Tektronix fundamentals of analysis schematic

TCZL/

I

LlX

SPECTRUM

ANALYZERS

26W-5360

FUNDAMENTALS OF

SPECTRUM ANALYSIS

Tektronix

COMMITTED TO

EXCELLENCE

CONTENTS:
Preface

Introduction

Primary Controls

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Nature of Measurement

Types of Measurement

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Amplitude

Scale Factor (Vertical Display)
Reference Level

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Typical Spectrum Analyzer Controls Photo

Frequency

Frequency Control
Span Control
Resolution Bandwidth

Secondary Controls

Sweep
Video Filter

Digital Storage
Frequency Range
Phase Lock
Preselector

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Time...............................................

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Applications

Amplitude Modulation

Harmonic Distortion
Intermodulation Distortion

Tracking Generator

Pulsed
Noise

Antenna Sweeps

Glossary

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RF(Radar)

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Measurements........................................

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Fundamentals of Spectrum

Analysis

was written by

Engineering Operations Manager, Frequency Domain

Bill

Benedict,

Instruments,

Tektronix, Inc.

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right © 1983, Tektronix, Inc. All rights reserved.

Preface

This handbook explains the funda­mentals, or basics, of spectrum
analysis. It describes the essential

controls and how to use them, how

to make elementary measurements,

and how to interpret the display.

There are other articles available

from Tektronix, Inc. and others, that
describe in more detail the opera­tion of an analyzer and the interpre­tation of the display. After reading
this handbook, an individual familiar
with basic electronics and primary
electronic communication theory will
be able to make basic measure-

ments with an analyzer.

For the best results, use an analyzer

when reading the text, especially the
section on Primary Controls. The

material will be much more mean-

ingful. Trying to duplicate the photos

is the most effective way to under­stand the function of each control.

A multi-function signal generator will

provide most of the signals used in
the photos. Gaining the basic know-

ledge of how to use a Spectrum

Analyzer will make it easier to
switch from one model analyzer to

another.

This text will not discuss all the con­trols of an analyzer as many of them

are for special functions and will vary

between analyzers and manufac-

turers. The operator's manual for a

particular analyzer should be con­sulted regarding the exact opera-

tion of all controls.

electrical signal during the mea-

surement interval with respect to

time. Likewise, a Spectrum Analyzer

permits observation of the amplitudes

and frequencies of the various dis-

crete sinusoidal signals during the

measurement interval. In both cases,
the results are displayed on a cath­ode-ray tube (crt) with the vertical
axis being the amplitude scale and
the horizontal axis being the time

scale for an oscilloscope or the fre-

quency axis for a Spectrum Analyzer.

Figures
waveforms as displayed on both an

oscilloscope and a Spectrum Ana-

lyzer.

1,

2, and 3 show various

In the first example, a sine wave is
displayed. The oscilloscope displays

the

peak-to-peak

voltage (vertical axis)

of the signal with respect to time

(horizontal axis). The Spectrum An-

alyzer shows the same sine wave

Oscilloscope Waveform: 3 MHz Sine Wave

Figure 1.

where the positive peak (vertical axis)

indicates the amplitude of signal and

the single signal (horizontal axis) indi­cates there is only one frequency or
sine wave present. [You will note the
presence of a zero hertz marker. It is
present due to system design of a
Spectrum Analyzer and is present
regardless of the input signal. All
signals to the left of the zero hertz
marker are not negative frequencies
as one might think; they are images
or reflections of those signals to the
right of the zero hertz

The second example (Fig. 2) is a

modulated carrier where both the
modulation frequency and carrier

frequency can be determined. The

Spectrum Analyzer indicates the
carrier as the larger signal and the

modulation as the two smaller sig-

nals (upper and lower sidebands).

Spectrum Analyzer Waveform: 3 MHz

Sine Wave

marker].

Introduction

Nature of Measurement

All electrical waveforms or signals

are composed of a combination of

sinusoidal signals of varying ampli­tudes and frequencies. The combi­nation of sine waves can be observed
in the time domain with an oscillo­scope, or in the frequency domain
with a Spectrum Analyzer. The os­cilloscope enables observation of

the amplitude and shape of an

Oscilloscope Waveform: Modulated Carrier
at 1 MHz, 15 kHz Modulation

Figure 2.

Spectrum Analyzer Waveform: Modulated

Carrier at 1 MHz,

15

kHz Modulation

The third example (Fig. 3) shows the
signal appearing on the oscilloscope
as a square wave. The Spectrum
Analyzer displays a "fundamental"

sine wave at the same frequency as

the square wave and the other fre­quencies of diminishing amplitude

(as the frequency increases) that

make up a square wave. These
other frequencies are identified as
the

3rd,

5th,

7th,

etc.

