System Simulation€¦ · EITF50: Introduction to Wireless Systems – Lab 1 2020 – Edition V 2...

Lund University

Electrical and Information Technology

Harsh Tataria

Mohammed Abdulaziz

Waqas Ahmad

Doruk Tayli

Göran Jönsson

August 2020 Fifth edition

Basic Wireless Communication Technique ETIF05

Laboratory Experiments 1:

System Simulation

Objectives

1. Get familiar with different tools and simulations ADS offers.

2. Understand the following:

a. Time based simulations and frequency-based simulations.

b. Selectivity.

c. Sensitivity.

d. Linearity.

3. AM receiver basic architecture.

4. FM double superheterodyne receiver architecture.

5. How to simulate noise performance.

6. How to simulate linearity performance.

7. Understanding the noise vs. linearity relation.

• All the preparation exercises should be solved before the laboratory exercises starts.

• Follow this manual carefully while doing the lab.

• All the questions in section 3 (lab work) must be discussed with the lab assistant in order for the lab to be approved!

Important: Have the ADS schematic ready before the lab, otherwise you will not be able to finish the lab in time.

Good luck!

EITF50: Introduction to Wireless Systems – Lab 1 2020 – Edition V

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1. Homework

1- Write the Friis formula, and explain the importance of having a low noise

amplifier connected to the antenna in the receiver chain.

2- In what sense a filter connected to the antenna is good for (filters have insertion

loss)?

3- Explain the function of the mixer and oscillator. What are they used for in the

receiver architectures?

4- Why do we need automatic gain control for the AM receiver?

5- Explain the following terms:

A. The two-tone test.

B. Sensitivity.

C. Selectivity.

D. 1 dB compression.

E. Intercept point.

6- You have the following components off shelf:

A. AMP1: Amplifier, G=18dB, NF=1.5.

B. AMP2: Automatic gain control amplifier Gmax=80 dB, NF=15 dB.

C. FILT1: Band-pass filter G =-2 dB, fo =10.7 MHz, BW3dB =12.5 KHz,

BWstop=25 KHz, Astop=40 dB.

D. FILT2: Tunable band-pass filter G=-2 dB, fo =118-138 MHz, BW3dB =3 MHz,

BWstop=10 MHz, Astop=40 dB.

E. FILT3: Band-pass filter G =-2 dB, fo =10.7 MHz, BW3dB =200 KHz,

BWstop=10 MHz Astop=40 dB.

F. FILT4: Low-pass filter G=-2 dB, f3dB =3 KHz BWstop=6 KHz Astop=40 dB.

G. FILT5: Low-pass filter G=-2 dB, f3dB =.5 Hz BWstop=1 Hz Astop=40 dB.

H. MIX1: Mixer G=-6 dB, NF=6 dB.

I. VCO1: Voltage controlled oscillator fLO =117.3 MHz, tuning range=20 MHz.

J. XO1: Crystal oscillator fLO =117.3 MHz.

K. AM_DEMOD1: AM demodulator.

- Design an AM superheterodyne receiver for aeronautical communication. The

received frequency range is 118-138 MHz and the channel separation is 25 KHz. The

baseband frequency range is 300-3000 Hz. You should use components from the list.

The design should be for minimum noise figure.

- If the AM_DEMOD1 cannot handle more than 0 dBm of power what is the maximum

input signal level at the antenna that can be received correctly (Assume automatic gain

control will react and make AMP2 G =0 dB).

- If the AM_DEMOD1 cannot handle less than -20 dBm of power what is the minimum

input signal level at the antenna that can be received correctly (Assume automatic gain

control will reacts and makes AMP2 G =80 dB).

- Calculate the noise figure of the receiver chain before the signal goes into automatic

gain control. Keep in mind that the filter’s 𝑁𝐹 = the attenuation.


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7- You have the following components off shelf:

A. AMP1: Amplifier, G=18 dB, NF=1.5.

B. AMP2: Amplifier G=40 dB, NF=3 dB.

C. 2×FILT1: Band-pass filter G =-2 dB, fo =463.4875 MHz, BW3dB =3 MHz

BWstop=5.95 MHz Astop=40 dB.

D. FILT3: Band-pass filter G =-2 dB, fo =21.4 MHz, BW3dB =15 KHz BWstop=150

KHz Astop=100 dB.

