Measurements in Electronics and Telecommunications – Lab 1
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Laboratory 1
Generating and viewing signals
Purpose: Familiarization with the basic functions of an oscilloscope and
of a signal generator. Adjusting and measuring specific parameters of signals.
Summary of theory
The oscilloscope (often abbreviated scope) is a device that allows viewing
the instantaneous value of a voltage v(t) as a function of time, and quantitative
measurements of voltage and time on the waveform, having broad applications
in the analysis of electrical signals. In this lab we use the Tektronix TDS1001
digital scope. The description that follows corresponds to this model.
The main parameters of a periodic signal
- waveform (deterministic) in the time domain
- period and frequency, f = 1/ T
- maximum value Umax
- minimum value Umin
- peak-to-peak value, the signal range UPP = Umax - Umin ;
- average (offset) value of the signal Uavg or UDC
- signal amplitude (if the signal is similar to a sine wave) U0 = Umax – UDC
= UDC - Umin
Remark: if UDC = 0, the amplitude, min and max values are equal in absolute
values.
- other parameters for certain waveforms, for example:
- rectangular wave: duty cycle η = τ/T (  represents the width of the
„high” pulse , and T is the signal period) , rise / fall time (trise , tfall)
- triangular wave: rising / falling slope
UDC
Umin
U0
t
T
a. Sine wave
Umax
Horizontally, the duration corresponding to the length of a division on
the screen is adjustable from the Cx knob (horizontal deflection coefficient). It
is labeled in units of time per division. The following relation exists between the
number of divisions N x that an event occupies and its duration Tx :
T x = Nx C x
Vertically, voltage U is applied from external input Y. Similar to the
reading on the X axis, one can read the value of voltage U that occupies N y
vertical divisions using the formula:
U = Ny Cy
Cy is called vertical deflection coefficient and is labeled in units of volts
per division.
Example: A period of the sine wave in Fig. 2 occupies 7 horizontal
divisions on the screen, and the time base is set at Cx=5ms/div. The signal’s
period is, thus, 35 ms. Vertically, the amplitude (peak value) occupies 3
divisions, which, for Cy= 2V/div, means 6V.
τ
U0
UDC
Umin
Measurements in Electronics and Telecommunications – Lab 1
Oscilloscope Settings
rev. 5
Umax
2
T
b. Rectangular wave
Figure 1. Periodic signals: sine, rectangular
t
Figure 2. Horizontal and vertical divisions
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Values Cx, Cy are called calibrated and have standard form (1,2,5)*10+/-K .
for Cy, and (1, 2.5, 5)*10+/-K. Calibrated coefficient values for usual
oscilloscopes are:
Cx  {5; 10; 20; 50; 100; 200; 500 mV/div; 1; 2; 5 V/div}
Cy  {5; 10; 25; 50; 100; 250; 500 ns/div; 1; 2,5; 5; 10; 25; 50; 100; 250; 500
μs/div;
1; 2,5; 5; 10, 25; 50; 100; 250; 500 ms/div, 1; 2,5; 5; 10 s/div}
Triggering the oscilloscope: An image that is stable on the display of the
oscilloscope is called triggered (synchronized). The physical meaning is the
following: when 2 successive images of a periodic signal start at the same
moment in time (relative to the signal period), the 2 images will overlap
perfectly, and so will happen for subsequent images. Thus, the eye perceives a
single stable image, although, in fact, we constantly have a new image
superimposed on the previous one. An example in case of displaying a rising
slope is given in Figure 3 (a). But if every display starts with some other
moment of time, the images will differ, and the eye will perceive many different
superposed images - Fig. 3 (b). In this case the image is called untriggered
(unsynchronized).
a) untriggered image
b) untriggered image
Figure 3: oscilloscope triggering
In order to obtain a triggered image, the operator must adjust the trigger
settings. The most important are: trigger source, its level and a slope at which
the displaying of the image should start. Usually, those adjustments are grouped
in a trigger menu.
