What is Analog Cro and its Manual Operation for Beginner

DUAL TRACE CRO

CRO OPERATION

The function of an oscilloscope is extremely simple: it draws a V/t graph, a graph of voltage against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis.

As you can see, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divisions on the horizontal axis. Usually, these squares are 1 cm in each direction:

Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the V/tV/t graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared. graph, so that you can display a clear picture of the signal you want to investigate.

FIRST TIME OPERATION:

1. Check that all push buttons in the out position, i.e., released.
2. TIME/DIV variable controls are hole off control, fully counter-clockwise their calibrated position.
3. Set all control in mid-range position.
4. select AC/DC switch which mode you want.
5. Set INDENS., (INDENSITY ) FOCUS controls for normal.

Setting up

1. Someone else may have been twiddling knobs and pressing buttons before you. Before you switch the oscilloscope on, check that all the controls are in their ‘normal’ positions.  

• all push button switches are in the OUT position
• all slide switches are in the UP position
• all rotating controls are CENTRE
• the central TIME/DIV and VOLTS/DIV and the HOLD OFF controls are in the calibrated, or CAL position

Check through all the controls and put them in these positions:
2. Set both VOLTS/DIV controls to 1 V/DIV and the TIME/DIV control to 0.2 s/DIV, its slowest setting:
3. Switch ON, red button, top centre:
The green LED illuminates and, after a few moments, you should see a small bright spot, or trace, moving fairly slowly across the screen.

4. Find the Y-POS 1 control:
What happens when you twiddle this?   The Y-POS 1 allows you to move the spot up and down the screen. For the present, adjust the trace so that it runs horizontally across the centre of the screen.

5. Now investigate the INTENSITY and FOCUS controls:
When these are correctly set, the spot will be reasonably bright but not glaring, and as sharply focused as possible. (The TR control is screwdriver adjusted. It is only needed if the spot moves at an angle rather than horizontally across the screen with no signal connected.)

6. The TIME/DIV control determines the horizontal scale of the graph which appears on the oscilloscope screen. With 10 squares across the screen and the spot moving at 0.2 s/DIV, how long does it take for the spot to cross the screen? The answer is 0.2 x 10 = 2 s. Count seconds. Does the spot take 2 seconds to cross the screen? Now rotate the TIME/DIV control clockwise:

With the spot moving at 0.1 s/DIV, it will take 1 second to cross the screen. Continue to rotate TIME/DIV clockwise. With each new setting, the spot moves faster. At around 10 ms/DIV, the spot is no longer separately visible. Instead, there is a bright line across the screen. This happens because the screen remains bright for a short time after the spot has passed, an effect which is known as thepersistence of the screen.

It is useful to think of the spot as still there, just moving too fast to be seen. Keep rotating TIME/DIV. At faster settings, the line becomes fainter because the spot is moving very quickly indeed. At a setting of 10 µs/DIV how long does it take for the spot to cross the screen?

7. The VOLTS/DIV controls determine the vertical scale of the graph drawn on the oscilloscope screen. Check that VOLTS/DIV 1 is set at 1 V/DIV and that the adjacent controls are set correctly:   Tht cro  built in source of signals which allow you to check that the oscilloscope is working properly.  A connection to the input of channel 1, CH 1, of the oscilloscope can be made using a special connector called a BNC plug, as shown below:

The diagram shows a lead with a BNC plug at one end and crocodile clips at the other. When the crocodile clip from the red wire is clipped to the lower metal terminal, a 2 V square wave is connected to the input of CH 1. Adjust VOLTS/DIV and TIME/DIV until you obtain a clear picture of the 2 V signal, which should look like this:

Check on the effect of Y-POS 1 and X-POS:

Y-POS 1 moves the whole trace vertically up and down on the screen, while X-POS moves the whole trace from side to side on the screen. These control are useful because the trace can be moved so that more of the picture appears on the screen, or to make measurements easier using the grid which covers the screen. You have now learned about and used the most important controls on the oscilloscope.

You know that the function of an oscilloscope is to draw a V/t graph. You know how to put all the controls into their ‘normal’ positions, so that a trace should appear when the oscilloscope is switched on. You know how the change the horizontal scale of the V/t graph, how to change the vertical scale, and how to connect and display a signal. What is needed now is practice so that all of these controls become familiar.

MAINTENANCE

1. Various important properties of the cro should be are fully checked at certain intervals.
2. The exterior the cro should be cleaned regularly with dusting brush.

• Frequency Counter
• Curve Tracer
• Function Generator
• Digital Voltmeter

Other oscilloscope control

screen: usually displays a V/t graph, with voltage V on the vertical axis and time t on the horizontal axis. The scales of both axes can be changed to display a huge variety of signals. on/off switch: pushed in to switch the oscilloscope on. The green LED illuminates.

X-Y control: normally in the OUT position. When the X-Y button is pressed IN, the oscilloscope does not display a V/t graph. Instead, the vertical axis is controlled by the input signal to CH II. This allows the oscilloscope to be used to display a V/V voltage/voltage graph. The X-Y control is used when you want to display component characteristic curves, or Lissajous figures. (Links to these topics will be added later.)

