Microwave Electronics - the Network Analyzer experiment. In this lab, we have used the Hewlett Packard HP8720 Network analyser to measure the S-Parameters for some simple devices.
School of
Electronics Engineering and Computer Science
ELE 569: MICROWAVE ELECTRONICS
Experiment 1: The Network Analyser
Contents
Abstract........................................................................3
Introduction...................................................................3
Background Theory..........................................................4
The S-parameters.....................................................4
The network analyzer...............................................5
Measurement of S-parameters..............................................7
Reflection and Transmission test set........................................14
Transmission Measurement.................................................14
Reflection Measurements 1.................................................18
Reflection Measurements 2.................................................21
Conclusion....................................................................23
References....................................................................24
2
ABSTRACT [1]
The main objective of this experiment is
??To provide a hands-on experience with the Microwave Network Analysis using
HP8720 Vector network analyser.
??To be familiarise in real life experience with the concepts of ELE569 course.
??To show the principles of swept frequency measurement
??Show how S-parameters are measured and reinforce the theory of S-
parameters in microwave circuit design
??Demonstrate principles of stub matching and show that it is a narrow
bandwidth method.
INTRODUCTION [1][3][6]
It is often convenient to measure the complete performance of a microwave
device. Just as h parameters are used in transistor design, S-parameters are used
in microwave design. S-parameters can be easily measured using an S-parameter
test set. In this lab, we have used the Hewlett Packard HP8720 Network analyser
to measure the S-Parameters for some simple devices.
The experiment is broken down into three parts. The first part is basically the
familiarisation with the various complexities of the equipment. Therefore, the
second part investigates the variation of the amplitude and the phase of S-
parameters. Different scenarios have been analysed, where either only Port 1 has
been used, or both Port 1 and Port 2 have been used.
In microwave engineering we frequently need to measure the phase response of
a component as well as its amplitude response. For this we use a Vector Network
Analyser, often referred to, without qualification, as a Network Analyser. This
instrument has two microwave inputs, the REFERENCE input and the TEST input,
and gives as output the relative amplitude (dB) and phase (deg.) between the two
inputs. By connecting the instrument to various microwave networks a wide
variety of precision measurements can be made [1].
And the final part is use of the reflection/transmission test set to observe
variations in the amplitude and the phase of the test device over a range of
frequencies . Various devices have been thus tested, including stub tuner, horn,
and micro strip patch antenna. The procedures are given in details in the next
sections of the report. The explanations given do not go into too much depth of
the theories of the subject because the experiments were aimed at only giving a
brief introduction of the functions of the equipment (the Network Analyser).
3
BACKGROUND THEORY
The concepts of S-parameters and Network Analysers are essential to
understanding this experiment and are briefly explained in relation to the
information required for this experiment.
Scattering Parameters [2][3][6]
This property is used to describe electrical behaviours of linear high frequency
electrical networks. It can be used to determine many electrical properties such
as gain, return loss, VSWR, reflection coefficient and amplifier stability. It uses
matched and un-matched loads to characterize a linear electrical network. The S-
matrix for an N-port contains a N2 coefficients (S-parameters), each one
representing a possible input-output path. Let us consider an N port network.
Such that the Vn is the amplitude of the incident wave and V'n is the amplitude of
the reflected wave. To calculate the output of the incident wave we need to
multiply it with the appropriate scattering parameter to get the amplitude of the
reflected signal. If the value of N is 2 and then the definitions of S-parameters are
like below:
2
Figure 1: Basic Definition of S-Parameters [2]
The no of ports might be higher than 2. Suppose if the value of N is 4 and is
represented like the following diagram,
4
Figure 2 : Measuring scattering parameters (4 ports)
Then,
S11 = V'n1 / Vn1 (nk = 0; nk =n1) S22 = V'n2 / Vn2 (nk = 0; nk =n2)
S21 = V'n2 / Vn1 (nk = 0; nk =n1) S12 = V'n1 / Vn2 (nk = 0; nk =n2)
Similarly the S-parameters for other ports can be calculated. The S- parameter is
also the gain of that port. For example S21 can be found by driving port 1 with the
voltage 1 and then calculating the output voltage at port 2 provided that, the
incident voltage is zero at all ports except port 1.
We will be investigating four S parameters in the first part which are as follows:
• S11 ...
