TAMPEREEN TEKNILLINEN YLIOPISTO
TAMPERE UNIVERSITY OF TECHNOLOGY
8005955, 8309505, 8402045
Laboratorio Courses
Modulation work
INTRODUCTION
The scope on this lab work is to introduce a usage of a spectrum analyzer
and after that you get familiar with AM- and FM modulation, RF
generator and some other basic RF principals.
BACKGROUND
Mixer is a component, which is used in receivers to shift frequencies. For
instance the local oscillator of the receiver shifts the RF- signal having a
frequency of 935 MHz to intermediate frequency (IF). This principal is
used in superheterodyne receivers. Widely used receivers (e.g. GSM
receivers) are double conversion receivers, which means that frequency
down-conversion takes place in two stages. The IF (or IF’s) is fixed and
by changing the frequency of the local oscillator, the channel is chosen.
The real channel selectivity is realized by the IF filter.
RF
IF1
LO1



IF2



demod
LO2
The radio frequency signal is shifted to a lower frequency, because it is
difficult to demodulate at high frequencies. The transmission of the
carrier is also difficult, so the information signal is transferred at both
sides of the high-frequency carrier, using modulation.
1. PRE-LAB ASSIGNMENT
1.1
In RF measurements power is usually given in dBm?
What does dBm mean?
Also calculate:
Which absolute power equals 33 dBm?
What would be the voltage generated by 0 dBm on a 50 load?
1.2
Draw a simple block diagram of a sweeping spectrum analyzer.
What are the following concepts when speaking about spectrum
analyzers:
SPAN, CENT FREQ, RBW, VBW, SWEEP TIME, ATTN, REF
LEVEL?
What does MEAS UNCAL warning mean? (It is related to RBW,
VBW and SWEEP TIME.)
1.3
Present the principal of mixing mathematically, i.e. multiply two
cosine signals.
What are the frequency components we get from the output of the
ideal mixer? What about practical mixer?
What frequency components do we get from the IF port of the
mixer, if the injection of the RF port is 71 MHz and that of LO
port is 70 MHz?
1.4
What are the different versions of AM- modulation?
What are the differences between them in power and spectral
efficiency?
Depict an AM-modulated signal in time domain and its spectrum,
when fc=10 MHz and fm=10 KHz.
What does modulation factor, modulation depth and
modulation percent mean? How you can define AM modulated
signal modulation percent
1) from time domain representation?
2) from output spectrum?
1.5
Consider output spectrum difference between narrowband FM- and
AM modulation if modulating signal is single tone ?
1.6
FM modulation output spectrum can be calculated, keeping in mind
that
xc(t)=Ac[cos(sinmt)cosct-sin(sinmt)sinct] and
cos(sinmt)=J0()+2Jn()cos(nmt)
and
sin(sinmt)=2Jn()sin(nmt).
Thus the representation of FM modulation in time domain shall be:
xc(t)=Ac-Jn()cos(c+nm)t.
This means that the output spectrum consists of the carrier
cos(c), plus
infinite number of sidebands at frequencies
(c+nm), i.e. fcnfm.
=modulation index (only for a sinusoidal signals)=Amf/fm.
Jn() gives amplitude of each frequency component
(from Matlab function besselj)
What is the spectrum of the output signal, if the frequency of the
modulated signal is fm=10KHz and the frequency deviation is
f=20KHz? The output spectrum can be found using the Bessel
functions.
2.
MEASUREMENTS
In this lab, we use the following instruments:
2 Rohde & Schwarz SMY02
1 HP ESA1500
1 HP 33120A
1 Mixer by MiniCircuits or HP
2.1
Spectrum analyzer basics training
Connect a signal with frequency of 70 MHz and level of +7 dBm to
the LO port of the mixer, and a signal with frequency of 71 MHz
signal and level of –30 dBm to the RF port of the mixer. Now we
use lower side injection. Measure the spectrum at the IF port of
the mixer, using a spectrum analyzer. In radio receivers the sum
and difference of the RF and LO signals are very useful, so
attention is paid to the vicinity of the sum of the frequencies. Set
CENT FREQ at the sum frequency and SPAN to 10 MHz. Try to
find even the weakest outputs of the mixer (how?).
Compare the frequency components found in this experiment with
those determined by theory. What is the conversion loss?
2.2
AM modulation
Set SMY 02 as carrier generator (frequency of 100 MHz)
and bring the modulating signal (100 kHz) from the HP generator.
Set the modulation to AM and the level of the modulating signal
1 Vpp (SMY notifies if the chosen level is not correct).
Change the modulation percentage (1%, 5%, 10%, 30%, 50% and
%100) from the generator and measure the carrier to sideband
ratio :
(Pc/PSB) -> Log scale: Pc [dBm]- PSB[dBm].
Are the measurement results consistent with the theory, i.e.
derive formula and calculate theoretical carrier to sideband
ratio.
2.3
FM modulation
Set the output frequency of the signal generator to 80 MHz, which
is now our carrier frequency, and the modulation to FM. The
modulating signal comes from HP 33120A, the level of which is 1
Vpp , then Am=1 and SMY modulates correctly.
The frequency of the modulating signal is hence 10 KHz and the
deviation is 20 KHz. Measure the output spectrum and compare
that with the results you got at 1.7.
Consider how to convert theoretical relative scale values into
dBm!
What is the difference between changing the deviation while
keeping the modulation signal fixed, and changing the frequency of
the modulating signal while keeping the deviation fixed?
3.
POST-LAB ASSIGNMENTS
3.1
Write down the answers to the questions, compare your results
with the theoretical values and explain what the sources of
possible differences are. Return the field book.
REFRENCES
MiniCircuits: RF Designer’s Handbook
Hewlett & Packard: Back to Basics
Carlson: Communications systems, Chapters:
modulation and Exponential CW modulation
A table containing the Bessel functions
Linear
CW