University of Tripoli

Faculty of Engineering

Electrical and Electronic Engineering Department

EE421 Electronic Laboratory

Spectrum Analyzer

Name : Motaz Mohamed zitoun

Student Number : 022162013

Experiment Instructor : Dr. Mustafa Alhamdi

Date of experiment 24-2-2022

1|Page
Objective :-
1)To be familiar with spectrum analyzer working.

2)To investigate a spectrum of some signals and their distortion (harmonics).

3)Display the frequency and power of radio channel on spectrum analyzer .

Abstract:
In this experiment we are going to investigate working of the spectrum analyzer, A the
spectrum analyzer measures the magnitude of an input signal versus frequency within the
full frequency range of the instrument. The primary use is to measure the power of the
spectrum of known and unknown signals. The input signal that a spectrum analyzer
measures is electrical; however, spectral compositions of other signals, such as acoustic
pressure waves and optical light waves, can be considered through the use of an
appropriate transducer. Optical spectrum analyzers also exist, which use direct optical
techniques such as a monochromator to make measurements, In this report, I’m going to
illustrate experiment theory, our experiment procedure, my notes and conclusion.

Introduction:
spectrum analyzer:
A spectrum analyzer is a device that displays signal amplitude (strength) as it varies by
signal frequency. The frequency appears on the horizontal axis, and the amplitude is
displayed on the vertical axis. To the casual observer, a spectrum analyzer looks like an
oscilloscope, and in fact, some devices can function either as oscilloscopes or spectrum
analyzers.
The electronics industry uses spectrum analyzers to examine the frequency spectrum of
radio frequency (RF) and audio signals. These devices display the individual elements of
these signals, as well as the performance of the circuits producing them. Through the use of
spectrum analyzers, organizations can determine what modifications may be needed to
reduce interference and thus improve the performance of Wi-Fi systems and wireless
routers.
Spectrum Analyzer is a measuring instrument that displays an electrical signal according to
its frequency, each frequency component contained in the input signal is displayed as a
signal level corresponding to that frequency.
When testing radio frequency circuits and systems in particular, it is useful to be able to see
signals in the frequency domain, i.e. signal amplitudes that appear at different frequencies.

2|Page
By looking at the amplitudes of signals at different frequencies it is possible to measure the
amplitudes of these signals, find what signals are present and the like.

Figure 1 (Spectrum analyzer displays signals in the frequency domain)

In this way it is possible to measure the frequencies of signals, and also check their levels.
With many modern-day signals occupying wide bandwidths, it is possible to measure the
signal bandwidths.
In this way, the spectrum analyzer is a particularly important item of test equipment for
anyone undertaking the test and measurement of circuits and systems involving radio
frequency or RF signals. In addition to this, spectrum analyzers may also be used for a
variety of other applications including audio analysis and the like.

How to calculating Total Harmonic Distortion

THD is defined as the ratio of the equivalent root mean square (RMS) voltage of all the
harmonic frequencies (from the 2nd harmonic on) over the RMS voltage of the
fundamental frequency (the fundamental frequency is the main frequency of the signal, i.e.,
the frequency that you would identify if examining the signal with an oscilloscope).
Equation 1 shows the mathematical definition of THD (note that voltage is used in this
equation, but current could be used instead):

3|Page
How Spectrum Analyzer work:
Most spectrum analyzers offer users the opportunity to set a start, stop and center
frequency. The center frequency is halfway between the stop and start frequencies and is
also the axis for the frequency used to determine the span -- the range between the start
and stop frequencies. With an RF spectrum analyzer, the analyzer measures the radio
"noise floor" and measures how close two signals can be while still being resolved into two
separate peaks.

Uses for spectrum analyzers:

A spectrum analyzer can be used to determine whether or not a wireless transmitter is
working according to federally defined standards for purity of emissions. Output signals at
frequencies other than the intended communications frequency appear as vertical lines
(pips) on the display. A spectrum analyzer can also be used to determine, by direct
observation, the bandwidth of a digital or analog signal.

Real-time:
Real-time analyzers collect real-time bandwidth and sample the incoming RF spectrum in a
limited span of time, converting the information using the fast Fourier transform (FFT)
algorithm. Because it's real-time data collection, there is no "blind time," and there are no
gaps in the calculated RF spectrum.