This is a preview of the whole essay
S21 = V'n2 / Vn1 (nk = 0; nk =n1) S12 = V'n1 / Vn2 (nk = 0; nk =n2)
Similarly the S-parameters for other ports can be calculated. The S- parameter is
also the gain of that port. For example S21 can be found by driving port 1 with the
voltage 1 and then calculating the output voltage at port 2 provided that, the
incident voltage is zero at all ports except port 1.
We will be investigating four S parameters in the first part which are as follows:
• S11 = reflection coefficient of the input
• S22 = reflection coefficient of the input
• S21 = forward transmission coefficient
• S12 = backward transmission coefficient
Network Analyser [1][2][5]
We motioned in the section above that in this experiment, in order to measure
the scattering parameters we used a vector network analyzer. Network analyzers
are used mainly at high frequencies in the range of 9 KHz to 110 GHz and they
come in two forms, scalar network analyzers (SNA) and vector network analyzers
(VNA). SNAs measure amplitude properties only while VNAs measure both
amplitude and vector properties.
A network analyzer measures S-Parameters as ratios of complex voltage
amplitudes and Figure 2 shows the basic architecture needed inside a 2 port VNA.
5
You should note that the reference plane is located inside the VNA as shown in
the diagram. The calculations using the complex voltage amplitudes lead to the
scattering parameters being calculated.
The mechanism of the Network Analyzer works using a bridge arrangement to
form sums and differences of the port currents and voltages. The phase angle is
found using synchronous detection with in-phase and quartered components.
Network Analyzers can be coupled with a computer-controlled oscillator to make
possible measurements over large frequencies. The S parameters can then be
plotted against a frequency range, either directly or on a Smith chart.
While using Network Analyzers care must be taken during the calibration
procedure. A Network Analyzer is presented with known scattering points from
matched loads or short circuits and then adjusts the S parameters taking into
consideration the defects of the transmission lines that connect the analyzer to
the network. Errors from measurements include losses, phase delays caused by
the effects of the connectors and cables. Some of these errors can be calibrated.
This can be done by using short or open circuits as well as matched loads.
Figure 3 : Network Analyser [1][2]
6
PROCEDURE, RESULTS AND DISCUSSION
PART 1: MEASUREMENT OF S-PARAMETERS
) To start the experiment, the system needs to be calibrated out of the effects of
the presence of the cable. This is referred to as to 'zero' the system. This was
done by connecting a coaxial short circuit to port 1 on the S-parameter test set.
And then pressing the DISPLAY button in the RESPONSE box, followed by DATA ->
MEMORY and then DATA/MEM from the screen menu. This enabled us to
measure the S11 (the reflection coefficient). Both amplitude and phase responses
were zeroed.
Figure 4 : While system is zero
Both amplitude and phase were then viewed. Phase was viewed by pressing
FORMAT button in the response box followed by selecting phase from the screen
menu. The display was reset to amplitude by pressing the FORMAT button in the
response box followed by LOG MAG from the screen menu.
The system is setup to sweep across all frequencies from 50MHz to 20GHz which
is the entire frequency band supported by the Network Analyser. This means that
measurements at every frequency across that band can be made in one
frequency pass. The actual number of frequencies that were computed can be
changed by altering the NUMBER OF POINTS. The default value for this is 201
points which is sufficient for most measurements.
7
2) Then we replaced the short circuit on port 1 with a matched coaxial load and the
frequency response was viewed as follows:
S11 Amplitude
0
0
-10
Return Loss (dB)
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
-20
-30
-40
Series1
-50
-60
Frequency (Hz)
Figure 5a: The S11 Frequency Response (Amplitude) of matched Coaxial Load
S11 Phase
2.00E+02
.50E+02
.00E+02
Phase (degree)
5.00E+01
0.00E+00
-5.00E+01
-1.00E+02
-1.50E+02
-2.00E+02
Frequency (Hz)
Series1
0
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
Figure: The S11 Frequency Response (Phase) of matched Coaxial Load
8
DISCUSSION
The typical value for S11 for a coaxial matched load is zero, which means that
there is no reflection due to the fact that the load is matched.
The frequency amplitude response is rippled because we are dealing with a
non ideal device. The signal can be distorted by noise or not correct
connection to the device. In real life there is no device without ripple effect.
According to the equation f=c/?, if the value of the ? changes then
proportionally the value of frequency will change as well. Hence in other
words the phase of the signal will change accordingly.
