Atten AT5000 Manual de Instruções - Página 15

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Introduction to

Spectrum Analysis

The analysis of electrical signals is a fundamental

problem for many engineers and scientists. Even if

the immediate problem is not electrical, the basic

parameters of interest are often changed into

electrical signals by means of transducers. The

rewards for transforming physical parameters to

electrical signals are great, as many instruments are

available for the analysis of electrical signals in the

time and frequency domains.

The traditional way of observing electrical signals is

to view them in the time domain using an

oscilloscope. The time domain is used to recover

relative timing and phase information which is

needed to characterize electric circuit behavior.

However, not all circuits can be uniquely

characterzed from just time domain information.

Circuit elements such as amplifiers, oscillators,

mixers, modulators, detectors and filters are best

characterized by their frequency response

information. This frequency informat5ion is best

obtained by viewing electrical signals in the

frequency domain. To display the frequency

domain requires a device that can discriminate

between frequencies while measuring the power

level at each. One instrument which displays the

frequency domain is the spectrum analyzer. It

graphically displays voltage or power as a function

of frequency only on a CRT (cathode ray tube).

In the time domain, frequency components of a

signal are seen summed together. In the frequency

domain, complex signals (i.e. signals composed of

more than one frequency) are separated into their

frequency components, and the power level at each

frequency is displayed. The frequency domain is a

graphical representation of signal amplitude as a

function of frequency. The frequency domain

contains information not found in the time domain

and therefore, the spectrum analyzer has certain

advantages compared with an oscilloscope.

The analyzer is more sensitive to low level

distortion than a scope. Sine waves may look in the

time domain, but in the frequency domain, harmonic

distortion can be seen. The sensitivity and wide

dynamic range of the spectrum analyzer is useful

for measuring low-level modulation. It can be used

to measure AM, FM and pulsed RF. The analyzer

can be used to measure carrier frequency,

modulation frequency, modulation level, and

modulation distortion. Frequency con-version

devices can be easily characterized. Such

parameters as conversion loss, isolation, and

distortion are readily determined from the display.

The spectrum analyzer can be used to measure long

and short term stability. Parameters such as noise

sidebands on an oscillator, residual FM of a source

and frequency drift during warm-up can be

measured using the spectrum analyzer's calibrated

scans. The swept frequency responses of a filter or

amplifier are examples of swept frequency

measurements possible with a spectrum analyzer.

These measurements are simplified by using a

tracking generator.

Types of Spectrum Analyzers

There are two basic types of spectrum analyzers,

swept-tuned and real-time analyzers. The swept-

tuned analyzers are tuned by electrically sweeping

them over their frequency range. Therefore, the

frequency components of a spectrum are sampled

sequentially in time. This enables periodic and

random signals to be displayed, but makes it

impossible to display transient responses. Real-time

analyzers, on the other hand, simultaneously display

the amplitude of all signals in the frequency range

of the analyzer; hence the name real-time. This

preserves the time dependency between signals

which permits phase information to be displayed.

Real-time analyzers are capable of displaying

transient responses as well as periodic and random

signals.

The swept-tuned analyzers of the trf ( tuned radio

frequency) or superheterodyne type. A trf analyzer

consists of a bandpass filter whose center frequency

is tunable over a desired frequency range, a detector

to produce vertical deflection on a CRT, and a

horizontal scan generator used to synchronize the

tuned frequency to the CRT horizontal deflection. It

is a simple, inexpensive analyzer with wide

frequency coverage, but lacks resolution and

sensitivity. Because trf analyzers have a swept filter

they are limited in sweep width depending on the

frequency range (usually one decade or less). The

resolution is determined by the filter bandwidth, and

since tunable filters don't usually have constant

bandwith, is dependent on frequency.

The most common type of spectrum analyzer differs

from the trf spectrum analyzers in that the spectrum

is swept through a fixed bandpass filter instead of

sweeping the filter through the spectrum. The

analyzer is swept through a narrowband receiver

which is electronically tuned in frequency by

applying a saw-tooth voltage to the frequency

control element of a voltage tuned local oscillator.

This same saw-tooth voltage is simultaneously

applied to the horizontal deflection plates of the

CRT. The output from the receiver is synchronously

applied to the vertical deflection plates of the CRT

and a plot of amplitude versus frequency is

displayed.

The analyzer is tuned through its frequency range

by varying the voltage on the LO (local oscillator).

The LO frequency is mixed with the input signal to

produce an IF (intermediate frequency) which can

be detected and displayed. and displayed. When the

frequency difference between the input signal and

the LO frequency is equal to the IF frequency, then

there is a response on the analyzer. The advantages

of the superheterodyne technique are considerable.

It obtains high sensitivity through the use of IF

amplifiers, and many decades in frequency can be

tuned.

Also, the resolution can be varied by changing the

bandwidth of the IF filters. However, the

superheterodyne analyzer is not real-time and sweep

rates must be consistent with the IF filter time

constant. A peak at the left edge of the CRT is

sometimes called the "zero frequency indicator" or

"local oscillator feedthrough". It occuts when the

analyzer is tuned to zero frequency, and the local

oscillator passes directly through IF creating a peak

on the CRT even when no input signal is present.

(For zero frequency tuning, FLO=FIF). This

effectively limits the lower tuning limit.

Spectrum Analyzer Requirements

To accurately display the frequency and amplitude

of a signal on a spectrum analyzer, the analyzer

itself must be properly calibrated. A spectrum

analyzer properly designed for accurate frequency

and amplitude measurements has to satisfy many

requirements:

Wide tuning range

Wide frequency display range

Stability

Resolution

Flat frequency response

High sensitivity

Low internal distortion

Frequency Measurements

The frequency scale can be scanned in three

different modes full, per division, and zero scan The

full scan mode is used to locate signals because the

widest frequency ranges are displayed in this mode.

(Not all spectrum analyzers offer this mode). The

per division mode is used to zoom-in on a particular

signal. In per division, the center frequency of the

display is set by the Tuning control and the scale

factor is set by the Frequency Span or Scan Width

control. In the zero scan mode, the analyzer acts as

a fixed-tuned receiver with selectable bandwidths.

Absolute frequency measurements are usually made

from the spectrum analyzer tuning dial. Relative

frequency measurements require a linear frequency

scan. By measuring the relative separation of two

signals on the display, the display, the frequency

difference can be determined.

It is important that the spectrum analyzer be more

stable than the signals being measured. The stability

of the analyzer depends on the frequency stability

of its local oscillators. Stability is usually

characterized as either short term or long term.

Residual FM is a measure of the short term stability

which is usually specified in Hz peak-to-peak. Short

term stability is also characterized by noise

sidebands which are a measure of the analyzers

spectral purity. Noise sidebands are specified in

terms of dB down and Hz away from a carrier in a

specific bandwidth. Long term stability is