Equipment needed from the stockroom: an analog universal meter (AVM), an analog oscilloscope with the manual, the manual for digital oscilloscope at your bench, leads.

1.1  Familiarize yourself with the instruments at your bench. 

Leave the digital scope for later. The dual dc power supply output consists of two independent units which can be connected in different configurations (such as series or dual polarity) or used independently. Check one of these units by connecting a DVM and AVM across its terminals. Turn the voltage adjustment knob to get several different voltage values (for example 1.5V, 14V, 30V. Compare readings on the power supply display with readings on the DVM and the AVM. Note readings of the voltmeters on different ranges of the instruments. 

Note: A DVM range is set by push buttons and an AVM range by its rotary switch. Make sure also that both instruments are set for DC measurement.

Which voltmeter range should be selected to read best a given voltage? Comment on the precision of voltage measurements on different ranges of the DVM and the AVM and note if its readings agree with the power supply display.

1.2  Configure the power supply.

Next, configure the power supply for dual polarity voltage, such as +15V and -15V with respect to   ground, which is required in many electronic circuits. Using the DVM, measure voltages between the ground (common) and "+", ground and "-" terminal, and also between "+" and "- " terminals.

We start with a simpler traditional analog oscilloscope, which is easier to understand and to operate. This will introduce us to the general oscilloscope measurement fundamentals which apply also to the digital scope.

Familiarize yourself with the analog oscilloscope. Following the scope manual identify two inputs, input switches (dc, ac, ground), vertical and horizontal adjustments and trigger functions. Note that one of the two input terminals of a scope is grounded (common); the one connected to the outer shield of the coaxial cable. Other instruments (e.g. a waveform generator) or points of a circuit you want to test may also be grounded. Be sure that in such a case you connect the common terminals together. Ignoring this rule may even result in destruction of a tested circuit! Can you see why?

Perform the following tests:

2.1.Time measurements

Observe a sine wave, a triangle and a square wave from the waveform generator at your bench. Measure a sinewave period and compare it with the generator frequency for a low frequency (between 50 Hz and 500 Hz), and a high frequency(>100 kHz). Expand the image of the sinewave by  selecting  appropriate    vertical (volts/division) and horizontal (time/division) gains.

Estimate the precision with which you can measure the period by considering a fraction of the horizontal scale divisions that you can read reliably.

Try internal and external triggering (for the latter use a "sync" or trigger output of the signal generator). Using internal triggering adjust the trigger level and observe its effect on the starting point of the displayed waveform. Make notes and sketch observed waveforms for your report.

2.2. Voltage measurements

Set the waveform generator to obtain a sinewave with a frequency of a few hundred hertz. Connect the oscilloscope to the generator (watch those ground leads! -polarity is important here!). Connect also the DVM to the same terminals. From the oscilloscope image measure the signal amplitude. Note the number of vertical divisions and volts/division scope setting. The latter should be selected so that the image is expanded on the screen for easier and more precise reading. The color inner knob on the range switch must be turned all the way to the "calibrated" position".

Note a reading on the DVM for the same signal that you observe on the scope. The meter should be set for an appropriate range of AC voltage measurement.

Next, without changing the scope settings or the generator frequency, repeat the measurements described above for a triangular wave and for a square wave.

Prepare a table showing the amplitudes (from the scope) and the DVM readings for the three waveforms. Do the scope and the voltmeter readings agree for the three waveforms? What does the voltmeter show? Compare appropriate values from the scope and DVM measurements by showing their difference in percent in the table.

2.3  Scope AC and DC inputs

A switch at the scope input has three positions: GROUND, DC, and AC. In GROUND position the scope internal circuit is disconnected from the input terminal and connected it to ground. This helps to find zero voltage level on the display. DC position connects the input terminal directly to the scope circuit while the AC setting makes this connection through a capacitor. Thus in the AC input mode any dc voltage, which may be present in the measured signal, does not show on the display (a capacitor represents an open circuit for dc).

