Agilent Technologies DSO-X-2024A Manuale utente

EE 201 Lab !

Agilent DSO-X-2024A digital oscilloscope !

In its basic mode of operation, an oscilloscope takes a rapid sequence of voltage measurements

and displays these in the form of a plot showing the voltage as a function of time. Imagine if you

and your lab partner were to take a series of voltage measurements at speciﬁed interval, say

every 3 seconds, and then enter your measurements into Excel. You could generate a plot of the

voltage as function of time, giving you an “image” of the time-varying waveform. This is what

the oscilloscope does, but signiﬁcantly faster than you and your lab partner. The voltage

measurements of the oscilloscope may be only 1 ns apart (1 billion measurements every second).

Thus, it provides a detailed view of a waveform from some part of circuit. The oscilloscope

works particularly well with periodic signals, because it can be “triggered” from the periodic

signal and offer a continuous display of the signal. However, when set up properly, the

oscilloscope can also capture aperiodic (or “one-shot”) waveforms. !

In some sense, the oscilloscope can be viewed as a “waveform microscope”. Using a biology lab

microscope, we peer through the lenses and then adjust the magniﬁcation to get clear images. If

we are trying to look at an amoeba, we would like to set the magniﬁcation so the image amoeba

will just ﬁll up the entire ﬁeld of view of the microscope. If the magniﬁcation is too low, the

image of the amoeba is too small to see details. If the magniﬁcation is too high, the amoeba

more than ﬁlls the ﬁeld of view and we may miss something interesting. In the same way, we

can adjust the time- and voltage-axis settings of the oscilloscope to get a clear graph of the

waveform. Sometimes we may want to reduce the scale in order to zoom in on some portion of a

waveform. Other times, we may need to increase the scale so that we can see several periods of

the waveform.!

The oscilloscope has an Autoscale function, which captures a waveform and then sets up the

display to give a reasonable graph. As you start out with the oscilloscope, you may use this

feature frequently, but as you become more proﬁcient with the “scope”, you should use

Autoscale less and less. Typically, we will use the Autoscale to get the preliminary measurement

and then make adjustments to the settings in order to look more carefully at things of interest.!

EE 201 rule of thumb: !

On any particular measurement on circuit, you should use the Autoscale no more than once,

right at the outset in order to get “close”. After that, all adjustments should be made with the

time and voltage settings knob. Excessive use of Autoscale is a clear indication that the user

is a rookie.!

The oscilloscopes in Rm 2014 are quad-channel, meaning that they can measure and display four

separate voltage waveforms simultaneously. This is very useful for seeing how a circuit modiﬁes

a signal. For example, we can apply a voltage signal from the function generator to the input of

an ampliﬁer or ﬁlter circuit and observe the source voltage with one channel. We can use another

channel to observe the output voltage waveform. If both waveform traces are displayed on the

oscilloscope screen, we can immediately see the ampliﬁcation or the signal ﬁltering action.!

EE 201 Lab !

" !

Figure 1. Some important items on the front panel of the Agilent DSO-X-2024A oscilloscopes

in the EE 201/230 laboratory.!

1. Power switch.!

2. Channel input terminals, 1 - 4. !

3. Vertical (voltage) scale and offset controls, one set for each channel. Usually, the ﬁrst

signal is applied to channel 1 and channels 2-4 are added as needed for the particular

measurement. Channels are activated automatically when an Autoscale is performed.

When a channel is active, its channel button will be lit and there will be a trace on the

display for that channel. In Fig. 1 above, only channel 1 is active –#the yellow “1” is lit

while all the other channel buttons are dim. Channels can be manually activated or de-

activated by pushing the channel buttons.!

4. Horizontal (time) scale knob.!

5. Autoscale button. This causes the oscilloscope to automatically acquire a voltage signal

and draw a trace on the display. It will activate each channel where a voltage signal is

sensed. Sometimes the Autoscale activates a channel even if there are not leads attached.

Other times, a channel that should be activated may not be. You can manually activate /

de-activate channels as described in item 3 above. !

