Sepectrum Tgv

Spectrum analyzer & Measurement

What is a spectrum?

A spectrum is a collection of sine waves that, when combined

properly, produce the time-domain signal under examination. Figure 1

shows the waveform of a complex signal. suppose that we were hoping

to see a sine wave. Although the waveform certainly shows us that the

signal is not a pure sinusoid, it does not give us a definitive indication of

the reason why. Figure 2 shows our complex signal in both the time

and frequency domains. The frequency domain display plots the

amplitude versus the frequency of each sine wave in the spectrum. As

shown, the spectrum in this case comprises just two sine waves. We

now know why our original waveform was not a pure sine wave. It

contained a second sine wave, the second harmonic in this case. Does

this mean we have no need to perform time-domain measurements?

Not at all. The time domain is better for many measurements, and some

can be made only in the time domain. For example, pure time-domain

measurements include pulse rise and fall times, overshoot, and ringing.

Figure 1. Complex time-domain signal

INSTITUTE OF ENGINEERING TECHNOLOGY


Figure 2. Relationship between time and frequency domain

Why measure spectra?

The frequency domain also has its measurement strengths. We

have already seen in Figures 1 and 2 that the frequency domain is

better for determining the harmonic content of a signal. People involved

in wireless communications are extremely interested in out-of-band and

spurious emissions. For example, cellular radio systems must be

checked for harmonics of the carrier signal that might interfere with

other systems operating at the same frequencies as the harmonics.

Engineers and technicians are also very concerned about distortion of

the message modulated onto a carrier. Third-order inter modulation

(two tones of a complex signal modulating each other) can be

particularly troublesome because the distortion components can fall

within the band of interest and so will not be filtered away.

Spectrum monitoring is another important frequency-domain

measurement activity. Government regulatory agencies allocate

different frequencies for various radio services, such as broadcast

television and radio, mobile phone systems, police and

emergency communications, and a host of other applications.

It is critical that each of these services operates at the assigned

frequency and stays within the allocated channel bandwidth.



Transmitters and other intentional radiators can often be required to

operate at closely

spaced adjacent frequencies. A key performance measure for the power

amplifiers and other components used in these systems is the amount

of signal energy that spills over into adjacent channels and causes

interference.

Electromagnetic interference (EMI) is a term applied to unwanted

emissions from both intentional and unintentional radiators. Here, the

concern is that these unwanted emissions, either radiated or conducted

(through the power lines or other interconnecting wires), might impair

the operation of other systems. Almost anyone designing or

manufacturing electrical or electronic products must test for emission

levels versus frequency according to regulations set by various

government agencies or industry-standard bodies. Figures 3 through

6 illustrate some of these measurements.

Figure 3. Harmonic distortion test of a transmitter

Figure 4. GSM radio signal and spectral mask showing limits of unwanted emissions



Figure 5. Two-tone test on an RF power amplifier

Figure 6. Radiated emissions plotted against CISPR11 limits as part of an EMI test

What is a Spectrum Analyzer?

A spectrum analyzer is a wide band, very sensitive receiver. It

works on the principle of "super-heterodyne receiver" to

convert higher frequencies (normally ranging up to several 10s of

GHz) to measurable quantities. The received frequency spectrum



is slowly swept through a range of pre-selected frequencies, converting

the selected frequency to a measurable DC level (usually logarithmic

scale), and displaying the same on a CRT. The CRT displays received

signal strength (y-axis) against frequency ( x-axis).

Obviously, signals that are weaker than the background noise could not

be measured by a spectrum analyzer. For this reason, the noise floor of

a spectrum analyzer in combination with RBW is a vital parameter to be

considered when choosing a spectrum analyzer. The received signal

strength is normally measured in decibels (dbm). (Note that 0 dBm

corresponds to 1 mWatt of power on a logarithmic scale). The primary

reasons for measuring the power (in dBm) rather than voltage in

Spectrum Analyzers are the low received signal strength, and the

frequency range of measurement. Spectrum analyzers are capable of

measuring the frequency response of a device at power levels as low as

–120dBm. These power levels are encountered frequently in microwave

receivers, and spectrum analyzers are capable of measuring the device

characteristics at that power levels

The super heterodyne spectrum analyzer

Figure 7 is a simplified block diagram of a super heterodyne

spectrum analyzer. Heterodyne means to mix; that is, to translate

frequency. And super refers to super-audio frequencies, or frequencies

abov the audio range. Referring to the block diagram in Figure 2-1, we

see that an input signal passes through an attenuator, then through a

low-pass filter ( later we shall see why the filter is here) to a mixer,

where it mixes with a signal from the local oscillator ( LO) . Because the

mixer is a non-linear d vice, its output includes not only the two

original signals, but also their harmonics and the sums and

differences of the original frequencies and their harmonics.If any

of the mixed signals falls within the pass band of the

intermediate-frequency (IF) filter, it is further processed(amplified



and perhaps compressed on a logarithmic scale) . It is essentially

rectified by the envelope detector, digitized, and displayed. A ramp

generator creates the horizontal movement across the display from left

to right. The ramp also tunes the LO so that its frequency change is in

proportion to the ramp voltage.

