CRO is an electronic peak voltmeter.
It works for both AC and DC signals.
CRO gives the variation of signal with respect to time.
CRO is an image plotter and it will display two types of images namely Y-t plot and X-Y plot.
Y-t plot: In general it is the amplitude variations of an unknown signal.
The peak voltage, rms voltage, DC voltage, frequency signal, time period, rise time, fall time, etc can be measured using CRO.
CRO consists of two sections namely a horizontal section and a vertical section.
CRO is widely used in industries.
Working of CRO:
Electrons are generated in the electron gun from the cathode electrode to the anode electrode the electron flows, and the electrons are accelerated towards the electron beam by the electron gun.
Electrons will bombard the fluorescent screen to emit light.
The intensity of the electron beam decides the brightness of the light emission.
When more electrons are accelerated, photons will be produced where the electrons move from the lower level to the higher level and then fall back.
The intensity control will be there in the CRO to control the brightness.
The top plate and the bottom plate in the CRO are vertical deflection plates and they will be placed horizontally.
When the signal voltage is positive the electrons flow from the electron beam attracts towards the positive signal and strike the top side of the fluorescent screen.
When there is no signal it strikes at the center.
When there is a negative signal then the electron from the electron beam repels and it strikes the bottom side of the fluorescent screen.
Two vertically mounted plates show horizontal deflection on the left side and the right side based on the signal.
When the Y input is positive it moves to the positive side(+y)
When the Y input is negative it moves to the negative side(-y)
When the X input is positive it moves to the positive side(+x)
When the X input is negative it moves to the negative side(-x)
The vertical deflection plates should be close to each other and kept distant from the screen to cover the entire screen.
When the beam strikes the phosphor-coated screen it emits light.
No phosphor coating will be there at the edges.
Primary electrons are the electrons that strike the phosphor.
The electrons which strike aquadag are the secondary electrons that are reflected back or grounded.
Deflection sensitivity:
Deflection sensitivity is defined as the amount of deflection produced per unit deflecting signal voltage.
The deflection factor is the reciprocal of deflection sensitivity.
DF = 1 / Sv
Sv (vertical sensitivity) = y/Vy
SH (horizontal sensitivity) = x/Vx
Sv = LD / 2sVa
Sv is the vertical deflection sensitivity
s is the distance between two VDPs
L is the length of each VDP
D is the distance between the center of the VDP and the screen
Va is the accelerating anode voltage.
Block diagram of CRO
In general, the probe is used to sense the unknown signal. The sensed unknown signal will be transmitted into the Vertical terminals by the cable. Based on this the specifications of the CRO will be decided.
The input resistance of CRO will be around 1Megaohm which causes less loading on the quantity to be measured.
Note: Electronic voltmeter offers high input resistance at the input side.
The output of the vertical amplifier which is the unknown signal will be delayed by the delay line as shown in the block diagram.
The synchronization has to be achieved between the unknown signal and the reference sweep signal in Y-t.
The unknown signal which has been sensed will be given the Y input to produce vertical deflection.
In X-Y mode the external known signal will be given to produce horizontal deflection.
The output of the vertical amplifier is fed to the triggering circuit. This triggering circuit is a link between the horizontal input and the vertical output.
The components of the horizontal deflection system are the triggering circuit, time base generator, and the horizontal amplifier.
X-input is the known signal or the ramp input
Y - input is the unknown signal
The horizontal amplifier is fed to the X- input of CRT (sweep signal or saw tooth signal will be driven to X-input).
The user or the operator can adjust the sweep frequency using the time base selector switch.