Lab 1(a) manual - Hong Kong University of Science and Technology

The Hong Kong University of Science and Technology
Department of Electronic and Computer Engineering
ELEC2400
ELECTRONIC CIRCUITS
SPRING 2014/15
LAB 1(a,b) –Instrumentation and Measurement
1. Objective
The objective of this lab session is to get you familiarized with the operation of various pieces of
electronic instrument that will be used in future lab sessions.
2. Component and Instrumentation
a.
b.
c.
d.
Digital Multimeter (AGILENT Technologies U3401A)
Oscilloscope (TEKTRONIX TDS1002 / TDS1012)
Synthesized Function Generator (SRS DS345)
DC Power Supply (TEKTRONIX PS280 / GWINSTEK GPS-3303)
3. Background Information
3.1 Digital Multimeter (AGILENT Technologies U3401A)
A Digital MultiMeter (DMM) is a handy tool for measurement. It can measure voltage, current, and
resistance. Some meters can even measure capacitance. The measured signal should be either dc
(direct current) or low-frequency ac (alternating current).
1) How does a DMM measure voltage, current and resistance?
The DMM converts the quantity to be measured into a voltage, which is then converted to a
digital number and shown displayed on the display. To measure voltage, the DMM scales the
voltage and converts the output to a digital number. To measure resistance, a known constant
current is injected into the measured device, and the voltage across the device is measured.
To measure current, the measured current is allowed to flow through a small fixed resistance
and the DMM measures the voltage across the resistance.
2) How accurate is the measurement?
It is not true that the least digit in the display represents the accuracy that can be achieved.
Always refer to the instruction manual for the accuracy specification. Generally speaking,
accuracy is increased as the smaller range is selected.
3) What value does a DMM display when measuring an AC signal?
When measuring the voltage or current of an AC signal, the DMM shows the rms value of
the quantity measured. Some good-quality DMM can measure true rms values by measuring
the power of the signal, while other DMMs just calculate the rms value by scaling the peak
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amplitude with a factor that applies only correctly to a sine wave, but the reading is
inaccurate if the signal is not a pure sine wave.
4) Can a DMM measure a high-frequency signal?
The frequency response or bandwidth describes the range of frequencies for which an
electronic instrument can measure reliably. Outside this range, the measurement shown on
the instrument will be inaccurate. Typically, electronic instruments can measure all the way
down to 0Hz (DC). Thus, if the bandwidth of a given instrument is 100MHz, it means that it
can measure signals from DC all the way up to 100MHz fairly accurately. Above 100MHz,
the measurement will be severely attenuated (i.e. it will be much smaller than the actual
value). The typical bandwidth of a DMM is around 10kHz. Normally, for the sake of
accuracy, a DMM is not used to measure a signal with a few hundred Hertz.
3.2 Oscilloscope (Tektronix TDS1002 / Tektronix TDS1012)
The oscilloscope (scope) or CRO (cathode ray oscilloscope) is a powerful tool for waveform capture
and measurement. Unlike the DMM, it can be used to study the time behaviour of a voltage and to
measure high-frequency signals. However, the scope can only measure voltage. Other quantities
must be converted to voltage in order to be displayed on the scope.
1) How do we choose a scope?
An analogue scope can only capture a periodic, long-lasting signal. It has no memory and is
much cheaper. A digital storage scope can capture transient and single-shot (one-time)
signals by digitizing and storing the waveform in its built-in memory (RAM). It can be used
as an analogue scope by disabling its storage function.
Select a scope with the appropriate bandwidth (e.g. 20MHz, 40MHz, and 100MHz.). Recall
from above that the bandwidth indicates the maximum frequency of the input signal to which
it can measure without severe attenuation. The attenuation of magnitude is about 0.707 at the
bandwidth frequency, and the attenuation increases as frequency increases, as shown above.
For example, if a 100MHz scope is used to measure a 100MHz 1V-amplitude sine wave, the
waveform captured/displayed will only be a 100MHz 0.707V amplitude sine wave. A scope
with large bandwidth can be used to measure high-frequency signals, such as those used in
radio. The scopes in our laboratory are 60MHz digital scopes.
2) What is triggering?
The trigger determines when the oscilloscope starts to acquire data and display a waveform.
The triggering circuitry detects the signal when it crosses through a given level called the
triggering level with a given slope (positive and negative) and starts the sweep at that time.
For example, the left figure below shows a 4Vpp (4V peak-to-peak) sine wave. If the
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triggering point is set to −1V and the triggering slope is set to be negative, then the waveform
displayed is as shown in the right figure below.
The simple scheme works for a large number of waveforms. The triggering level can be
changed accordingly. The duration of the display frame can be computed from the time base
setting of the scope. There are 10 divisions in total across the screen, and multiply the time
base setting (in time-unit per division) by 10 gives the duration of the triggering frame.
3) Why do the waveforms sometimes appear to be unstable on the screen?
A major cause is that the input signal is not periodic and the waveforms in the triggering
frames are not identical, resulting in an unstable waveform which is the overlap of many
non-identical waveforms.
A second possible cause is that the signal is periodic but has a complex shape. In this case,
the triggering circuitry may find more than one triggering point that satisfies the triggering
slope and triggering level requirement, within any one cycle of the input signal. The situation
is shown below.
A third possible cause is that the input signal is contaminated by external noise, which again
leads to a complex waveform. The situation is illustrated below. The solution is to remove
the source of noise or select the triggering point with a steepest slope, which can help to
decrease the noise sensitivity.
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4) What is a ×10 probe used for?
