Lab Exercise 3: Transmission Line Basics
Contents
3-1
3-2
3-3
3-4
3-5
P RE - LAB A SSIGNMENT . . . . . . . . . .
I NTRODUCTION . . . . . . . . . . . . . . .
U SEFUL EQUATIONS . . . . . . . . . . . .
E QUIPMENT . . . . . . . . . . . . . . . . .
E XPERIMENT . . . . . . . . . . . . . . . .
3-5.1 Role of Wavelength . . . . . . . . . .
3-5.2 Standing Waves On The Slotted Line
3-5.3 Network Analyzer . . . . . . . . . .
3-6 L AB WRITE - UP . . . . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
35
35
35
36
36
36
42
48
56
Objective
To examine basic transmission-line concepts. In addition, the network
analyzer will be introduced as a measurement tool, and will be used to
study the reflections caused by various load terminations.
General concepts to be covered:
• Network analyzer
• Role of wavelength
• Standing waves on transmission lines
34
3-1
3-1
PRE-LAB ASSIGNMENT
35
P RE -L AB A SSIGNMENT
3-1.1 Read Sections 2-1 through 2-6 of the text.
3-1.2 To be entered into your lab notebook prior to coming to lab:
Summarize the experimental procedure (1 paragraph per section) of:
(a) Section 3-5.1: Role Of Wavelength
(b) Section 3-5.2: Standing Waves On The Slotted Line
(c) Section 3-5.3: Network Analyzer
3-2
I NTRODUCTION
With the ever increasing role of communications in our daily lives, it has become important
to connect remote sites together to share data and information. One method for providing
links between remote sites is to use transmission lines. Some everyday examples include
telephone lines, electrical lines, and cable television. In order to provide the best link
possible, it is necessary to understand how signals propagate on transmission lines.
In basic electronic circuits, it is assumed that if a voltage is applied at the input of a
circuit, the output voltage appears instantaneously at the output of the circuit. For circuits
where the line lengths are much smaller than the wavelength of the signal, this assumption
is acceptable with only negligible consequences. However, when the length of the wires or
transmission lines are an appreciable fraction of the wavelength or longer, the output signal
changes phase compared to the input signal, and at impedance discontinuities, reflections
can occur.
In this lab, you will explore experimentally the role of wavelength, standing waves on
transmission lines, and the reflections caused by different load terminations. In addition,
you will be introduced to a new tool for analyzing transmission line systems (or networks),
the network analyzer.
3-3
U SEFUL E QUATIONS
λ=
up
f
(m)
1 + |Γ|
1 − |Γ|
S−1
|Γ| =
S+1
¯
¯
¯1+Γ¯
¯
|ZL | = Z0 ¯¯
1−Γ¯
SW R = S =
(Ω)
36
LAB EXERCISE 3: TRANSMISSION LINE BASICS
3-4
E QUIPMENT
Item
Cables & connectors
Calibration Kit
Network Analyzer
Oscilloscope
Printer
Scanner Antenna
Signal source
Slotted Line
3-5
E XPERIMENT
3-5.1
Role Of Wavelength
Part #
——
HP 85032E
HP 8712C
HP 54645A
HP DeskJet 400
——
HP 8648B
HP 805A
In this experiment, you will investigate the role of wavelength in circuits. Recall that it
is the ratio of the line length to the wavelength that determines whether transmission-line
analysis is required. If
l
> 0.01
(3.1)
λ
then the system must be analyzed as a transmission-line system. To demonstrate this, you
will compare the output of a voltage divider that is fed by a wire of length l to the input
signal at several frequencies.
Setup
This experiment uses the signal source, oscilloscope, and voltage divider.
Set up the experiment:
• Connect the BNC “Y” to channel 1 of the oscilloscope.
• Connect the input of the voltage divider to one of the arms of the BNC “Y” connector
using a BNC plug to BNC plug adapter.
• Connect the BNC jack to N plug adapter to the RF output of the RF signal source.
• Use a 2400 piece of BNC co-axial cable to connect the signal source to the open arm
of the “Y”.
