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The University of Texas at Arlington
Electrical Engineering Department
Electric Circuit Lab
The Oscilloscope
Agilent 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
Hints
The Digitizing Oscilloscope
• 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.
• 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 and position
- Turn up signal brightness with intensity control
Digital Oscilloscope Block
Diagram
The Digitizing Oscilloscope
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: Coupling
- Check Trigger Source 1 DC AC
- Set Mode Auto
The Digitizing Oscilloscope
Status bar:
Vertical sensitivity Autoscale Measure: Vrms, tr, f ,f
Sweep speed Print Trace /Setup Memory
Trigger time reference Calibrate Display: grid, vectors,
Channel(s) on averaging
Run/stop
1 1.00V 0s 2 us/ 1 RUN
Storage: Auto-store,
1
Erase, RUN/STOP
Softkey ----Setup memory----
Undo Default
labels Triggering:
Modes: Auto, Normal,TV
Sources: Ch1, Ch2, Ext, Line
Calibration source
Power ON/OFF
Screen brightness Vertical position, Horizontal:
sensitivity Main Sweep Speed,
Math on/off Sweep Modes:
Delayed, XY, Roll
t = 0 Reference
Getting Started
Getting Started: 4
Line 2
1 (Power ON)
0 1
Default
Setup (All settings 5
2 Setup 3
to default )
1
3 Connect probe to calibrator
Auto-
4 scale
1 1.00V 200 1 RU
0.00s
us/ N
5 Compensate probes:
1
Adjust screw to eliminate: Default
Setup
Overshoot
Undershoot
1 1.00V 0.00s 200 1 RU
Set probe attenuation factor:
us/ N Press Channel key for
1
selected probe.
Probe Toggle softkey for setting
1 10 100
1
that matches probe
Probe
1 10 100
10:1
Match
Trigger slope
What the display says:
is positive
Blinks if
(rising edge)
no trigger
Vertical sensitivity of CH1
is 1Volt per major division Time=0
t<0 t>0
Trigger source
1 1.00V 0.00s
200us/ 1 RU is Channel 1
N Scope is ready
for trigger
1 Ground (V=0)
[If dc part of CH1 signal
is too big, ground
arrow points off-screen.
1 Probe If this happens, adjust
Off On 1 10 100 vertical sensitivity]
Channel 1 is ON
Horizontal sweep speed is Readings scaled
200 msec per major division for a 10:1 probe
What the main controls do:
Horizontal Trigger level Line is only
Grid full delay visible when you turn the
knob.
Display: When signal is smaller than
Vectors off trigger level, scope stops
sampling.
Time/division
(sweep speed)
Vertical Vertical
Volts/division Position
Other main controls:
Trigger holdoff
Signals with multiple zero crossings per
cycle cause unstable displays.
Holdoff ignores the false triggers for a
given length of time.
Holdoff
Trigger
Holdoff=11.5
us
+
1 Math Functions
- Enable channel -Add waveforms
- Set up probe -FFT (if equipped with
module)
Horizontal Controls
Vertical Controls
Trigger Controls
Display
Display (continue)
Other Functions
Other Functions (continue)
Other Functions (continue)
Other Functions (continued)
Other Functions (continued)
Other Functions (continued)
Making Measurements: Vp-p
Measure Vp-p, using cursors:
1 Hook Calibrator signal to CH1
Grid Cursor adjustment knob
2 Display None
3 Cursors Clear Clears any cursors already on
Cursor the screen
s
Source
4 1 Set for the correct channel
|- Active Cursor -| Toggle to highlight
5 V1 V2 T1 T2 the V1 cursor; Rotate
knob for waveform
minimum D
V1= 31.25mV V2= 5.031V V= 5.000V
|- Active Cursor -|
5 Change to V2 cursor;
V1 V2 T1 T2 Use knob to set to
waveform minimum Answer
OR: Measure Vp-p, the easy way:
|- Voltage Measurements -| Simply select Vp-p from the
1 Voltage Vp-p Vavg Vrms Voltage menu.
Time
Making Measurements:
RISETIME
Next
1 Time Menu
|----------- Time Measurements -----------| Rise(1) <4.000 us
2 +Width -Width RiseTime FallTime
Risetime Answer
If answer needs more resolution:
Time/Div 90% line
3
Rotate for best display: 10% line
Rise(1) 920 ns
Making Measurements:
RISETIME, Using DELAYED SWEEP:
Time/Div
1
Rotate to show multiple
cycles on screen
Rise(1) <4.000 us
2 Main |-------Horizontal Mode --------| Main/Delayed
Delayed Main Delayed XY Roll
Time/Div
3 Rotate. See how
upper bracketed part is
exploded into lower Rise(1)
Delayed Cntr
920 ns
window
Time/Div
Making Measurements: FFT To do FFT, a
(Frequency Domain) Measurement/Storage
Module must be installed
1 Setup: Default on back of scope.
Auto-
2 scale
+ |-- Function 2 --|
3 Off On Menu
|-- Function 2 --|
4 Off On Menu (Hit Menu key)
Operation
5 FFT
Hint: To look ONLY at FFT signal
without time domain signal, turn
channel off: 1
1 Off On
Use Time/Div to set
FFT resolution
Hint: To return to FFT menu at any time,
use Math key +
Auto-
Storing Waveforms: store
AUTOSTORE
Auto- Displays all waveforms
1 store (Good for looking at jitter,
noise, glitches)
2
Example: Calibrator signal in CH 1.
3 Setup Default. AUTOSCALE. Touch
AUTOSTORE.
Now move horizontal delay knob and
4 watch all waveforms stay on screen,
making an overlapping pattern.
Auto- Touching Autostore again returns
5 store display to normal mode.
Storing Waveforms:
TRACE MEMORY
1 Example: Calibrator signal in CH 1.
Setup Default. AUTOSCALE.
Trace Save to
2 Trace 1
Trace 1
3 Off On
Now move waveform with horizontal Vertical position
4 delay and vertical position knobs.
Note that Trace 1 still stays on screen,
allowing comparison with stored Horizontal delay
waveform.
