3B Scientific Training Oscilloscope - Manual (Page 2)

2
respect to the cathode potential, and partly by gas
constriction as a result of the tube being filled with
neon at a pressure of 0.01 mm Hg that also renders
the beam visible inside the tube.
There are also two opposing deflection plates in the
tube, oriented parallel to the beam, which can be
connected to the integrated saw-tooth generator, or
to an external voltage supply. The generator supplies
saw-tooth voltage waveforms with a frequency range
of 3.5 to 650 Hz and an amplitude of 100 V relative to
the anode potential.
Fig. 1: Cathode ray tube
Tube:
17 Deflection plates
18 Anode
19 Wehnelt cylinder
20 Cathode
21 Heater
22 Fluorescent screen
3. Technical data
Anode voltage: 250 V DC
Anode current: 1 mA max.
Heater voltage: 6...8 V AC/DC
Wehnelt voltage: -50...0 V DC
Size of deflection plates: 12 x 20 mm²
Distance of deflection plates: 14 mm
Deflecting coils: 300 + 300 turns
R
i
= 4.2
Ω
L = 6 mH
Saw-tooth voltages: V
pp
= 100 V
f = 3.5..650 Hz
4. Operation
4.1 To start operation
In order to supply current to the demonstration oscil-
loscope, power supplies are required that can output
the following voltages:
+250 V DC,
0-50 V DC, regulated,
6-8 V DC, regulated.
Power supplies U8521371 and U33000, which can
supply all of these voltages, are particularly well
suited to this end.
• Turn off the power supply.
• Connect the inputs of the demonstration oscillo-
scope to the outputs of the power supply, that
supply the specified voltages.
• Adjust the voltage so as to not to exceed the lim-
its.
• Turn on the power supply.
After 10-30 sec, a green spot appears on the fluores-
cent screen, which denotes where the electron beam
is hitting the screen. In order to keep the tube as
simple and clear as possible for educational purposes,
it was decided to do without additional apparatus for
secondary acceleration and focusing of the beam. For
this reason, it is not usually possible to focus the
beam to the sharpness seen in conventional oscillo-
scopes.
• Vary the Wehnelt voltage until the spot reaches its
minimum dimensions.
The electron beam is visible as a reddish thread inside
the tube, but only in a darkened room because of its
low intensity.
4.2 Deflecting apparatus
4.2.1 Electrical deflection
Using the deflecting plates located inside the tube,
the electron beam can be deflected horizontally by
applying a voltage of up to 100 V. For most applica-