• Use a 18000 piece of BNC co-axial cable to connect the output of the voltage divider
to channel 2 of the oscilloscope.
• Configure the oscilloscope to send output to the printer (see Lab Exercise 2).
The setup is shown in Fig. 3-1. Block diagrams of the setup and voltage divider
configuration are shown in Fig. 3-2
3-5
EXPERIMENT
37
Oscilloscope
Signal Source
(a) General setup
180" BNC Cable
or
12" BNC Cable
(to Channel 2)
BNC ’Y’
24" BNC Cable
(from Signal Source)
Voltage Divider
(b) Close-up of oscilloscope
Figure 3-1: Setup for Section 3-5.1
Station
X
1
2
3
4
5
R1 (Ω)
1689
1484
997
2212
1004
1488
R2 (Ω)
1887
1527
1495
1808
2196
2682
Table 3-1: Voltage divider resistor values.
Procedure
Note: When recording the value of the phase, record the value only to the ones or tens place.
For example, if the phase value is jumping between 400 m◦ and −300 m◦ , record the phase
as 0◦ . If the phase value is jumping between 130◦ and 134◦ , record the phase as 132◦ .
18000 BNC Cable
1. Record the two resistor values (R1 ,R2 ) given in Table 3-1.
Turn the signal generator on and set the frequency to f1 = 100 kHz.
38
LAB EXERCISE 3: TRANSMISSION LINE BASICS
Signal Source
Oscilloscope
Cable 1
Cable 2
Voltage Divider
(a) General setup block diagram
Outer conductor
Inner conductor
R1
+
+
v In
R2
-
v Out
-
BNC Connector
(b) Voltage divider block diagram
Signal +
Source -
Cable 2
Cable 1 R1
+
+
+
V2
V1 R2 Vout
24" BNC
Cable
-
180" BNC Cable
or 12" BNC Cable
(c) v1 and v2 are measured by the oscilloscope
Figure 3-2: Block diagrams of (a) the setup for Section 3-5.1, (b) the voltage divider, and
(c) the overall circuit configuration.
3-5
EXPERIMENT
39
¤
¡
• Press £Power ¢to turn on the RF signal source. Wait for the RF signal
source to perform its self check.
¤
¡
• Press £Mod On/Off ¢until the modulation is turned off
¤
¡
¤ ¡¨
¥
• Press £Amplitude ¢followed by £0 ¢ MHz
dB(m) to set the amplitude to 0 dBm.
§
¦
¤
¡
¤ ¡¤ ¡¤ ¡¨ ¥
kHz
• Press £Frequency ¢ followed by £1 ¢ £0 ¢ £0 ¢ §
mV ¦to set the output
frequency to 100 kHz.
¤
¡
• Press £RF On/Off ¢until the RF output is On
¤
¡
If at any point you make a mistake entering a number, you can use the £← ¢
key to delete the last number entered.
Set the oscilloscope to show both channel 1 and channel 2. Adjust the oscilloscope
display so that two periods are shown on each of the channels.
¤
¡
• Press £Autoscale ¢
2. Record the signal amplitudes v1 (channel 1) and v2 (channel 2), and the relative phase
shift between channel 1 and channel 2 of the oscilloscope (∆φ).
To measure voltage:
¤
¡
• Press £VOLTAGE ¢
• Press the Clear Meas softkey to clear the last measurement
• Press the Source softkey until channel 1 is selected
• Press the Next Menu softkey until the VAMP softkey is displayed
• Press the VAMP softkey to display the amplitude of the waveform
• Repeat for channel 2 (Don’t clear the last measurement)
40
LAB EXERCISE 3: TRANSMISSION LINE BASICS
To measure the
¤ phase
¡ difference between channel 1 and channel 2:
• Press £TIME ¢
• Press the Next Menu softkey until the Define Thresholds softkey
is displayed on the far left side of the screen
• Press the Measure Phase softkey
Note: Negative phase indicates the displayed signal on channel 2 is
leading the signal on channel 1. Add 360◦ to obtain the value of ∆φ.
If the signals displayed are noisy and the measured values are jumping around, you
can use averaging to reduce the effects of the noise.