To see the effect of different input modes perform these tests:

a) Set the input switch to GROUND and by adjusting the VERTICAL POSITION knob move the image (a straight horizontal line) to the middle position on the display grid.

b) Switch the scope input to DC and observe a sine wave or a triangular wave. Check if the wave is centered on the zero (ground) level adjusted previously. Now add some dc bias to your signal, adjust the dc level, and observe the waveform position on the scope display. (There is a knob on the waveform generator for setting dc or "offset" voltage).

c) Switch to AC input and observe if changing dc bias affects the image.

d) Switch the waveform generator to produce a square wave (without dc bias) at high frequency (>5 kHz) and observe it in both DC and AC input modes. Repeat the same at low frequency (about 50 Hz).

Make notes of your observations and sketch waveforms seen in tests b) and d). Explain what you see. Why the square wave looks "strange" only in AC mode at low frequency?

2.4  Frequency range of instruments

Measuring instruments are designed to operate within certain voltage and frequency range and should not be used outside their design specifications. The oscilloscopes in our laboratory are capable of operating at fairly high frequency (tens of megahertz). What about the DVM and the AVM? To check their useful frequency range, do the following:

· Connect an oscilloscope, DVM, and AVM to the waveform generator at the same time.

· Set the waveform generator to a sine wave with the amplitude of a few volts. Start with a frequency of a few tens of hertz and measure the voltage with all three instruments.

· Increase frequency by a decade (a factor of ten) and measure the voltage with the three instruments again. Repeat this procedure until you reach the maximum frequency of the waveform generator.

You should notice that readings of voltmeters drop with increasing frequency while the scope indicates almost the same voltage (the generator output voltage may somewhat vary with frequency too). Adjust frequency to find its values where the readings of DVM are only about 5% below the voltage indicated by the scope. Repeat the same for AVM. This way you can find the useful frequency ranges for each voltmeter, defined here as the frequency range where a voltmeter reading is different by no more than 5% from the "true value" assumed to be shown by the oscilloscope.

A digital oscilloscope can be a great tool, provided that you know how to use it. It has many functions and menus which have to be programmed before the desired mode of operation is obtained. This may be daunting for a beginner who has to spend time studying the manual before making any measurements. Like with a computer, it is best to start with simple operations and move to more complex tasks as you gain experience. Familiarity with an analog scope should help, because front panels and basic functions of digital oscilloscopes are made to resemble those of analog scopes.

Here are some of the most common problems that beginners have with digital scopes:

· It is often not obvious how to access various functions of the instrument, many of which are programmable from the front panel buttons. Consult the manual.

· The waveform you see on the screen does not necessarily represent the input signal. It may be a waveform stored from a prior measurement.

· You may be easily "fooled" by a digital scope if you let it "think" for you by selecting the "auto" mode in which the voltage and time scales are set by the instrument program. On occasions a noise may look like a nice signal or a momentary transient voltage like a continuous wave.

All this can be sorted out and clarified with experience which you are about to acquire.

Repeat measurements 2.1 and 2.2 from the previous section using the digital scope at your bench. Note that you do not need to count the scale divisions to get the signal amplitude or the period; the instrument displays this information for you. You can also activate the vertical and horizontal cursors and check the values of amplitude and period using the cursor position displays.

Describe briefly the measurement procedure and the results, including sketches of waveforms. Include the table of voltage vs. frequency from part 2.4. Address the topics and answer the questions printed in bold letters in the manual.

In particular discuss these problems:

• Do the scope and DVM measurements agree (to what extend - give % values)?

• What value does a DVM measure for a sine wave? What about a square wave and a triangular wave?

• What is the useful frequency range of the DVM and the AVM?

Add any observations or conclusions you wish to make; they enhance your report.

Do not forget to number figures and tables and to give them captions (titles).

Number all pages of the report.

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