These are free to download and to share with others provided credit is shown. Phase difference (φ phi) =( 2 π x 0.6)/3.1 = 1.2 rads.īesides frequency, Lissajous figures can also be used to identify phase differences.Īll downloads are covered by a Creative Commons License. To measure the phase difference in radians, the horizontal length of one wave (in cm) must also be known (the period).Įxample: The phase difference is 0.6 cm and the wavelength is 3.1 cm. On a double beam oscilloscope the phase difference in cm is be measured along the time-line between similar points on each wave. In this way an unknown frequency can be identified, provided it is in a simple ratio with the first frequency. We then have the case where we have two AC signals, one across the X-plates and the other across the Y-plates.ĭifferent ratios of frequency give different images. These images are obtained by switching off the time-base and inputting an AC signal into it. If the time-base is 2ms/cm (2 x 10 -3 sec./cm), then the period (T) is:Īnother way of measuring frequency is by Lissajous figures. The period T is inversely proportional to frequency f :Įxample: In the example above, measuring from the first crest to the second horizontally, the distance is 3.3 cm. Once T has been measured, the frequency f can then be found. In other words to find the period ( T) of the wave (the time interval for one complete oscillation). The peak-to-peak voltage ( V PP) is the vertical displacement between the minimum and maximum values of voltage.įrequency is measured by finding the 'wavelength' of the waveform along the time-line. The peak voltage ( V 0) is the maximum vertical displacement measured from the time-line. It is important to know the names of vertical measurements on a waveform. It has units of ' volts/cm' or ' volts/division'. This controls the vertical deflection of the dot. The control has units of ' time/cm' or ' time/division', with settings in the range 100 ms - 1 μs per cm/division. With higher settings the dot appears as a horizontal line. With the time-base switched off, the dot appears static in the centre. This controls how fast the dot moves across the screen. These ' collimate' the beam into a narrower, faster stream of electrons, producing a smaller, sharper dot on the screen. The electron beam is focussed by passage through several annular anodes. of the grid can then be varied to accelerate electrons through it or repel some back to the cathode. This beam current is controlled by having a grid over the cathode. The heating element heats the cathode in the 'electron gun' to produce electrons forming a beam current. The brightness of a CRO display is a measure of the numbers of electrons impacting the screen. The graphite layer also catches rebounding electrons that are back scattered from impact with the screen. Without this feature, the accumulated charge would reduce the numbers of electrons arriving at the screen, reducing brightness. To reduce this effect, the graphite is electrically connected to 'earth' (zero volts). This fluoresces (gives out green light) when electrons impact its surface.Ī layer of graphite is painted on the the inside of the vacuum tube close to the fluorescent screen.Īs a result of impacting electrons, the screen acquires a negative charge. The display screen is coated on the inside with a very thin layer of a phosphor called cadmium sulphide. With the time-base on, a sine wave is displayed. across the X-plates (the time-base) switched off, a sinusoidal signal makes the dot go up and down, executing simple harmonic motion. applied to the Y-plates is the signal to be examined. The curve has the general shape of a ' saw-tooth' and is often referred to by this name. The applied voltage is called the time-base. The effect is to move the dot very quickly from right to left (fly-back). The voltage is then reversed and increased rapidly. On the CRO screen this translates as an illuminated dot moving from left to right. This causes the electron beam to be slowly repelled from the left-hand plate and attracted towards the right-hand plate. To display a waveform, a repetitive reversing voltage is applied to the X-plates. The deflection system consists of two pairs of parallel plates called X-plates and Y-plates. The anodes accelerate the electrons and collimate them into a narrow beam. The cathode ray oscilloscope consists of three main elements:Įlectrons are produced by thermionic emission.Įssentially a cathode(negative electrode) is heated and electrons boil off the surface to be attracted by a series of anodes (positive electrodes). Home > Electricity, cathode ray oscilloscope
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