Usually, in order for an image to be synchronized, the Trigger Level has
to be between the [minimum, maximum] level of the signal. A smaller or larger
level means that the Trigger Level does not meet the signal, therefore it cannot
trigger a display.
In the measurements that will be performed next, the following trigger
settings will especially be used: Source = CH1, slope = Rise, Mode = AUTO.
For automatically adjusting the Trigger Level the Set To 50% button has to be
pushed.
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Measurements in Electronics and Telecommunications – Lab 1
Measurements
1. Automatic settings of the oscilloscope
Using the function generator, generate a sine wave with the following
parameters:
- WAVE key – sine wave
- medium amplitude (AMPL knob rotated halfway to the right – the
position indicator on the knob is in the middle)
- no DC level (OFFSET knob is pushed)
- frequency f1 written on the blackboard (keyboard: press keys
corresponding to the frequency then press the [kHz] button). (fig. B1
from annex B).
Connect the OUTPUT 50Ω of the generator to channel 1 of the scope
using a coaxial cable (or two crocodile clips).
On the scope, press CH1 MENU in order to display the settings of
channel 1 (at repeated presses, channel 1 is alternatively turned on and off).
Because a simple cable is used, not an attenuator probe, press the soft-key Probe
until the indication is 1x (there are probes that contain a divisor that attenuates
the signal 10...100 times, in which case settings 10x, 100x would be used). Press
the AUTOSET key (for obtaining automatically setting the adjustments for
displaying and triggering).
Notice how many periods of the image there are on the screen. Write
down the value the oscilloscope has automatically set for Cx (the M - Main
indication at the bottom of the screen – see Fig. 2). Measure the period by
counting the divisions and subdivisions Nx of a period and use the formula
Tmeas  C x  N x . Calculate the frequency fmeas=1/Tmeas and verify that it corresponds
with the value indicated by the generator.
Repeat the previous steps for the vertical adjustment. What is the value
for the vertical deflection coefficient Cy? (the indication CH1 at the bottom of
the screen). By counting the vertical divisions and subdivisions Ny and applying
the formula U=NyCy, measure the amplitude of the signal (peak value UP). In the
same way, measure peak-to-peak value UPP. Calculate the ratio between UP and
UPP (measured values). What is the theoretical value of the ratio?
Remark: for identifying settings and indications of the oscilloscope and
signal generator, use annexes A and B
2. Computing the display settings
Pay attention! This exercise is only for computing the settings. You do
not have to display signals on the scope!
a) Compute how many divisions, NX și NY, will the amplitude and period
of the signal occupy on the scope display for the following signals and settings::
Measurements in Electronics and Telecommunications – Lab 1
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a1. sine wave with amplitude UP1=4V and frequency f1=20kHz.
Oscilloscope settings: CX1=10μs/div and CY1=1V/div.
a2. sine wave with amplitude UP2=6V and frequency f2=8kHz.
Oscilloscope settings: CX2=25μs/div and CY2=2V/div.
Indication 1: Use the following T = NX∙CX and UP = NY∙CY . You can
compute T from the frequency value.
Indication 2: Use as an example the solved exercises at the end of the
laboratory work. Same goes for (b) and (c).
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Measurements in Electronics and Telecommunications – Lab 1
Starting from (a), set CY to the value of CY2=2V/div. Measure the amplitude,
UP2meas, and compare it with UP1. Explain the relationship between UP2meas and
UP1..
c) Influence of CX on the displayed image
Starting from (b), modify CX to the value of CX3=50 μs/div. Measure the period,
T 3, and compare it with T1. Explain the relationship between T3 and T1.
