The University of Texas at ArlingtonElectrical Engineering Department

Electric Circuit Lab

The OscilloscopeAgilent 54621A

One of the most often used instruments in the electrical engineering lab is the oscilloscope which not only you can measure the waveform quantities, but also it allows you to display the waveform as a function of time.

The oscilloscope (or simply, “scope”) consists of a display tube on which one can trace the waveform. An electron beam which is generated by electron gun accelerate toward the part of the display and is deflected by electric fields, writes figures on the fluorescent screen.

Following figure shows the general principle and major subsystems of an oscilloscope

General principle of an oscilloscope showing the display tube and the deflection system.

There are two types of scopes, the analog and the digital ones. Digital scopes have more features than the analog scopes. Digital scopes can process the signal and measure its amplitude, frequency, period, rise and fall time. Some of them have built-in mathematical functions and can do fast Fourier transforms in addition to capturing the display and sending it out to a printer. The oscilloscopes in the EE Undergraduate Lab are Agilent 54621A type digital oscilloscopes which have most of the above functions built-in.

Following figure shows a typical probe used in measuring by oscilloscope. A probe is a high quality coaxial cable that has been carefully designed not to pick up stray signals originating from radio frequency (RF) or power lines. They are used when working with low voltage signals or high frequency signals which are susceptible to noise pick up. Also a probe has a large input resistance which reduces the circuit loading. The connections to the scope's will be made through a X 1 probe (where X 1 means that the probe does not attenuate, or reduce the signal; if your probe has a X 1/ X 10 selector, set it to X 1).

If the voltage source to be measured has one terminal grounded, make sure that the ground of the probe is connected to the ground of the voltage source and not vice versa; otherwise, the source will be shorted and may be damaged.

Review the front Panel of the scope

Review the Rear Panel of the scope

HintsThe Digitizing Oscilloscope• The BNC shield is at earth ground. Use only the probe TIPfor measuring high voltages. "Floating" the BNC shield orconnecting it to a high voltage could cause a safetyhazard.• Make sure probes are compensated.• If you can't get the signal on screen:- Check probe connection- Touch: SETUP, Default Setup- Touch: AUTOSCALE- Check for offset (ground symbol). If offscale, adjustvertical sensitivity and position- Turn up signal brightness with intensity control

Digital Oscilloscope Block Diagram

Hints:* The BNC shield is at earth ground. Use only the probe TIP for measuring high voltages. "Floating" the BNC shield or connecting it to a high voltage could cause a safety hazard.* Make sure probes are compensated and set to proper scale (X1,X10,X100).* If you can't get the signal on screen:- Check Probe connection- Touch: SETUP, Default Setup- Touch: AUTOSCALE- Check for offset (ground symbol). If offscale, adjust vertical sensitivity. If still offscale, Touch: - Check Trigger Source- Set Mode Auto

The Digitizing Oscilloscope

1CouplingDC AC

The Digitizing Oscilloscope

1 0s 2 us/1RUN1.00V

1

----Setup memory----

Undo Default

Status bar:Vertical sensitivitySweep speedTrigger time referenceChannel(s) onRun/stop

Softkeylabels

Calibration sourcePower ON/OFFScreen brightness

Measure: Vrms, tr, ,fTrace /Setup Memory Display: grid, vectors, averaging

Triggering: Modes: Auto, Normal,TVSources: Ch1, Ch2, Ext, Line

Vertical position, sensitivityMath on/off

Storage: Auto-store, Erase, RUN/STOP

Horizontal:Main Sweep Speed, Sweep Modes:

Delayed, XY, Rollt = 0 Reference

AutoscalePrintCalibrate

Getting Started

1 0.00s200 us/

1 RUN

1.00V

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DefaultSetup

Connect probe to calibrator

Adjust screw to eliminate:

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Getting Started:

Compensate probes:

(Power ON)

Setup

DefaultSetup (All settings

to default )

Auto-scale

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Line0 1

OvershootUndershoot

Press Channel key for selected probe.

