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Working Principle of an Oscilloscope
An oscilloscope is a device which allows the amplitude of electrical signals (current, voltage, power etc.) to be display as a function of time.
Oscilloscope Parts and their Function
An oscilloscope consists of following major parts (see the block diagram) −
Cathode Ray Tube (CRT) – The CRT displays the quantity being measured. It generates and accelerates an electron beam, deflects the beam to create the image and contains a phosphor screen where the electron beam eventually becomes visible.
Vertical Amplifier –The vertical amplifier amplifies the waveform of the signal to be viewed.
Horizontal Amplifier – The horizontal amplifier is fed with a saw-tooth voltage which is then applied to horizontal deflection plates.
Sweep / Time Base Generator – It produces the saw-tooth voltage waveform, which is used for horizontal deflection of the electron beam.
Trigger Circuit – The trigger circuit produces trigger pulses to start horizontal sweep.
High & Low Voltage Supply −
Low Voltage (LV) Supply – It supplies the required operating voltages (of the order of few hundred volts) to the all circuits of the oscilloscope.
High Voltage (HV) Supply – It supplies the high voltages (of the order of few thousand volts) required by CRT, for acceleration as well as relatively low voltage for the heater of the electron gun, which emits the electrons.
Delay Line – The purpose of the delay line is to delay the vertical signal enough to keep it from reaching the CRT deflection plates before the horizontal circuits are running. This line presents in the vertical amplifiers of high frequency oscilloscopes.
Oscilloscope Working Principle
The signal is to be viewed on the screen being applied across the Y-plates of CRT. To see the waveform of the input signal, it is essential to spread it horizontally from left to right, which is done by applying a saw-tooth voltage wave to X-plates. Under these conditions, the electron beam would move uniformly thereby graphing vertical vibrations of input signal with respect to time.
Due to repetitive tracing of the viewed waveform, a continuous display is obtained because of persistence of vision. To get a stable stationary display on the screen, the input signal across the Y-plates must be synchronised with the horizontal sweeping of the beam.
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