Instruction manual

HB 08-18-2010 2
deflection plates. A voltage across one pair of deflection plates deflects the electron beam in
the vertical direction, while a voltage across the other pair of deflection plates deflects the
electron beam in the horizontal direction. After passing through the deflection plates the
electron beam strikes a phosphor material that covers the inside surface of a flat portion of
the vacuum tube. Light is emitted where the electron beam strikes the phosphor and some
of this light passes to the outside of the tube and can be observed. If the electron beam is
suddenly cut off, the light from the phosphor does not stop immediately but decays in less
than a second. The flat part of the tube looked at is called the screen. When you are looking
at the screen the electron beam is coming toward you. By applying time dependent voltages
to the deflection plates the beam can be moved around the screen creating a pattern of light
on the phosphor often called a “trace.” As the light from the phosphor decays quickly the
trace must be continually refreshed by the electron beam so that the image on the screen
can be observed. When the trace is “stationary” or “frozen” the electron beam continually
refreshes the trace by repetitively executing the same path on the screen. The screen has
a grid or graticule, usually in cm, which enables voltages (vertical deflections of the trace)
and times (horizontal deflections of the trace) to be measured.
The image on a television screen that uses a CRT is formed the same way, except the
deflection of the beam is done magnetically rather than electrostatically. Many modern TV’s
do not use a CRT.
The most basic way in which a scope is operated is to apply the voltage you want to
examine (the input voltage) to the vertical deflection plates. The vertical deflection of the
electron beam and the trace will then be proportional to the input voltage. A linear ramp
voltage, produced by the scope, is applied to the horizontal deflection plates. This ramp
voltage sweeps the electron beam horizontally across the screen at a uniform rate. If the
input voltage is periodic, and the horizontal ramp voltage is also made periodic with an
appropriate frequency, the scope trace will be a graph of the time dependent input voltage,
with the vertical axis the input voltage and the horizontal axis the time.
The voltage V
T
is the trigger voltage. It is the value of the voltage that tells the electron
beam to start the beginning of the curve. The horizontal time scale of the curve is equivalent
to how fast the electron beam is swept across the phosphor. This is determined by the “time
base” of the scope. If the voltage has a very high frequency the time base needs to sweep
the electron beam across the phosphor quickly. A good scope can display voltages with
frequencies in the MHz range. If the frequency is not high, the electron beam needs to move
across the phosphor slowly. By varying the time base it is possible to put many cycles of the
voltage on the scope or just part of one cycle.
The BK scope is actually capable of showing two traces (known as a dual trace or 2
channel scope). When used this way one can input two voltages and observe how each input
voltage produces its own trace. In later experiments you will see how useful this feature
can be. The BK scope can also be used as an x-y scope. When used this way, one input
voltage is applied to the vertical deflection plates and the other input voltage is applied to
the horizontal deflection plates. The trace then shows the plot of one voltage against the
other. For the time being we will not discuss these two capabilities and will use the BK
scope as a single trace scope with the horizontal deflection voltage supplied by the scope.