4. Generating a sine wave with a DC component
b) Compute the deflection coefficients (CXcalc, CYcalc) that should be set for
displaying a sine wave with the frequency of f3=1kHz and amplitude UP3 =2V,
such that the amplitude occupies two divisions and the period occupies four
divisions.
a) Until now the signal did not have a DC component or DC level (also
named offset), being symmetrical from 0 V. Generate a sine wave (button
WAVE from the generator), with the frequency of f1=20kHz, amplitude UP =2V
and DC component UDC1= -1V. Use coefficients CX1=25μs/div and CY1=1V/div.
c) Repeat (b) for a signal with f4=500kHz și UP4=8V, if the amplitude
occupies four divisions, and the period 10 divisions (CXcalc, CYcalc)
u(t) = UDC1 + UV sin ωt [V]
3. Generating a triangular wave
a) Generate a triangular wave, with no DC component (OFFSET knob pushed),
with frequency f1= 10 kHz and amplitude UP1 =2V. For the oscilloscope choose
settings: CX1 = 25μs/div, CY1 = 1V/div.
a1. What is the measured period of the signal (T1)? What is the theoretical
period of the signal (T1calc) (computed form the value of the frequency) and how
many divisions does it occupy on the scope (NXT) ?
a2. How many divisions does the signal amplitude occupy (NYmas)?
a3. Draw the image and write down, on the graph, the 0V (Ground) level
and the values of Cx and Cy. The Ground level is displayed by a tiny arrow
(indication 6 from fig. A3) and the channel number (1 in our case), on the left.
Indication- follow these steps:
- set the values of CX1 (indication M) and CY1 (indication CH1), the
waveform (button WAVE) and the frequency of the signal (from the generator
keyboard) to the desired values;
- adjust the image vertically (Vertical position – fig. A3 from annex A)
so that the arrow corresponding to the Ground level is in the middle of the
display.
Remark: Since for the generators in the lab, buttons AMPL and OFFSET are
not marked with numerical values, these parameters cannot be adjusted directly
to a certain value without measuring it using the oscilloscope.
b) Influence of CY on the displayed image
If it isn’t already, adjust the 0V (Ground) level in the middle of the screen, using
Vertical Position, and set CH1 MENU->Coupling ->DC.
In order to set the DC level, pull out the OFFSET knob from the signal
generator, then rotate it. Please notice that, while adjusting the OFFSET knob,
the sine wave image moves down/up on the screen.
a1. Draw the image on the scope and specify the ground level. Use CH1
MENU->Coupling ->DC. Write down on the graph the position of the Ground
level and the Cx and Cy values. (Same remark for a2.)
a2. Draw the image when using AC coupling (CH1 MENU->Coupling >AC). What happens to the image? Explain.
Indication for adjusting the DC level:
- knowing CY1=1V/div, compute how many divisions and subdivisions
NY1 , taking also the sign into account, correspond to the value of UDC1 .
- set DC coupling (CH1 MENU->Coupling ->DC) and adjust the DC
component of the signal (pull out the OFFSET knob from the signal generator,
then rotate it) until the position of the maximum (or minimum) moves upwards
(for +) or downwards (for -) with exactly the computed number of divisions for
UDC1 . If needed, adjust the trigger value (SET TO 50%) so that the image is
triggered.
-DC coupling means that the signal is applied directly, without altering the
possible DC component of the signal.
-AC coupling means that there is a capacitor inside the oscilloscope on the
signal path; it blocks the DC component, and only allows the AC part of the
signal.
Measurements in Electronics and Telecommunications – Lab 1
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b) repeat (a) for a DC component of UDC2=2V.
5. Generating a rectangular wave
Generate a rectangular wave (WAVE button), no DC component
(OFFSET button pushed), with the amplitude of UP5=2V, frequency of f5=
2kHz. View the signal with CY5 = 1V/div, CX5=100μs/div and use coupling
CH1 MENU->Coupling ->DC.
Adjust the duty cycle of the signal (see fig. 1b) successively to the values
of η1=50%, η2=20% as follows: on the signal generator press (successively, not
simultaneously) buttons Shift and 7 (Duty), then adjust the numerical values
from the keyboard or from the rotating knob (button 10 in fig. B1, annex B) then
press Hz % (fig B1, annex B).
- Measure (in divisions) the values of T and τ for both cases (see fig 1b).
