Experiment #1
Introduction to Laboratory Electronics
ENPH 259
Prof. David Jones
July 10, 2013
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Objective
The purpose of this experiment is to gain familiarity with the equipment you
will use in many of the subsequent experiments. This will involve performing
a number of simple measurements with the prototype boards, oscilloscopes,
function generators and digital multimeters (DMMs).
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Connectors and Cables
In this lab (and in electronics in general) the test and measurement equipment generally has two types of connectors: banada connectors and Bayonet
Neill-Concelman (BNC) connectors. A third connector used is alligator clips to
conveniently clip wires. You have access to all three types and well as cables
and converters that will interchange the connector type. The banana connector
and alligator clip are shown on the far left in Fig. 1 and the banana is used
on the DMMs. The coaxial BNC connector is shown in the middle in Fig. 1
and is used on the function generator and oscilloscope. Note that the BNC is
effectively a two pin (or two wire) connector. Thus a BNC cable inherently has
two parallel wires in it, with one wire by default being ground. A convertor
Figure 1: Left: banana connector and alligator clip; middle: BNC connector;
right: banana-BNC converter
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between the two types of connectors is shown on the far right of Fig. 1. The
nubbin on the upper banana pin signifies that it is the ground pin. Using this
fact, obtain such a converter from the front of the lab and using your DMM
operating in ohmmeter mode, determine where the ground wire and signal wire
on the BNC connector/cable are located and sketch it in your lab notebook.
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Prototype Board
When building circuits often you want to quickly try a design with discrete
components. Rather than using a printed circuit board, one uses a prototype
board to quickly construct and troubleshoot a circuit. You will be using this approach to build most all circuits in this and future electronic laboratory courses.
Shown in Fig. 2 is a prototype board with two example circuits using integrated
circuits (ICs), resistors and capacitors.
Figure 2: Prototype board with two example circuits. Which circuit would be
easier to troubleshoot?
Electrical connections and thus circuits are made by using jumper wires between rows and columns of the board. To familiarize yourself with the prototype
board, use a digital multimeter (DMM) in ohmmeter mode to determine which
rows/columns of the board are connected, and which are not. Make a quick
sketch of this information in your lab book. Hint: by putting the DMM in a
”diode” or continuity check mode, it will beep when the two leads are shorted
together, thus making it easy to determine what is electrically connected and
what isn’t.
When making connections with jumper wires, it is important that stripped
portion of the wire is long enough to engage the spring located in each hole
of the prototype boards. Otherwise, you might not get electrical connection
even though it may appear physically connected. However, you don’t want the
stripped portion of the wire to be too long or it will easily short with other wires
in the vicinity, In addition, there could be holes/springs in your prototype board
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that do not grab the wires when inserted and thus make a ”flakey” connection
that depend on the angle of the wire. See if you can observe either of these
problems with your board. And keep these possible troubles in mind for all
future labs
Finally, notice the two example circuits shown on the prototype board in
Fig. 2. Which one would be easier to troubleshoot? This should be a clue that
going forward, it will be very helpful (in terms of minimizing time in the lab
and frustration) if you take a little bit of extra time in the beginning to build a
tidy circuit: it will save you much heartache and head-banging.
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Introduction to the Oscilloscope and Function
Generator
You should take some time to explore all of the controls on the oscilloscope
and the function generator - the manuals are located at the front of the Lab if
you need additional help with some feature/function. The following points will
provide some guidance in getting started with these instruments. In your lab
notebook record your observations. For example, DO NOT, just say ”we investigated the vertical settings.” Rather describe what happens to the displayed
waveform as you change the vertical settings.
To become familiar with the oscilloscope triggering and display options begin
by setting up this arrangement:
• Set the function generator to output a sine wave of approximately 1kHz.
• Connect the output from the function generator to the Channel 1 of the
oscilloscope.
• Connect the SYNC output of the function generator to Channel 2 as well
as the EXT input of the scope. Use a BNC splitter (available at the front)
to split the SYNC signal. The SYNC output is located on the back of the
function generator.
• Ensure that the selected output channel from the function generator is
ON. Note that channel 1 is the bottom BNC on the front panel.
In order for the scope to display something useful, it must be triggered. The
trigger is a signal that tells the scope when to display the voltage measured on
the input channels. Explore the horizontal and vertical settings and the trigger
menu by performing the following tasks and answering the following questions
in your lab notebook:
• Set the trigger to channel 1, then try changing the slope and trigger level.
What effect do these have on the displayed waveform? Describe in your
own words what the trigger is doing with respect to the slope and level.
