Lab 3: Smith Chart and Single-Stub Tuning
NAME_______________NAME_______________NAME_______________
Introduction:
In this experiment you will measure the impedance as well as the reflection
coefficient of a 24-ohm low frequency load resistor at different microwave
frequencies using a network analyzer HP 8753A, and also match the load
impedance to 50 ohms at a single frequency 500 MHz using single-stub
tuning techniques.
The system, an HP 8753A network analyzer measures the complex one port
reflection coefficient S11 or simply Γ . Calibration of the instrument is
done with a standard open load, a standard short load and a standard 50-ohm
load (all from the calibration kit). Calibration is required before the
measurement if the settings are changed on the instrument.
This system uses real time analog signal processing exclusively.
Measurements can be done over wide frequency sweeps (linear frequency
sweep from 0.3 MHz - 3 GHz) to see overall trends and then individual
frequency points (CW frequency) examined for details.
By plotting the component input impedance on a Smith Chart we can
rapidly match the component to 50 ohms. This involves selecting the correct
lengths of transmission lines to act as impedance transforming elements, as
described in section 2-10 of the text.
Network Analyzer Calibration Procedure:
1. Turn on the instrument.
2. Press “preset” on the front panel
“format” on the front panel Æ “Smith chart” softkey
“cal” on the front panel Æ “calibrate menu” softkey
Æ “S11 1-port” softkey
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3. Connect the standard open load from the cal kit and press the softkey
“open”.
4. After the “open” calibration, change the load to the standard short
load from the cal kit and press the softkey “short”.
5. After the “short” calibration, change the load to the standard 50-ohm
load from the cal kit and press softkey “load”.
6. After the calibration process, press “done 1-port cal” at the bottom
and then “save reg. 1”. Now the calibration data are saved in register
1 on the instrument. To recall this state at any time, press “recall” on
the front panel Æ “recall reg. 1” softkey. The “preset” state sweeps
over the full range of frequencies of the 8753A from 300 kHz to
3 GHz. The instrument needs to be recalibrated, if the operating
frequency is changed to CW frequency (single frequency) or a
smaller frequency span, as will be performed later in this lab.
Procedures:
Use Smith chart to measure the frequency response of a resistor
1. Connect a 24-ohm resistor to port 1 and look at the frequency
response of this resistor on the Smith chart. To measure quantitatively
the impedance of this resistor as a function of frequency, you need a
frequency marker. Press “mrk” on the front panel Æ “marker mode
menu” softkey Æ “smith mkr menu” softkey Æ “R+jX mkr” softkey.
The marker (an arrow) should appear on the screen. The real part (R)
and imaginary part (X) of the impedance Z = R+jX and the frequency
appear on the top of the screen. Turn the large knob on the front panel
to change the frequency. The equivalent inductance or capacitance
associated with the reactance X is also shown on the top of the screen.
Based on the marker reading, please find the frequencies at which the
resistor is resistive (R), inductive (R+jX) and capacitive (R-jX).
Resistive: ______________________
Inductive: _____________________
Capacitive: ____________________
2. To read the magnitude and angle of the reflection coefficient S11 = Γ
associated with the impedance Z, press “lin mkr” softkey on the top.
The magnitude is given in “mU” meaning “milliunits”, with the outer
circle on the Smith chart representing one “unit”. So if the reading is
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700 mU, it means Γ = 0.7 . The phase is given in degrees following
the magnitude. Find the normalized impedance and the reflection
coefficient at the frequencies listed in the table. A specific frequency
can be typed in by using the number keys followed by the unit key on
the front panel.
Frequency
r = R/Z0
x = X/Z0
|Γ|
Θ (degrees)
1 MHz
750 MHz
2 GHz
What is the relation between r, x and |Γ|? _________________________
Single Stub Matching:
This procedure is done at a single frequency, so new calibration is necessary
before the measurement. Take the 24-ohm resistor off.
1. To set the instrument to a single frequency at 500 MHz, press “menu”
on the front panel Æ “CW freq” softkey, and use the number keys on
the front panel to input “500” and then press “M/ µ”.
2. You must recalibrate the instrument for this new setting.
Press “cal” on the front panel Æ “calibrate menu” softkey
Æ “S11 1-port” softkey
Connect the standard open load from the cal kit and press the
softkey “open”.
After the “open” calibration, change the load to the standard short
load from the cal kit and press the softkey “short”.
After the “short” calibration, change the load to the standard 50-ohm
load from the cal kit and press softkey “load”.
After the calibration process, press “done 1-port cal” at the bottom
and then “save reg. 2”.
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3. Insert a GR TEE connector to port 1. Put the 24-ohm load together
with an adjustable line on one side of the TEE and a stub on the other
side of the TEE. The measured impedance should show up as a dot on
the screen. What is the impedance for a matching condition? _______
Is this a parallel matching or series matching? ____________
Where do you expect the dot to be on the Smith chart for the
matching condition by adjusting the distance between the 24-ohm
resistor and the TEE and the length of the stub?
____________________
Can you match the load at 500 MHz? ________________________
If you want to use the same adjustable line and the same stub to
match this 24-ohm resistor at a different frequency, would the
matching be easier to achieve at higher frequencies or lower
frequencies? Why? (hint: how does the wavelength change as
increasing or decreasing the signal frequency?)
4. Examine the frequency bandwidth of this matching.
Press “menu” on the front panel Æ “sweep type” softkey
Æ “LIN freq” softkey.
Press “start” on the front panel and type in “450 M”.
Press “stop” on the front panel and type in “550 M”.
A new calibration needs to be done since this is a new frequency
setting. Take the entire TEE off port 1 but keep the matching
condition. Use the cal kit to repeat the “open”, “short” and “load”
calibration procedure. Once you are done, put the TEE back to port
1. Is the matching maintained for frequencies off 500 MHz? If we
define the matching bandwidth as SWR<1.5, what is the bandwidth
in this measurement?
Matching bandwidth _____________________________________
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