6. Run-stop and Single measurement buttons. In run mode, the oscilloscope continuously

measures and displays the voltage at the inputs. Pushing the button once causes the

measures to stop and the display will freeze with the last measured cycle of the input.

Single is used to capture a voltage transient that happens only once. !

7. Softkeys for selecting various options during oscilloscope set up. The function of the

button changes depending on what you are trying to do.!

EE 201 Lab !

8. Multi-function Entry knob. Adjusts various settings on the oscilloscope.!

9. Meas and Cursors keys. Use these to obtain new measurements from the channel traces or

to use cursors to look at speciﬁc points on the traces.!

10. Save/Recall key. Use this to enter the Save/Recall menu when you would like to capture a

screen image of the graph and data to include in a report. !

11. USB port.!

Basic measurements!

A typical oscilloscope measurement involves a small number of steps that are used almost every

time.!

1. Power up!

2. Make connections from your circuit to the oscilloscope.!

3. Perform an Autoscale so that the oscilloscope will automatically acquire a trace and draw a

it on the display.!

4. Make manual adjustments to the time scale and voltage scales in order to set up for any

measurements you would like to perform on the voltage traces.!

5. (Optional) Use cursors and measurements to obtain speciﬁc voltage levels, voltage

differences and time differences from the voltage traces.!

6. Save a screen image of the graph, if needed.!

Each item is described below. Most of the real work occurs during step 3. However, since the

oscilloscope is doing most of that work, the step requires little explanation. Ironically, the most

mundane step, making connections, requires the longest explanation below.!

1. Power up!

When the oscilloscope is powered up, it will go through a boot-up cycle, enters the default

state, and waits for you to initiate a measurement. There will be a horizontal yellow line on

the display indicating that Channel 1 is turned on, but there is no voltage applied.!

2. Connections!

To measure voltages, you must attach leads from your circuit to the channel inputs. In

principle, you can use any channel, but typically, we make the ﬁrst connection to Channel 1

and sequentially move down the line as we need more measurement points. (It will be a rare

occasion when we have an EE 201 circuit that is so complicated that we will need 4

simultaneous measurements.)!

There are two methods for making oscilloscope connections in EE 201 lab. The ﬁrst is to use

scope probes, which are designed speciﬁcally for use with an oscilloscope. (Using the scope

probe is the way the professionals connect the oscilloscope to circuits.) A scope probe has a

BNC connector on one end that connects to the input jack of the oscilloscope, and pair of leads

at the other end for connecting into the circuit. One lead is a small alligator clip that is the

negative connection. The other lead is either a sharp probe that can be used to contact a point

EE 201 Lab !

in the circuit or small clip that can be hooked onto a wire in the circuit.!

The other method for making connections is to use a BNC to banana-plug adapter on the

oscilloscope input and then use the superstrip leads that we regularly use for the multimeter

and power supply connections.!

The scope probes are the preferred approach for making connections. They are robust and

provide good contact with minimal loss and stray capacitance – important when making

measurements on high-frequency signals. They also provide some protection to the

oscilloscope input circuitry, which can be damaged if large voltages are applied. They can be

broken if mishandled and are expensive to replace. On the other hand, the breadboard leads

are easy to use, cheap to replace. For our relatively simple EE 201 measurements, the simple

leads may be adequate, but in general, you should avoid using them, because they tend to act

like antennas and pick up all sorts of extraneous signals.!

Whenever you connect a probe, you must check and set the probe scaling factor. Most of the

scope probes have a 10x attenuator meaning that the incoming voltage is reduced by a factor

of 10 on its path to the input circuitry of the oscilloscope. This is how the scope probes

provide protection. (Some probes in lab have 100x attenuation.) You must “tell” the

oscilloscope that the signal is being attenuated so that it can compensate and provide accurate

readings. If you do not set this properly, all of your voltage readings for a given channel will

be off by some factor, usually 10. !

