Instruction manual

HB 01-26-09 Oscilloscope (1) Lab 5 2
time interval plotted is P = t
2
− t
1
. In Fig. 1 the curve to the right of t
2
is dotted. No
plotting is done for a time interval N but the actual voltage is following the dotted curve. At
time t
3
= t
2
+ N , when the voltage has positive slope and the value V
T
(the same values at
time t
1
) the plotting on the paper is resumed not at horizontal axis point t
3
but at horizontal
axis point t
1
. Again the plotting is stopped at horizontal axis point t
2
. This second plotting
exactly overlays the first one and “refreshes” the curve, which otherwise would fade away.
This process is continued. Every time the plotting on the paper stops at point t
2
it is resumed
after time N at point t
1
when the voltage has positive slope and reaches the value V
T
. The
plotting always starts at the same point in the cycle. The “over” drawing of the same curve
is repeated again and again to get around the vanishing ink problem. (Note: It has been
assumed that for one cycle the same slope and V
T
occur only once. This will be true of all
the voltages you examine. Can you construct a periodic voltage where this is not true?)
If the voltage is periodic it should be noted that no more than one cycle of the voltage
is necessary to know everything there is to know about the voltage. In Fig. 1 the plotted
(solid) curve is more more than one cycle but less than two cycles.
Fortunately there is an instrument that does all this. It is the oscilloscope, or scope.
In the analogy, the “pen” is a pencil beam of energetic electrons and the “paper” is a
material known as a phosphor. When hit by the electrons the phosphor emits light at the
point where the electrons strike the phosphor. When the electron beam moves across the
phosphor the curve it traces can be observed by looking at the light emitted by the phosphor.
But in analogy with the vanishing ink, the light from the phosphor fades with time when
the electrons are not striking it. To keep the curve visible on the phosphor it is necessary for
the electron beam to retrace the same curve on the phosphor again and again. One sweep
of the electron beam across the phosphor takes the time P. The electron beam is shut off or
“blanked” for a time N.
The voltage V
T
stands for 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.
3 Analogue Oscilloscope Basics
The heart of an analogue scope is a cathode ray tube (CRT). See Fig. 2. A cathode ray is
an historical name for a beam of electrons. The “tube” refers to the glass vacuum envelope.
Electrons are thermally emitted from a hot cathode. The electrons are accelerated (to 2 kV
in the BK scope) by an electrode and then focused into a thin beam by electrostatic lenses.
A vacuum is necessary so that the electrons will not be scattered by air molecules and so
that the cathode will not burn up. The electron beam is passed through 2 pairs of 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