Amplitude and Frequency Modulation Experiment

This assignment details an experiment investigating Amplitude (AM) and Frequency (FM) modulation. Students analyze waveforms from both time domain (oscilloscope) and frequency domain (spectrum analyzer) perspectives. They calculate modulation indices, compare theoretical with measured frequencies, and discuss the advantages of FM over AM in terms of signal-to-noise ratio and bandwidth. The experiment provides a hands-on understanding of these modulation techniques.

Running head: AM AND FM EXPERIMENT 0
AM AND FM EXPERIMENT
Name of Student
Institution Affiliation

AM AND FM EXPERIMENT 2
AIM
To compare and contrast the amplitude modulation and Frequency modulation for both the
frequency domain and time domain.
Introduction
In a radio system, there are higher frequency signals of the sinewave which are modulated by
a message signal. These signals are as well referred to as carrier wave. This is achieved through
modulating it with a signal having a relatively lower frequency. This is very important because it
is challenging to convey the message signal in a straight line from the transmitter to the receiver.
Modulating signal may vary in three different types. All the types of the modulation are analog
and are employed in the transmission of the signal through wireless. And for any analog
modulator, there is always two input with just one output. These two inputs are the carrier signal
waveform and the analog signal to be transmitted while the output is the modulated output. We
will hence discuss these two types of modulations;
1. Frequency Modulation (FM)
2. Amplitude Modulation ( AM)

AM AND FM EXPERIMENT 3
Frequency Modulation (FM)
In this form of modulation the carrier frequency changes due to the analog baseband
info signal ready to be conveyed via wireless devices. This modulation is essentially treated to be
better than the AM because it of better noise immunity it has and its capacity to reject the
interfering signals because of the capture effect. The equation below help to obtain modulation
depth
m= Δf
fm ……………………………………………………………………………………..i
Where Δf is the frequency deviation and fm is the frequency of the modulating signal.
For the FM the value of m defines the amplitude of different frequency component in the FM
spectrum.The amplitudes in the FM are related to the Basel function values. Figure 1 below
illustrates the spectrum of FM signal;
Fig 1: Showing the spectrum of FM signal

AM AND FM EXPERIMENT 4
The utmost common technique of applying modulation to a signal is by overlaying the audio
signal onto the amplitude of the carrier. When employing frequency modulation, extra advancements in
signal to noise ratio can be realized only if the audio signal is stressed. To obtain this there must be an
amplification of lower level high frequency sounds to a higher degree than the lower frequency sounds
before they are conveyed. As soon as the receiver the signals are passed via a network with the reverse
effect to reinstate a flat response frequency.
The transmitter and receiver networks must match one another. For broadcasting in North
America values of 75μs with a break frequency of 2.1 kHz is used. Frequency modulation is used in a
wide variety or radio communications applications from broadcasting to two-way radio communications
links as well as mobile radio communications. It possesses many advantages over amplitude modulation
and this is the reason for its widespread use. Nowadays, many digital forms of radio communications are
being introduced, but despite this the use of frequency modulation, FM will undoubtedly continue for
many years to come in many areas of radio communications.
Equipment
1. Spectrum Analyser- Rhode & Schwarz HMS-X
2. Oscilloscope- Keysight Infiniivision DSO- X 2002A, 70 MHz, 2GGSa/sec, Digital
Storage Oscilloscope (DSO).
3. Waveform Generator – Keysight 33500B.
Procedure
A carrier frequency of 500 kHz for the modulated and frequency of the 50 kHz audio signal. The
amplitude of the FM signal was confirmed that it was not varying. The deviation of frequency

AM AND FM EXPERIMENT 5
was adjusted to 5,25,50,100 andc120kHz, equivalent to modulation depth of m= 0.1, 0.5,1,2 and
2.4. The spectrum screenshots were saved using USB memory stick and the amplitude of each
frequency component was measured in dBm (dB milliWatt).
Results
Fig6: Showing Spectrum of FM signal

AM AND FM EXPERIMENT 6
Fig 7: Spectrum analyzer of FM modulated signals.
The value of the power ratio can be calculated using the formula below
¿)2 for each power ratio and the result was compared with the theoretical values of the power
ratios from the Basel coefficients as shown;
For the modulation depth of m= 0.1
There is only one sideband as shown below;
Power ratio = −32
−5 = 6.4 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 6.4
20 = 2.089 2 ≈ 4

