CRT Features – Electrostatic CRTs are available in a number of types and sizes to suit individual requirements. The important features of these tubes are as follows.
Size refers to the screen diameter. CRTs for oscilloscopes are available in sizes of 1, 2, 3, 5, and 7 inches. 3 inches is most common for portable instruments.
For example a CRT having a number 5GP1. The first number 5 indicates that it is a 5 inch tube.
Both round and rectangular CRTs are found in scopes today. The vertical viewing size is 8 cm and horizontal is 10 cm.
The screen is coated with a fluorescent material called phosphor. This material determines the colour and persistence of the trace, both of which are indicated by the phosphor.
The trace colours in electrostatic CRTs for oscilloscopes are blue, green and blue green. White is used in TVs, and blue-white, orange, and yellow are used for radar.
Persistence is expressed as short, medium and long. This refers to the length of time the trace remains on the screen after the signal has ended.
The phosphor of the oscilloscope is designated as follows.
- P1 – Green medium
- P2 – Blue green medium
- P5 – Blue very short
- P11 – Blue short
These designations are combined in the tube type number. Hence 5GP1 is a 5 inch tube with a medium persistence green trace.
Medium persistence traces are mostly used for general purpose applications.
Long persistence traces are used for transients, since they keep the fast transient on the screen for observation after the transient has disappeared.
Short persistence is needed for extremely high speed phenomena, to prevent smearing and interference caused when one image persists and overlaps with the next one.
P11 phosphor is considered the best for photographing from the CRT screen.
3. Operating Voltages
The CRT requires a heater voltage of 6.3 volts ac or dc at 600 mA.
Several dc voltages are listed below. The voltages vary with the type of tube used.
- Negative grid (control) voltage – 14 V to – 200 V.
- Positive anode no. 1 (focusing anode) – 100 V to – 1100 V
- Positive anode no. 2 (accelerating anode) 600 V to 6000 V
- Positive anode no. 3 (accelerating anode) 200 V to 20000 V in some cases
4. Deflection Voltages
Either ac or dc voltage will deflect the beam. The distance through which the spot moves on the screen is proportional to the dc, or peak ac amplitude. The deflection sensitivity of the tube is usually stated as the dc voltage (or peak ac voltage) required for each cm of deflection of the spot on the screen.
5. Viewing Screen
The viewing screen is the glass face plate, the inside wall of which is coated with phosphor. The viewing screen is a reetangular screen having graticules marked on it. The standard size used nowadays is 8 cm x 10 cm (8 cm on the vertical and 10 cm on horizontal). Each centimeter on the graticule corresponds to one division (div). The standard phosphor colour used nowadays is blue.