(odd)

har-

monics of the fundamental fre-

quency.

Types of Measurements

Composite voltage waveforms are

displayed by an oscilloscope. The

Spectrum Analyzer, as the name

implies, analyzes the composite

form and displays the individual fre-

quency components and the relative
power each component contributes

to the total waveform.

Since the Spectrum Analyzer has
this characteristic, it is well suited
for work that involves oscillators, RF
carriers, RF spectrum surveillance,
etc. With an analyzer, it is possible
to observe:

wave-

• an oscillator

• RF carrier

• amount and frequency of modu­lation

• unexpected modulation

• carrier suppression in single

sideband radio

• harmonic level of oscillators and

RF carriers

With a sweeping oscillator or "Track­ing Generator", filter response,
amplifier frequency response, and

antenna standing wave ratio (SWR)
can all be checked, along with other

measurements described in the Ap-

plications section dealing with the

Tracking Generator.

Primary Controls

(Refer to front panel photo on pages

4 and 5 for typical Spectrum

Analyzer controls).

Oscilloscope Waveform: 100 kHz Square
Wave

Amplitude

The Spectrum Analyzer has two

major amplitude controls. The first
controls the scale factor

dB/div) and the second determines

what input signal amplitude is nec­essary to produce a signal display

up to the top line on the crt, which

is called the Reference Level.

Scale Factor (Vertical Display)

Most oscilloscope graticules are

divided vertically into eight major

divisions. Each major division is fur­ther divided into five minor divisions.
Thus, a signal of one minor division

in amplitude can be accurately

measured and another signal of

eight divisions in amplitude can be

measured and compared to deter­mine the larger one as being

8

div (5 minor div/div)

1 minor division

= 40 times greater than

the smaller signal.

To determine this ratio in dB, use

= 20 log ^ = 32dB.

Since many Spectrum Analyzers are

capable of displaying ratios of 80 dB
on screen, either a different scale
factor is required or a crt display
with 2,000 major vertical divisions is
required! The obvious solution is to

use a logarithmic scale of 10 dB/div

(volts/div

or

Figure 3.

Spectrum Analyzer Waveform: 100 kHz

Square Wave

with the standard eight division
screen to display 80 dB of range.

As an example, with 80 dB of on-

screen range, two signals can be

measured simultaneously; one of 1 W

( + 30 dBm) and the other of 0.01

(-50

dBm). That is a voltage ratio

of

10,000:1 , far greater than the

/WV

40:1

ratio possible with the oscilloscope.
Before going further, note the basic

equations that can be used to con-

vert to dB,

dBm,

dBV, and

dBmV.

Once you begin to use the Spectrum

Analyzer, you will find that most mea-

surements will be in dB or dBm and

no conversion will be necessary. It is

not important that you conquer these
equations before going further.

Signal ratios are expressed in dB:

or1 0 log

Power into a known load (50, 75,

600 ohms, etc.) is expressed in:

dBm

* (at specified impedance)

dBV = 20

dBmV = 20log

* (volts are

The obvious problem with having a

RMS

*

Power (1)

= 10 log

log

volts)

Power*

1

mW

1 V

vm

1 mV

scale factor that allows such a large
range of signals on screen simulta­neously is that two signals appear-

ing close in amplitude may in reality
vary significantly in amplitude. As an

example, assume there is one signal

of 1

mW

and another signal of 2

power. Using the equations, it is

mW

apparent they are

+ 30 dBm MAX

STEP

ATTENUATOR

OPTIMUM INPUT

LEVEL

-30

( + 13 dBm

dBm

MAX)

10 log

1 mW

=10log2 =

apart in amplitude, or 1.5 minor divi-

sions with a scale factor of 10 dB/div.

To allow accurate measurements of
signals of close amplitudes, an ana-

lyzer typically has a Display Mode of

2 dB/div where, as in the previous
example of two signals being 3 dB
apart

(2X),

the display would indicate

1.5 major divisions of separation. A

third common display mode is linear

scale factor, where the

RMS

value

of the signal is displayed with a cali-

bration of

volts/div.

Reference Level

The Reference Level is one of the

three main controls of a Spectrum
Analyzer. The purpose of this control
is to obtain an adequate display of
signal amplitude on screen. This

control sets the level of the signal

necessary to produce a full-screen
deflection (i.e., the top of the screen
is the Reference Line). Thus, if the
Reference Level control is set for 0
dBm with a Vertical Display of

10

dB/div, a 0 dBm signal would rise

to the top

-20

crt

graticule marking. A

dBm signal would be 2 divi-

sions down from the top [0 dBm

- 2 div

(1 0 dB/div) = - 20

dBm].

