LAB1_Tranceiver_Blocks_Simulation_Autumn2009

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Autumn 2009: Radioelectronics (TSEK-02) 1/14

Electrical Engineering Department (ISY) Linkping University, Sweden

Date: ________Student Name: _________________________ Lab Supervisor: ___________

Personal Number: - Signature: _______________

LAB-1

Simulation of Radio Transceiver

Building Blocks

Prepared by

Rashad.M.Ramzan and Henrik Fredriksson

{rashad}{henrik}@isy.liu.se


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Introduction:

In this lab you will investigate the characteristics and behaviour of some functional blocks

used in RF transmitters and receivers. The models have been implemented in SimulinkTM

,which is a popular tool for functional level modelling and simulation of various systems at

the block-level of abstraction. For our purpose we have mainly used the Communication

Blockset, which offers a variety of RF models. This LAB has five modules which requirea preparatory work to be completed by you before starting the LAB. To complete all the

program, we assume it should not take more than the scheduled four hours if you have

prepared for the LAB. The LAB consists of the following five modules:

1. AM Modulation/Demodulation2. FM Modulation/Demodulation3. Two Stage up-conversion4. Heterodyne Receiver5. Zero-IF Receiver

1. Instructions

1.1. The LAB manual and Simulink files are available on course web page http://www.ek.isy.liu.se/courses/tsek02/

1.2. You must hand over the Preparatory Exercises (PE) to the lab assistant before thestart of the LAB to get the credit of this LAB. Very brief but to the point answersare desirable. Please, do not waste eleventh word if the answer in ten words and a

figure is possible.

1.3. If you are not able to complete the LAB during the scheduled hours, you can do itin your spare time and once everything is in order and complete, show the results

to the teaching assistant.

2. Preparatory Exercise

Q1: a) What is the difference between the standard amplitude modulation, linear

addition, and multiplication of signals?

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b) Why is it desirable to have the modulation index of an AM signal as large aspossible without overmodulation? Why is overmodulation undesirable?

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c) Consider a BB signal with a bandwidth between 500Hz and 5kHz to be

amplitude modulated using a carrier of 200 kHz. Plot the AM spectrum in twocases: standard AM is used, SSB-SC AM is used. Compare the bandwidth occupied

by them.

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Q2: FM Definition: Consider a carrier signal at frequency 200 =f and amplitudeA0,

being frequency modulated by a sinusoidal signalxBB(t)= em(t)=Em cos (mt+ 1800).

The frequency of the modulated signal is thenfFM(t) =f0 kfEm cos mt. This can be

expressed as a function of time as:

( )( ) ( )( )

( ) ( )( )tmtAtEk

tA

dttEktAdttfAts

mfm

m

mf

mmfFMFM

sinsinsin2

sin

cos2sin2sin)(

0000

000

+=

+=

==

Which also can be expressed as a series of the Bessel functions of the first kind as:

{ ) ( ) ) ( )( ) ( )( )[ ]

( ) ( )( ) ( )( )[ ] }...2sin2sin

sinsinsin)(

002

001000

++

+=

ttmJ

ttmJtmJAts

mmf

mmffFM

whereJn are Bessel functions ofnth order.

a) Define the modulation index of the FM signal (see lecture 3).

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A phase modulated signal can be expressed as: ( )( )txktAts BBPPM += 00 sin)( wherexBB (t) is the modulating signal.b) What is the relationship between the phase modulation and frequencymodulation? Consider the FM signal sFM(t) as a PM signal and derive the

corresponding phase modulation signalxBB(t).

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c) What is the relationship between the modulating signal and bandwidth of the FM

signal? (Hint: Carsons rule)

Derive also a formula for the FM bandwidth for narrowband modulation (mf


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Q4: a) Heterodyne receivers have typically: the band selection filter, image rejection

filter, and channel filter. Explain the function of each of these filters.

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b) A heterodyne receiver is designed to receive a signal at 1.1 GHz using a 0.9 GHz

LO frequency. Calculate the IF frequency and image frequency. Explain what theimage frequency means.

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c) Explain the purpose of a two-tone test.

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Q5: a) What is the main advantage of a zero-IF receiver?

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b) What kind of operation is needed to retrieve the wanted signal (and toreject the image /mirror/) in the zero-IF receiver?

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c) Looking at the diagram on the title page, list the problems associated with the

zero-IF receiver?

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3. References

[1] J. Dbrowski,Radioelectronics, LiU-Tryck, 2009

[2] R. Blake,Electronic Communication Systems, Delmar, Thomson Learning, 2002

[3] L. E. Frenzel, Principles of Electronic Communication Systems, McGraw-Hill,

2006.

[4] B. Razavi,RF Microelectronics, Prentice Hall, 1998.


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LAB Exercises

3.1. AM Modulation/Demodulation

Fig-1: AM Modulation/Demodulation

Open the file am.mdl. Simulate the model and observe the time domain signals onthe scope, and frequency domain signals on the FFT analysers.

Change the Modulation Index by changing gain of the block Modulating signal.

Use the following values: 0.2, 0.5, 1, and 2. Observe the output. Does thedemodulator recover the signal correctly in all cases? Comment on the problem.

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What would happen if a simple envelope-detector (diode and RC low-pass filter)was used as a demodulator for the modulation index equal 2?

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Compare the Rx-OUT signal, with and without the Band-Select Filter.