E. FILT4: Low-pass filter G=-2dB, f3dB =3 KHz BWstop=6 KHz Astop=40 dB.

F. 2×MIX1: Mixer G=-6 dB, NF=6 dB.

G. VCO1: Voltage controlled oscillator fLO =440.6 MHz, tuning range=3 MHz.

H. XO1: Cristal oscillator fLO =20.945 MHz.

I. FM_DEMOD1: FM demodulator.

-Design a narrow band FM double superheterodyne receiver for public service

communication. The received frequency range is 462-464.975 MHz. The baseband

frequency range is 300-3000 Hz. You should use components from the list. The

design should be for minimum noise figure.

-What is the advantage of using a double superheterodyne receiver having in mind

that more active components are needed for such an approach?

2. Getting started with ADS

ADS stands for Advanced Design System from Agilent Technologies (now Keysight

Technologies) as the owner company defines it, it is “electronic design automation

software for RF, microwave and high-speed digital applications”.

Including a full set of integrated of system behavioral models, circuits and electro-

magnetic tools and simulators, to enable full characterization and design of complete

systems. It also provides dynamic help utilities and also application specific design

guides.

2.1. Running ADS

In the terminal window first, create a working directory and then descend into that

directory. This directory will hold all the design and simulation files.

Open a terminal and run the command:

‘source /usr/local-eit/cad2/keysight/ads2016/setup.efd’

Run the command:

‘inittde ads2016’

Then run the command

‘ads’


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Figure 1 ADS License Selection Window

If the software is opened for the first time a Product Selection screen appears

(Figure 1 ADS License Selection Window). On this screen select the “ADS All Inclusive”

license and click OK.

ADS main window appears as shown in Figure 2, in this window you can create a

workspace or a design; browse the help documents and many other tasks.

Browse the different menus and get familiar with the different commands and

menus in this window.

Figure 2 ADS Main Window

2.2. Create a workspace

In the main menu click File->New->Workspace, if an Introduction page opens click

Next. The “Workspace Name” window should open specify the wanted location and


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name of the workspace, click Next and the “Add Libraries” window opens deselect

the DSP library and click Finish. Make sure that the Analog/RF library is selected.

Figure 3 Add Library Window

After that, download the AM receiver library from the course home page (this

library includes the AM demodulator and AGC amplifier) and copy it into a folder. In

the ADS main window click on File->Manage Libraries and the Manage Libraries

window will open, click on Add Library and select the “AMrec_lib” file that was

extracted (Figure 3), click OK. You will now have two libraries in the Manage Libraries

window. Close this window by clicking Close. The folders AGC_AMP_LAB1,

AM_DEMOD will then be visible in the main window under the Folder View tab and

will also be available for use. When you later need to use the copied designs just drag

them to the schematic window.

To add a new schematic, from the main menu click on File->New->Schematic, specify

the name of the schematic and deselect the “Enable Schematic Wizard” option. In the

main window you should now have another folder with the schematic name that was

created.

2.3. Schematic

The schematic window is shown in Figure 4. The textboxes explain the most frequently

used functions during the design time, in ADS the dynamic help utility gives hints about

the function of each command by just placing the mouse over the wanted button, and a

textbox appears with the explanation of the button’s function.

For example try to place the mouse arrow over the simulate button (a yellow help

box should appear).


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Figure 4 Schematic Window

3. Lab work

In this lab you have to simulate the designed receivers in preparation exercises 6 and 7.

Basic simulations will be done to test systems sensitivity and linearity characteristics.

Follow the steps and discuss with the lab assistant different results and conclusions.

In order to find a component either you choose it from the component category and then

click on the palette to the left on the wanted component or search for the component in

the component library list then drag and drop the component by the left button of

the mouse.

3.1. AM receiver

In this exercise the schematic diagram of the AM receiver designed in the homework

will be created in ADS. The designed receiver will be simulated to verify its different

properties.

1. Interconnect the different building blocks to create the schematic of the receiver as

shown in Figure 5. The filters chosen have the Butterworth response since it has flat

amplitude output and so the data carried in the amplitude is not affected. The I_probe

component is used to sense the current going through it without changing impedance

of that particular node connected to it. This will prove useful for doing the noise

calculations of the system. So between two components an I_Probe should be there

Component

category

Data Display

Window

Simulate

Name a signal

Wire to connect

components

Descend into

component

Insert a

variable

Insert ground

Components

history


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incase noise figure is calculated. For maximum power transfer the input and output

impedances of these components are the same as Z=50Ω.

2. Set the required parameters for each building block.

Figure 5 Schematic of the AM receiver [I_Probe components not shown]

3. After adding I_Probe between the components, name all the wires with descriptive

names, as they will be used to plot the results. To name a signal click on the name

button, write the wanted name and then click on the wire to be named. Finally for

the mixer and the amplifier clear the values found for the following parameters:

SOI, TOI, Psat, GainCompSat, GainCompPower, GainComp.

