Studying CRO-CathodeRayOscilloscope

Q. What is Cathode Ray Oscilloscope? 

A. Cathode-ray oscilloscope, an electronic-display device containing a cathode–ray tube (CRT) that generates an electron beam that is used to produce visible patterns, or graphs, on a phosphorescent screen.

The graphs plot the relationships between two or more variables.With Horizontal axis normally being a function of time and Vertical axis normally being a function of the voltage generated by input or output signal. We take voltage as the vertical axis because we can almost convert any physical quantity to it’s corresponding electrical voltage. This makes the OSCILLOSCOPE a versatile tool for measuring various quantities and seeing the respective graph formations.

THE GERMAN PHYSICIST FERDINAND BRAUN DEVELOPED THE FIRST CATHODE-RAY-OSCILLOSCOPE IN 1897.

The central component of the device is the CRT(Cathode-Ray-Tube). It is the heart of the CRO that gives you the graph on the phosphorescent screen.

Cathode-Ray-Tube consist of the evacuated glass container with the phosphorescent coating at one end and an electron gun at the other end with a focusing, accelerating and deflecting system in between them.

General INFO of a general purpose CRO:-

1. Screen Size– 10 cm X 8 cm. graticule.(most common).
2. Phosphor– The screen is coated on the inside with a fluorescent material called ‘Phosphor’. It decides the colour and the persistence of the trace.
   • Persistence-is the length of time the trace remains on the screen after the signal has ended. It is expressed as SHORT,MEDIUM and LONG.
   • Short Persistence- is used for extremely high-speed phenomena.
   • Medium Persistence- are used for general purpose applications.
   • Long Persistence- are used for transients, since they keep the fast transient on the screen for observation after the transient as disappeared.
3. Operating_Voltage– CRT requires a heater voltage of 3V-6V AC/DC at 600mA.
4. Deflection_Voltage- AC or DC both voltages deflect the beam. The distance through which the point on the screen moves is propotional to the DC voltage or the AC peak voltage.
5. Vertical Amplifier- determines to characteristics, SENSITIVITY and FREQUENCY BANDWIDTH response.  The attenuator used at the input stage of the verticle amplifier gives the V/div control on the front panel.
6. Delay Line- is used to delay the signal in the vertical section. The signal in the horizontal has to pass through the TRIGGER PICK OFF circuit, TIME BASE generator and the HORIZONTAL AMPLIFIER before reaching to the horizontal deflection system. Thus a delay line circuit is use to delay the vertical signal so that we get a perfect trace on the screen.
   • The time taken for the signal to pass through the Horizontal Section is 80ns and the Delay Line used is 200ns so that the Horizontal Sweep signals start in time.
7. Time_Based_Generator-is used to generate the sawtooth voltage required to deflect the beam in the horizontal section. The circuit used is CONTINUOUS SIGNAL GENERATOR. As this circuit can not withstand (follow) the variations of fast varying signals the circuit is modified and is now called TRIGGERED SWEEP GENERATOR. The timing of the time base generator gives the Time/div control on the front panel of CRO.
8. Horizontal_Amplifier- is used to amplify the sawtooth voltage before it is applied to the horizontal deflection system. This block consist of PUSH-PULL AMPLIFIER.
9. Trigger_Circuit- is used to convert the incoming signal into triggered pulse, so that the input signal and the sweep frequency can be synchronised. It is activated by variety of signals which are then converted to trigger pulse of the uniform amplitude. If the trigger is to low the circuit will not operate and if it is to high the UNI-JUNCTION TRANSISTOR (UJT) will conduct for too long and the part of the leading edge of the trace signal would be lost. It is operated by a three position switch INTERNAL, EXTERNAL and LINE on the front panel.
10. Power_Supply- CRO uses two power supplies, A NEGATIVE HIGH VOLTAGE and A POSITIVE LOW VOLTAGE. 

• Advantages of the high negative supply:
  • less insulation is required between the positioning controls and the chassis.
  • the accelerating anodes and the deflection plates are close to the ground potential. this ground potential protects the user from the high voltage shocks when making connections to the plates.
  • the deflection voltage are measured with respect to the ground potential hence high voltage blocking or coupling capacitors are not needed.

FRONT PANEL CONTROLS:-

1. Power:-Puts the instrument to main supply or cut it off with a LED indication.
2. Intensity:-Controls the brightness of the trace.
3. Focus:-Controls the sharpness of the trace.
4. Time Base:-18 step switch (normally) which enables the selection of 18 calibrated sweeps ranging from 0.5 microseconds/division to 0.2 s/div.
5. Time Base Variable:-Extends or holds the sweep speed of the wave in clockwise direction.
6. Hold :-makes the signal steady.
7. X Position:- Moves the trace left or right along the horizontal axis.
8. Y Position:- Moves the trace up and down along the vertical axis.
9. Level:- Controls variable speed and multiple waveform and selects the trigger point.
10. Auto/Normal:- In auto mode CRO displays a wave in absence of any input signal.
11. Ext/Int:-
    • EXT:-displays trigger derived from any external source.
    • INT:- displays trigger derived from CH1,Ch2, and LINE.
12. Line:-triggers the trace from power line frequency.
13. +/-:-Selects trigger point on either positive or negative slope of the displayed waveform.
14. AC/DC:-Selects trigger signal compiling.
15. HF rej:- Introduces the low pass filter in the trigger compiling.
16. 0.2 V, 1 kHz:-200mV(peak to peak), 1kHz square wave calibration.
17. GND:-selects input grounding.
18. EXT.TRG:- input BNC for external trigger signal. Checks if CRO is working properly or not.

~Jay Mehta

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