Cathode Ray Oscilloscope
Front Panel
Sweep Generator
CRO Operation
Introduction
The cathode-ray oscilloscope (CRO) is a common laboratory instrument that provides accurate
time and
amplitude measurements of voltage signals over a
wide range of frequencies .
Its reliability, stability, and ease of operation make it suitable as a general purpose
laboratory instrument. The
heart of the CRO is a
cathode-ray tube shown schematically
in Fig.1 .
The device consists mainly of a vacuum tube which contains a
cathode ,
anode ,
grid ,
X & Y-plates,
and a
fluorescent screen . When the cathode is
heated (by applying a small potential difference
across its terminals), it
emits electrons . Having a potential difference between the
cathode and the anode (electrodes),
accelerate the emitted electrons towards the anode,
forming an electron beam, which passes to fall on the screen.
When the fast electron beam
strikes the fluorescent screen, a bright visible spot is produced. The grid, which is
situated between the electrodes, controls the amount of electrons passing through it
there by controlling the intensity of the electron beam. The X & Y-plates, are responsible
for deflecting the electron beam
horizontally and
vertically .
A
sweep generator is connected
to the X-plates, which moves the bright spot horizontally across the screen and repeats
that at a certain frequency as the source of the signal. The voltage to be studied is
applied to the Y-plates. The combined sweep and Y-voltages produce a graph showing the
variation of voltage with time, as shown in Fig. 2
The front panel of the CRO is shown in Fig.2 .
In the most common use of the oscilloscope the signal to be studied is first
amplified
and then applied to the vertical (deflection) plates to deflect the beam vertically and
at the same time a voltage that
increases linearly with time is applied to the horizontal
(deflection) plates thus causing the beam to be deflected horizontally at a
uniform
constant rate . The signal applied to the verical plates is thus displayed on the screen
as a function of time. The horizontal axis serves as a
uniform time scale .
The linear deflection or sweep of the beam horizontally is accomplished by use of a sweep
generator that is incorporated in the oscilloscope circuitry. The voltage output of such
a generator is that of a
sawtooth wave as shown in Fig. Application of one cycle of
this voltage difference, which increases linearly with time, to the horizontal plates
causes the beam to be deflected linearly with time across the tube face. When the voltage
suddenly
falls to zero , as at points (a) (b) (c), etc...., the end of each sweep - the
beam flies back to its initial position. The horizontal deflection of the beam is
repeated periodically, the frequency of this periodicity is adjustable by external controls.
A simplified
block diagram of a typical oscilloscope is shown below in Fig. In general,
the instrument is operated in the following manner.
The signal to be displayed is by the
vertical amplifier and applied to the verical
deflection plates of the CRT.
A portion of the signal in the vertical amplifier is applied to the sweep trigger
as a
triggering signal . The sweep trigger then generates a pulse coincident with
a selected point in the cycle of the triggering signal. This pulse
turns on the
sweep generator, initiating the sawtooth wave form. The sawtooth wave is amplified
by the horizontal amplifier and applied to the horizontal deflection plates.
Usually, additional provisions signal are made for appliying an external
triggering signal or utilizing the
60 Hz line for triggering. Also the sweep
generator may be
bypassed and an external signal applied directly to the horizontal amplifier.
