Atten AT5010 Operating Instructions Manual - Page 15
Browse online or download pdf Operating Instructions Manual for Measuring Instruments Atten AT5010. Atten AT5010 20 pages. Spectrum analyzers
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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