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Hi all, lately I've been interested in oscilloscopes a lot and i wanted to represent the basic moments of a CRT oscilloscope simply. An oscilloscope basically converts electrical signals into visual ones, for analysis and measurement. The way a CRT oscilloscope works is that you have a cathode ray tube to represent the visual variations of the wave and control circuitry to manage the CRT. The CRT has several electrodes:
- Filament - heats up the cathode. Typically works with low voltage high current power supply.
- Cathode - generator of thermionic emission. Basically a heated metal plate that emits electrons. Typically is charged very negatively, sometimes up to a few kV negative voltage.
- Control grid - Controls the acceleration of electrons thus the screen luminosity. Typically is charged positively up to a few hundred volts. There are usually a few consecutive anodes which further accelerate the beam.
- Focusing Anode - controls the focus of the beam, by narrowing or expanding the stream of electrons.
- Vertical deflection plates - these are the plates that deflect the beam up and down. For typical measurement applications this is where the amplified input signal is applied. They controls the Y axis of the scope and show the amplitude of the studied wave
- Horizontal deflection plates - these plates usually have a sawtooth generator controlling them at the same frequency or a multiple of that frequency as the input signal. I'll get further into that in a minute. They control the X axis of the scope and show the period of the studied wave.
- Phosphorus screen - a phosphorus coated screen that illumimates at the point of which the electron beam hits it. The intensity of the beam is directly proportional to the luminosity of the screen. Typically the screen would have multiple layers of coating some of which lead coat, to prevent x ray leakage.
Now how it works. - The heater heats up the cathode, which is very negatively charged. It starts emitting electrons, that are neither focused nor accelerated yet. The beam passes through several plates which direct it and accelerate it towards the phosphorus screen. The vertical deflection plates represent the Y coordinate in an oscilloscope. They move the beam up and down proportionally to the input signal. This still gets us nowhere because the only thing you'll see at this point is a vertical line on the screen. We need a time base, something to show the variation of the signal through time. This is where the horizontal plates come in. They are usually driven by a sawtooth generator with the same frequency or a multiple frequency of the input signal. The way it works is that the sawtooth wave moves the beam from left to right while the vertical plates move it up and down. After the horizontal sweep reaches the end of the screen it quickly has to return to the beginning of the screen and start the cycle again. The return time is usually not instantaneous, so newer oscilloscopes usually use some sort of disabling circuit, to negate the return path from showing on the screen (you can see the return path here around the 0V). In order for it to work the sawtooth wave must have either the same or a multiple of the frequency of the input wave. For instance if we have a 1:1 ratio the screen will show only one cycle for the wave. A ratio of 1:2 will show 2 cycles and so on. Here I've made it with a ratio of 1:10 which gives us 10 complete cycles on the scope. You can play around with the ratios but the sawtooth frequency must always be a multiple of the input frequency. You can try to use a sawtooth wave with a different frequency than the original or the multiple and see how it affects the visualisation. Try 2.3ms for the sawtooth. Typically all deflection plates are driven by carefully trimmed voltage amplifiers, and all other plates have potentiometers to vary their impact on the beam, like intensity, focus, center position of the X and Y axis.
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