Download Oscilloscope 2

Oscilloscope
Oscilloscope
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Electron guns
Electron beams
Focusing coils
Deflection coils
Anode connection
Mask for separating beams
for red, green, and blue
part of displayed image
7. Phosphor layer with red,
green, and blue zones
8. Close-up of the phosphorcoated inner side of the
screen
Oscilloscope
• Referred to as cathode-ray oscilloscope, it is a
universal instrument that displays wave forms
on a phosphor coated "screen" of a cathoderay tube (CRT).
• The oscilloscope represents a two dimensional
graph of signals VS time or in general, other
signals (instead of time).
oscilloscope
• The general purpose of an oscilloscope is
displaying voltage signals and other electrical
signals or non-electrical signals.
• There non-electrical signals would have to be
converted to voltage signals (using
transducers).
• NOTE: many of the oscilloscope controls can
be adjusted to suit the individual performance
required by the operator.
oscilloscope
• When using an oscilloscope:
1. You can determine the time and voltage value
of given signal.
2. You can calculate the signal frequencies.
3. You can tell malfunction in the oscilloscope is
the component is destroying the signal.
4. You can find out how much of the signal is DC or
AC .
5. You can tell if there is some noise with the signal
Oscilloscope
• NOTE: oscilloscopes come in many shapes and
sizes, hence, beside the power supply, the
basic oscilloscope has different subsystems.
• Now, on the screen a spot of light is formed
where the electron beam strikes and leaves a
glowing trail as the beam moves across the
screen (fades unless traced over)
• The beam passes first between one pair of
vertical and then horizontal deflection plates
when the voltage is applied to the vertical
deflection plates this produces an electrical
field between them, which deflect the
electron beam either up or down similarly the
electrical field in the Horizontal plates deflects
the beam left or right.
CRT operation:
• The signal displayed on the screen passes
through the calibrated attenuator.
• The attenuator matches the high impedance
of the scope to the low impedance of the
vertical pre-amplifier; it also scales the input
signal to a level that the vertical pre-amps can
handle.
• Then the signal is magnified by the vertical
amplifier, which provides the required
deflection voltage.
• The trigger circuit:
1. Trigger level: selects the voltage of which the
input signal initiates a sweep.
2. Slope: determines whether a sweep begins
on the positive or negative ongoing slope of
the wave form.
• The horizontal deflection plates provide a
similar action where its input maybe switched
between two plates of inputs:
1. External input (x-y)
2. internal input: (time-base0generator) or
(sweep generator)The circuit initiating the
sweep is called the trigger circuit.
• The heater cathode emits electrons. They are
accelerated to the first accelerating anode when
the amount of the cathode current controls the
intensity of the spot.
• The beam is focused and then sent to be given
some additional energy by the 2nd accelerating
anode.
• Focusing the beam prevents the electrons from
being diverged and producing an ill-defined spot
on the phosphoric screen.
• The electron beam is focused with an
electrostatic lens.
• This lens requires three elements with the center
elements at a lower potential that the two outer
elements.
• NOTE:
1. The force on the electron is in the direction
normal to the equal-potential surface.
2. Only electrons passing through the exact
centre will experience no force.
3. Electrons that displayed away from the
center would experience a force and thus
would be focused (deflected) with other
electrons.
• For the (S) and (G) , note that the highly
accelerated beam possesses more kinetic
energy and thus produces a brighter image on
the screen.
• The light on the screen depends on the
amount of energy that is transferred into the
screen by the beam.
Post deflection acceleration:
• If the velocity of the electron beam slows
down then the light on the screen will drop
off. Therefore great acceleration of the beam
is desired in fast oscilloscopes.
• However, fast acceleration makes it more
difficult to deflect the beam. To overcome this
problem we have two ways:
1) Use of scan expansion mesh:
• Accelerating the electron beam to a relatively
low velocity (through a few 1000s volts) then,
after the deflection, the beam is farther
accelerated to the desired final velocity
(through 10 kV or more) this is performed by a
metallic mesh which acts as a magnifying lens
that causes the deflection to be further
increased.
• NOTE: the second acceleration doesn’t affect
the deflection sensitivity
• The disadvantages:
1. The mesh tends to defocus the electron
beam (due to conducting some of the
electrons away from the screen), which
reduces the beam current and thus reduces
the spot intensity on the screen.
2. The electron beam tends to be defocused in
the area of the deflection plates, this is
caused by the repulsion from charge
distributions within the beam.
2 ) Meshless scan expansion :
•In this method the focus electrodes are
constructed from individual metal wafers which
allow for different focusing characteristics.
• Now the beam is accelerated to final velocity by
aquadrapole lens which to do it without
distorting or defocusing the beam , now this
causes an increase in the deflection sensitivity
(here the typical values are 2.3 v/cm (vertical
deflection), 3.7 v/cm ( horizontal deflection)
where the difference between horizontal and
vertical is due to the occurrence of (VD) at low
velocity .
• The mesh-less electron gun can be used for
oscilloscopes that operate at frequencies
higher than 100 MHz , using integrated circuits
with (40 –50)V for deflection
• Note: The mesh-less tube makes smaller and
lighter oscilloscope.
Oscilloscope screens:
For the inner surface of the screen,
• It is phosphoric which has the property of
emitting light when stimulated by radiation
(kinetic energy), this property is referred to as
fluorescence.
• The screen (phosphor) can continue the
emission of light after cutting off the source;
this property is known as phosphor sense.
• The time of continuing emission of light is
referred to as persistence of phosphor.
• The persistence of phosphor is measured in
terms of the time required for the image to
decay to ascertain percentage (10%) of the
original light output.
• The intensity of the image on the screen is
known as “luminance”, which depends on:
1. The number of electrons strikes the
screen/second.
2. energy with the electrons, which depends on
the accelerated voltage.
3. The physical characteristics of the phosphor
screen .
• (in general ,phosphor (p31)) is known to be
the best for general purpose viewing since it
has high luminance and medium persistence.
• Notes:
• Excessive current density may cause a permanent
damage to the phosphor through burning.
• The process (controlling the current density)
depends on two factors:
1. Beam density which controlled by :
a) Intensity
b) Focus
c) Astigmatism
2. Duration of excitation: which controlled by the
sweep (time/div).
• Note: the striking electrons release secondary
emission electrons. These electrons having low velocity
are called by conductive coating known as aquadag on
the inside of the glass tube.
• The CRT must be supplied with several dc potentials to
provide proper acceleration controls and focusing. The
1st requirement is a law voltage for the cathode heater.
The 2nd requirement is the total accelerating voltage
which is applied in two halves:1. High negative potential applied to the cathode grid.
2. High positive potential applied to the post deflection
acceleration electrode.
• This process prevents the output of the deflection
amplifier from being at high potential and simplifies
the design of the circuit.
• The required operating voltage are derived from
voltage dividers with the following controllers :1. Intensity: - this controller varies the potential
between the cathode and the control grid, and
simply adjusts the beam current.
• For the intensity control an increase in the beam
current increase the # of electrons striking the
phosphoric screen and adjusts the light output.
2. Focus: - this controller adjusts the focal length of
the electron lens
3. Astigmatism: - this controller adjusts the
potential between the deflection plates and the
1st accelerating electrode used to produce a
focused spot.
• Note: since the deflection sensitivity depends
on the value of the accelerating voltage this
voltage is usually regulated.