function in time is one that repeats itself over and over again - such as a sine wave or a square wave. A scope can display a voltage that is periodic in exactly the same way.  Every scope has a signal generator or time base oscillator.  When the ramp voltage across the horizontal deflection plates is VL, the electron beam is at the left of the screen.  When the ramp voltage is Vr, the beam is at the right of the screen center of the screen.   When the ramp voltage is zero the beam is in the horizontal center of the screen - when the ramp voltage is Vr the beam is at the of the screen.  The TIME/DIV controls the electron beam.

If a “trigger” pulse is applied to the circuit the electron beam is turned on and the time base oscillator applies the ramp to the horizontal deflection plates. The input voltage is applied to the vertical deflection plates and the electron beam is swept across the screen from left to right at a constant speed, which will take a time P. At the...
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