Toggle softkey for setting that matches probe

Set probe attenuation factor:

10:1

1 0.00s

200 us/

1 RUN

1.00V

1

Probe1 100 10

Match

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Probe1 100 10

1 0.00s200us/

1 RUN

1.00V

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Probe1 100 10

What the display says:

Vertical sensitivity of CH1is 1Volt per major division

Horizontal sweep speed is200 sec per major division

Channel 1 is ON

Ground (V=0)[If dc part of CH1 signalis too big, groundarrow points off-screen.If this happens, adjustvertical sensitivity]

Trigger slope is positive(rising edge)

Trigger source is Channel 1

Scope is ready for trigger

Time=0

Blinks ifno trigger

t > 0t < 0

Readings scaledfor a 10:1 probe

1Off On

What the main controls do:

VerticalVolts/division

VerticalPosition

Horizontaldelay

Time/division(sweep speed)

Trigger level Line is onlyvisible when you turn the knob.When signal is smaller than trigger level, scope stops sampling.

Display:

Grid full

Vectors off

Other main controls:

+Math Functions

-Add waveforms-FFT (if equipped with module)

Trigger holdoffSignals with multiple zero crossings per cycle cause unstable displays.Holdoff ignores the false triggers for a given length of time.

- Enable channel- Set up probe

Trigger

Holdoff

Holdoff=11.5 us

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Ho

rizo

nta

l Co

ntr

ols

Vertical Controls

Trigger Controls

Display

Display (continue)

Other Functions

Other Functions (continue)

Other Functions (continue)

Other Functions (continued)

Other Functions (continued)

Other Functions (continued)

ClearCursors

Making Measurements: Vp-p

Source 1

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GridNone

Hook Calibrator signal to CH1

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Cursors Clears any cursors already onthe screen

Set for the correct channel

|- Active Cursor -|V1 V2 T1 T2

Toggle to highlight the V1 cursor; Rotateknob for waveformminimum

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|- Active Cursor -|V1 V2 T1 T2

Change to V2 cursor;Use knob to set to waveform minimum

Measure Vp-p, using cursors:

Answer

V1= 31.25mV V2= 5.031V V= 5.000V

Cursor adjustment knob

OR: Measure Vp-p, the easy way:

1 Voltage|- Voltage Measurements -|

Vp-p Vavg VrmsSimply select Vp-p from the Voltage menu.

Display

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Risetime Answer

Making Measurements: RISETIME

1 NextMenu

If answer needs more resolution:

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Time

|----------- Time Measurements -----------|+Width -Width RiseTime FallTime

Rotate for best display:3

Time/Div

Rise(1) 920 ns

90% line

10% line

Time

Rise(1) <4.000 us

Rotate to show multiple cycles on screen

Time/Div

Rise(1) <4.000 us

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1

Main/Delayed

Time/Div

MainDelayed

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Main Delayed XY Roll|-------Horizontal Mode --------|

Rise(1) 920 nsDelaye

dCntr

Rotate. See how upper bracketed part is exploded into lower window

Time/Div

Making Measurements: RISETIME, Using DELAYED SWEEP:

(Hit Menu key)

To do FFT, a Measurement/Storage Module must be installedon back of scope.

Making Measurements: FFT (Frequency Domain)

Setup: Default

+

Auto-scale

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|-- Function 2 --|Off On Menu

|-- Function 2 --|Off On Menu

OperationFFT

Use Time/Div to set FFT resolution

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Hint: To look ONLY at FFT signal without time domain signal, turn channel off: 1

Off On

Hint: To return to FFT menu at any time, use Math key +

Auto-store

Storing Waveforms:AUTOSTORE

Displays all waveforms (Good for looking at jitter, noise, glitches)

Auto-store

Example: Calibrator signal in CH 1. Setup Default. AUTOSCALE. Touch AUTOSTORE.Now move horizontal delay knob and watch all waveforms stay on screen, making an overlapping pattern.

Auto-store

Touching Autostore again returns display to normal mode.

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Storing Waveforms:TRACE MEMORYExample: Calibrator signal in CH 1. Setup Default. AUTOSCALE.

Trace

Now move waveform with horizontal delay and vertical position knobs. Note that Trace 1 still stays on screen, allowing comparison with stored waveform.

Save to Trace 1

Trace 1 Off On

Vertical position

Horizontal delay

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