The oscilloscope is the basic instrument for the study of all types of waveforms and measuring parameters. The cathode ray oscilloscope consists of

Transcription

1 Oscilloscopes 1

2 Contents: 1-1 Cathode Ray Tube Oscilloscope 1-2 Waveform Display 1-3 Measurement of Voltage, Frequency, and Phase 1-4 Pulse Measurements 1-5 Dual Trace Oscilloscope 1-6 Special Oscilloscopes 2

3 The oscilloscope is the basic instrument for the study of all types of waveforms and measuring parameters. The cathode ray oscilloscope consists of 1- Cathode ray tube (CRT), 2- Power supply, 3- Input circuitry (deflection amplifierstime base circuit), and 4- Oscilloscope controls. 3

4 In the CRT, electrons generated at a heated cathode are shaped into a fine beam and accelerated toward a screen. The screen glows at the point at which the electrons strike. The electron beam is easily deflected vertically and horizontally by voltages applied to deflection plates. 4

5 5

6 CRT consists of three basic sections: Electron gun a)triode section b) Focusing section Deflection section Screen 6

7 1- Electron Gun: The electron gun is responsible for generating a fine beam of electrons. This beam of electrons is then directed to the screen causing the phosphors coating on the screen to glow where the electrons strike. It consists of two sections: a) Triode section, and b) Focusing section. 7

8 a) Triode Section: Consists of a cathode, a grid, and an anode. Electrons emitted from the cathode toward the hole in the grid. Cathode heating is provided by a filament. The grid-cathode potential controls the electron flow (control the number of electrons reaches the screen (brightness). 8

9 b) Focusing Section: A1 provides the accelerating filed to draw the electrons from the cathode. The hole in A1 limits the initial cross section of the electron beam. The ve potential on A2 tends to slow down the electrons, but they are accelerated again by A3. 9

10 2- Deflection Section: If the vertical and horizontal deflecting plates were grounded, the beam of electrons would strike the center of the oscilloscope screen (bright glowing point). When one plate of a pair of deflecting plates has a +ve voltage applied to it, and the other one has a ve potential, the electrons in the beam are attracted towards the +ve plate and repelled from the ve plate. 10

11 When the upper plate potential is +E/2 Volt and the lower plate potential is E/2 Volt, the potential difference between the plates is E Volt and there will be a deflection on the screen that proportional to E. 11

12 1- Deflection Sensitivity: Is the deflection produced by 1 Volt (cm/v). 2- Deflection Factor: Is the voltage required to produce one division (one cm) of deflection at the screen (V/cm). 12

13 A grounded isolation shield is situated between the V. and H. deflecting plates. This prevents the electric fields of one set of plates from influencing the other pair of plates. 13

14 Post Deflection Acceleration: For maximum brightness, the electrons should be accelerated to its greatest velocity. However, if the electron velocity is very high when passing through the deflection plates, the deflecting voltages will have a reduced influence and the deflection sensitivity will be poor. 14

15 It is for this reason that the post deflection acceleration is provided; that is the electrons are accelerated again after they pass between the deflecting plates. A helix of resistive material is deposited on the inside of the tube from the deflecting plates to the screen and is connected to a very high voltage (12 kv) at the screen and zero at the other end. Thus, the electrons leaving the deflecting plates experience a continuous accelerating force all the way to the screen. 15

16 3- Screen: The screen of a CRT is formed by depositing a coating of phosphor material on the inside of the tube face. When the electron beam strikes the screen, electrons within the screen material are raised to a higher energy level and emit light as they return to their normal levels. The color of the light depends on the material employed. 16

17 A graphite coating termed aquadag is used around the neck of the tube, so that the negative potential does not accumulate on the screen (if the accumulated electrons increase two much, this will repel the electron beam. Also the coating provide a heat think, conducting away heat that might damage the screen. 17

18 Waveform Display If there is no potential difference applied to both the horizontal deflecting plates (H.D.P) and the V.D.P, the electrons strike the origin (center) of the screen. 18

19 If a sawtooth potential difference is applied to the H.D.P and no signal is applied to the V.D.P, the beam will be swept left to right at a uniform rate, causing a horizontal line in the middle of the screen. 19

20 When an alternating voltage is applied to the V.D.P, and there is no signal at the H.D.P, the beam is deflected vertically. This produces a vertical line centered on the screen. 20

21 If a sawtooth potential difference is applied to the H.D.P and the input signal is applied to the V.D.P, the beam will trace out a plot of the vertical signal (as a function of time). 21

22 22

23 23

24 CRT Oscilloscope Block Diagram 24

25 A basic Cathode Ray Oscilloscope (CRO) consists of: 1- Cathode ray tube 2-Time base (sweep) generator 3- Horizontal amplifier 4-Trigger circuit 5- Vertical amplifier 6-Delay Line 25

26 1- The CRT is used to trace an input signal on the screen. 2- The time base generator generates the saw tooth voltage required to deflect the beam horizontally. 3- The horizontal amplifier amplifies the saw tooth voltage, before it is applied to the horizontal section. 26

27 4- The trigger circuit is used to convert the incoming signal into trigger pulses, so that the input signal and the sweep frequency can be synchronized. 5- The vertical amplifier amplifies (or attenuates) the input signal to make it seen properly on the screen. This process is controlled by volt/div control. 6- The delay line is used to delay the signal in the vertical section (since the signal takes longer to reach to the H.D.P as compared to the vertical section). 27

28 Oscilloscope controls Intensity (Brightness): It controls the number of electrons per beam by varying the grid bias. Focus: It controls the sharpness (concentration) of the beam (thickness of the electron beam). Time base control: It controls the frequency (period) of the generated sawtooth signal. Amplitude control: It controls the amplitude of the input voltage (V/div). X-Shift (horizontal position control): Controls the x-position of the spot on the screen. 28

29 Y-Shift (vertical position control): It controls the y-position of the spot on the screen. Synchronize selector and Trigger selector: A sync selector, with the use of INT-EXT-Line switch (trigger selector) can synchronize the sweep to internal signals from the vertical amplifier, external signals or the line supply. DC - GND AC: Is a 3-position switch used to adjust the level when DC or AC is measured. To adjust for zero level, the switch has to be in the GND position. 29