RU2234708C2 - Panoramic acoustic-optical receiver-wavemeter - Google Patents

Abstract

FIELD: radio-measuring equipment engineering.

SUBSTANCE: device has laser, collimator, controlling acoustic-optical deflector, optical system for image transferring, signal acoustic-optical deflector, optical synchronization system, wide-aperture photo-receiver, device for measuring time intervals, impulse forming device, controlling signals generator, light divisor, photo-receivers row, video-amplifiers set, threshold devices set, solver and adder.

EFFECT: higher precision of radio signals frequency measurement.

10 dwg

Description

The present invention relates to radio-measuring equipment and can be used as a high-precision meter of the frequency of radio signals in broadband communication systems, radar and radio reconnaissance.

A well-known (see figure 1) acousto-optical (AO) analyzer of a sequential type spectrum with spatial integration (published in the book: Holography and information processing // Edited by Prof. SB Gurevich. - L.: Science. - 1976. - 196 p .; on page 110), which includes a laser connected in series over the world, a low-frequency AO deflector controlled by a low-frequency (LF) signal generated by the control device, a high-frequency AO deflector, to the electrical input of which a measured radio signal is supplied, integrating lens recording stroystvo - a photodetector which is used as a photomultiplier tube, and measuring time intervals from the control device; an oscilloscope was used as a time interval meter. In this analogue, the frequency of the input radio signal is judged by the oscilloscope measuring the duration of the time interval, the beginning of which is determined by the control device, and the end of the measured time interval is the detected light signal taken from the output of the photodetector.

The reason that impedes the achievement of the claimed technical result is the insufficient accuracy of frequency measurement and the resolution of the AO analyzer. In this analogue, the accuracy of frequency measurement is related to the accuracy of recording the corresponding time interval, which, in turn, depends on the shape, duration, and amplitude of the detected light pulse, which is a “reaction” to the input radio signal. As for the frequency resolution, this parameter is determined by the number of angular positions of the laser beam specified by the low-frequency acousto-optical deflector, which for modern deflectors does not exceed N≤ (100-500) units; in analogue, the frequency resolution approximately cannot exceed a value equal to Δf _p = ΔfΣ / N, where ΔfΣ is the operating frequency band of the analyzer.

Signs of an analogue that coincide with the features of the present invention are a laser connected in sequence, a low-frequency AO deflector controlled by a signal generated by a control device, a high-frequency AO deflector, the measured radio signal being fed to its electrical input, a photodetector recording device, and a time interval meter with control device.

Known AO spectrum analyzer with reading diffracted light distribution using an auxiliary (mirror, bimorph) deflector; it is published in the book: Acousto-optical devices for spectral and correlation signal analysis // S.V. Kulakov. - L .: Science. - 1978. - 144 p. - on page 38. An analog device (see figure 2) contains a laser, a collimator, an AO signal deflector, an integrating lens, an auxiliary deflector and a photodetector sequentially located on the optical axis, the photodetector connected to the input of the time interval meter, which an oscilloscope is used, which carries out a scan of the intensity of the light distribution on the screen, which carries information about the square of the spectral density module of the analyzed signal. In this analogue, the temporary location (coordinate) of the maximum spectral power density of the input radio signal is uniquely related to its central frequency. The features of this analogue, common with the claimed invention, are a series-connected laser, a collimator, a signal AO deflector, to the electrical input of which a measured radio signal is supplied, an integrating lens, an auxiliary deflector controlled by a signal generated by the control device, a photodetector and a time interval meter, one of the inputs which is connected to the control device.

The reason that impedes the achievement of the required technical result is the lack of accuracy in measuring the frequency, which is mainly due to the use of an oscilloscope in this analog as a time meter.