It is a good practice to use a ×10 probe to measure signals. The ×10 probe is used to increase
the equivalent input impedance of the scope. The input impedance of the scope is about
0.89MΩ measured at 1kHz and decreases down to about 73kΩ measured at 100kHz. The
decrease of input impedance at high frequency (due to probe capacitance) implies the current
will flow from the circuit into the input circuitry of the scope. The scope becomes a
significant load to the measured circuitry and leads to an inaccurate result.
The ×10 probe has an internal passive circuitry and can increase the input impedance of the
scope by 10 times. Thus a ×10 probe is often used in high-frequency measurements. Also,
when the impedance of the measured circuit is comparable to the scope input impedance, a
×10 probe can be used to reduce the loading effect. However, when using a ×10 probe, the
measured voltage will only be one-tenth of the voltage when using ×1 probe. So, read the
range values specially designated for use of a ×10 probe. A ×10 probe needs to be calibrated
from time to time. A ×1 probe is just a conducting wire, but includes a lot of capacitance,
which increases the load on the measured circuit at high frequencies.
3.3 Synthesized Function Generator (SRS DS345)
A function generator is an electronic signal source that can generate different kinds of waveforms,
such as sine, triangular and rectangular waves. The frequency and the duty cycle of the generated
waveform can be adjusted via panel controls.
3.4 DC Power Supply (Tektronix PS280 / GPS-3303)
A DC power supply rectifies and regulates the AC input voltage to give a DC output voltage. A dual
power supply provides two outputs, which can usually operate independently.
1) What is the difference between the "−" output and ground?
To obtain a voltage output from the power supply, we connect the "+" and "−" terminals, but
not "+" and "ground". The "ground" is in fact the “chassis ground” but not the lower potential
point of the output. The chassis is the metallic casing enclosing the electronic circuit. The
chassis ground is completely isolated from the "−" and "+" terminals, or it is connected to the
"−" output via a very small capacitor for the purpose of electromagnetic compatibility. For
measurements that are affected by electromagnetic compatibility problems, it is good practice
to connect the "−" to the chassis ground with a heavy conducting wire.
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2) What is the current-limit knob used for?
The current-limit knob on the panel is used to set the maximum current that can be drawn
from the output, regardless of the output voltage. It is used to protect the circuit being tested
from damage due to excessive currents, e.g., in the case of a short circuit.
3) What is tracking?
Tracking is used to obtain two outputs with identical voltage magnitude. By enabling the
tracking function, one of the outputs becomes master and the other becomes its slave. By
adjusting the voltage-level knob of the master output, the slave output level will follow (track)
the master output.
4. Prelab
In this section, you are required to read through the background of this lab before the lab session.
Since we will always use these electronic instruments in future lab sessions, you have to get
familiar with their operation.
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The Hong Kong University of Science and Technology
Department of Electronic and Computer Engineering
ELEC2400
ELECTRONIC CIRCUITS
SPRING 2014/15
LAB 1(a) –Instrumentation and Measurement
5. Procedure
5.1 Phase Measurement
Because the SCOPE can display the time behavior of a waveform, we can use it to measure the
time between events. In this experiment, we use the SCOPE to measure the phase difference
between two sinusoidal waveforms.
VA(t)
Use C =1 µF , R = 1kΩ
Figure 1
1. Connect the circuit shown in Figure 1. Apply a 1kHz 4Vp-p (+/− 2V) sine wave from the
function generator to V(t). Use the SCOPE to ensure that the frequency of the waveform is
correct.
2. Connect node ' A ' to channel 1 and ' B ' to channel 2 of the scope.
3. Press [AUTO SET] button in the scope to produce a usable display of the input signals.
4. Turn the [SEC/DIV] knob such that a full cycle of a sine wave is displayed on the screen.
5. Switch the triggering source to 'ch1', view the screen, and then switch it to 'ch2'. <Press [TRIG
MENU] button, and select the ‘Source’ option to change the triggering source>
6. Measure the phase difference in time t (shown in Figure 2). Then find the phase difference θ
between the sine waves using θ = t * 360o / T.
Q1. What is the phase difference θ between the sine waves? θ = ______________
Vm
0
θ
-Vm
Figure 2
T
t
7. Press [AUTO SET] button in the scope to produce a usable display of the input signals.
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8. Press [DISPLAY] button in the scope to change the appearance of the entire display.
9. Press the 3rd option button to select the ‘Format’ option, and keep pressing the button until the
‘Format-XY’ type is selected. <The ‘XY’ format displays a dot each time a sample is
acquired on channel 1 and channel 2. Channel 1 voltage determines the X coordinate of the dot
(horizontal) and the channel 2 voltage determines the Y coordinate (vertical) >.
10. Press [CH1 MENU] button in the scope, then press the 1st option button to select ‘Coupling’
option, and keep pressing the option button until the ‘GND’ is selected, and turn the
[CURSOR1 POSITION] knob to set the CHANNEL 1 offset to zero.
11. Press [CH2 MENU] button in the scope, then press the 1st option button to select ‘Coupling’
option, and keep pressing the option button until the ‘GND’ is selected, and turn the
[CURSOR2 POSITION] knob to set the CHANNEL 2 offset to zero.
12. Find the phase difference θ using the Lissajous plot (use X-Y mode of scope) shown in Figure
3, using sin θ = a/b.
Q2. What is the phase difference θ using the Lissajous plot? θ = ______________
Figure 3.
Q3. Sketch V(t) and VA(t) on a graph with clear amplitudes and phases labeled.
Q4. What is VA(t), when V(t) is 2*cos(2π1kt) V?
Q5. Show the waveforms to TA.
TA signature : _________
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