3) After that, the coaxial matched load was removed from Port 1 and an open circuit
was left. The amplitude and phase responses were as follows:
S11 Amplitude
5
4
3
Return Loss (dB)
2
Series1
0
-1
-2
-3
-4
Frequency (Hz)
0
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
Figure 6a: The S11 Frequency Response (Amplitude) of Open Circuit
9
S11 Phase
0
0
-1
-2
Phase (degree)
-3
-4
-5
-6
-7
-8
Frequency (Hz)
Series1
2E+09 4.5E+09 8E+09 1.5E+10 1.3E+10 1.45E+10 1.7E+10 2.05E+10
Figure 6b: The S11 Frequency Response (Phase) of Open Circuit
4) Then, one end of the long RF coax cable was connected to port 2 on the S-
parameter test set and the other end was connect to port 1. S21 measurement
(the forward transmission coefficient) was selected by pressing the MEAS button
in the RESPONSE box followed by S21 from the screen menu. The reference value
needed to be set to 0dB. To scale the plot, the DISPLAY button in the RESPONSE
box was pressed followed by DISPLAY DATA from the screen menu. The SCALE REF
button in the response box was pressed then REFERENCE
VALUE from the screen menu was selected. Finally, '0' (zero) was pressed
followed by 'x1' from the ENTRY box. To change the scale factor for the display to
5dB per division, SCALE/DIV was selected from the screen menu and then '5' was
pressed followed by 'x1' from the ENTRY box.
0
The following results were achieved:
Amplitude response of S21 to coax attenuator
-18
0
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
-18.5
-19
Amplitude (dB)
-19.5
-20
-20.5
-21
-21.5
-22
-22.5
-23
Frequency (Hz)
Series1
Figure 7 : The S21 Frequency Response (Amplitude) of Short Circuit
DISCUSSION
It is evident from the result that this is the forward transmission coefficient but
there are some losses in the signal since the transmission part of the graph is
supposed to be a straight line.
5)
Next, looked at a specific microwave component, the coaxial attenuator. For that,
we performed another zeroing to calibrate out the effects of the presence of the
cable which should remain connected. We did it in the same way of step 1.
One end of the coaxial attenuator was connected to port 1 and the other end to
port 2 via the coaxial cable, were used in the section. All four S-parameters (S11,
S12, S21, S22) were measured, switching between the measurements following a
similar procedure to the above section. I changed the scale on the display (as
described above) to see the results. The results had been plotted as follows:
1
S11 Amplitude
0
0
-10
Return Loss (dB)
-20
-30
-40
-50
-60
Frequency (Hz)
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
Series1
Figure 8 : The S11 Frequency Response (Amplitude) of Coaxial Attenuator
S12 Amplitude
0
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
-1
-3
Return Loss (dB)
-5
-7
Series1
-9
-11
-13
-15
Frequency (Hz)
Figure 9 : The S12 Frequency Response (Amplitude) of Coaxial Attenuator
2
S21 Amplitude
-1
0
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
-3
R etu rn L o ss (d B )
-5
-7
Series1
-9
-11
-13
-15
Frequency (Hz)
Figure 10 : The S21 Frequency Response (Amplitude) of Coaxial Attenuator
S22 Amplitude
0
-10
Return Loss (db)
-20
-30
-40
-50
-60
Frequency (Hz)
Series1
0
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
Figure 11 : The S22 Frequency Response (Amplitude) of Coaxial Attenuator
3
S11,S12,S21,S22 Amplitudes
0
0
2E+09 4E+09 6E+09 8E+09 1E+10 1.2E+10 1.4E+10 1.6E+10 1.8E+10
-10
Return Loss (dB)
-20
Series1
-30
-40
-50
-60
Frequency (Hz)
Series2
Series3
Series4
Figure 12 : The Frequency Response comparison of all the S-parameters
DISCUSSION
S21 is a measure of the attenuation of the wave it is the backward
transmission coefficient.
According to the diagrams, the frequency response for S22 shows more
prominent effects compared to S11. On contrary, S22 shows a better
impedance matching value when compared to S11.
PART 2: REFLECTION & TRANSMISSION TEST SET
In microwave engineering we frequently need to measure the phase response of
a component as well as its amplitude response. For this we use a Vector Network
Analyser, often referred to, without qualification, as a Network Analyser. This
instrument has two microwave inputs, the REFERENCE input and the TEST input,
and gives as output the relative amplitude (dB) and phase (deg.) between the two
inputs. By connecting the instrument to various microwave networks a wide
variety of precision measurements can be made [1].