The closest in technical essence to the claimed device is a prototype device: panoramic AO receiver-frequency meter described in the article: V. Rozdobudko Acoustooptic microwave frequency counter of sequential type // Radio engineering. - 1991. - No. 12. - p. 81-86. The prototype device (see Fig. 3) in its composition contains a laser located sequentially in the light, a collimator, a control AO deflector, an optical image transfer system, a signal AO deflector, an optical matching system and a wide-aperture photodetector connected to one of the inputs of the time interval meter as well as a pulse shaper and a control signal generator, which is used as a linear frequency-modulated (LFM) signal generator tunable in the ΔΩ frequency band; In the prototype device, the input radio signal is supplied to the electrical input of the signal AO deflector and to the input of the pulse shaper, the output of which is connected to the input of the generator of control signals connected to the electrical input of the control AO deflector and simultaneously to the second input of the time interval meter.

The features of the selected prototype, common with the claimed invention, are a laser, a collimator, a control AO deflector, an optical image transfer system, a signal AO deflector, an optical matching system and a wide-aperture photodetector loaded at the input of the time interval meter, as well as a pulse shaper and control signal generator.

Specifically, the operation of the prototype device is associated with the use in its structure of a signal AO deflector operating in the so-called auto-detuning mode, the theory of which is described in the works: V. Rozdobudko Interaction band in acousto-optical deflectors in the auto-tuning mode // Izv. Universities of the USSR - Radioelectronics. - 1991. - No. 8. - p. 53-56; Barenboim A.B., Nevsky Yu.E. Auto-tuning mode of the Bragg angle in acousto-optic cells with multi-element piezoelectric transducers // Radiophysics. - 1989. - No. 1. - p. 125-128.

The prototype device operates as follows. The leading edge of the measured input radio signal generates a start pulse of the generator of control (LFM) signals and the meter of time intervals. Under the influence of FM oscillations, the controlling AO deflector scans the angle of incidence of light and, accordingly, reconstructs the frequency domain AO of the interaction of the signal AO deflector. The wide-aperture photodetector “responds” when the frequency of the input signal coincides with the tunable narrow-band frequency region of the AO interaction. The output of the wide-aperture photodetector is connected to the second input of the time interval meter. In this case, the duration of the recorded time interval is counted from the time the meter starts time intervals until the response from the output of the photodetector. Thus, in the prototype, a time interval is formed proportional to the frequency of the input radio signal, which is recorded by the time interval meter.

The problem to which the invention is directed, is to increase the accuracy of measuring the frequency of radio signals.

The technical result achieved by the implementation of the invention is to increase the accuracy of frequency measurement by an order of magnitude in comparison with the prototype and analogues.

The claimed technical result is achieved due to the fact that the following elements are additionally introduced into the panoramic acousto-optical receiver-frequency meter after the optical matching system in the direction of the diffracted light beam: a beam splitter, a line of photodetectors (FPU), a set of video amplifiers, a set of threshold devices, a resolving device and an adder. The functional purpose of the set of the listed elements is to clarify the frequency of the measured radio signal within the range of operating frequencies specified by the control deflector for one of the angles of incidence of light on the signal acousto-optic deflector.

To achieve a technical result, a panoramic acousto-optic receiver-frequency meter containing a laser connected in series through the light, a collimator, a control acousto-optic deflector, an optical image transfer system, an acousto-optic deflector, an optical matching system and a wide-aperture photodetector connected to one of the inputs of the time interval meter, and also sequentially included pulse shaper and a generator of control signals supplied to the input of the control ac a stooptic deflector, and the input radio signal in the receiver-frequency meter is fed to the electrical input of the signal acousto-optic deflector and simultaneously to the input of the pulse shaper, the output of which is loaded on the second input of the time interval meter, a beam splitter is connected in series after the optical matching system in the direction of the diffracted light beam, ruler photodetectors, a set of video amplifiers, a set of threshold devices, a deciding device and an adder, the outputs of the photodetector line ISRC connected in series with a set of video amplifier, and a set of threshold devices solver whose output, and output time slots meter connected to an input of the adder.