Figure 7. simplified block diagram of a superheterodyne spectrum analyzer

Spectrum analyzer is an invaluable item of electronic test

equipment used in the design, test and maintenance of radio frequency

circuitry and equipment. Like an oscilloscope Spectrum analyzer is also

basic test equipment used for observing signals. However, where

oscilloscopes look at signals in the time domain, spectrum analyzers

look at signals in the frequency domain. Thus a spectrum analyzer will

display the amplitude of signals on the vertical scale, and the frequency

of the signals on the horizontal scale.

At IET I worked with the Advantest U3741 spectrum analyzer.

A figure of the analyzer is shown figure 8. I participated for field

strength measurements which were done using this spectrum

analyzer.

PRODUCT DESCRIPTION

Minimum Frequency 9 kHz



Maximum Frequency 3 GHz

Frequency span Range 5 kHz to Full, zero span

Resolution bandwidth Range: 100 Hz to 1 MHz (1 to 3 steps)

Measurement range: Displayed average noise level to

+30 dBm Displayed average noise

level to 134 dBµV

Sweep timeSetting range 20 ms to 1000 s (spectrum

mode)

50 µs to 1000 s (zero span)

RF input Connector: N-type female

Frequency reference input Connector BNC 50 Ω

Impedance 50 Ω (nominal)

VSWR Input attenuator ≥ 10 dB



Figure 8 Advantest U3741 spectrum analyzer

In view of the way in which a spectrum analyzer displays its

output, it is widely used for looking at the spectrum being generated by

a source. In this way the levels of spurious signals including harmonics,

inter modulation products, noise and other signals can be monitored to

discover whether they conform to their required levels. Additionally

spectrum analyzers can make measurements of the bandwidth of

modulated signals can be checked to discover whether they fall within

the required mask. Another application of a spectrum analyzer is in

checking and testing the response of filters and networks. By using a

tracking generator -a signal generator that tracks the instantaneous

frequency being monitored by the spectrum analyzer, it is possible to

see the loss at any given frequency. In this way the spectrum analyzer

makes a plot of the frequency response of the network.

The display

The purpose of a spectrum analyzer is to provide a plot or trace of

signal amplitude against frequency. The display has a graticule which

typically has ten major horizontal and ten major vertical divisions.

The horizontal axis of the analyzer is linearly calibrated in

frequency with the higher frequency being at the right hand side of the

display. The vertical axis is calibrated in amplitude. Although there is

normally the possibility of selecting a linear or logarithmic scale, for

most applications a logarithmic scale is chosen. This is because it

enables signals over a much wider range to be seen on the spectrum

analyzer. Typically a value of 10 dB per division is used. This scale is

normally calibrated in dBm (decibels relative 1 milliwatt) and therefore

it is possible to see absolute power levels as well as comparing

the difference in level between two signals. Similarly when using

a linear scale is used, this is often calibrated in volts to enable

absolute measurements to be made using the spectrum analyzer.



Setting the frequency

To set the frequency of a spectrum analyzer, there are two

selections that can be made. These are independent of each other. The

first selection is the centre frequency. As the name suggests, this sets

the frequency of the centre of the scale to the chosen value. It is

normally where the signal to be monitored would be located. In this way

the main signal and the regions either side can be monitored. The

second selection that can be made on the analyzer is the span, or the

extent of the region either side of the centre frequency that is to be

viewed or monitored. The span may be give as a given frequency per

division, or the total span that is seen on the calibrated part of the

screen, i.e. within the maximum extents of the calibrations on the

graticule. Another option that is often available is to set the start and

stop frequencies of the scan. This is another way of expressing the span

as the difference between the start and stop frequencies is equal to the

span.

Adjusting the gain

There are many other controls on a spectrum analyzer. Most of

these fall into one of two categories. The first is associated with the gain

or attenuation of sections within the spectrum analyzer. If sections are

overloaded, then spurious signals may be generated within the

instrument. If this occurs then false readings will be giving. To prevent

this happening it is necessary to ensure that the input stages in

particular are not overloaded and an RF attenuator is used. However if

too much attenuation is inserted, additional gain is required in the later

stages (IF gain) and the background noise level is increased and

this can sometimes mask lower level signals. Thus a careful

choice of the relevant gain levels within the pectrum analyzer is

needed to obtain the optimum performance.