To turn averaging
on:
¤
¡
• Press £Display ¢
• Press the Average softkey
• Press the # Average softkey until 4 averages are selected.
To turn averaging off:
¤
¡
• Press £Display ¢
• Press the Normal softkey
Print the display.
¡
¤
• Press £Print/Utility ¢
• Press the Print Screen softkey
3. Change the frequency of the signal source to f2 = 100 MHz
.
¤
¡
¤ ¡¤ ¡¤ ¡¨ ¥
MHz
• Press £Frequency ¢followed by £1 ¢£0 ¢£0 ¢§
mV ¦
Adjust the oscilloscope display so that two periods are shown for both the input and
output signals. Record the signal amplitudes v1 and v2 , and the relative phase shift
between channel 1 and channel 2 of the oscilloscope. Print the display.
3-5
EXPERIMENT
41
1200 BNC Cable
4. Replace the 18000 BNC co-axial cable with a 1200 BNC co-axial cable.
5. Change the frequency of the signal source to f1 = 100 kHz. Adjust the oscilloscope
display so that two periods are shown for both the input and output signals. Record
the signal amplitudes v1 and v2 , and the relative phase shift between channel 1 and
channel 2 of the oscilloscope. Print the display.
6. Change the frequency of the signal source to f2 = 100 MHz. Adjust the oscilloscope
display so that two periods are shown for both the input and output signals. Record
the signal amplitudes v1 and v2 , and the relative phase shift between channel 1 and
channel 2 of the oscilloscope. Print the display.
Measured Data
Copy the following chart into your lab book and fill in the measured data. If you are missing
any data, please repeat the necessary parts of this experiment before proceeding.
R1
R2
=
=
(Ω)
(Ω)
Frequency
f1 =100 kHz
f2 =100 MHz
Cable
1200
18000
1200
18000
v1 (V)
v2 (V)
∆φ (◦ )
Analysis
1. Compute the wavelength for frequencies f1 and f2 . Assume εr of the cable is 1.9.
Arrange the results in tabular form. Record these values.
2. Compute the ratio of l to λ for the 1200 BNC cable and the 18000 BNC cable for
frequencies f1 and f2 . Arrange the results in tabular form.
3. If we were to ignore the cables in Fig. 3-2(c), then vout would be the same as v2
measured by the scope. The output of the voltage divider is given by:
vout = v1
R2
(V)
R1 + R2
(3.2)
Calculate this value for each combination.
4. Compare the computed output voltage vout from the previous step to the recorded
output voltage v2 (channel 2) for both the 1200 and 18000 BNC cables at each of the
frequencies: f1 and f2 . Comment on your results.
5. Compare the phase of the input signal to that of v2 for both the 1200 and 18000 BNC
cables at each of the frequencies: f1 and f2 . Comment on your results.
6. Comment on the role of wavelength in circuits using the data collected in this
experiment. This can be two or three sentences saying: As X increases, A analysis
fails and we need B. This happens because . . . 1) . . . 2) . . .
42
LAB EXERCISE 3: TRANSMISSION LINE BASICS
Questions
1. Would you expect the output voltage of the RF signal source to be constant over
the frequency range examined? Is this what you observed? If not, explain why the
voltage on channel 1 was not constant over the frequency range.
2. Which of these systems needs to be treated as a transmission line system? Why?
Justify your answer quantitatively. Indicate any assumptions that you are making.
(a)
(b)
(c)
(d)
Integrated circuit at high frequencies (500 MHz→1 GHz)
Electrical lines running through your house
Electrical lines connecting cities separated by hundreds of kilometers
VHF antenna leads from a rabbit ear antenna to your television
3. Why is it necessary to treat lines that have a length to wavelength ratio greater than
0.01 as transmission-line systems? Explain in terms of the phase of a co-sinusoidal
signal given by A cos(ωt − βz), where A is the peak amplitude of the signal, z is the
position on the line, and β = 2π
λ . (hint: what are the major assumptions of a wire
in DC analysis and how are those violated on a system requiring transmission line
analysis?)
