precautions while using cathode ray oscilloscope

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Precautions while using Cathode Ray Oscilloscope

The cathode-ray oscilloscope (CRO) is a common laboratory instrument that provides accurate time.
1. Cathode ray oscilloscope
The cathode ray oscilloscope CRO is a common laboratory instrument that provides accurate time and aplitude 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.
The cathode ray is a beam of electrons which are emitted by the heated cathode negative electrode and accelerated toward the fluorescent screen. The assembly of the cathode, intensity grid, focus grid, and accelerating anode positive electrode is called an electron gun. Its purpose is to generate the electron beam and control its intensity and focus. Between the electron gun and the fluorescent screen are two pair of metal plates one oriented to provide horizontal deflection of the beam and one pair oriented ot give vertical deflection to the beam. These plates are thus referred to as the horizontal and vertical deflection plates. The combination of these two deflections allows the beam to reach any portion of the fluorescent screen. Wherever the electron beam hits the screen, the phosphor is excited and light is emitted from that point. This coversion of electron energy into light allows us to write with points or lines of light on an otherwise darkened screen.
2. Cro operation
A simplified block diagram of a typical oscilloscope is shown in Fig. 3. In general, the instrument is operated in the following manner. The signal to be displayed is amplified 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.
3. Cro controls
The controls available on most oscilloscopes provide a wide range of operating conditions and thus make the instrument especially versatile. Since many of these controls are common to most oscilloscopes a brief description of them follows.
4. Switching the cro on
O.K. this may seem simple, but the first step is simply the act of locating the power switch and turning the CRO on.Seems simple but just keep it in mind that somebody is going to have trouble doing this.
5. Ensuring the cro is calibrated correctly
Rotate both Gain Variable Controls and the Time Base Variable Control completely clockwise, or untill an audible click becomes apparent.Taking the action mentioned above allows us to be sure that the CRO is properly calibrated and any readings taken are accurate.Where can I find diagrams showing the location of these controls.
6. Adjusting the trace position and appearance
First we set the AC GND DC for both Channels A and B to GND.this should allow us to see a horizontal line across the display The next thing that we do is use the Mode Selector to allow us to view channel A on the display.Now that we can see the position of the trace we use both the Intensity and Focus controls to achieve a clear appearing trace.Finally we move the trace into the center of the display using the both the Horizontal Position Control and the Vertical Position Control corresponding to Channel A.Use the Mode Selector to view the trace of Channel B on the display.Repeat d .Finally to check that the above steps have been carried out correctly we can set the Mode Selection Switch to DUAL which will allow us to view both traces on the displays.s
7. Checking that the cro is triggered correctly
Before any measurements can be made we must ensure that the CRO is correctly triggered. The best way to do this is to follow the steps listed below. First check that the Trigger Level Control is set in the AUTO position. This is done by pulling the knob towards you.Now we put the Slope Control to the Positive selection.Ensure that the Sync Selector is in the A.C. position.Finally make sure that unless otherwise advised, leave the Source Selector in the INT position.
8. Connecting the cro to the circuit
It is most important, when connecting the CRO to make a circuit measurement, to remember that the CRO can only take readings of VOLTAGES. This means that the CRO should always be connected in PARALELL to the circuit element over which we are trying to find the voltage.We now need to select which device we will use to connect the CRO to the circuit. The two devices that we have to choose from are the X10 Probe and the Coaxial Cable. In almost all cases we will use the Coaxial Cable as the X10 Probe is only required when we wish to make extremely accurate readings.
9. Measuring the input signal
O.K. now we that the CRO has been connected to the circuit we can actually adjust the view of the signal on the display to allow us to make an accurate reading This is done by following the instructions listed belowSelect the input channel that we wish to view using the Mode Selector.i.e. if Channel A is required the Mode Selector would be in the CH A position, Channel B the CH B position and to see both of the input signals the Mode Selector would be in the DUAL position.
After selecting the input we wish to the AC GND DC Selector is moved from the GND position to the D.C. position.NOTE It may seem that if an A.C. signal is being measured, the AC GND DC Selector should be set in the A.C. position. This is not neccessary unless there is a large D.C. offset in the A.C. input. So please avoid the temptation to use the A.C. selection.At this point the Vertical Gain must be adjusted. This is adjusted to allow the signal to be viewed on a scale that is convienient. Show me examples of how the Vertical Gain Controls Work..
10. Objective
To learn how to operate a cathode ray oscilloscope.


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