Audio:
Spectrum analyzers can also be used in the audio spectrum, displaying volume levels of
frequency bands audible to humans. This method is aimed at analyzing the harmonics of an
audio signal. Once known as wave analyzers, these types of spectrum analyzers are widely
used by sound engineers and can run on almost any computer equipped with a sound card.

Advantages and disadvantages:

Swept-tuned spectrum analyzers face tradeoffs between how rapidly the display can
update a full frequency span and the resolution. Sometimes, if engineers are working with a
very weak signal, a preamplifier is needed before analysis can begin. On the other hand,
FFT analyzers can strain the capabilities of analog-to-digital converters and require
significant processing power, limiting the possible frequency range that can be analyzed.
Real-time FFT analyzers can offer good resolution while reducing potential gaps in
sampling.

4|Page
Equipment:
 Spectrum Analyzer.
 Function generator.
 Dual trace oscilloscope Inc. probes.
 Coaxial wires.
 Adapters to interface between the coaxial wires & the spectrum analyzer.

Procedures and Results:

First, we should power on the spectrum analyzer and wait for it to warm up, we set the
span at 3MHz by pressing start then 0 then start again then stop then 3 then stop again, or
we can press span button and set it to 3MHz and then adjust the center to 1.5MHz, we
repeated this procedure for 10MHz and 100MHz without inputting any signal from the
signal generator.

Note that the smaller span the more details we can see in the spectrum, so if accuracy is
our priority then we should make the span as small as possible.

Fig (2) - The span of the spectrum analyser at 3 MHz

5|Page
 Fig (3) - The span of the spectrum analyser at 10 MHz

Fig (4) - The span of the spectrum analyser at 100 MHz

6|Page
Then we make sure that the input signal is less than 1 Vpp so we protect the
spectrum analyzer, input signal is shown in figure 5:

Fig(5) -The Input Signal

Using Oscilloscope, we found that the input frequency is 1.03MHz.

Then we apply it to a spectrum with a span of 1.5MHz, as shown below:

Fig( 6)- Spectrum Analyzer with 1.5MHz Span

7|Page
The input frequency is 1.037MHz as shown in figure 6 above.
It can be seen that there is an error between Oscilloscope and Spectrum Analyzer which is
0.68% which is acceptable.
Power= -10.08 dbm
(Note: dbm is the gain of the measured power relative to 1mw).

When we extend the span to 15MHz we see a lot of harmonics (n*1MHz) then the
frequency and power of the first ten harmonics was measured and recorded in
Table1.
Harmonic (n) f [MHz] N [dBm]
1 1.009 8
2 2.019 -16
3 3.020 -31.6
4 4.027 -37.6
5 5.036 -45.2
6 6.040 -48
7 7.050 -50
8 8.050 -51.6
9 9.058 -52.4
10 10.062 -52.8
Table 1: The harmonic frequency and their power component for sin wave signal.

We can measure the distortion by subtracting the biggest harmonic power from the
fundamental frequency power, we can see the distortion
which is kind of acceptable (the acceptable range is 40dbm and above).

The power difference between the fundamental harmonic and the second is:

N 1−N 2 =8−(−16)=24 dB

The power difference between the fundamental harmonic and the third is:

N 1−N 3 =8−(−31.6)=39.6 dB

8|Page
Then we changed the signal type to square wave with a period T = 1μs (ƒ = 1MHz) as shown in
figure 7 and the measured values stored in Table 2.

Harmonic (n) f [MHz] N [dBm]

1 1.050 7.6
2 2.010 -9.6
3 3.010 -7.6
4 4.015 -14
5 5.020 -20
6 6.035 -19.6
7 7.035 -33.2
8 8.045 -24.4
9 9.040 -45
10 10.040 -30
Table 2: The harmonic frequency and their power component for square wave signal

Figure (7) - The spectrum analyser’s display for the square wave signal.

9|Page
We notice that the odd harmonics are bigger than the even harmonics, and that’s because the
input signal is an odd signal so odd harmonics only should be presented, the even harmonics
are nothing but harmonic distortion.

