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Custom-designed
Real-Time Frequency Analyzers 90-140 GHz


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.