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• Change the output level and frequency on the function generator. What
happens to the output and the SYNC output (amplitudes and frequencies)?
• Set the trigger mode to External. How does this change the triggering?
Based on the last two results, describe in your own words what functionality the SYNC provides.
Often at the beginning of your analysis or if you get the scope into a strange
configuration you can press the AUTO button. This action will cause the scope
to attempt to set reasonable parameters for the vertical and horizontal scales, as
well as the trigger level. However, you should always check the resulting setting
to make sure they are sensible. For example, would you want the horizontal
scale to be 5 ns/div if you are looking for a 1kHz signal?
Next, you will investigate the cursor functions. When taking measurements
on the scope it is always best to make the signal as large as possible (by adjusting
the vertical scale of the scope) while still displaying all features of interest. This
is done to maximize the measurement sensitivity:
• Press the CURSOR button to bring up the cursor options.
• Use the cursors to measure the period and peak-to-peak amplitude of
the signals on channel 1 and 2. Measure by hand using the timebase
and volts/division, as well as the automatic measurement supplied by the
scope under the measurement menu. Remember to include uncertainties,
both from the act of measurement and the scope itself (the latter can be
found in the binder).
• Now change the amplitude and frequency of the function generator and see
if the settings on the function generator always match what you measure
on the scope (in this case you are measuring the Channel 1 of the function
generator, not the SYNC, right?). Hint: check high frequencies. If they
don’t always match , which one would you trust more?
In the coming weeks, you will need to take electronic data from the scope.
Using a USB drive, you can download data from the scope as a two column data
file (voltage and time). However, these scopes use a FAT16 file system so the
maximum USB drive size one can use is 4 GigBytes (you can thank Microsoft
for this limitation). If you don’t have this small of a USB drive, there should
be some you can borrow in the front of the lab.
• First save a bitmap copy of the scope screen displaying the sine wave on
your USB and insert it into your lab notebook.
• Next, save the data from the scope in raw format as comma separated
values (CSV). Load this data into gnuplot/Matlab/etc (BUT NOT EXCEL) and construct a graph of the saved data and put it into your lab
notebook.
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Digital Multimeter (DMM)
Using the green binder located on your lab bench find the following information
and record it in your lab book: (i) the input resistances of the oscilloscope and
both DMM’s (operating in Voltmeter mode); (ii) the AC (alternating current)
frequency ranges over which both of the DMM’s operate. Note that the input
resistances of the oscilloscope and DMMs are different and you will need to take
this into account when evaluating circuits in the coming weeks.
5.1
Voltmeter, ampmeter and ohmmeter
Using one of the DMMs, measure the resistances of two resistors (one being
10 Ohms) and compare the results with the values indicated on them by the
resistor colour code. Next, determine the resistance of the 10 Ohm resistor by
using two DMMs: use one DMM to measure both the current through a 10
Ohm resistor and the second to measure the voltage difference across it. An AC
current can be generated by connecting the function generator to the resistor.
The DMM used as an AC voltmeter is placed in parallel with the resistor,
and the one used as an AC ampmeter is placed in series with the resistor.
From the data, together with Ohms Law, infer the value of the resistance.
Compare this result with your prior direct measurement of the resistance using
the DMM in Ohmmeter mode. Remember - a meaningful comparison
requires an estimation of the uncertainties in the measurements. You
should estimate the uncertainties using the procedure (calculus-based) discussed
in tutorial. If the numbers disagree by more than your uncertainty estimate,
try to explain why.
5.2
AC measurements
When set for AC operation, your DMMs (Keithley and HP) are designed to
display the true rms value of the input signal, regardless of the shape of the
waveform selected. This applies only for the AC component of the input signal.
If you don’t know what AC or DC means with regard to a signal, ask one of the
TAs before starting the measurements below.
• Using both a DMM and the oscilloscope connected in parallel (make a
circuit diagram) to measure the signal generated by the function generator
(with frequency set at some intermediate value, say 1 kHz), compare the
reading on the DMM with the amplitude observed on the oscilloscope for
a sine wave, a square wave, and a triangular wave. For the sine wave
and square wave show that these measurements are in agreement (within
uncertainties) with the definition of rms (root mean square - is the square
root of the mean of the square of the waveform). The material we covered
in tutorial will help you complete this measurement.
• For the square wave signal, use the DC offset function of the signal generator to add sufficient DC offset to the signal so that the waveform on
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the oscilloscope varies between zero and a positive level (the scope must
be set to DC coupling for this purpose). Measure both the AC and DC
component of this waveform using the appropriate settings on your DMM.
Observe what happens when you change the input coupling of the oscilloscope from DC to AC.
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