AM AND FM EXPERIMENT 7
Power Ratio Theoretical ( Basel) Measured (Spectrum)
¿)2 4.5 4
For the modulation depth of m= 0.5
There are two sidebands as shown below;
First sideband
Power ratio = −18
−5 = 6.4 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 3.6
20 = 1.513 2 ≈ 2
Second sideband
Power ratio = −35
−5 = 7 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 7
20 = 2.28 2 ≈ 5
Power Ratio Theoretical ( Basel) Measured (Spectrum)
¿)2 2.15 2
¿)2 4.9 5

AM AND FM EXPERIMENT 8
For the modulation depth of m= 1
There are three sidebands as shown below;
First sideband
Power ratio = −14
−8 = 1.75 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 1.75
20 = 1.223 2 ≈ 1.5
Second sideband
Power ratio = −25
−8 = 3.125 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 3.125
20 = 1.433 2 ≈ 2
Third sideband
Power ratio = −40
−8 = 5 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 5
20 = 1.77 2 ≈ 3

AM AND FM EXPERIMENT 9
Power Ratio Theoretical ( Basel) Measured (Spectrum)
¿)2 1.0 1.5
¿)2 2.2 2.0
¿)2 3.2 3.0
For the modulation depth of m= 2
There are three sidebands as shown below;
First sideband
Power ratio = −11
−19 = 0.57 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 0.57
20 = 1.068 2 ≈ 1.0
Second sideband
Power ratio = −15
−19 = 0.78 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 0.78
20 = 1.0951 2 ≈ 1.2
Third sideband

AM AND FM EXPERIMENT 10
Power ratio = −25
−19 = 1.315 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 1.315
20 = 1.16 2 ≈ 1.3
Power Ratio Theoretical ( Basel) Measured (Spectrum)
¿)2 1.0 1.0
¿)2 1.0 1.2
¿)2 1.2 1.3
For the modulation depth of m= 2.4
There are three sidebands as shown below;
First sideband
Power ratio = −12
−54 = 0.222222 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 0.2222
20 = 1.0259 2 ≈ 1.0
Second sideband
Power ratio = −15
−54 = 0.27777 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.

AM AND FM EXPERIMENT 11
Power ratio ( Mw ) = antilog 0.27 777
20 = 1.032 2 ≈ 1.0
Third sideband
Power ratio = −20
−54 = 0.37 ( dBm)
Power ratio (dBm) = 20 log Power ratio value.
Power ratio ( Mw ) = antilog 0.37
20 = 1.04 2 ≈ 1
Power Ratio Theoretical ( Basel) Measured (Spectrum)
¿)2 1.0 1.0
¿)2 1.1 1.0
¿)2 1.2 1.0
2. Amplitude Modulation (AM)
AM is a type of modulation where carrier amplitude varies based on the baseband of the analog
info signal which is about to be conveyed via the wireless device. This form of modulation is
typically used in radios. In figure 8 below the three components of the AM signal is highly
conspicuous, two sidebands (side frequencies) having amplitude mVc
2 and the one carrier signal
having amplitude Vc. The depth of modulation m shows how much the carrier is modulated by
the message signal. And this can be obtained from figure 9 through the following equations

AM AND FM EXPERIMENT 12
m= Emax−E min
Emax + Emin ………………………………………………………………………… ii
The two signals can be represented by;
i. Message ( modulation )
VM(t) =Vm cos ῳmt …………………………………………………………………….iii
ii. Carrier signal
Vc (t)v= Vc Cos ῳc t .................……………………………………………………………iv
If the amplitude of the carrier signal is varied, the below equations are got;
VAM (t) = ( VC +VM Cos ῳm t ) Cos ῳ c t ……………………....………………………….v
The equation v can be modified to obtain equation vi as below
VAM (t)= VC Cos ῳC t + MV C
2 Cos (ῳC +ῳm)t + mV c
2 Cos (ῳC - ῳm)t………………………..vi
m is the modulation depth which is given by Vm
Vc
The frequency spectrum can be shown in the following figure;

AM AND FM EXPERIMENT 13
Fig 8: Showing the frequency spectrum of AM signal.
After the modulation of the AM, its time domain signal can be shown below;
Fig 9: Time domain for AM signal.
Applications of AM
Quadrature amplitude modulation:
This modulation is employed for the conveying data in the whole thing from short-range wireless links
like Wi-Fi to cellular telecommunications. Successfully it is made by having two carriers 90° out of
phase.
Air band radio:

AM AND FM EXPERIMENT 14
Very high-frequency transmissions for most airborne applications still use AM. It is basically used for
two-way radio links for ground staff and ground to air radio communications.
Broadcast transmissions:
This modulation is usually employed for broadcasting on the SW, MW and LW wave bands. It is simple to
demodulate and this infers that radio receivers are able to demodulate amplitude modulation as they
are cheap and simple to manufacture.
Advantages of AM modulation
1. AM receivers are very cheap as no specialized components are needed as compared to FM
2. AM is so simple to implement as compared to FM
3. AM can be demodulated using a circuit consisting of very few components
Equipment
1. Spectrum Analyser- Rhode & Schwarz HMS-X
2. Oscilloscope- Keysight Infiniivision DSO- X 2002A, 70 MHz, 2GGSa/sec, Digital
Storage Oscilloscope (DSO).
3. Waveform Generator – Keysight 33500B.
Procedure

AM AND FM EXPERIMENT 15
The waveform generator was connected to the oscilloscope, then the carrier frequency on
the generator was put to 500 kHz with the modulating frequency set to 50 kHz. The result of the
generator was attuned to offer the modulation depth of 30%, 40%, and 50%. The result of the
signal generator was attuned as well so that the output voltage was not more than 0.5 V peak- to-
peak. This helped the spectrum analyzer from being damaged. The spectrum analyzer is
damaged if relatively higher voltages are used. The time domain and frequency domain was
saved in a USB memory stick then the modulated depth was calculated from the DSO and then
compared with that got from the spectrum analyzer. The frequency obtained from the analyzer
was checked if it matches with was expected. The frequency of the carrier was varied and the
observation was made. The frequency of the modulated signal was as well varied and it was
confirmed that the sidebands move further apart but closer together.
Results

AM AND FM EXPERIMENT 16
Fig 3: AM modulation with the carrier and low/higher sidebands from Spectrum analyzer.
Fig 4: Showing AM modulation from an oscilloscope.
When the modulation index is 30% or 0.3, m is given by ;
m= Emax−Emin
Emax+ Emin
m= 102.1−57.25
102.1+ 57.25
m= 0.28
When the modulation index is 30% or 0.3, m is given by ;
m= Emax−Emin
Emax+ Emin

AM AND FM EXPERIMENT 17
m= 109.5−48
109.5+48
m= 0.39
When the modulation index is 30% or 0.3, m is given by ;
m= Emax−Emin
Emax+ Emin
m= 117.25−40.25
117.25+40.25
m= 0.48
Questions
1. What will happen to the number of sidebands if you reduce the amplitude of the modulating
signal so that m is less than 0.3?
When the amplitude of the modulating signal is reduced to m is less than 0.3, there will be only
one sideband of the signal. This is true due to the fact that the modulation depth m is so low
that will give one sideband.
2. What happens to the spectrum if you increase the amplitude of the modulating signal so that
m is very large.
If the amplitude of the modulating signal increased, the signal will make the J0m of the
spectrum analyzer will reduce and it can be even invisible if it is reduced further.
Discussion

AM AND FM EXPERIMENT 18
At the end of the experiment, the frequency obtained was plotted in the analyzer which
was crossed checked against the theoretical values, the values were checked if they match. The
result of the frequencies of the Am and FM modulation matched the expected ones. This is
evident by the waveforms in the spectrum and the calculation made for the power ratios for each
modulation depth which were cross-checked against the theoretical values.The difference
between the measured values and the theoretical values were very minimum. This difference was
due to errors encountered during the measuring process. These errors can be caused by the
inappropriate voltage fed into the spectrum which could affect the normal operation of the
analyzer. Even though FM has a larger bandwidth it is still employed since it has a better signal
to noise ratio that is it is perfect in reducing the noise signal as compared to AM.
Conclusion
After the analysis, the results found perfectly match the theory of both AM and FM, and
the theory shown were very correct as it can be seen that the FM has a larger bandwidth as
compared to AM. And with that, it was possible to compare the amplitude modulation and
Frequency modulation for both the frequency domain and time domain. Advantages of the FM
are; For AM several conveyed power is not important but in FM all conveyed power is
important, it is probably to reduce the noise in FM by increasing the deviation which is not
possible in AM, because the amplitude in FM is constant this makes it autonomous of the
modulation depth but in AM m guides the transmitted power.