Atten AT5010 Instrukcja obsługi - Strona 17
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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 (AT5010 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 configuration is use for
making selective and sensitive precise measurement
of frequency, by tuning to the signal and switching
to zero scan.
In the closed-loop configuration, the tracking
generator signal is fed into the device under test and
the output of the device under test is connected to
the analyzer input.
In this configuration, the spectrum analyzer/tracking
generator becomes a self-contained, complete
(source, detector, and display) swept frequency
measurement system. An internal leveling loop in
the tracking generator ensures a leveled output over
the entire frequency range. The specific swept
measurements that can be made with this system are
frequency response (amplitude vs. frequency),
magnitude only reflection coefficient, and return
loss. From return loss or reflection coefficient, the
SWR can be calculated. Swept phase and group
delay measurements cannot be made with this
system; however, it does make some unique
contributions not made by other swept systems, such
as a sweeper/network analyzer, a sweeper/spectrum
analyzer, or a sweeper/detector oscilloscope.
Precision tracking means a every instant of time the
generator fundamental frequency is in the center of
the analyzer passband, and all generator harmonics,
whether they are generated in the analyzer or are
produced in the tracking generator itself, are outside
the analyzer passband. Thus only the tracking
generator fundamental frequency is displayed on the
analyzer's CRT. Second and third order harmonics
and intermodulation products are clearly out of the
analyzer's CRT. Second and third order harmonics
and intermodulation products are clearly out of the
analyzer tuning and, therefore, they are not seen.
Thus, while these distortion products may exist in
the measurement set-up, they are completely
eliminated from the CRT display.
The 1dB gain compression level is a point of
convenience, but it is nonetheless considered the
upper limit of the dynamic range. The lower limit,
on the other hand, is dictated by the analyzer
sensitivity which, as we know, is bandwidth
dependent. The narrowest usable bandwidth in turn
is limited by the tracking generator residual FM and
any tracking drift between the analyzer tuning and
the tracking generator signal.
Mainly Performance
and Calibration of
AT5010/5011 Series
Spectrum Analyzer
1. Structural integrity
No obvious impairment and gilding damage, all
outside components are well installation, reliable
fasten and flexible operability. Warm up for one
hour to inspect the performances.
2. Inspection of Y-POS
Adjust Y-POS knob, occurs hand feeling by obvious
locating point. Failures in adjusting the locating
point will lead to all measured amplitudes are
invalid.
3. Accuracy of frequency readout
Not exceed
2% spectrum width +100kHz
4. Inspection of frequency range
Output 0.15MHz 1050MHz frequency signal from
a signal generator, screen should display the
complete signals.
inspect frequency range of tracking signal
generator: connect output port with frequency
counter, both output attenuation and bandwidth are
zero, adjust CF knob, to check the signal generator
could output 0.15MHz 1050MHz signal or not.
5. Inspection of reference level
2dB at 500MHz
Output 500MHz frequency and -27dBm level
signal, from signal generator to spectrum analyzer.
Set spectrum analyzer CF at 500MHz, scanwidth
2MHz/DIV, center frequency 400kHz, with zero
input attenuation. And signal amplitude should up to
top line.
Inspect the output level of tracking generator:
connect input to output with a coaxial cable, with
CF 500MHz, scanwidth 0MHz/DIV, input
attenuation 30dB, maximum output, then amplitude
should lower the top line than one graticule.
6. Frequency Response
It means signal response of spectrum analyzer in
measuring equal level and different frequency.
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.
7. Inspection of attenuators
(0~40dB)4 10dB step,
1dB/10dB,
2dB(40dB).
8. Residual response
andinterference immunity
Short circuit the input by connect a 50
terminal
load to input port, then signal response should not
appears in screen. Otherwise, it may seriously affect
the ability of spectrum analyzer in analyzing small
signals.
Replace the 50 terminal load off input port, noise
base line should not raise, especially the tracking
source cannot affect the instrument. In a common
room, no signal response appears in screen.
9. Inspection of sensibility
Sensibility should better than -95dB.
Output a -95dBm signal with a standard signal
generator, set CF in the frequency, with IF 20kHz,
0.1MHz/DIV scanwidth, video filter on. Spectrum
analyzer is able to measure the signal.
10. Grass level
The typical value is 15dB when IF bandwidth is
400kHz, otherwise, the sensibility of he unit is too
inferior to analyze smaller signals.