4. The system is now ready to be simulated. ADS offer different simulation methods

for different properties. However all simulations we need are either frequency based

or time based.

Time simulations in principle consume a lot of memory and processing power since

a time domain signal is fed and then the output in time domain is examined. It is

recommended to use such simulations in the final design stage where accurate

figures are needed. It must be mentioned that some time based simulations e.g.

envelope simulations are fairly fast.

On the other hand frequency based simulations are faster since simulation is done

in the frequency domain, it can be faster because the number of harmonics to be

processed is controlled and also almost all of the components we are going to use

are behavioral models. Basic frequency domain simulations like AC simulation do

not provide any information about the system linearity however nonlinearities can

be simulated efficiently in ADS using Harmonic Balance simulation. Harmonic

balance simulation is a frequency domain simulation and it is suitable for noise and

linearity simulations. To do harmonic balance simulations:

1- In the component category panel click on Simulation-HB the click on H B and

click on the schematic.

2- Double click on the placed H B simulation, a window like the one in Figure 6

appears and type the input frequencies including the LO frequency. The LO

frequency should have higher order (5 is enough) as it will be multiplied with RF

signal and produce new frequencies, the RF can have lower order to reduce

simulation time (2 is a good option).


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Figure 6 Harmonic balance simulation window

3- Click on the output tab and add all the signals already named to be saved as an

output, see Figure 7.

Figure 7 Add outputs to be displayed after simulation

4- P_nTone port can feed more than one tone to the system by just clicking on the

property (Freq and P) and then click add a new source is added as shown in Figure

8. Add two tones (sidebands) around the RF with an offset of 1.5 KHz. The power

level of the sidebands should correspond to a modulation index m=1. Choose low

power level for the carrier (like -40dBm).


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Figure 8 P_nTone port settings window

5- Click the simulate button, after the simulation ends the data display window

appears see Figure 9. If the simulation takes very long time or the simulator halts to

respond then click on Simulate-> Stop and Release Simulator.

Figure 9 The data display window

6- Click on rectangular display button, a window appears see Figure 10 the saved

outputs and the probe signals will be seen here, on the Plot options tab the X-axis

and Y-axis ranges should be adjusted to best show the signal.

Rectangular display

Insert equation

Insert marker


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Figure 10 Plot Traces and Attributes window

7- Plot the input signals, the mixer output, the filtered mixer output and finally the

baseband output. Has the signal been successfully demodulated? Do other

harmonics exist, if so what is the source of this nonlinearity?

3.1.1. Noise

In this part you will simulate the noise performance of the designed receiver; we will

simulate the noise performance of the front end before the signal goes into the

demodulator and the noise figure of the whole system. This means two I_Probe

components must be inserted one after the IF filter and one at the output to measure the

noise at these two points.

a. From the Simulation-HB category place ‘Options’, and three noise controllers

‘NoiseCon’. The ‘Options’ is used to control the simulation run properties like

simulation temperature, convergence and many other things see Figure 11.

The noise controllers allow the simulation of noise at one specific frequency. Since

in this system there are three operating frequencies which are RF, IF and

BASEBAND, three noise controllers are needed to specify each node and the

associated frequency. Double clicking on the noise controller opens a window like

the one shown in Figure 12.

In the ‘Freq’ tab we insert the corresponding frequency and in the ‘Nodes’ tab we

select the nodes working at that frequency and in the ‘Misc’ tab we leave the noise

bandwidth as the default 1Hz. Fill the data for the other two controllers where they

have different operating frequency and of course the associated nodes.

Finally double click on the HB component and go to the Noise tab and add the three

noise controllers one by one.


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Figure 11 Options window simulation temperature is set to NoiseTemp

(a): Freq tab to set the evaluation

frequency

(b)Nodes to set which nodes to

evaluate the noise at

(c)Misc tab where the noise

bandwidth is set

Figure 12 Noise Controller

After successful simulation the data display window appears, however

unfortunately noise simulations do not directly calculate the noise figure at the

stated nodes so further calculations are needed. Each expression is inserted as an

equation, and to insert an equation click the ‘Eqn’ button. Then these equations can

be plotted listed or used in other equations.

b. In order to proceed we need to know:

1- The index at which the harmonic data resides in the array, for example

IF_Index=find_index(freq,10700000)

will find the index where harmonic of 10.7MHz resides.