3-5.2
Standing Waves On The Slotted Line
In this experiment, you will use the oscilloscope and the slotted line to examine the
standing-wave pattern caused by various loads. The measurements you will be making
are uncalibrated measurements. In order to correct for this, the first step is to measure the
standing-wave pattern for a known load to use as a reference.
The reference load that will be used is the short termination since we know the standingwave pattern that should be produced by the short termination. By measuring the slotted
line terminated with various loads and calibrating against the reference measurement, the
standing wave pattern of the loads can be determined.
The slotted line consists of a piece of metal tubing (which is an unshielded transmission
line), a probe, and a detector. The probe measures the electric field present in the line and
uses a detector to convert the measured field to a voltage. The probe and the detector are
housed on a mount which can slide down the line. On the side of the mount is a scale that
can be used to measure the position of the probe. The slotted line is shown in Fig. 3-3.
Detector mount
BNC cable
Load
RF IN
Position adjustment knob
Figure 3-3: The slotted line.
3-5
EXPERIMENT
43
Setup
This experiment uses the signal source, oscilloscope, slotted line, and various loads and
cables.
Setup the experiment as shown in Fig. 3-4:
• Attach the source end of the slotted line to the signal source using a patch cord.
• Attach the output of the probe to channel 1 of the oscilloscope using the 18000 piece
of BNC co-axial cable.
• Configure the signal source to output a 10 dBm, 1 GHz, 1 kHz 50% AM modulated
RF signal.
¤
¡
¤ ¡¤ ¡¨
¥
¤
¡
¤ ¡¤ ¡¤ ¡¤ ¡¨
¤
¡
– Press £Amplitude ¢followed by £1 ¢£0 ¢ MHz
dB(m)
§
¦
¥
– Press £Frequency ¢followed by £1 ¢£0 ¢£0 ¢£0 ¢ MHz
dB(m)
§
¦
– Press £RF On/Off ¢until the RF output is On
¨
¥
INT
– Press §
1 kHz ¦
¤
¡
– Press £Mod On/Off ¢until modulation is On
¤
¡
¤ ¡¤ ¡¤ ¡
£ ¢
– Press £AM ¢followed by £5 ¢£0 ¢ %µV
• Adjust the settings on the oscilloscope so at least 2 periods of the wave-form are
shown.
• Configure the oscilloscope to measure the voltage amplitude.
You may want to use AC coupling to keep the waveform from shifting position on the
screen. Be sure to keep adjusting the voltage scale on the oscilloscope to best fit the
wave on the screen.
44
LAB EXERCISE 3: TRANSMISSION LINE BASICS
Oscilloscope
RF Signal Source
BNC cable
Detector Mount
Slotted line
Patch cord
Position adjustment knob
(a) Setup for Section 3-5.2
Signal Source
Oscilloscope
1
2
BNC Cable
Slotted Line
Load
Patch Cord
(b) Block diagram of setup
Figure 3-4: Setup and block diagram for Section 3-5.2
Station
1
2
3
4
5
R (Ω)
22
100
10
68
39
C (pF)
39
10
39
15
1
Table 3-2: Resistor and capacitor values for the resistive and capacitive loads.
3-5
EXPERIMENT
45
BNC connection (to oscilloscope channel 1)
Tuning Knob
Position indicator
Probe height adjust knob
Figure 3-5: Slotted line probe mount
Procedure
The two knobs (black and silver) on top of the probe shuttle allow
you to tune the position and electrical characteristics of the probe
needle. The goal of tuning is to have the needle adequately sample
the standing wave without distorting the measured signal.
The figure to the left shows two needle positions. The first signal
(solid) is larger, but suffers from distortion. Therefore the second
location is a better choice despite the lower signal amplitude
Probe Voltage
Tuning the slotted line
0.6
0.4
0.2
0
Time
1. Connect the short termination to the end of the slotted line. Move the detector mount
to a position where a strong signal is shown on the oscilloscope. To move the probe,
push in and turn the black knob (see Fig. 3-4) on the side of the slotted line.