Spectrum Analyzer Vs. Oscilloscope



a. A spectrum analyzer displays received signal strength (y-axis)

against frequency (x-axis). An Oscilloscope, displays received

signal strength (y-axis) against time (x-axis).

b. Spectrum analyzer is useful for analyzing the amplitude response

of a device against frequency. The amplitude is normally

measured in dBm in Spectrum Analyzers, where as the same is

measured in volts when using Oscilloscopes.

c. Normally, Oscilloscope can not measure very low voltage levels

(say, -100dBm) and are intended for low frequency, high

amplitude measurements. A spectrum analyzer can easily

measure very low amplitudes (as low as -120dBm), and high

frequencies (as high as 150GHz).

d. The spectrum analyzer measurements are in frequency domain,

whereas the oscilloscope measurements are in time domain.

e. Also, a spectrum analyzer uses complex circuitry compared with

an Oscilloscope. As a result of this, the cost of a spectrum

analyzer is usually quite high.

Key Features to Consider When Buying a Spectrum Analyzer:

Resolution bandwidth Frequency range Frequency stability AC/DC Operation Service warranty

Resolution bandwidth: This is an important parameter to consider

when buying a Spectrum Analyzer. The sensitivity of the spectrum

analyzer is directly dependent on the resolution bandwidth of the

analyzer. If your measurements are over a wide band, a 3 KHz RBW is

normally sufficient. If you need to make very narrow band

measurements (such as filters), then consider a 300Hz or even a

10Hz RBW spectrum analyzer. Obviously, a spectrum analyzer

with lower RBW costs more than a spectrum analyzer with 3 KHz

RBW.



Frequency range: This is the range of frequencies that you need to

make measurements. Spectrum analyzers are available from 100 Hz to

50 GHz range. If you require measurements up to, say IF to 2.4 GHz, a

spectrum analyzer from 10MHz-2.4 GHz would be suitable.

Frequency Stability: Frequency stability is the ability of the spectrum

analyzer to maintain the frequencies within a specified accuracy. The

frequency stability is dependent on the Local Oscillator stability of the

spectrum analyzer. For narrow band measurements, this is a very

important parameter. Spectrum analyzers do not normally have very

high stability clock. If high accuracy of measurement is required,

consider buying a spectrum analyzer with provision for external

frequency reference. In such an event, the accuracy of the spectrum

analyzer is as good as the external reference.

Input Power Range: This is the range of input power that could be fed

to the spectrum analyzer input connector. Normally, this ranges from -

100 dBm to +10 dBm. Beyond the lower limits, the spectrum analyzer

may not be able to identify the signal from back ground noise. If you

feed signals beyond the maximum specified range, it is possible that the

input mixer is saturated and the reading shown on the spectrum

analyzer may not represent the actual power levels accurately. There is

also a likelihood of damaging the front-end component of the spectrum

analyzer. Use an external attenuator if it is required to measure power

levels beyond the specified limits. Please note that spectrum analyzers

are available for various input signal power levels.

Harmonics: The frequency harmonics is a measure of accuracy of the

spectrum analyzer. Normally, the harmonics are greater than 30 dB

below the desired signal. The harmonics add to the measurement

uncertainty, and should be kept to the minimum.

AC/DC operation: If you need to make measurements out-

doors, you may require DC operation. Check if it is

available.Service warranty: Normally, spectrum analyzers are



very expensive. A comprehensive warranty is recommended when

buying a spectrum analyzer. Also ensure that the rf input connection

has dc protection.

Spectrum Analyzer Applications

Device Frequency Response Measurements: You can use spectrum analyzers for

measuring the amplitude response (typically measured in dbm) against frequency of

device. The unit of frequency is Hertz. 1000Hz=1KHz, 1000Kz=1MHz, 1000MHz=1GHz.

The device may be anything from a broadband amplifier to a narrow band filter.

Microware Tower Monitoring: You can measure the transmitted power and received

power of a Microware tower. Typically, you use a directional coupler to tap the power

without interrupting the communications. In this way, you can verify that the frequency

and signal strength of your transmitter are according to the specified values.

Interference Measurements: Any large RF installations normally require site survey. A

spectrum analyzer can be used to verify identify and interferences. Any such interfering

signals need to be minimized before going ahead with the site work. Interference can be

created by a number of different sources, such as telecom microwave towers, TV stations,

or airport guidance systems etc.

Other measurements that could be made using spectrum analyzer include the following:

Return-loss measurement

Satellite antenna alignment

Spurious signals measurement

Harmonic measurements

Inter-modulation measurements

Field strength measurement

Antenna gain measurement

Functions for evaluating frequency characteristics



The normalize function enables direct measurement of cable loss

and filter characteristics. The frequency offset function of the tracking

generator enables measurement of frequency characteristics and

conversion loss characteristics of mixers and other frequency

conversion devices.

Function for return loss measurement

The SWR bridge can be used to measure reflection characteristics of an

antenna or filter. It can determine the return loss and evaluate

the VSWR.