To check and set the probe scale factor for a particular channel, start by pushing the channel

button (section 4 in Fig. 1.) Press the Probe softkey that appears – pressing it once causes a

pop-up menu to appear. Continue to press the Probe softkey until the Ratio menu item is

selected. Use the Entry knob to adjust the ratio until it matches the channel’s probe. For the

scope probes, the attenuation ratio is printed on the probe case – most in our lab are 10:1. Of

course, the breadboard leads have no attenuation, so the ratio is 1:1.!

The connections for the four channels are not completely independent. The negative terminals

of all four are in common. There are really only ﬁve available connections: positive for Ch1,

positive for Ch2, positive for Ch3, positive for Ch4, and the one negative that serves all four

channels. The common negative connection has two consequences. First, you only need one

negative connection from the oscilloscope to your circuit. (This is good since it cuts down on

wires.) Secondly, you can only have one negative connection in your circuit – all four

channels must reference the same negative connections. (This is bad because it somewhat

limits what you can measure.) If you try to connect the two negative leads from two channels

to different points in the circuit, you will mess up your measurement. Connecting negative

leads to different points in the circuit effectively shorts out the part of the circuit between the

two the leads! Be careful.!

3. Autoscale!

Once the connections are made, you can press the Autoscale button to initiate measurements.

The oscilloscope will measure the waveforms at each channel. If a repetitive waveform with a

measurable amplitude is present, the channel is turned on, and the waveform trace will be

drawn on the display. The display time scale is automatically adjusted to show a two (or a

EE 201 Lab !

few) period of the waveform. The voltage scaling for each channel is automatically adjusted

to that the amplitude of the trace is an appreciable fraction of the total display.!

You can use Autoscale at the beginning of any measurement. It is also helpful once you have

manually messed up all of the settings and can no longer see any traces. Just hit Autoscale

again, the oscilloscope will take you back to where you began.!

4. Making manual adjustments!

Autoscaling is nice, but usually it does not provide the trace in exactly the form that you

would like. You can manually ﬁne-tune the traces on the oscilloscope to get exactly the view

that you want.!

You can adjust the time scale by turning the Horizontal control knob (item 4 in Fig. 1.)

Turning this knob adjusts the time / division of the graph on the display, expanding or

contracting the trace in the horizontal direction. The current time / division setting is listed at

the top left corner of the graph.!

The vertical scaling and position for each channel trace are adjusted using the vertical controls

(section 3 in Fig. 1). Each channel has its own vertical scaling, and is adjusted using the

vertical scale knob (larger knob above the channel buttons). The scaling (V/division) for each

channel is given along the top edge of the graph. In addition, the traces can be moved up and

down on the graph using the vertical position knobs (smaller knobs below the channel

buttons). The zero point for each trace is indicated along the left side of the graph by a small

“ground” symbol. You can move two or more traces so that they have the same zero point in

order to make direct comparisons. Or you can move traces away from each for clearer

viewing.!

5. Measurements & Cursors!

There are various kinds of measurements that can be made on the waveforms. The values for

each type of measurement can be displayed for each channel waveform in the area just to the

right of the graph. The default measurements are peak-to-peak amplitude and frequency for

each active channel. To add more measurements, press the Meas key to display the

measurements menu in the softkey area. Use the Source softkey to choose which channel

trace you would like to do the measurement on. Use the Type softkey and the Entry knob to

choose the type of measurement. Then use the Add Measurement softkey to add the

measurement to the displayed list.!

Cursors are extremely useful for comparing different points on one trace or points on different

traces. You can use cursors to ﬁnd time delays and voltage changes in waveforms. There are

two x-cursors (time cursors), each of which can be moved to any point along the time axis.

There are two y-cursors (voltage cursors), each of which can be moved to any point along the

voltage axis. The value of the trace at the cursor point is listed, along with the difference

between the two cursors.!

6. Saving traces!

Lastly, you will probably want to save a copy of your measured traces to include in a report.

Plug a ﬂash drive into the USB port and press the Save/Recall key to enter the Save/Recall

EE 201 Lab !

menu in the softkey area. You can then save a screen capture image to a ﬂash drive.!

