B+K precision 2165A, Instruction Manual, Page: 17 / 28

17
Introduction to Spectrum Analysis
General
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 informa-
tion which is needed to characterize electric circuit behavior.
However, not all circuits can be uniquely characterized 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 information 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 on a CRT
(cathode ray tube).
In the time domain, all 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 good 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
conversion 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 analyzers
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 are usually 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 de-
flection. 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 do not usually have
constant bandwidth, 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 basically 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).