Atten AT6010, Руководство пользователя

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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
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 characterized by the frequency
drift of the analyzers Los. Frequency drift is a
measure of how much the frequency changes
during a specified time
(i.e., Hz/hr)
Frequency Measurements
Before the frequency of a signal can be
measured on a spectrum analyzer it must first
be re-solved. Resolving a signal means
distinguishing it from its nearest neighbors.
The resolution of a spectrum analyzer is
determined by its IF bandwidth. The IF
bandwidth is usually the 3dB bandwidth of the
IF filter. The ratio of the 60dB bandwidth
(in
Hz) to the 3dB bandwidth (in Hz) is known as
the shape factor of the filter. The smaller the
shape factor, the greater is the analyzer's
capability to resolve closely spaced signals of
unequal amplitude. If the shape factor of a
filter is 15:1, then two signals whose
amplitudes differ by 60dB must differ in
frequency by 7.5 time the IF bandwidth before
they can be distinguished separately.
Otherwise, they will appear as one signal on
the spectrum analyzer display.
The ability of a spectrum analyzer to resolve
closely spaced signals of unequal amplitude is
not a function of the IF filter shape factor only.
Noise sidebands can also reduce the
resolution. They appear above the skirt of the
IF filter and reduce the offband rejection of the
filter. This limits the resolution when
measuring signals of unequal amplitude.
The resolution of the spectrum analyzer is
limited by its narrowest IF bandwidth. For
example, if the narrowest bandwidth is 10kHz
then the nearest any two signals can be and
still be resolved is 10kHz. This is because the
analyzer traces out its own IF band-pass shape
as it sweeps through a CW signal. Since the
resolution of the analyzer is limited by
bandwidth, it seems that by reducing the IF
bandwidth infinitely, infinite resolution will be
achieved. The fallacy here is that the usable IF
bandwidth is limited by the stability
(residual
Fm) of the analyzer. If the internal frequency
deviation of the analyzer is 10kHz, then the
Resolution
narrowest bandwidth that can be used to
distinguish a single input signal is 10kHz. Any
narrower IF-filter will result in more than one
response or an intermittent response for a
single input frequency. A practical limitation
exists on the IF bandwidth as well, since
narrow filters have ling time constants and
would require excessive scan time.
Sensitivity
Sensitivity is a measure of the analyzer's
ability to detect small signals. The maximum
sensitivity of an analyzer is limited by its
internally generated noise. The noise is
basically of two types: thermal
(or Johnson)
and nonthermal noise. Thermal noise
power can be expressed as:
P =K . T . B
n
Where:
P = Noise power in watts
n
K = Boltzmanns Constant
-23
(1.38*10 Joule/K)
T = absolute temperature, K
B = bandwidth of system in Hertz
As seen from this equation, the noise level is
directly proportional to bandwidth. Therefore,
a decade decrease in bandwidth results in a
10dB decrease in noise level and consequently
10dB better sensitivity. Nonthermal noise
accounts for all noise produced within the
analyzer that is not temperature dependent.
Spurious emissions due to nonlinearities of
active elements, impedance mismatch, etc. are
sources of nonthermal noise. A figure of merit,
or noise figure, is usually assigned to this
ninthermal noise which when added to the
thermal noise gives the total noise of the
analyzer system. This system noise which is
measured on the CRT, determines the
maximum sensitivity of the spectrum analyzer.
Because noise level changes with bandwidth it
is important, when comparing the sensitivity