- Compute the ratio τ/T
- Draw the two signals.
- Measure the two DC levels UDC1 și UDC2 (similar to 4)
Indication: alternatively, in order to measure the DC level of a displayed signal,
modify the coupling - from AC to DC. The vertical movement (NYdc) of the
maximum or minimum of the signal indicates the values of the DC component
(|UDC| = CY ∙NYdc), and the direction of the movement determines the sign of
UDC: upwards  positive value, downwards  negative value.
Explain how the sign of the DC component is determined when switching from
DC to AC.
Come back to a duty cycle of 50%.
Important remark: the duty cycle is a parameter that has meaning only for
a rectangular wave, according to the definition in Fig. 1b. Do not try to set a
duty cycle for a sine or triangular wave, you will receive an error!
6. Automatic measurements using the oscilloscope
Digital scopes can perform some of these measurements automatically
(but only for the displayed image).
a) Check the measurements from section 5 (η1=50%) using the
MEASURE button on the oscilloscope. There are 5 “simultaneous”
measurements that can be programmed on the 5 soft-keys (keys on the edge of
the screen). Press a soft-key, select SOURCE=CH1, and depending on TYPE
you can measure the following: Freq (frequency), Period, Mean (mean value,
for this symmetrical signal it should be almost 0), Pk-Pk (Peak-to-peak), Min,
Max (negative and positive peak value, meaning the amplitude), τ (positive
width). Measurements are done for two values of CX: CX1=1ms/div and
CX2=250μs/div.
b) Repeat the measurements for η2=20%.
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Measurements in Electronics and Telecommunications – Lab 1
Compare with the values from section 5. What do you notice?
Explain the value for Mean in the two cases, knowing that any real signal
is also corrupted by noise (so a value of mV is negligible for a signal with an
amplitude of V).
Recap question: with respect to the function generator in the lab, which
of the following signal parameters can be adjusted using only the generator,
without looking at the oscilloscope display? (choose between amplitude U,
frequency f, period T, offset U0, duty factor η).
Solved exercises
1. A sine wave with a frequency of f=2kHz and amplitude of UP=4V is displayed
using an oscilloscope. The settings are: CY=1V/div, CX=250 μs/div. Determine how many
divisions the amplitude and period of the signal occupy on the display of the oscilloscope.
Solution:
The number of divisions that the amplitude occupies on the display can be determined
using
UP=NY∙CY 
NY = UP/CY = 4 [V] / 1[V/div] = 4 div
The period of the signal is T = 1/f = 1/2000Hz = 500 μs
The number of divisions that the period of the signal occupies on the display is
T = NX∙CX

NX = T/CX = 500∙10-6[s] / 250∙10-6 [s/div] = 4 div
2. A sine wave with a frequency of f=2MHz and amplitude of UP=6V is displayed
using an oscilloscope. Determine the values of CX and CY such that the amplitude occupies 3
divisions (NY=3 div), and the period occupies two divisions (NX=2div).
Solution:
The equations in ex. 1 can be used:
The period of the signal is T = 1/f =1/2∙106s = 0,5 μs.
CY = UP/NY = 6V / 3 div = 2 V/div
CX = T/NX = 0,5 μs / 2 div = 0,25 μs/div
3. A sine wave is displayed using an oscilloscope. When the coupling is switched from
AC to DC, the sine wave moves downwards by NY = 2 div. The vertical deflection coefficient
is CY=5V/div. Determine the DC level of the signal.
Solution:
The DC level value determines the downward movement of the signal image by a
value that is equal to the value of the offset (for DC coupling). The direction of the movement
determines the sign of the offset: upwards - positive; downwards - negative.
Based on these observations, we can determine the DC component:
UDC = - NY∙CY = -10V
4. A sine wave is displayed using an oscilloscope. When the coupling is switched from
AC to DC, the sine wave moves downwards by NY = 4 div. The vertical deflection coefficient
is CY=1V/div. Determine the DC level of the signal
Solution:
The difference from ex 3 is that now we switch from DC to AC. Because, after the
elimination of the DC component (AC coupling), the signal moves upwards, this means that it
Measurements in Electronics and Telecommunications – Lab 1
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(the DC component) was “pulling” the signal downwards (in DC coupling). This means that
the value of the offset is negative.