Electric field strength

Electric field strength is a quantitative expression of the intensity

of an electric field at a particular location. The standard unit is the volt

per meter (v/m or v · m -1 ). A field strength of 1 v/m represents a

potential difference of one volt between points separated by one meter.

Any electrically charged object produces an electric field. This

field has an effect on other charged objects in the vicinity. The

field strength at a particular distance from an object is directly

proportional to the electric charge, in coulomb s, on that object.

The field strength is inversely proportional to the distance from a

charged object. The field-strength-vs-distance curve is a direct



inverse function, and not an inverse-square function, because electric

field strength is specified in terms of a linear displacement (per meter)

rather than a surface area (per meter squared).

An alternative expression for the intensity of an electric field is electric

flux density . This refers to the number of lines of electric flux passing

orthogonally (at right angles) through a given surface area, usually one

meter squared (1 m 2 ). Electric flux density, like electric field strength,

is directly proportional to the charge on the object. But flux density

diminishes with distance according to the inverse-square law, because it

is specified in terms of a surface area (per meter squared) rather than a

linear displacement (per meter).

Sometimes the strength of an electromagnetic field ( EM field ) is

specified in terms of the intensity of its electric-field component. This is

done by engineers and scientists when talking about the radio-

frequency field strength at a certain location arising from sources such

as distant transmitters, celestial objects, high-tension utility lines,

computer displays, or microwave ovens. In this context, electric field

strength is usually specified in microvolt per meter (µV/m or µV · m -1 ),

nanovolts per meter (nV/m or nV · m -1 ), or Pico volts per meter (pV/m

or pV · m -1 ).



TRAINING EXPERIANCES

ANTENNA GAIN MEASUREMENT

This measurement is used for measure the gain of an antenna.

Before issuing an antenna to the market it has to be checked for

appropriate gain because gain is the most important feature of the

antenna.

THE EQUIPMENTS THAT WE USE TO MEASURE THE GAIN OF AN

ANTENNA

Spectrum analyzer

Signal Generator

Standard Dipole Antenna

Transmit Antenna

Co-Axial Cable

Cable Connectors

Anechoic Chamber

Tripods

PROCEDURE

The dipole lengths of the receiving and transmitting antennas

were adjusted according to the specimen. Above antennas were

connected to tripods and placed them in line of sight.

Then the transmitting antenna was connected to the signal

generator’s RF output and the receiving antenna was connected to

spectrum analyzer’s RF input using RG55 cables as shown in below

Figure-09.



SPECTRUM ANALYZER & ANECHOIC CHAMBER

SIGNAL GENERATOR

STANDERD ANTENNA RECEIVING

ANTENNA

An 800MHz RF signal was supplied to the transmitting antenna

from the signal generator while keeping the Reference signal strength

at ‘0’dB level. Then the strength

(dBm value) of the receiving signal (according to the above supplied

frequency) was measured using the spectrum analyzer. After that

the supplied frequency was increased by 2MHz steps until it

reached to 900MHz (800MHz-900MHz). All the receiving dBm

values were measured for every supplied frequency and note

down.


SPECTRUM ANALYZERSIGNAL GENERATOR

RF IN

RF OUT

TRANSMITTING ANTENNA RECEIVING ANTENNA


The above procedure was repeated to the specimen and the

antenna gain was calculated using two value sets. All the above

measurements were taken under room temperature.

OBSERVATIONS

At 810MHz supplied frequency, the receiving dBm values for,

# Slandered Dipole = -54dBm

# Specimen = -47dBm

SPECIMEN CALCULATION

The antenna gain = -47dBm-(-54dBm)

= 7dBm



Figure-01

ANTENNA GAIN REPORT

(800 MHz – 900 MHz)

Communication Division

Arthur C Clarke Institute for Modern Technologies,



Katubedda,

Moratuwa.

Test report for antenna

Used equipments :-

RF Signal Generator (Agilent)

Spectrum Analyzer (Agilent)

Standard Dipole Antenna

RF Cables

Anechoic Chamber

Frequency Range :- 800 MHz – 900 MHz

Environmental Conditions :- Ambient temperature = 30 C

Test carried out inside an anechoic chamber.

Gain was measured with respect to Standard Dipole Antenna.



Attachments :-

Frequency & Gain Table.

Frequency (MHz) Antenna Gain (dBi)

800 6

802 6

804 6

806 6

808 7

810 7

812 7

814 7

816 7

818 7

820 7

822 7

824 8

826 8

828 8

830 8

832 10

834 10

836 10

838 9

840 9

842 9



844 10

846 10

848 10

850 10

852 11

854 11

856 11

858 10

860 10

862 10

864 10

866 9

868 9

870 9

872 10

874 10

876 9

878 9

880 9

882 10

884 10

886 10

888 10

890 10

892 11


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