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.
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.
Video Filtering
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 AT6010/
AT6011)
+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
theanalyzer itselt. Most of these are caused by
the non-linear behavior of the input mixer. For
the AT6010/AT6011 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).
Frequency Response
The frequency response of an analyzer is the
amplitude linearity of the analyzer over its
frequency range. If a spectrum analyzer is to
display equal amplitudes for input signals of
equal amplitude, independent of frequency,
then the conversion
(power)
loss of the input
mixer must not depend on frequency. If the
voltage from the LO is too large compared to
the input signal voltage then the conversion
loss of the input mixer is frequency dependent
and the frequency response of the system is
nonlinear. For accurate amplitude
measurements, a spectrum analyzer should be
as flat as possible over its frequency range.
Flatness is usually the limiting factor in
amplitude accuracy since its extremely
difficult to calibrate out. And, since the
primary function of the spectrum analyzer is to
compare signal levels at different frequencies,
a lack of flatness can seriously limit its
usefulness.
Tracking Generators
The tracking generator
(AT6010 only)
is a
special signal source whose RF output
frequency tracks
(follows)
some other signal
beyond the tracking generator itself. In
conjunction with the spectrum analyzer, the
tracking generator produces a signal whose
frequency precisely tracks the spectrum
analyzers tuning. The tracking generator
frequency precisely tracks the spectrum
analyzer tuning since both are effectively
tuned by the same VTO. This precision
tracking exists in all analyzer scan modes.
Thus, in full scan, the tracking generator
output is a start-stop sweep, in zero scan the
output is simply a CW signal.
The tracking generator signal is generated by
synthesizing and mixing two oscillators. One
oscillator is part of the tracing generator itself,
the other oscillator is the spectrum analyzer's
1st LO. the spectrum analyzer/tracking
generator system is used in two
configurations: open-loop and closed-loop. In
the open-loop configuration, unknown
external signals are connected to the spectrum
analyzer input and the tracking generator
output is connected to a counter. This