UDC = - NY∙CY = -4V
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Measurements in Electronics and Telecommunications – Lab 1
ANNEX 1. Tektronix TDS1001 Oscilloscope
1
2
3
4
5
6
Exercises
1.
An oscilloscope is set to Cy=0,5V/div. The amplitude of a signal measured on the
oscilloscope screen is 3.8div. What is the signal amplitude in volts?
2.
An oscilloscope is set on Cx=20ms/div. The measured period of a sine wave on
the screen of the oscilloscope is 5 div. Determine the frequency of the sine wave.
3.
Given a sinusoidal signal of frequency 10kHz and amplitude 4V, determine the
values of the horizontal and vertical deflection coefficients so that the amplitude and the
period of the signal can be precisely measured on the screen.
4.
A sinusoidal signal is viewed with an oscilloscope. When the coupling button is
set from AC to DC position the sinusoidal signal moves on the screen, downwards by 3
divisions. Cy=1V/div. Determine the DC level of the signal.
5.
A symmetrical rectangular signal, having amplitude A=1V, no DC level and
frequency of 1kHz, is applied at the input of an oscilloscope. The oscilloscope has
Cy=0,5V/div, Cx=0,2ms/div, trigger level Utrig=0,5V and SLOPE=falling. Draw the image.
Figure A1: front panel of the oscilloscope
The front panel of the oscilloscope is represented in Fig. A1. The interface
of the oscilloscope contains the following elements:
1. The screen of the oscilloscope
Figure A2: Informations and symbols that are displayed on the screen of the
oscilloscope
Measurements in Electronics and Telecommunications – Lab 1
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The screen contains an area for displaying the grid of the screen, the area
of the control menu (right side of the grid area) and the area for displaying
parameters (above and below the grid zone).
The graticule area is formed from Nx=10 units horizontally and Ny=8
units vertically, and it is used for displaying the image.
Beside the image there are several parameters of the oscilloscope or of the
waveform that are displayed according to the selected working mode. On Fig.
A2, the most relevant ones are: (1) acquisition time (normal, averaging, etc)
(2) Trig’d = triggered = synchronized (3) Trigger moment, it can be swept
using the adjustment HORIZONTAL POSITION
(5) Trigger Level, it is
adjusted using TRIGGER LEVEL (6) identifier of traces 1 and 2, its position
is swept with VERTICAL POSITION
(8) Cy values of both channels (9)
BW= Bandwidth Limit (it limits the maximum frequency of the oscilloscope to
20MHz) (10,11) Cx values for the main time base (Main) and the secondary
time base (Window) (12,13) the source and the slope of the trigger (17) the
measured frequency of the signal.
2. Control keys - they allow changing the control fields displayed on the
screen of the oscilloscope. They are called soft keys because those fields change
according to the selected menu/mode.
3. Channel Y adjustments (vertical adjustments) - there is a set of
adjustments for each of the two channels of the oscilloscope.
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Measurements in Electronics and Telecommunications – Lab 1
 CH1 MENU – the result of pressing the button is the display in the
control area of fields that allow the control of the displaying on the
vertical axis, for channel 1 (CH1).
 The following fields will be displayed:
o Coupling - it selects the coupling type AC/DC/Ground
(alternating current/continuous current/zero level)
o BW Limit – band limiting at 20MHz instead of 40 (option
ON/OFF)
o Volts/Div –
calibrated (Coarse) or noncalibrated (Fine)
adjustment. For the calibrated adjustment the vertically deflection
coefficient can only have values Cy={1,2,5}x10kV/div.
o Probe- the type of probe used (x1/x10/x100/x1000). The value
must be the same as that used at the probe of signal.
o Invert – reverses the image when ON.