2. Tune the slotted line detector and probe. Turn the silver probe knob on the detector
mount to the right until the probe is as close to the line as possible (see Fig. 3-5).
Adjust the selectivity by turning the black knob on the probe mount to the right and
left until a peak is observed on the oscilloscope.
Short Termination
3. Locate the first minimum nearest the load. Record the voltage amplitude reading on
the oscilloscope and the position on the slotted line. Use the position indicator on
the detector mount to read the value in millimeters from the scale that runs along the
slotted line. Use the 0 marker on the position indicator when reading the position.
46
LAB EXERCISE 3: TRANSMISSION LINE BASICS
You will use this position as the load plane reference for the remainder of this
λ
experiment. In other words, each of the other terminations will begin their 20
steps
at this measurement point, and not their first zero location. This will allow us to
determine the phase between the standing waves.
4. Locate the first maximum by moving the probe toward the generator (away from the
load). Record the voltage amplitude reading on the oscilloscope and the position on
the slotted line.
5. Locate the second minimum from the load. Record the voltage amplitude reading on
the oscilloscope and the position on the slotted line.
6. Return the probe to the load plane reference. Measure the voltage amplitude on the
λ
line over a half-wavelength interval in steps of 20
. You should be able to calculate
λ from knowledge of the signal frequency and εr =1. If you’re unsure of your
calculation, please check your result with your GSI. Record the voltage amplitude
and position of each measurement.
Open Termination
7. Connect the open termination to the end of the slotted line. Place the probe at the load
reference position (i.e. the same starting point as the first short measurement) and
record the voltage amplitude. Measure the voltage on the line over a half-wavelength
λ
interval in steps of 20
. Record the voltage and position of each measurement.
8. Locate the first minimum nearest the load reference position. Make sure that you
move the probe toward the generator. Record the voltage amplitude and position of
the first minimum.
Matched-load Termination
9. Connect the matched termination to the end of the slotted line. Place the probe at
the load reference position and record the voltage amplitude. Measure the voltage
λ
amplitude on the line over a half-wavelength interval in steps of 20
. Record the
voltage amplitude and position of each measurement.
Resistive Termination
10. Connect the resistive termination to the slotted line. Record the resistor value listed
in Table 3-2. Place the probe at the load reference position and record the voltage
amplitude . Measure the voltage amplitude on the line over a half-wavelength interval
λ
in steps of 20
. Record the voltage amplitude and position of each measurement.
11. Locate the first minimum nearest the load reference position. Make sure that you
move the probe toward the generator. Record the voltage amplitude and position of
the first minimum.
Capacitive Termination
12. Connect the capacitive termination to the slotted line. Record the capacitor value
listed in Table 3-2. Place the probe at the load reference position and record the
3-5
EXPERIMENT
47
voltage. Measure the voltage amplitude on the line over a half-wavelength interval in
λ
steps of 20
. Record the voltage amplitude and position of each measurement.
13. Locate the first minimum nearest the load reference position. Make sure that you
move the probe toward the generator. Record the voltage amplitude and position of
the first minimum.
Measured Data
Copy the following charts into your lab book and fill in the measured data. If you are
missing any data, please repeat the necessary parts of this experiment before proceeding to
the analysis section.
Short Termination
Location
1st minimum
1st maximum
2nd minimum
|v| (V)
Position (mm)
Loads
Resistive termination value
Capacitive termination value
=
=
(Ω)
(pF)
Probe Position [λ]
Load
0
λ
20
2λ
20
3λ
20
4λ
20
5λ
20
6λ
20
Minimum
7λ
20
8λ
20
9λ
20
10λ
20
Short
Open
Matched
Resistor
Capacitor
Probe Position [mm]
Notes:
• Position 0 refers to the load reference position
• Minimum refers to the exact location of the first minimum unless specified
otherwise. Record both the voltage amplitude and position with greater precision
λ
than 20
steps.
Pos.
Vol.
48
LAB EXERCISE 3: TRANSMISSION LINE BASICS
Analysis
1. Using the measured minima and maxima positions for the short termination, compute
the distance between minima (in wavelengths, use εr =1). Record this distance.
