CRO/DSO

An oscilloscope is a test instrument which allows us to look at the shape of electrical signals by displaying a graph of voltage against time on its screen.

It is like a voltmeter with the valuable extra function of showing how the voltage varies with time.

A graticule with a 1cm grid enables us to take measurements of voltage and time from the screen.

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Figure

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Procedure

1. Switch on the oscilloscope to warm up (it takes a minute or two).
2. Do not connect the input lead at this stage.
3. Set the AC/GND/DC switch (by the Y INPUT) to DC.
4. Set the SWP/X-Y switch to SWP (sweep).
5. Set Trigger Level to AUTO.
6. Set Trigger Source to INT (internal, the y input).
7. Set the Y AMPLIFIER to 5V/cm (a moderate value).
8. Set the TIMEBASE to 10ms/cm (a moderate speed).
9. Turn the timebase VARIABLE control to 1 or CAL.
10. Adjust Y SHIFT (up/down) and X SHIFT (left/right) to give a trace across the middle of the screen.
11. Adjust INTENSITY (brightness) and FOCUS to give a bright, sharp trace.

A calibration point is used to calibrate the CRO.
It gives a steady square wave at a particular set frequency and voltage.
The standard calibration signal is 0V–2V at 1 kHz.

After connecting the oscilloscope to the circuit you wish to test, set the controls to obtain a clear and stable trace on the screen.

Most oscilloscopes have a BNC socket for the Y input and the lead is connected with a push and twist action.

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Control Knobs

• Y AMPLIFIER (VOLTS/CM): Determines the height of the trace. Choose a setting so the trace occupies at least half the screen height, but does not disappear off the screen.
• TIMEBASE (TIME/CM): Determines the rate at which the dot sweeps across the screen. Choose a setting so the trace shows at least one cycle of the signal across the screen.

  Note: A steady DC input signal gives a horizontal line trace for which the timebase setting is not critical.

• TRIGGER Control: Usually best left set to AUTO.
• AC/GND/DC Switch:
• Normal setting is DC for all signals (including AC signals).
• Switching to GND connects the y input to 0V to quickly check the position of 0V.
• Switching to AC inserts a capacitor in series with the input to block DC and pass only AC signals.
• Position Control: Controls horizontal position of trace on screen.
• DUAL BUTTON: Allows display of both channel signals on the screen at the same time (DUAL MODE).

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Measurement of Peak-to-Peak Voltage and Peak Voltage

To measure the AC voltage of a sinusoidal waveform:

1. Apply the input AC signal from the signal generator to a channel of CRO.
2. Adjust the voltage/div switch (Y-plates) and time base switch (X-plates) such that a steady picture of the waveform is obtained.
3. Measure the vertical height L (peak-to-peak height).
4. Multiply this height L by the voltage/div sensitivity n.

$$ V_{pp} = L \times n = 2V_o $$

Measurement of Frequency

Using Time-Period

1. From the trace, the time period of the input signal is equal to t × (time/div setting).
2. Suppose the time period of the input signal is $T$.
3. Then the frequency of the signal is:

$$ f = \frac{1}{T} $$

Digital Storage Oscilloscope