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The non-linear block, following the Up-conversion Mixer, models odd nonlinearity.Double click on the block and change it to an even-order non-linearity by changing"u - 0.001*u*u*u" to "u - 0.001*u*u". Simulate and observe what happens to the

TX-OUT signal.

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Remove the carrier from the AM signal. For this purpose set the value in theConstant block to 0. In this way a two-tone signal is generated by the Up-

Converter. To emphasise the effect of the 3rd

order nonlinearity redefine theNonlinearity block as u - 0.1*u*u*u.

Simulate and observe the available spectra. What are the power levels of the desired

signal, the in-band and out-of-band spectral content evoked by the nonlineardistortion? (The band is defined by the Band-select filter between 7 and 13 kHz).

For comparison you can also reduce the nonlinearity. Explain the differences.

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3.2. FM Modulation/Demodulation

Fig-2: FM Modulator/Demodulator Model

Open the file fm.mdl. Simulate the model and observe the response of the timedomain signals (scope) and frequency domain signals. Carsons rule can be used to

determine the bandwidth of the FM signal (see the preparatory exercise Q2c).Compare the results from the simulated model with this rule. Do they match?

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Change the modulating frequency and amplitude of the modulating signal. Observethe effect of these changes on the BW? Compare with the Carsons rule.

Increasing Modulating Frequency: {Increases / Decreases / No effect} BW

Increasing Amplitude of Modulating Freq: {Increases / Decreases / No effect} BW

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3.3. Two Stage up-conversion

Fig-3: Two step up-converter model

Open the file two_up.mdl and run the simulation. In this model there are two architectures to compare, the quadrature and non-

quadrature one. Both are designed to create a SSB modulated signal. How muchsuppression of the unwanted sideband can you see in these two models?

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Explain why there is a difference.

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Observe that the BPF used in the quadrature up-converter does not affect the

spectrum signal significantly. In practice, however, this filter is used anyway.What could be the possible reason for using this filter?

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A very selective BPF at high frequency is difficult to achieve. How can youimprove the lower sideband rejection in a non-quadrature up-conversion without

changing the filter? (Hint: Look at the LO frequencies in the first and second stage)


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Verify the improvement through simulation. How much suppression of the lowersideband have you achieved in the modified model?

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3.4. Heterodyne Receiver

Fig-4: Heterodyne Receiver model

Open the file he_Rx.mdl and run the simulation. This file models a heterodynereceiver for a 1.1 GHz RF signal and IF of 200 MHz. The band selection and image

rejection filters are band-pass filters with a pass band between 1 and 1.2 GHz. The

nonlinearity of the amplifier is chosen to be very weak. The channel selection filter(BPF-IF) has a pass-band between 190 and 210 MHz. For each of the signal

sources, identify the corresponding components in the IF spectrum. Explain the

differences in the signal levels in IF band. Compare the spectra at the input and theoutput of the IF filter.

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Which one of the input signals can be considered the image if the wanted signal isat 1.1 GHz? How much suppression does it achieve? (a small offset frequency is

used to make the image visible)

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To emphasise the image problem change the LO frequency to 800 MHz. Now thewanted signal is at IF equal 300 MHz, respectively. (In fact, as the IF frequency is

changed the IF filter should also be changed accordingly, but we skip the IF filter

problem in this investigation). Run the simulation to see the wanted signal andidentify its image (esp. look at the input of the IF filter). Which input signal makes

the image and how much suppression does it achieve in this case? Compare the

image suppression with the previous case when LO was tuned to 900 MHz andexplain the difference.

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Disconnect all RF-signals except the 1.1GHz source. Increase the amount of non-linearity in the LNA by clicking on the symbol and change 10*u[1]-0.0001* u[1]*

u[1]-0.0001* u[1]* u[1]* u[1] to 10*u[1]-1* u[1]* u[1]-1* u[1]* u[1]* u[1]

Open the zero order hold-block and change the sampling time to 1/10e9 as thethird harmonic will reach triple frequency and also be up-converted.

Simulate and identify the harmonics in the receiver.

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Change the model to perform a two-tone test. Use only two signal sources, one at1.09 GHz and other at 1.11 GHz and change the sampling time to 1/1e9 for the

zero order hold-block at the IF filter input (there will be much aliasing in the FFT

spectrum but the part of interest will not be affected and IM3 can be measured

well). Calculate the frequencies of the 3rd order intrmodulation products.

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Measure the relevant spectral components at the input of the IF filter and calculateIIP3 of the receiver.

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3.5. Zero-IF Receiver

Fig-5: Zero-IF (Direct Conversion) Receiver Model

Open the file Zero_if.mdl.

A standard zero-IF architecture is modelled as shown in Fig-6. Here the receiver is

used to down-convert a two-tone signal of 1500 Hz and 1700 Hz. To show theeffect of the mirroring at IF (which is zero) the LO frequency is chosen at 1550 Hz.

The IQ mismatch and nonlinearities of the Rx blocks are minimized.

Run the simulation and observe the instruments. Identify the down-converteddesired and undesired signal components after the Hilbert transform and addition.

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Change the amplitude of signal S2 inside the Test generator block from 1/4 to1. Measure, with the instruments, the mirror rejection ratio (image rejection)

obtained at the output.

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Open the LO dialog box. Add a small phase mismatch, say 3 deg. Run thesimulation. What is the image rejection now?


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LAB1_Tranceiver_Blocks_Simulation_Autumn2009

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