The above sections give only a brief overview of some of the capabilities of the oscilloscope. As

with most things in lab, the best way to become proﬁcient in using an oscilloscope is to use it

frequently. Don’t be afraid to try different things and poke around in some of the menus. If you

press and hold a key, a help screen will pop up with some information about the function

associated with the key. Also, an extensive manual is available on the web site. http://

tuttle.merc.iastate.edu/ee201/lab/agilent2024A.pdf. Reading a thick oscilloscope manual is

certainly not a leisure-time activity. However, there is a nice “Getting Started” section at the

beginning of the manual. If you want to get all the details on some particular function or feature,

you can then dig deeper into the manual.!

A simple exercise!

Set up a voltage divider like the one shown in Fig. 2 below. For the source, set up the function

generator set to provide a sinusoid with a 5-VRMS amplitude and a frequency of 1 kHz. (See the

function generator tutorial for details of setting up the function generator.) Measure the voltages

on the resistors using the multimeter set to AC volts to ensure that the circuit is working properly.!

" !

Figure 2. Simple voltage divider circuit for oscilloscope exercise.!

1. Power up the oscilloscope.!

2. Make connections using scope probes (or use breadboard leads, if no probes are available).

Use Channel 1 to measure the source voltage – oscilloscope channel negative to the

negative side of the source and oscilloscope channel 1 positive to the positive side of the

source. Use Channel 2 to measure the voltage across R2– channel 2 positive lead

connected to the node between R1and R2. Remember: You do not need to connect the

negative side of Channel 2.!

3. Press the Autoscale key. After a few seconds, the two traces should appear on the

oscilloscope display.!

4. Adjust the time base (horizontal) so that there are about two full periods displayed on the

graph. Note the time scale listed in the upper left-hand corner of the display.!

5. Adjust the vertical position of the two traces so that they are both located at the center line

of the display’s graph. Adjust vertical scaling for Channel 1 so that trace ﬁlls as much of

the graph as possible without extending outside the graph. Adjust the vertical scaling of

Channel 2 to match that of Channel 1. Now you can directly compare the source voltage

–

vR2

1 kΩ

EE 201 Lab !

and the resistor voltage.!

6. Turn on the cursors by pressing the Cursors key. Push the cursor’s knob (push like it was a

button) and then turn the knob to select X1 in pop-up menu. Push the cursor knob again to

select X1. Turn the cursor knob to move cursor X1 across the graph. The time and voltage

values of the point where the cursor intersects the trace are displayed in the “cursor box”.

Move the cursor to one of the peaks in the Channel 1 trace. Use the same process to select

and move cursor X2 to an adjacent peak in the Channel 1 trace. Note the time difference

between the two cursors – it should be one period of the sinusoid. Use the same process to

choose and move cursor Y1 to the minimum (trough) of the Channel 1 sinusoid, and then

move cursor Y2 to the minimum of the Channel 2 sinusoid. Again, the difference is just the

difference in voltage between the input and the output of the voltage divider.!

7. Save a screen image of the graph with the two traces and the cursors. Insert a ﬂash drive

into the USB port of the oscilloscope. Press the Save/Recall key. The Save/Recall Menu

will appear in the softkey area. Press the Save softkey. The menu changes – press the

Format softkey, and the Entry knob to choose the format –#24-bit .bmp (bitmap) or 24-

bit .png (portable network graphics) are are probably good choices. Then use the softkey in

the second position to navigate to the desired location on the ﬂash drive for storing your

ﬁle. Under the Settings softkey, you can choose the option Invert Grat to change the

background color of the graticule from black to white – this might save you a bit of ink

when printing out screen images. Finally, press the Press to Save softkey to save the

image. You should receive a message conﬁrming that the ﬁle was stored.!

8. Now, play around a bit. Change the type of waveform, the frequency, and the amplitude on

the function generator and observe the changes in the oscilloscope traces. Change the time-

and voltage-scale settings on the oscilloscope. Poke around a bit and try out some of the

different functions of the oscilloscope.!

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