Transmission Measurements
In this section, we investigated the transmission properties of a microwave
waveguide device. This is important in the design of a microwave system to
ensure that the device operates correctly.
4
The apparatus is setup as in the figure below:
Figure 13: Transmission Test Set
)
In this part, at first we recalibrated (zero) the system, but this time using 2 cables,
but first we made a few changes to the setup of the equipment.
Since we were to investigate a waveguide device, we observed that waveguide
has a restricted operational frequency band so sweeping from 50MHz to 20GHz is
unnecessary. For the waveguide we are using the operational region is in the X
band, so restricted the range of frequencies to be from 8GHz to 12GHz. To change
the frequency range, we selected START from the STIMULUS box followed by '8'
and 'G/n' from the ENTRY box. To set the stop frequency, we selected STOP from
the STIMULUS box followed by '1', '2' and 'G/n' from the ENTRY box. The
frequency range was then set.
In order to get a better resolution in the sweep band, we increased the number of
sample frequencies by increasing the number of sample points to 401. We
pressed the SWEEP SETUP button in the STIMULUS box followed by 401 and 'x1'
in the ENTRY box.
After that we took the two coaxial cables and joined them with an SMA joiner,
one end was connected to port 1 and the other end to port 2. To perform the
calibration (zeroing), we set the system to measure S21 and followed the zeroing
procedure as previously described.
Then we removed the SMA joiner and insert the device as illustrated in figure 2,
2)
3)
4)
5)
5
making sure that the attenuator was set to 0dB.
W-guide to Coax Transition
W-guide to Co-ax Trans
Attenuator
Cable
Phase
Shifter
Cable
Figure 14 : Configuration for Transmission Measurements [1]
6)
In order to be able to trace the measured values accurately at a desired
frequency, we did set a marker. We set a marker for a frequency of 10GHz. To do
this, we pressed the MARKER button in the RESPONSE box followed by 10, 'G/n' in
the ENTRY box.
Note the value of attenuation at 10GHz.
DISCUSSION
The attenuation at 10GHz is -2.1dB.
This value is supposed to be zero in an ideal situation but it is less than zero in this
case due to the losses in cables and connectors.
7)
We varied the attenuator setting of the test device and noted the amplitude reading
at 10GHz when the attenuator is set to values of 10dB and 20dB.
DISCUSSION
Amplitude reading when at attenuation is set to 10dB and 20dB for a 10GHz
frequency is 1.25dB and 2.3dB respectively.
We varied the phase shifter micrometer setting and noted down any variation that
we observed in attenuation.
DISCUSSION
If the phase shifter micrometer reading is varied, very minor changes in the
attenuation is observed.
6
8)
We finally reset the test device adjusters to zero. We plotted a calibration curve
of phase shift as read off the display using the marker against micrometer setting
of the phase shifter.
Micrometer Settings (mm)
0
1
2
3
4
5
6
7
8
9
10
11
Phase Shift (deg)
94.5
88
89.7
96.6
112.7
129
164.8
-148.4
-49.6
17
79
109.7
Micrometer Settings vs Phase Shift
200
50
Phase Shift (degree)
00
50
0
-50
-100
-150
-200
Micrometer Settings (mm)
2
3
4
5
6
7
8
9
0
1
2
Series1
Figure 15 : Phase Shift vs. Micrometer Settings
DISCUSSION
From the graph it is evident that the phase shifter is not a linear device although it
does some sense of linearity at certain points of the graphs proving that at certain
micrometer settings the phase shifter is a linear device.
7
Reflection Measurements 1
The apparatus is setup as in the figure below:
Figure 16 : Reflection Test Set
)
In this part of the experiment, firstly we removed everything attached to the
ports of the Network Analyser and attached to port 2, the short length of coaxial
cable, connected to the bench mounted waveguide and terminated with a
waveguide matched load, as shown in the figure below:
Figure 17 : Configuration for Reflection Measurements 1
2)
3)
We set the Network Analyser to measure S22 and set the number of sample points
to 201.
We then performed a simplified waveguide calibration where the return loss from
the matched load is effectively zeroed. To do this, we pressed DISPLAY in the
RESPONSE box, followed by DATA TO MEMORY and DATA-MEM from the screen
menu.
After that, we removed the waveguide matched load and replaced with a stub
tuner / horn combination making sure that the stub screws were not protruding
into the waveguide.
