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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 ona CRT, anda
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 dont 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 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}. The LO
frequency is mixed with the input signal to produce an IF
(intermediate frequency) which
can be detected
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 occurs 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, F, ,=F,,). 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
. Linear and Logarithmic display modes (voltage and dB)
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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 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
Subject to change without notice
M7 5005/5006
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