Instruction manual

HB 01-26-09 Oscilloscope (1) Lab 5 3

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 BK scope is actually capable of showing two traces (termed 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.

4 Description of How a Scope Displays a Single Periodic Voltage

A periodic function in time is one that repeats itself again and again, such as a sine wave or

a square wave. A graph of such a function, with time on the horizontal axis and the function

on the vertical axis, repeats itself every period of the function. A scope can display a voltage

that is periodic in time in exactly the same way. The periodic voltage is connected to the

vertical input of the scope, and if the scope is adjusted correctly, a graph of the voltage as

a function of time appears as a stationary trace on the screen of the scope.

The ramp voltage that is applied to the horizontal deﬂection plates of the scope is usually

supplied by the TIME BASE oscillator unit of the scope. We consider that case here. (If

the scope has “X-Y” capability the voltage to the horizontal deﬂection plates can also be

supplied from an external voltage source.)

Every scope has a signal generator or time base oscillator. This oscillator produces a

voltage that is a linear ramp as a function of time. Fig. 3 shows the ramp voltage for one

sweep of the electron beam. This linear ramp, when applied to the horizontal deﬂection

plates, sweeps the electron beam from the left side of the screen to the right side of the