Some analyzers separate the Refer-

ence Level control into two individ-

ual controls. Together they represent
the Reference Level, but separately
each controls an individual section

of the analyzer. The two independent
sections of the analyzer are the RF

Attenuator control and the IF Gain
control.

The RF Attenuator control selects

the amount of RF attenuation the

signal experiences just after it enters
the analyzer. For optimum analyzer
performance, the input signal must
be attenuated to a level specified by

CONTROL

Figure 4. Spectrum Analyzer Input Indicating Point of Optimum Input Level.

the manufacturer for the

(optimum input level, see Fig. 4). For

example, Tektronix 490 Series Spec­trum Analyzers have an optimum
signal level for the
dBm. Therefore, if the signal being

measured with the analyzer is

dBm in level, the RF attenuator
should be set for 20 dB of attenua-

tion. The first mixer would then see:

-10

dBm (input)

tion)

= -30 dBm

-20

(1st

The IF Gain control selects the pro­per amount of gain within an ampli­fier stage to keep the instrument
within amplitude calibration. This

control does not have any restric­tions for proper operation.

Some analyzers, like the Tektronix
490 Series, contain a microprocessor

that selects the proper ratio of RF at­tenuation and IF gain, depending on
the Reference Level selected. This

eases operator responsibility, be-

cause the operator is only required
to keep the signal at or below the
top graticule line by selecting an

appropriate Reference Level.

All analyzers have a maximum input

level that must be observed. Typically,

this level is

+30

dBm (1

tremely important to observe this

limit, because extensive and ex-

pensive damage may occur to the

input circuitry. Usually, both the RF

attenuator and the 1 st mixer have a

maximum input level, and quite often

they are not the same level. The RF

attenuator can handle a significantly

larger signal level than the

1st

mixer

1st

mixer of -30

dB (attenua-

mixer level).

W).

It is ex-

1st

-10

mixer

without damage. Therefore, if you
are unsure of the level of the input
signal, select the largest RF atten­uation available. Once the signal is
displayed on the screen, the atten-

uation can be removed one step at

a time to bring the largest signal to

the top of the screen. Typically, if

the input is less than 0 dBm, the an­alyzer will not be damaged regard­less of how the Reference Level

controls are set.

Most RF power meters indicate the
total amount of power available at
the head of the power meter from

all signals present on the cable.
Thus, if there are many discrete

sinusoidal signals present on the

cable, the amplitude of any one

signal cannot be determined with
the power meter. The Spectrum Ana­lyzer allows each signal to be viewed
separately for both amplitude and
frequency. However, the input (at-

tenuator and

1st

mixer) circuitry is

like the power meter in that it is ex­posed to all signals present. There­fore, the rules regarding maximum
input level apply to the sum of all
signals present on the input, re-

gardless of whether they are all be-

ing displayed on the screen or not.

As an example, if two signals of

dBm and one of -50 dBm are
sent on a cable, the input circuitry

is actually being exposed to over

+ 23 dBm. Remember (from a pre-

vious example), if you double the

power, you have a signal level 3 dB

higher

[(+

20 dbm) +

( + 23

dBm)].

With over

(+

20 dBm)

+23

+20

pre-

>

dBm on

TRIGGERING:

SELECTS MODE OF

TRIGGERING SWEEP

VERTICAL DISPLAY:

SCALE FACTOR FOR

VERTICAL DEFLECTION (i.e.

Y-AXIS SCALE FACTOR)

F
D

V\

0

TIME/DIV:

SELECTS RATE AT WHICH

FREQUENCY SPECTRUM IS

ANALYZED

PROVIDES ACCURATE

SIGNAL FOR AMPLITUDE

AND FREQUENCY

CALIBRATOR:

CALIBRATION

REFERENCE LEVEL:

DEFINES AMPLITUDE (LEVEL)

OF SIGNAL NECESSARY FOR

FULL SCREEN DEFLECTION

FREQUENCY:

DEFINES FREQUENCY

WHICH REFERENCE DOT
ON SCREEN REPRESENTS

FREQUENCY RANGE:

SELECTS FREQUENCY BAND

OF ANALYSIS (INDICATED

ON LOWER SCREEN

READOUT)

FREQUENCY

SPAN/DIV:

CONTROLS MAGNITUDE OF

FREQUENCY SPECTRUM BEING

ANALYZED (i.e., X-AXIS

SCALE FACTOR)

RESOLUTION BANDWIDTH:

DEFINES ABILITY OF

ANALYZER TO IDENTIFY
ADJACENT SIGNALS

DIGITAL STORAGE:

SELECTS MODES OF

ACQUIRING AND DISPLAYING
SIGNALS FOR PRESENTATION

ON SCREEN