Compare to the expected theoretical value.
2. Using the measured minima and maxima positions for the short termination, compute
the distance between the first minimum and maximum (in wavelengths). Record this
distance. Compare to the expected theoretical value.
3. Normalize all of the measured data by the maximum value recorded for the short
termination (i.e. divide all measurements by max([Vshort ]))
Plot the standing wave pattern for each of the measured loads using the normalized
data. Compare the patterns and comment on the results.
Questions
1. What would you expect the standing-wave pattern to look like if the slotted line was
1
terminated in an inductor (assume ωL = ωC
)?
2. Why was it acceptable to define the load reference position to be at a location other
than the load?
3-5.3 Network Analyzer
In this experiment, you will familiarize yourself with the network analyzer and measure the
reflection coefficient of three standard impedances.
The most vulnerable parts of the network analyzer are the RF connectors and the
calibration kit components. Ask the lab instructor to demonstrate how to make connections
and handle the calibration standards. As far as pushing the knobs and keys on the
instrument, no special care is necessary.¤ The worst
¡ thing that can happen is the instrument
will lock up. If that should occur, press £PRESET ¢or shut the instrument off and turn it back
on.
The network analyzer is a measurement tool for making phase and magnitude
measurements. The network analyzer you will use is capable of making phaser (magnitude
and phase) measurements in the frequency range from 0.0003→1.3 GHz (109 Hz).
Setup
This experiment uses the network analyzer, calibration kit, patch cord, scanner antenna, and
printer.
Setup the network analyzer as follows:
3-5
EXPERIMENT
49
¤
¡
• Press £PRESET ¢
• Turn channel 2 Off.
¤
¡
– Press £MEAS 2 ¢followed by the Meas Off softkey
• Set the display to show only channel 1.
¤
¡
– Press £DISPLAY ¢
– Press the More Display softkey
– Press the SPLIT Disp full SPLIT softkey until only channel
1 is displayed (FULL will be displayed in all caps and split
will be displayed in all lowercase letters).
• Set the start frequency to 100 MHz.
¤
¡
– Press £FREQ ¢
– Press the Start softkey.
¤ ¡¤ ¡¤ ¡
– Enter £1 ¢£0 ¢£0 ¢using the key pad and press the MHz softkey.
• Set the stop frequency to 1000 MHz.
¤
¡
– Press £FREQ ¢.
– Press the Stop softkey.
¤ ¡¤ ¡¤ ¡¤ ¡
– Enter £1 ¢£0 ¢£0 ¢£0 ¢using the numeric key pad and press the
MHz softkey.
• Recall the uncalibrated instrument state.
¨
¥
SAVE
– Press §
.
RECALL ¦
¤ ¡
¤ ¡
– Use the £↑ ¢and £↓ ¢keys to select the file nocal.cal from the
root directory.
– Press the Recall State softkey
50
LAB EXERCISE 3: TRANSMISSION LINE BASICS
Configure the Display of the network analyzer as follows:
• Set the network analyzer to display magnitude data in logarithmic
format.
¤
¡
– Press £FORMAT ¢
– Press the Log Mag softkey
• Set the network analyzer to measure the reflections on channel 1.
¤
¡
– Press £MEAS 1 ¢
– Press the Reflection softkey
• Autoscale the display.
¤
¡
– Press £SCALE ¢
– Press the Autoscale softkey
Configure the network analyzer to display a title on the screen as follows:
• Display the date and time on the screen.
¤
¡
– Press £DISPLAY ¢
– Press the More Display softkey
– Press the Title and Clock softkey
– Press the Show Clock on Line 2 softkey
• Since each plot should contain a descriptive title, you can write a
descriptive title on any printouts using pen, or you can configure
the network analyzer to embed a title on each plot. If you choose
to manually write a title on each plot, be sure to clear out any titles
retained in the machine from the previous groups.
– Press the Enter Line 1 softkey
– To clear the previous titles, the Clear softkey
– Alternatively, enter Sec. x # y where x is the last digit of your
three digit lab section number and y is the lab station number.