4)
8
5)
By adjusting the stub tuner screws, located a suitable null around the centre of
the screen. The amplitude level of the null should be in the region of -60dB. When
a suitable null had been located, we pressed MARKER in the RESPONSE box and
then use the dial in the ENTRY box to move the marker to the null.
The frequency that we have chosen is 9.7GHz.
Figure 18 : Amplitude of stub tuner
Figure 19 : Phase of stub tuner
9
6)
Without disturbing the stub tuner screws, we removed the horn and replaced
with the matched load.
The amplitude and phase readings at frequency 9.7GHz are -23.1dB and -98deg
respectively.
We removed the matched load and replaced the horn and returned to where we
were before.
Then we fully retracted the stub tuner screws from the interior of the waveguide.
The amplitude and phase readings at our chosen frequency (9.7GHz) are -21.4dB
and -83.2deg respectively.
The aim of the experiment is to show that the phase measured with the stub
tuner/horn combination and the stub tuner/matched load combination cancel
each other and produce a real input impedance at the throat of the horn.
Amplitude Difference between the two sets of readings
= (-21.4dB) - (-23.1dB)
= 1.7 dB
Phase Difference between the two sets of readings
= (-83.2deg) - (-98deg)
= 14.8 deg
7)
8)
DISCUSSION
The experiment with the stub tuner/horn has better impedance matching that is
why it has more magnitude of amplitude than the matched waveguide
experiment. The phase for the first one shows more deviation in terms of angle
where as the phase for the second part shows more stability.
20
Reflection Measurements 2
)
As a final measurement, we would measure the input reflection coefficient for the
microstrip patch antenna shown in the figure below. The antenna has a quarter
wavelength matching section at its input.
?/4 Matching
Section
Coaxial SMA
Connector
Patch Antenna
Microstrip
Line
Figure 20 : Configuration for Reflection Measurements 2 [1].
The screenshot below shows the actual antenna used during the experiment. This
antenna was connected to port 1 of the network analyzer.
Figure 21 : Microstrip Patch Antenna used in the experiment.
2)
Based on the procedures previously defined, we perform the necessary steps to
compute the input reflection coefficient for the patch antenna over a
frequency band of 0.5GHz to 12GHz.
21
STEPS:
. We zeroed the system first with short circuit coaxial cable.
2. Then we changed the frequency range to 0.5GHz-12GHz.
3. After that we connected the antenna to Port 1.
4. We took the frequency response (amplitude) readings for S11.
The resonant frequency of this antenna is 5.771 GHz and the return loss is -
3.2dB.
Figure 22: Resonance frequency if the antenna on the VNA.
The result achieved is as follows:
Figure 23 : The S11 Frequency Response of Microstrip Patch Antenna
22
CONCLUSIONS
To conclude, we can say that the network analyzer is an important instrument
when it comes to measuring different microwave readings. The first part of the
experiment deals with the S-parameters using matched load, coaxial open circuit,
a long RF coaxial cable and a coaxial attenuator. The coaxial matched load
produced distortion and attenuation, the open circuit produced higher noise and
the attenuator produced an improved system with less noise interference.
The second part of the experiment dealt with the reflection and transmission
measurements using the network analyzer. The first stage of the experiment
provided proof on the fact that the calibrations of the system had slight errors
present in them. Measurement of phase offset, attenuation and amplitude were
also carried out over a certain range of frequency. This part also proved that the
phase shifter is a non-linear device. The next part dealt with the reflection
measurement using a stub horn, which showed a very low return loss at the
frequency.
The discussions and explanations on the relevant theories clearly helped us to
realize some very important aspects of microwave engineering field, which acts as
the foundation of today's world of advanced technology.
23
REFERENCES
[1] ELE569, Microwave Electronics, Lab 1: The Microwave Network Analyzer Lab
sheet
[2]https://www.student.elec.qmul.ac.uk/courseinfo/ele569/documents/3_Netwo
rk.pdf
[3] Pozar, David M, Microwave and RF wireless systems, Wiley, 0471322822.
[4] Combes et al, P.F, Microwave components, devices and active circuits
Wiley1987; ISBN 0471912778.
[5]http://www.microwaves101.com/encyclopedia/networkanalyzermeasurement
s.cfm#networkanalyzers
[6] http://en.wikipedia.org/wiki/Scattering_parameters
24