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When millimeter-wave applications came into many domains of
human activity, a need for precise measurements of signal
frequency spectrum has become critical. This need is quite
understandable when, for example, it concerns to exploring
extraterrestrial radio-sources or micro/mm-wave background
emission in radio-astronomy, or measuring chemical composition
of the atmosphere through the molecular emission of different
its components, plasma diagnostics or many military applications.
Moreover, in some cases the spectrum measurements have to
be implemented in the real time scale. For example, the high
temperature plasma physics research is just the case because
of short plasma life time in experimental devices and poor
plasma parameter reproducibility from discharge to discharge;
also it is important for plasma feedback control purposes.
As well, the real time spectrum measurements are very needed
in electronic warfare applications, such as object recognizing
or electronic countermeasures to millimeter-wave weapons.
However, real time broad band analyzers with direct frequency
spectrum measuring continue to be a rare bird in catalogues
of leading world manufacturers of mm-wave products. ELVA-1 fill this need for the frequency range of 90 to
140 GHz with their device.
Composition
Whole the system consists
of millimeter-wave sweep generator G4-143f (Fig.
1), which is the product recently elaborated and now provided
by the firm, heterodyne receiver, connected to the generator
with waveguide line through attenuator, external IBM PC equipped
with data acquisition card and software.
How
it operates
Our device is typical
swept-frequency spectrum analyzer and uses the classic heterodyne-based
architecture (Fig. 2). Besides
the double-side-band (DSB) heterodyne receiver the system
incorporates the sweep generator with 90 to 140 GHz frequency
band as local oscillator (LO) of the receiver. The main part
of the generator is back wave oscillator (BWO). Sweeping its
output frequency is realized by means of saw-tooth modulation
of BWO accelerating potential. Being supplied to the receiver,
the swept wave thus allows scanning input mm-wave signal over
the frequency during each sweep period, so spectra repetition
rate is equal to the sweep frequency. At each time point of
the each period output video signal of the receiver is proportional
to power of the input signal at given frequency within its
spectrum. This implies that both frequency and sensitivity
calibration of the analyzer has to be carried out. The frequency
calibration of the receiver may be performed at any time when
it is needed with aid of internal harmonic generator built
in the receiver and providing 9 equidistant frequencies covering
the band from 90 to 138 GHz. As to the power calibration is
concerned, it is fulfilled by the manufacturer using external
calibrated mm-wave source which imitates the input signal.
Result of the calibration is provided in form of the calibrating
curves in the package with the device (Fig 3). Maximum spectral
sensitivity of the device is defined by the minimum input
signal power, approximately 0.001 mW, and intermediate frequency
(IF) bandwidth of the receiver. With the latter being equal
to 12 MHz (from 4 to 16 MHz), it gives the maximum sensitivity
to be equal to about 10-7 W/MHz. The sweep time for the whole
frequency band can be varied within a wide range: 10ms, 20ms,
40ms, .., 4 s, 8 s, 10 s - 13 steps totally, that defines
the frequency sweeping rate to be equal, respectively, to
5 GHz/ms, 2.5 GHz/ms etc.. The analyzer operates in two basic
regimes. At the first of them, digital to analogue convertor
(DAC) of the generator produces 4096 conversions for the whole
frequency band, from 90 to 140 GHz. It implies that frequency
resolution at this regime is about 12 MHz which is the minimum
one and corresponds to the IF bandwidth of the receiver. Of
course, it is not obligatory to scan over the whole band,
user may narrow the band decreasing, respectively, the sweep
time and increasing sweep repetition rate, but the number
of the conversions will be decreased proportionally keeping
the same resolution. At the second, "zoom" or "lens"
regime, the user chooses a narrowed band, 2 GHz, 1 GHz, 0.5
GHz or 0.25 GHz, centered about a frequency in any place of
the whole band. Then the 4096 conversions is made for this,
narrowed, band. Thus, the maximum frequency resolution of
the analyzer may be carried to the value as high as 60 kHz.
Control of whole the analyzer, visualization of the spectra
obtained (Fig. 4) and saving
them on hard disc are realized by means of the external IBM
PC. The generator is governed with aid of a microcomputer,
which is built in the generator, in accordance with instructions
received from the main computer. Current status of the generator
is monitored with the internal computer and displayed both
on its liquid crystal display (LCD) on the front panel of
the generator housing and on the IBM PC monitor. On Fig.
5 observation of a spectrum evolution performed with aid
of the analyzer is presented as an example of the analyzer
application to the high temperature plasma research.
Features
and Specifications
- External IBM PC: Pentium
200MHz, Win95/LabView4.0, RS-232 serial port.
- Built-in BWO protection
against current overloading, anode voltage drop and cathode
overheating. Internal automatic frequency calibration of
the receiver.
- Pulsed or CW input
signal.
- Spectrum measurement
over whole the frequency range or over a frequency window,
with the minimum window width of 0.25 GHz.
- Operational modes:
spectrum measurement during a single sweep and measurement
of the spectrum averaged over several sweeps with visualization
of the spectra on the IBM PC monitor;
- Real time spectrum
measurements with continuously repeated sweeping and accumulating
the output data on the IBM PC hard disc and consequent visualization
of the specter;
- Basic frequency resolution
of 12MHz and "zoom" mode with the resolution of
60 kHz.
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