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In this section we describe first the cathode-ray tube. Based on that the setup of an oscilloscope will be presented and finally its use explained.
The oscilloscope is a basic tool to display periodic, time dependent voltages, which will be presented on its screen. There are two versions, analog and digital oscilloscopes. The incoming signal can be stored by a digital oscilloscope. By that technique, for example, very slow processes, which may happen only once, can be displayed.
The main part of an electron beam oscilloscope is the so called cathod-ray tube. It consists out of an evacuated glass tube, whose front end is coated internally by a fluorescent thin film. The glass tube contains the following elements in linear order: cathode, Wehnelt cylinder, electron optics, anode, capacitor plates to deflect electrons.
Fig. 1: Setup of the cathod-ray tube.
The cathode (negatively charged electrode) will be heated by the wire and consequently releases electrons, which are accelerated towards the anode (positively charged electrode). If the electrons pass the following electric components a beam of electrons will be formed. The electrons are passing the Wehnelt cylinder, which is negatively charged with respect to the cathode. According to the size of this voltage more or less electrons will be furtheron accelerated towards the anode i.e. more or less electrons will hit the screen. If the electrons hit the fluorescent thin film, we can see a bright green spot. Therefore, the brightness of this spot on the screen can be varied by the voltage of the Wehnelt cylinder.
With the use of the electron optics the beam of electrons can be focused and the diameter and contrast of the spot can be adjusted. The electron optics works like an electric lense, which is generated by proper oriented, geometrical arranged electric fields.
The anode is like a ring, so the electrons can pass. The controlled deflection of the electron beam is produced by plates; each pair of plates is oriented parallel and pairwise vertical. If voltages are applied on each plate, the electron beam can be deflected in vertical and horizontal direction separately.
To display time dependent voltages by such a cathod-ray tube respective voltages U(t) must be applied to the suited electrodes and some more components for tuning and control must be added.
Fig. 2: Scheme of a working electron beam oscilloscope.
The signal voltage to be displayed will be applied via an amplifier on the vertically oriented plates (deflection in y-direction). If the applied signal voltage would be, for example, a sinusoidal alternating voltage then the image on the flourescent screen would be a vertical bar.
To demonstrate its time dependence U(t), the electron beam must also be deflected in x-direction. This can be achieved by a saw tooth generator, whose saw tooth voltage will be applied on the horizontally oriented plates (deflection in x-direction). The increase of the saw tooth voltage moves indirectly the spot in x-direction. After a full period of saw tooth oscillation this spot jumps back to its starting position (time base). If both voltages - saw tooth voltage and signal voltage - superimpose properly we can recognize the picture of the sinusoidal alternating voltage.
To achieve a standing picture of a time dependent voltage (e.g. sinusoidal alternating voltage) on the screen of the oscilloscope the signal voltage and the saw tooth voltage must be synchronized. The trick is that the deflection in x-direction always starts in phase with exactly the same value of the signal voltage: this technical procedure is called triggering. The saw tooth generator is started by a periodic rectangular voltage of the trigger unit. The rectangular voltage, on the other hand, is produced if the signal voltage has reached a certain choosable value (called trigger level). If one whole period of the saw tooth voltage produced by its generator has passed, this generator is not producing the next period, unless the signal voltage has reached the same value in phase as in the first cycle. The next time deflection is triggerd by the rectangular voltage and the whole process is repeated.
Fig. 3: Time evolution of voltage of the time deflection generator.
If the trigger level is tuned the starting point to display the signal voltage can be varied. If the trigger level is higher than the signal voltage no synchronization of both voltages is achievable. As a consequence the displayed signal voltage seems to be shifted in horizontal x-direction on the oscilloscope screen.
This real oscilloscope in that RCL can be used directly via the image of the oscilloscope. The few elements, which can be used in the lab section, will be briefly described next.
Fig. 4: Front panel of the oscilloscope with knobs for handling.
Left up one can identify the power switch; touching its upper part the oscilloscope is switched on, touching its lower part it is switched off.
Here the signal can be shifted in x-direction and in y-direction ( x ≡ time; y ≡ voltage). The upper and the lower part of the knobs is additionally subdivided in an internal and external section; touching the internal one produces a small shift, touching the external one a larger shift.
The amplification of the signal in y-direction can be varied with that knob, i.e. ratio of voltage and scale unit (box size on screen is 1x1 cm2). On the oscilloscope screen one can read Volts/Div. as units. This function allows to zoom in y-direction.
With that controller one can vary the unit for the time scale, i.e.Time/Div. (box length on screen is 1 cm). This will be technically realized in that oscilloscope in fitting the frequency of the saw tooth generator. This function allows to zoom in x-direction.
This controller allows to choose the above described trigger level.
In the following we would like to describe and explain all elements of the display. A possible picture of the oscilloscope screen may be:
Fig. 5: Display of the oscilloscope screen.