To prove the existence of a causal relationship between the claimed features and the achieved technical result, we consider the principle of operation of the prototype and, in particular, the operation of the signal AO deflector, which operates in the so-called auto-detuning mode, characteristic of acousto-optical deflectors in which sound is excited by an antiphase system (array) converters, including interdigital converters (IDT). In the latter case, piezoelectric crystals are used as sound ducts in AO deflectors, and ultrasound is excited by a system of metal electrodes directly from the surface of the piezocrystal (see: V. Rozdobudko, A.V. Study of the frequency response of an acousto-optical microwave deflector with sound excitation by an interdigital transducer system // Izv. Universities of the USSR - Radio Electronics. - 1991. - No. 9. - p. 42-46).

Consider an equidistant IDT transducer located on the surface of an isotropic piezoelectric with the crystallographic axes X, Y and Z, assuming that its width L is many periods - d (see figure 4), and the height in the direction of the Y axis is much greater than its other geometric sizes.

Each pair of IDT electrodes can be considered as a source of volumetric acoustic waves, and each pair of electrodes excites a sound wave phase-shifted 180 ° compared to the neighboring one.

In this case, these electrodes are a system of out-of-phase non-directional emitters (found in radio navigation devices) with a large base, since d >> Λ, where Λ = ϑ / f is the wavelength of ultrasonic vibrations.

In this case, the total power of body waves generated by IDT will be relatively large if the waves excited by individual electrodes are summed coherently. If d is the period of the location of IDT electrodes, then this condition will be satisfied for a body wave propagating at an angle θ to the IDT surface, so that the wavelength generated is Λ = dcosθ.

Hence f = ϑ / dcosθ, where f is the frequency, ϑ is the velocity of the body wave. The angular distribution of the amplitude of the sound field S (φ, z) emitted by the IDT lattice in the far zone is described by the expression

in which S _n are the amplitudes of the generated volumetric sound waves in the directions φ _n ;

these areas are defined as

определяется из условия

а амплитуды S_n изменяются по закону 2/nπ.their total number

determined from the condition

and the amplitudes S _n vary according to the law 2 / nπ.

In this case, the amplitudes S _n corresponding to n = ± 1 significantly exceed the others, therefore, expression (1) can be written in the form:

Where

S ₀ - the amplitude of the sound wave in the plane of the IDT;

Each of the two lobes of the radiation pattern (ID) S (φ, z) of the IDT within δφ≈Λ / L = ϑ / fL, along the directions φ ± changing with frequency as φ ± = ϑ / 2df, contains (2 / π) ² total power P _ak = (S ₀ ) ² sound excited in the plane of the IDT.

In turn, in the approximation "flat-field" relationship between the power of the acoustic oscillations P _ak and E _x tangential component of the electric field E acting on the upper face of the piezoelectric crystal may be expressed by

where k is the electromechanical coupling coefficient,

ε is the dielectric constant of the crystal in the direction of the applied field,

(m-1) ab is the radiation area with a field E _x ,

b and a are the length and width of IDT electrodes.

The latter can be calculated by the formula

Where

G (Δ) is the parameter defined by the expression

- в данном случае эллиптический интеграл первого рода;

- in this case, an elliptic integral of the first kind;

P _m (cosΔ) is the Legendre polynomial.

Thus, from the consideration undertaken, it follows that the IDT in the far zone excites two ultrasonic beams with the same power P _ak , divergence equal to δφ = ϑ / fL, the deviation angles of which vary with the frequency of the input signal in accordance with φ ± = ϑ / 2df.

When applying to one of the sound rays of laser radiation, the normalized AFC of the AO interaction can be described by the relation

in which the function sincχ = sinχ / χ;

m - in this case, the number of electrodes in IDT;

F = (ff ₀ ) / f ₀ - dimensionless frequency;

f ₀ is the central frequency of the AO interaction, selected from the condition θ _B (f) = φ ₊ (f);

Θ _P is the angle of incidence of light on the side face of the AO deflector normalized to the Bragg angle at a frequency f ₀

From the analysis of relation (4) it follows that when the AO deflector is in auto-detun mode, i.e. with an incidence angle θ _n of light when the change of frequency of light incidence angle θ _R and the angle of deviation of rays Nam IDTs are changed in opposite directions, the center frequency f _q AO interaction equal

where the signs ± correspond

до

rebuilds within two octaves: from

before

In this case, the AO interaction band in the first approximation, equal to ΔF = f ₀ / (m-1), which also follows from relation (4), retains its constant value in the entire tuning range, i.e. at all angles of incidence of light. The diffracted light is localized in space in the direction θd ₁ = λf ₀ / nϑ, independent of the frequency of the input signal, and the interval of variation of the diffraction angle, within which the light intensity is nonzero, is