TV-separation: Oscilloscopes are often used to investigate waveforms inside television systems. This control allows the display to be synchronised with the television system so that the signals from different points can be compared. You must not try to investigate television systems because of the dangerously high voltages inside. The correct postion for this control is OFF.

TIME / DIV: Allows the horizontal scale of the V/t graph to be changed.

Trigger controls: This group of controls allows the oscilloscope display to be synchronised with the signal you want to investigate. When the AT/NORM button is in the OUT position, triggering is automatic. This works for most signals. If you change the AT/NORM button to its IN position, the most likely result is that the signal will disappear and the oscilloscope screen will be blank. However, if you now adjust the LEVEL control, the display will be reinstated. As you adjust the LEVEL control, the display starts from a different point on the signal waveform. This makes it possible for you to look in detail at any particular part of the waveform.The EXT button should normally be in its OUT position. When it is pushed IN, triggering occurs from a signal connected to the trigger input, TRIG INP, socket. The slide switch to the left of TIME/DIV gives additional triggering options. AC is the normal position and is suitable for most waveforms. In the DC position, you use the LEVEL control to select a particular DC voltage on the signal waveform where triggering will occur. The +/- button gives triggering on the upward slope of the signal waveform in the OUT position, and triggering on the downward slope in the IN position.

Intensity and focus: Adjusting the INTENSITY control changes the brightness of the oscilloscope display. The FOCUS should be set to produce a bright clear trace. If required, TR can be adjusted using a small screwdriver so that the oscilloscope trace is exactly horizontal when no signal is connected.

X-POS: Allows the whole V/t graph to be moved from side to side on the oscilloscope screen. This is useful when you want to use the grid in front of the screen to make measurements, for example, to measure the period of a waveform. .X-MAG: In the IN position, the horizontal scale of the V/t graph is increased by 10 times. For example, if TIME/DIV is set for 1 ms per division and X-MAG is pushed IN, the scale is changed to 0.1 ms per division.

CAL outputs: The top terminal gives a 0.2 V peak to peak square wave, while the lower terminal gives a 2 V peak to peak square wave, both at 50 Hz. The signals from these outputs are used to confirm that the oscilloscope is correctly calibrated.

component tester: The output socket provides a changing voltage which allows component characteristic curves to be displayed on the oscilloscope screen. When the button is IN, the oscilloscope displays a V/V graph, with the component tester voltage connected internally to provide the horizontal axis. To get normal V/t graph operation the component tester button must be in the OUT position.

Y-POS I and Y-POS II: These controls allow the corresponding trace to be moved up or down, changing the position representing 0 V on the oscilloscope screen. To investigate an alternating signal, you adjust Y-POS so that the 0 V level is close to the centre of the screen. For a pulse waveform, it is more useful to have 0 V close to the bottom of the screen. Y-POS I and Y-POS II allow the 0 V levels of the two traces to be adjusted independently.

Invert: When the INVERT button is pressed IN, the corresponding signal is turned upside down, or inverted, on the oscilloscope screen. This feature is sometimes useful when comparing signals.

CH I and CH II inputs: Signals are connected to the BNC input sockets using BNC plugs. The smaller socket next to the BNC input socket provides an additional 0 V, GROUND or EARTH connection. VOLTS / DIV: Adjust the vertical scale of the V/t graph. The vertical scales for CH I and CH II can be adjusted independently.

DC/AC/GND slide switches: In the DC position, the signal input is connected directly to the Y-amplifier of the corresponding channel, CH I or CH II. In the AC position, a capacitor is connected into the signal pathway so that DC voltages are blocked and only changing AC signals are displayed. In the GND position, the input of the Y-amplifier is connected to 0 V. This allows you to check the position of 0 V on the oscilloscope screen. The DC position of these switches is correct for most signals.

GENERAL INFORMATION:

• Cathode Ray Tube: Rectangular medium short persistence.
• Accelerating potential: 2000 v.
• Display: 8.10 cm.
• Trace rotation: Adjustable on front panel.
• Calibrator: Square wave generator 1KHz 0.2 v.
• Z Modulation: TTL level Stabilized.
• Power Supply: All operating voltages including the EHT.
• Mains voltage: 220 v-240 v, 50Hz.
• Mains fluctuations: 10 (per.)
• Power consumption: 33va
• Weight: 7.5Kg
• Operating temperature: 0- 40 degree.

SERVICE GUIDE

The instruments fails completely, the first and most important steps;

1. Check mains/line voltage and power fuse.
2. Will be to measure the deflecting plate voltages of the Crt in almost any case the fault section can be located.

The section represent,

• Vertical deflection.
• Horizondal deflection.
• CRT circuit.
• power supply.
• Sweep section.
• pre-amp.,

WARNING:

Any interruption of the protective conductor inside or outside the instruments or disconnection of the protective earth terminal is likely to make the instruments dangerous. Intentional interruption of the protective earth connection is prohibited. The mains/line plug should be inserted before connections are made to measuring circuits.