the Basic Features of a CRT,about Triggered Sweep CRO
- The Basic Features of a CRT:
The important features of these tubes are as follows.
1. Size: 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 5GPI . 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 l0 cm.
2. Phosphor: The screen is coated with a fluorescent material called phosphor. This material
determines the color and persistence of the trace, both of which are indicated by the phosphor.
The trace colors in electrostatic CRTs for oscilloscopes ale blue, green and But 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.
Pl --Green medium
P2--Blue green medium
P5--Blue very short
P11--Blue short
These designations are combined in the tube type number. Hence 5GPl 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
600mA.
Several dc voltages are listed below. The voltages vary with the type of tube used.
(i) Negative grid (control) voltage 14 V to - 200 V.
(ii) Positive anode no. 1 (focusing anode) -100 V to - ll00 V
(iii) Positive anode no. 2 (accelerating anode) 600 V to 6000 V
(iv) Positive anode no. 3 (accelerating anode) 200 v to 20000 V in some cases
4. Deflection Voltages: Either ac or dc voltages 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 rectangular screen having graticules marked on it. The
standard size used nowadays is 8 cm x l0 cm (8 cm on the vertical and 10 cm on horizontal).
Each centimeter on the graticule corresponds to one division (div). The standard phosphor color
use d nowadays is blue
- Triggered Sweep CRO:
The continuous sweep is of limited use in displaying periodic signals of constant
frequency and amplitude. When attempting to display voice or music signals, the pattern falls in
and out of sync as the frequency and amplitude of the music varies resulting in an unstable
display.
A triggered sweep can display such signals, and those of short duration, e.g. narrow pulses. In
triggered mode, the input signal is used to generate substantial pulses that trigger the sweep.
Thus ensuring that the sweep is always in step with the signal that drives it. As shown in Fig. 2.1
resistance R3 and R 4 form a voltage divider such that the voltage Vo at the cathode of the diode
is below the peak voltage i/o for UJT conduction. When the circuit is switched on, the UJT is in
the non-conducting stage, and C, charges exponentially through RT. towards VBB until the
diode becomes forward biased and conducts; the capacitor voltage never reaches the peak
voltage required for UJT conduction but is clamped at Vo. If now a –ve pulse of sufficient
amplitude is applied to the base and the peak voltage Vp is momentarily lowered, the UJT fires.
As a result, capacitor C. discharges rapidly through the UJT until the maintaining voltage of the
UJT is reached; at this point the UJT switches off and capacitor CT charges towards VBB, until
it is clamped again at VD fig 2.2 shows the output waveform
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