Thus, the presence of a causal relationship between the claimed features and the achieved technical result is determined by the fact that in the proposed receiver-frequency meter, the value of the frequency of the input signal within the coarse frequency discrete is further clarified by including in the receiver circuit the FPU line, a set of video amplifiers, threshold devices and solver. Moreover, the listed set of elements functions only because, as shown above, diffracted light in the entire range of operating frequencies is localized in space and changes its direction only within Δθ _d1 or within the coarse frequency discrete ~ ΔF.

Figure 5 presents a structural diagram of the inventive panoramic AO receiver-frequency meter, in which a review of the operating frequency band ΔfΣ is carried out sequentially during the time T _review with discreteness ΔF, and if a radio signal is detected in the ΔF band, its frequency is specified - it is measured by the parallel part of the receiver with increased accuracy.

The positions in FIG. 5 indicate: 1 — laser, 2 — collimator, 3 — control AO deflector, 4 — control signal generator, 5 — pulse shaper, 6 — optical image transfer system, 7 — signal AO deflector, 8 — optical matching system , 9 - a beam splitter, 10 - a photodetector, 11 - a time interval meter, 12 - a line of photodetectors, 13 - a set of video amplifiers, 14 - a set of threshold devices, 15 - a decisive device, 16 - an adder.

The proposed panoramic AO receiver-frequency meter operates as follows.

A laser light source 1 with a wavelength λ through a collimator 2, the purpose of which is to form a light beam of a given geometry, falls at a Bragg angle onto the controlling AO deflector 3.

Upon arrival at the input of the panoramic AO receiver-frequency meter of the measured signal, its leading edge by the pulse shaper 5 initiates the start of the time interval meter 11, which starts the countdown. At the same time, the pulse shaper 5 starts the control signal generator 4, which automatically starts the sequence of radio pulses with different frequencies Ω ₁ , Ω ₂ , Ω ₃ ... filling and output power P ₁ , P ₂ , P ₃ ....

The set of frequencies and powers - Ω _n , P _n generated by the control signal generator 4, is such that when they are fed to the piezo transducer of the control AO deflector 3, it sequentially generates an angle-tunable light beam, which, passing through the optical image transfer system 6, falls at different angles θ ₁ , θ ₂ , θ ₃ .... These angles of incidence correspond to different areas of AO interaction in the signal AO deflector 7. The choice of frequencies Ω _n and angles of incidence θ _n is such that the corresponding frequency regions of the same width ΔF = f ₀ / (m-1) overlap at the same level of their unevenness.

In this case, the equality of the intensities of diffracted light at the output of the signal AO of the deflector 7 is ensured by selecting (setting) the power levels of the signals P ₁ , P ₂ , P ₃ .... The last is illustrated in Fig.6, on the upper part of which - Fig.6, and - shows a sequence of radio pulses generated by the control signal generator 4 with filling frequencies Ω _n , and FIG. 6, b shows the frequency response of the signal AO of the deflector 7 taking place at different angles of incidence of light θ _n on its side face.

Thus, the signal AO deflector 7 provides the possibility of forming a frequency-tunable narrowband region of AO interaction, the shape and parameters of which do not change in the range of operating frequencies.

When the frequency of the measured microwave signal hits one of these areas, the laser radiation diffracts once more and, using the optical matching system 8, passes through the beam splitter 9 to the photodetector 10. The detected light signal taken from the output of the photodetector 10 stops the time interval meter 11.

The meter 11 forms a time interval, the duration of which is accurate to Δt / 2 (or ΔF / 2) is proportional to the frequency of the measured signal.