2- Gain of the system at that point can be calculated as:


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IF_Gain=10*log(abs(0.5*(IF_voltage[IF_Index]*conj(I_probe1.i[IF_Index]))))

+30-RF_power_dBm

Where IF_Voltage is the same name of the voltage signal at the required node and

I_probe1 is the name of I_Probe instance placed at the required node.

3- Since the noise is simulated as a voltage signal, we need to calculate the

impedance and then calculate the noise power and finally we refer it back to the

input (antenna).The node impedance is simply calculated as:

Z_IF= IF_voltage[IF_Index]/I_probe1.i[IF_Index]

and so the noise figure is:

NF_IF=10*log(abs((pow(IF_voltage.noise,2))*real(1/Z_IF)/((10**(IF_Gain/10

))*(boltzmann*kelvin*Noise_BW))))

Where kelvin is the Kelvin scaled simulation temperature=300K at room

temperature, the Noise_BW is the noise bandwidth=1Hz and boltzman is the

Boltzmann constant and it is a built in expression and there is no need to specify it

we just write it and it will be resolved automatically.

c. Display the noise figure at the IF frequency. Does it agree with the calculations?

Display the noise figure of the whole system. What happened to the noise figure?

Estimate the noise figure of the demodulator. What can be done to improve the

noise performance of the system?

3.1.2. Linearity

To simulate the linearity of the system i.e. the compression point and the intercept points

a two-tone test is the choice. First we need to define the dynamic range of the active

components, the amplifier and the mixer. The models of these two components are in

fact based on realistic models that use polynomial fitting to approximate a realistic

behavior of how an ordinary component will act like. For this lab we will specify the

TOI (output Third Order Intercept Point) and then the other parameters will be directly

approximated.

Change TOI of the amplifier closest to the RF input to 10 (it means 10dBm). In

order to proceed with this simulation a variable input power is needed to sweep the

power.

a. Click on the ‘VAR’ button then double click on the variable component window

like the one in Figure 13, name the power as ‘PRF= -60 _dBm’ which will give it a

-60 dBm of default value for example.

b. Apply a closely spaced (1.5 kHz of spacing is a good option) two tones at the RF

input with the same input power as PRF.

c. In the harmonic balance window in the ‘freq’ tab update the frequencies to be

simulated, and on the sweep tab fill the parameter to sweep box with PRF, sweep

type as linear start from -30 to 10 with a step size of 1.

d. Deactivate the noise controllers since we are not interested in simulating the noise,

by clicking on the activate deactivate button and the clicking on the noise

controllers.


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e. Click on the simulate button and the data display window will appear. An equation

can be written to find the index of the fundamental and the third order

intermodulation harmonic like:

IF_Index=find_index(freq[0,::],IF_freq) where IF_Freq is the frequency of the

fundamental.

f. We need to plot the fundamental together with intermodulation term to estimate the

third order intercept point and the compression point. In the plot ‘Traces and

Attributes’ window change the signal indices as shown in Figure 14.

g. Note the value of the intercept point then clear TOI of the amplifier and change TOI

of the mixer to 10 and simulate. What is the value of the intercept point? Why is it

different? Explain. Which component causes the system to compress first and how

can this be solved?

Figure 13 Setting the variable value

Figure 14 Plotting a two dimensional data with referring to one of its coordinates


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3.2.FM receiver (optional)

The double superheterodyne FM receiver designed in the homework (exercise 7)

will be implemented now, make a new schematic file and use the same

components used in section 3.1.

3.2.1. Noise

Do the same noise simulations as done in subsection 3.1.1 for the two IF points in the

receiver.

a. Simulate the noise performance with Amp2 placed in between the mixers.

b. Simulated the noise performance with Amp2 placed after the second mixer. Note

the noise performance results and compare.

c. Explain the difference in performance, and how does it agree with the

calculations.

3.2.2. Linearity

The same linearity simulation as done in subsection 3.1.2 will be done here.

a. Change the TOI of Amp1 to TOI=10.

b. Place the Amp2 in front of the second mixer then simulate and calculate the

output referred third intercept point of at the output of the second mixer at the

second intermediate frequency.

c. Place the Amp2 after of the second mixer then simulate and calculate the output

referred third intercept point of at the output of the second mixer at the second

intermediate frequency.

d. What is the difference between the two values, explain the results.

e. Draw conclusions about noise vs. linearity performance in terms of gain increase

and decrease based on what the simulation results.

System Simulation€¦ · EITF50: Introduction to Wireless Systems – Lab 1 2020 – Edition V 2...

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