* Use the knob on the front panel to highlight the desired
character
* Press the Sel Char softkey
* Repeat for all characters. To enter a space, press the
Space softkey. To move back a character, press the
Backspace softkey.
* Press the Enter softkey when done
3-5
EXPERIMENT
51
The network analyzer should retain the printer setting from the previous section, but if
you’re having difficulty printing, you can configure the network analyzer to send output to
the printer as follows:
¨
¥
HARD
• Press §
COPY ¦
• Select the parallel output port and printer type
– Press the Select Copy Port softkey
¤ ¡
¤ ¡
– Use the £↑ ¢and £↓ ¢keys to select the configuration which
corresponds to:
Device Type: HP Printer
Language: PCL
HardCopy Port: Parallel Port
– Press the Select softkey
– Press the Prior Menu softkey
• Configure the graph and printer settings
– Press the Define Hardcopy softkey
– Press the Graph Only softkey
– Press the Prior Menu softkey
– Press the Define Printer softkey
– Press the Monochrome softkey
– Press the Landscape softkey
• Set the printer resolution to 150 dpi.
– Press the More Printer softkey item Press the Printer
Resolution softkey
¤ ¡¤ ¡¤ ¡
– Enter £1 ¢£5 ¢£0 ¢
– press the Enter softkey
Set the printer switch box to output channel B.
Procedure
Note: For the short, open, and matched terminations, you will need to attach the N jack
to N jack adapter to the end of the patch cord. Remove the adapter before attaching the
antenna. (i.e. don’t use a male-to-male adaptor to a female-to-female adaptor, when neither
is necessary)
Calibration
1. Connect the patch cord to the Reflection RF Out port on the network analyzer
(channel 1). Attach the short termination to the end of the patch cord. Be sure to
52
LAB EXERCISE 3: TRANSMISSION LINE BASICS
Autoscale the display. Print the magnitude response (see next page for instructions).
Place a marker at 500 MHz:
¤
¡
• Press £MARKER ¢
• Press the Marker 1 softkey
• Enter the desired frequency (500) in MHz using the numeric key
pad and press the MHz softkey
The marker should be located at the entered frequency and the value at
that point displayed in the upper corner of the screen.
• Add a descriptive title (in addition to the title you added
previously).
¤
¡
– Press £DISPLAY ¢
– Press the More Display softkey
– Press the Title and Clock softkey
– Press the Enter Line 1 softkey
– Add the descriptive title (i.e., Short Uncal)
* Use the knob on the front panel to highlight the desired
character
* Press the Sel Char softkey
* Repeat for all characters. To enter a space, press the
Space softkey. To move back a character, press the
Backspace softkey.
* Press the Enter softkey when done
¨
¥
HARD
• Press §
COPY ¦
• Press the Start softkey
Record the magnitude of the reflection coefficient at 500 MHz. Change the display
to show the phase of the reflection coefficient.
¤
¡
• Press £FORMAT ¢
• Press the Phase softkey
Record the phase of the reflection coefficient at 500 MHz.
3-5
EXPERIMENT
53
You should have seen that the response was not at all what you expected. This is
attributed to the fact that the network analyzer had not been calibrated. To make
accurate reflection measurements, you must first calibrate the network analyzer by
performing a single-channel calibration.
¤
¡
• Press £FORMAT ¢followed by the Log Mag softkey
¤
¡
• Press £CAL ¢
• Press the One Port softkey
• Connect the open standard from the calibration kit to the end of the
patch cord. Press the Measure Standard softkey
• Remove the open standard and connect the short standard from the
calibration kit. Press the Measure Standard softkey
• Remove the short standard and connect the matched (50 Ω)
standard from the calibration kit. Press the Measure Standard
softkey
54
LAB EXERCISE 3: TRANSMISSION LINE BASICS
Save the computed calibration coefficients as follows:
¨
¥
SAVE
• Press §
RECALL ¦
• Press the Define Save softkey
• Press the Cal softkey until On is selected
• Press the Data softkey until Off is selected
• Press the Prior Menu softkey
• Press the File Utilities softkey
• Press the Directory Utilities softkey
¤ ¡
¤ ¡
• Use the £↑ ¢and £↓ ¢keys to select the EECS230 directory
• Press the Change Directory softkey
¤ ¡
¤ ¡
• Use the £↑ ¢and £↓ ¢keys to select the SECxxx directory, where xxx is
your three digit lab section number.
