2-27
the center of the vertical axis. Because the electron beam is still moving horizontally, electron 3 will
appear to the right of and above electron 2. At time 4 (T4), the sine wave applied to the top vertical-
deflection plate is at its maximum positive value. This attracts electron 4 toward the top deflection plate.
The upward deflection of electron 4 is increased by the negative-going sine wave (at time 4) applied to
the bottom deflection plate. This negative voltage repels electron 4 upward. Thus, electron 4 strikes the
CRT face to the right of and above electron 3. Finally, at time 5 (T5) both input sine waves are again at
zero volts. As a result, electron 5 is not deflected vertically, only horizontally. (Remember, the beam is
continually moving from right to left.)
While this discussion is only concerned with five electrons, vertical scanning, or deflection, involves
millions of electrons in a continuous electron beam. Instead of seeing five spots on the CRT screen, you
will actually see a visual presentation of the sine wave input. This was, as you remember, described
earlier as the unique feature of the CRT. You may have wondered why so much space in this chapter was
taken up with the discussion of the CRT. There are two reasons for this. First, the field of electronics is in
a constant state of evolution. Transistors replaced most vacuum tubes. Transistors are being replaced by
integrated circuits (ICs). As you progress in your career in electronics, you will find that the equipment
you work on will follow this evolution, from transistors to IC chips. Of all the tubes discussed in this text,
the CRT is the least likely to be replaced in the near future. Thus, in all probability, whether your career
in electronics lasts for only the time you spend with this text or 20 years, the CRT will be your constant
companion and co-worker.
The second reason for this rather extensive coverage of the CRT is that, while the CRT has a unique
ability, it operates exactly like all the tubes previously discussed.
SUMMARY OF THE CRT
This summary will not only review the CRT, but will also point out the similarities between the CRT
and other tubes.
Look at figure 2-28. Here you see both a schematic diagram and a pictorial representation of a CRT.
Each element is identified by a circuit number. We will review briefly the function of each element in a
CRT and its similarity to elements in conventional tubes. This summary will help you tie together
everything you have learned about the CRT and electron tubes in general.
