Atten AT5011 Руководство по эксплуатации - Страница 16
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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)
Resolution
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:, 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 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
-23
K = Boltzmanns Constant (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 of two analyzers, to compare sensitivity
specifications for equal bandwidths. A spectrum
analyzer sweeps over a wide frequency range, but is
really a narrow band instrument. All of the signals
that appear in the frequency range of the analyzer
are converted to a single IF frequency which must
pass through an IF filter; the detector sees only this
noise at any time. Therefore, the noise displayed on
the analyzer is only that which is contained in the IF
passband. When measuring discrete signals,
maximum sensitivity is obtained by using the
narrowest IF bandwidth.
Video Filtering
Measuring small signals can be difficult when they
are approximately the same amplitude as the
average internal noise level of the analyzer. To
facilitate the measurement, it is best to use video
filtering. A video filter is a post-detection low pass
filter which averages the internal noise of the
analyzer. When the noise is averaged, the input
signal may be seen. If the resolution bandwidth is
very narrow for the span, the span, the video filter
should no be selected, as this will not allow the
amplitude of the analyzed signals to reach full
amplitude due to its video bandwidth limiting
property.
Spectrum Analyzer Sensitivity
Specifying sensitivity on a spectrum analyzer is
somewhat arbitrary. One way of specifying
sensitivity is to define it as the signal level when
signal power = average noise power. The analyzer
always measures signal plus noise. Therefore, when
the input signal is equal to the internal noise level,
the signal will appear 3dB above the noise. When
the signal power is added to the average noise
power, the power level on the CRT is doubled
(increased by 3dB) because the signal
power=average noise power.
The maximum input level to the spectrum analyzer
is the damage level or burn-out level of the input
circuit. This is (for the AT5010/11) +10dB for the
input mixer and +20dB for the input attenuator.
Before reaching the damage level of the analyzer,
the analyzer will begin to gain compress the input
signal. This gain compression is not considered
serious until it reaches 1dB. The maximum input
signal level which will always result in less than
1dB gain compression is called the linear input
level. Above 1dB gain compression the analyzer is
considered to be operating nonlinearly because the
signal amplitude displayed in the CRT is not an
accurate measure of the input signal level.
Whenever a signal is applied to the input of the
analyzer, distortions are produced within the
analyzer itselt. Most of these are caused by the non-
linear behavior of the input mixer. For the
AT5010/11 these distortions are typically 70dB
below the input signal level for signal levels not
exceeding -27dBm at the input of the first mixer. To
accommodate larger input signal levels, an
attenuator is placed in the input circuit before the
first mixer. The largest input signal that can be
applied, at each setting of the input attenuator, while
maintaining the internally generated distortions
below a certain level, is called the optimum input
level of the analyzer. The signal is attenuated before
the first mixer because the input to the mixer must
not exceed -27dB, or the analyzer distortion
products may exceed the specified 70dB range. This
70dB distortion-free range is called the spurious-
free dynamic range of the analyzer. The display
dynamic range is defined as the ratio of the largest
signal to the smallest signal that can be displayed
simultaneously with no analyzer distortions present.
Dynamic range requires several things then. The
display range must be adequate, no spurious or
unidentified response can occur, and the sensitivity
must be sufficient to eliminate noise from the
displayed amplitude range.
The maximum dynamic range for a spectrum
analyzer can be easily determined from its
specifications. First check the distortion spec. For
example, this might be "all spurious products 70dB
down for -27dBm at the input mixer". Then,
determine that adequate sensitivity exists. For
example, 70dB down from -27dBm is -97dB. This
is the level we must be able to detect, and the
bandwidth required for this sensitivity must not be
too narrow or it will be useless. Last, the display
range must be adequate.
Notice that the spurious-free measurement range
can be extended by reducing the level at the input
mixer. The only limitation, then, is sensitivity. To
ensure a maximum dynamic range on the CRT
display, check to see that the following requirements
are satisfied.
The largest input signal does not exceed the
optimum input level of the analyzer (typically-
27dBm with 0dB input attenuation).
The peak of the largest input signal rests at the
top of the top of the CRT display (reference level).