 MATH MENU – allows applying some mathematical functions on
signals (add, subtract, Fourier Transform).
4. Digital Functions Menu – the effect of pressing a button from this area is
displaying on the screen a menu that contains functions which are specific to
digital oscilloscopes (saving, measurement, acquisition, cursors, utilities,
display). For the laboratory the menu DISPLAY is the one that is interesting. It
contains the control field Format, RUN/STOP and SINGLE SEQ buttons.
Figure A4: Menus for digital functions
a) adjustments of the Y channel
b) adjustments of the X channel
Figure A3
 POSITION – it allows moving the image vertically
 VOLTS/DIV – knob used for changing the vertical deflection coefficient.
Its value is displayed in the bottom side of the screen (the area for
displaying parameters).
 DISPLAY
o Format – selecting operating mode y(t) (YT) or x(y) (XY)
 RUN/STOP – in RUN mode the oscilloscope acquires signal
continuously. In STOP mode the acquisition is stopped, the image which
is displayed being the last image before pressing the STOP button.
 SINGLE SEQ – the oscilloscope acquires a single image (that
corresponds to a single course on the screen) and it waits for a new press
of the button. Pressing the button is equivalent to a RESET.
5. Adjustments for X channel of the oscilloscope (horizontal axis
adjustments)
 SEC/DIV – the adjustment of the horizontal deflection coefficient Cx. Its
value is displayed in the bottom side of the screen as M 10ms, which is
equivalent to Cx=10ms/div.
Measurements in Electronics and Telecommunications – Lab 1
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 HORIZ MENU – displays the menu for control of the displaying on
horizontal axis.
o MAIN- selects the display of the image for main time basis (the
usual working mode).
o Window zone – selects the display of the image for secondary time
basis (in fact, secondary time basis is the name from analogical
oscilloscope. In this context it is a portion of the image that is
"dilated" on horizontal axis)
o Window – adjustment of the temporal window for the secondary
time basis.
o Trigger Knob – it allows selecting the function of the LEVEL
button from the TRIGGER buttons area: implicitly it represents the
adjustment of the trigger level; when the holdoff adjustment is
selected, the LED bellow LEVEL button is on (lighting).
 POSITION – moves the image horizontally
 SET TO ZERO – restores the image to its original position (horizontal
displacement deleted)
6. Adjustments/Settings for the synchronizing circuit (TRIGGER) – For
the TDS 1001 oscilloscope, the trigger moment corresponds to the middle
of the screen.
 LEVEL – it allows to set the trigger level and the holdoff time.
 TRIG MENU – enables the menu for control of synchronization
(trigger). It contains the following control fields:
o Type – selects the type of the trigger: Edge – triggering on the slope
of the signal, Video - triggering on a video signal, Pulse - triggering
on pulses.
o Source – the source of the triggering signal (CH1, CH2, EXT,
EXT/5, AC signal from the mains)
o Slope – type of the front: positive or negative (Rising/Falling)
o Mode – synchronizing mode (Auto/Normal):
 AUTO: if trigger conditions are not satisfied, the oscilloscope
automatically generates, after a certain time, a signal to trigger
the display. This way, when the input signal is not present a
horizontal line can be observed on the screen. It represents the
zero level. This is the default mode if not specified otherwise !
 Normal – This time, the display is triggered only if the trigger
conditions are met. Otherwise, the oscilloscope does not display
any image. The trigger level can also be set outside the limits of
the signal. There exists the possibility that, although on the input
of the oscilloscope a signal is present, the signal is not displayed
because the trigger conditions are not met.
o Coupling – the coupling mode of the synchronization signal: AC –
eliminates the DC level from the trigger signal. DC – the trigger signal
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Measurements in Electronics and Telecommunications – Lab 1
has nonzero DC level. Noise Reject – the noise is eliminated from
trigger signal. HF REJ (High Frequency Reject) – it eliminates high
frequencies from trigger signal. LF REJ – it eliminates low
frequencies from trigger signal.