Then we repeated the above procedures as shown:

The power difference between the fundamental harmonic and the second is:

N 1−N 2 =7.6−(−9.6)=17.2 dB

The power difference between the fundamental harmonic and the third is:

N 1−N 3 =−7.6−(−7.6)=15.2 dB

Then we change the input signal to a triangular wave with period of 1μs (ƒ = 1MHz)
the measured values stored in Table 3.

Harmonic (n) f [MHz] N [dBm]

1 1.050 -12.4
2 2.10 -52.4
3 3.010 -33.6
4 3.99 -60.4
5 4.99 -40
6 5.98 -62.8
7 6.98 -44.8
8 7.97 -62
9 8.97 -48
10 9.95 -64.8
Table 2: The harmonic frequency and their power component for the triangular wave signal.

The power difference between the fundamental harmonic and the second is:

N 1−N 2 =−12.4−(−52.4)=40 dB

The power difference between the fundamental harmonic and the third is:

N 1−N 3 =−12.4−(−33.6)=21.2 dB

10 | P a g e
Then we took the cable from the roof The spectrum analyser connected to the dipole
antenna. to measure the frequencies of TV Channels and we measured the 10 powers
as listed in Table4 below:
Harmonic (n) f [MHz] N [dBm]
1 88.812 -69.6
2 90.39 -47
3 92.449 -66.4
4 93.78 -71.6
5 96.9 -58
6 99.295 -42
7 100.29 -46.8
8 101.196 -69
9 102.13 -72
10 102.96 -70.4

Table 3: The ten highest power radio channels.

Figure (8) - The spectrum analyser’s display for the dipole antenna.

11 | P a g e
Now we change the signal to the Triangular wave and repeat the all pervious steps

The following pic when the span 1.5 MHz

12 | P a g e
When the span 15 MHz

Harmonic F(MHz) P(dBm)

First harmonic 1 -13.4
Second harmonic 3 -31.8
Third harmonic 5 -40.6
Fourth harmonic 7 -46.2
Fifth harmonic 9 -49
6th harmonic 11 -52.4
7th harmonic 13 -54.7
8th harmonic 15 -55.9
9th harmonic 17 -57.2
10th harmonic 19 -59.3

13 | P a g e
There’s a dipole antenna on the roof and we connected it to the spectrum analyzer , then
we measured the frequency and power of the best channel broadcasting that have "Highest
power in the spectrum analyser"

The harmonic in the spectrum analuzer is show in the following pic :

Frequency and power in the following table :

F(MHz) Name of radio P(dBm)

88.8 Alshabibia -75.8

91.1 -------- -82.6

93.4 Libya -75.8

93.8 Lebda -84.2

94.1 Karawan -81

95.5 Almadina -84.6

96.26 -------- -43.2

99.9 Al-Iman -78.6

101.69 University -31.8

106.56 --------- -66.1

14 | P a g e
Conclusion:
In this experiment we learned how to use spectrum analyzer, how to measure power of
signal of different frequencies, and how to measure distortion, Spectrum Analyzer is a
measuring instrument that displays an electrical signal according to its frequency, each
frequency component contained in the input signal is displayed as a signal level
corresponding to that frequency, most spectrum analyzers offer users the opportunity to
set a start, stop and center frequency. The center frequency is halfway between the stop
and start frequencies and is also the axis for the frequency used to determine the span --
the range between the start and stop frequencies. With an RF spectrum analyzer, the
analyzer measures the radio "noise floor" and measures how close two signals can be while
still being resolved into two separate peak, It was a useful and interesting experiment and
Each signal has the fundamental frequency and the other component is known as the
harmonics which is the multiple integer number of the fundamental frequency So, a purely
sinusoidal signal has no distortion while a square wave, which is periodic but does not look
sinusoidal at all, will have lots of harmonic distortion .

References:
• Wikipedia: https://en.wikipedia.org/wiki/Spectrum_analyzer#Theory_of_operation

• Search Networking: https://searchnetworking.techtarget.com/definition/spectrum-

analyzer

• Electronics Notes: https://www.electronics-notes.com/articles/test-methods/spectrum-

analyzer/spectrum-analyser-overview.php

15 | P a g e