• Press the Change Directory softkey
• Press the Prior Menu softkey twice
• Press the Save State softkey. A new file name of the form
STATEy.STA, where y is an incremental counter starting at 0 will
appear in the current directory.
• Press the File Utilities softkey
• Press the Rename File softkey
• Press the Clear Entry softkey
• Enter the desired file name. The file name must be a legal DOS file
name. Press the Enter softkey
Measurements
2. Now make the following measurements:
¤
¡
Note: After changing a load, press £SCALE ¢followed by the Autoscale softkey. Be
sure to change the descriptive title before printing.
(a) Connect the short termination to the end of the patch cord. Print the magnitude
response. Record the magnitude and phase of the reflection coefficient at
500 MHz.
Note: If this still does not look close to the expected response, repeat the
calibration.
3-5
EXPERIMENT
55
(b) Connect the open termination to the end of the patch cord. Print the magnitude
response. Record the magnitude and phase of the reflection coefficient at
500 MHz.
(c) Connect the 50 Ω (matched) termination to the end of the patch cord. Print
the magnitude response. Record the magnitude and phase of the reflection
coefficient at 500 MHz.
(d) Remove the matched termination from the patch cord. Connect the scanner
antenna to the end of the patch cord. Be sure to completely extend the antenna
before making measurements.
(e) Change the display format of the network analyzer to SWR.
¤
¡
• Press £FORMAT ¢
• Press the SWR softkey
(f) Print the resulting display. Place a marker at 100 MHz. Using the position knob
on the front panel of the network analyzer, move the marker to the frequency
where the SWR is a minimum. Record this frequency and the corresponding
SWR. Using the marker, locate the two frequencies nearest the minimum where
the SWR becomes 2.5. Record these two frequencies.
Measured Data
Copy the following charts into your lab book and fill in the measured data. If you are
missing any data, please repeat the necessary parts of this experiment before proceeding.
Reflection Coefficients
Load
Short (uncal)
Short (cal)
Open
50 Ω (matched)
|Γ|
∠Γ
Scanner Antenna SWR
Frequency of SWR minimum
Minimum SWR value
(Lower) Frequency of SWR = 2.5
(Higher) Frequency of SWR = 2.5
(MHz)
(MHz)
(MHz)
Analysis
1. Compare the printouts of the short termination before and after calibration. What was
the effect of the calibration? There are two types of error, systematic and random.
How does calibration affect these two types of error?
56
LAB EXERCISE 3: TRANSMISSION LINE BASICS
2. For each of the loads measured (short, open, matched), compute the theoretical
reflection coefficient and compare it to the measured reflection coefficient. Since
the network analyzer performs a power measurement, the conversion to a linear scale
is:
|Γlinear | = 10( |ΓdB |/20 )
(3.3)
Comment on your results.
3. Compute the magnitude of the antenna input impedance at the frequency where the
SWR minimum occurred. Record this value.
Note: For your calculations, assume that that Γ is essentially real.
Questions
1. Is the antenna that you measured good for broad-band communication systems
(20 MHz → 2 GHz)? Why or why not? (Hint: recall the connection between the
reflection coefficient and the SWR)
2. Using the computed input impedance for the antenna at the frequency of minimum
SWR, which of these systems would the antenna work well with?
(a) 50 Ω transmission line system
(b) 75 Ω transmission line system
(c) 300 Ω transmission line system
Use the definition that in order for the antenna to work well, the SWR must be ≤ 2.5.
3-6
L AB W RITE - UP
For each section of the lab, include the following items in your write-up:
(a) Overview of the procedure and analysis.
(b) Measured data where asked for.
(c) Calculations (show your work!).
(d) Any tables and printouts.
(e) Comparisons and comments on results.
(f) A summary paragraph describing what you learned from this lab.