6-10
CRT GRATICULE
A GRATICULE was used in our previous discussion in figure 6-10. It is simply a calibrated scale
(made of clear plastic) of amplitude versus time that is placed on the face of the CRT.
The graticule can be used to determine the voltage of waveforms because the DEFLECTION
SENSITIVITY of a CRT is uniform throughout the vertical plane of the screen. Deflection sensitivity
states the number of inches, centimeters, or millimeters a beam will be deflected for each volt of potential
difference applied to the deflection plates. It is directly proportional to the physical length of the
deflection plates and their distance from the screen and inversely proportional to the distance between the
plates and to the second-anode voltage. Deflection sensitivity is a constant that is dependent on the
construction of the tube.
Deflection sensitivity for a given CRT might typically be 0.2 millimeters per volt. This means the
spot on the screen will be deflected 0.2 millimeters (about 0.008 inch) when a difference of 1 volt exists
between the plates. Sometimes the reciprocal of deflection sensitivity (called DEFLECTION FACTOR)
is given. The deflection factor for the example given would be 125 volts per inch (1/0.008).
Q-13. What term is used to describe the reciprocal of deflection sensitivity of a scope?
In the above example, 125 volts applied between one set of plates would deflect the beam 1 inch on
the screen. This means that the deflection caused by small signals would likely not be observed. For this
reason, the deflection plates are connected to amplifiers that magnify the signals applied to the vertical
input of the scope.
Assume, for example, that a peak-to-peak value of a known voltage applied to the oscilloscope
indicates that each inch marking on the graticule is equal to 60 volts. Each of the 10 subdivisions will,
therefore, equal a value of 6 volts. Most oscilloscopes have ATTENUATOR controls to decrease or
GAIN controls to increase the strength of a signal before it is placed on the deflection plates. Attenuator
and gain settings must not be disturbed after the calibration has been made. For maximum accuracy, you
should recalibrate the graticule each time a voltage is to be measured.
CRT DESIGNATIONS
Cathode-ray tubes are identified by a tube number, such as 2AP1, 2BP4, or 5AP1A. The first number
identifies the diameter of the tube face. Typical diameters are 2 inches, 5 inches, and 7 inches. The first
letter designates the order in which a tube of a given diameter was registered. The letter-digit combination
indicates the type of phosphor (glowing material) used on the inside of the screen. Phosphor P1, which is
used in most oscilloscopes, produces a green light at medium PERSISTENCE. Persistence refers to the
length of time the phosphor glows after the electron beam is removed. P4 provides a white light and has a
short persistence. If a letter appears at the end, it signifies the number of the modification after the
original design.
OSCILLOSCOPE CONTROL COMPONENTS
Although the CRT is a highly versatile device, it cannot operate without control circuits. The type of
control circuits required depends on the purpose of the equipment in which the CRT is used.
There are many different types of oscilloscopes. They vary from relatively simple test instruments to
highly accurate laboratory models. Although oscilloscopes have different types of circuits, most can be