Part of the laser radiation deflector 7 diffracted on the measured signal in the AO signal through the beam splitter 9 enters the line of photodetectors 12. Using a set of video amplifiers 13, a set of threshold devices 14 and a resolving device 15, the number of the central from the group of “illuminated” photodiodes in the line of photodetectors 12 is determined. Since the line 12 with the total number of photodiodes NΣ "has" a working frequency band equal to ΔF, the accuracy of recording the frequency of the input radio signal is 0.5 frequency samples or

The aforesaid is also illustrated in Fig.6b, where a diffracted spot of light is shown falling on a line of photodetectors 12; the location of the maximum of the diffracted spot of light and corresponds to the carrier frequency of the measured radio signal. From the output of the time interval meter - 11, the signal in the form of a code for the coarse frequency value and from the output of the resolver - 15, the signal in the form of a code for the exact frequency value is supplied to the inputs of the adder 16, from the output of which the frequency parameters of the measured radio signal are presented to the consumer.

In conclusion, we evaluate the degree of increasing the accuracy of measuring the frequency and resolution of the proposed device.

In the prototype and analogs, this accuracy, determined by the number of frequency areas of AO interaction - ΔF, falling within the operating frequency band of the receiver ΔfΣ, is equal to ~ 0.5ΔF.

In the proposed frequency meter, the accuracy of determining the frequency depends on the number NΣ of photodiodes in the FPU line used and in absolute terms is

Thus, an increase in accuracy will be equal to As for the frequency resolution, in the proposed AO receiver, the value of this parameter is determined by the size of the time aperture of the signal AO deflector, which can be a value equal to units of MHz and, thus, tens of times higher than the frequency resolution of the prototype, equal to 10-80 MHz.

The proposed panoramic acousto-optical receiver-frequency meter includes a set of elements whose parameters are characteristic of the current industrial level of development of acousto-optics. In particular, for the frequency range (500-2000) MHz, the frequency meter can be made on the basis of the following domestic acousto-optical element base.

Laser 1, it is advisable to use a gas Ne-He, for example, types LGN-219, LGN-223, LGN-208, or semiconductor visible (ILPN-207) or infrared. The signal acousto-optical deflector for the specified frequency range can be made on the basis of such material as LiNbO ₃ . The sound exciter in this deflector must be a system of antiphase converters, for example, made on the basis of ZnO, or the sound can be excited directly from the surface of LiNbO _{3 by a} system of IDT type electrodes. Examples of the practical implementation (including industrial) of such signal deflectors are known: see, for example, V. Rozdobudko Investigation of the frequency response of an acousto-optical microwave deflector with sound excitation by a system of interdigital transducers // Radioelectronics. -1991. - No. 9. - p. 42-46.

AO control deflector 3 - low-frequency and high-performance can also be made on the basis of LiNbO ₃ or TeO ₂ ; the possibility of using industrial AO deflectors, for example, such as ML-201, etc.

As the photodetector 10, it is advisable to use p-i-n or avalanche photodiodes of the LFD-2 type.

The line of photodetectors 13 can be implemented on the basis of domestic photomatrix types FPU-14 or MF-14. As for the time interval meter 11, its implementation is possible, for example, on the basis of a calibrated pulse counter. In this case, the start of the reference time interval is carried out by the pulse shaper 5, and the end of the measured time interval is a short pulse taken from the output of the photodetector 10. By the number of clock pulses generated by the counter, one can judge the duration of the time interval and, accordingly, the frequency of the radio signal at the panoramic input AO receiver-frequency meter.

To the optical elements included in the claimed device, there are no special requirements: a collimator 2, an optical image transfer system 6, an optical matching system 8 and a beam splitter 9 can be made by standard technology, for example, from K8 glass. As a collimator 2, the use of a standard lens is possible.

Claims (1)

A panoramic acousto-optical receiver-frequency meter containing a laser sequentially located throughout the world, a collimator, a control acousto-optical deflector, an optical image transfer system, an acousto-optic deflector, an optical matching system and a wide-aperture photodetector connected to one of the inputs of the time interval meter, as well as a pulse shaper, which starts the generator of control signals supplied to the input of the control acousto-optical deflector, and the input radio the signal in the receiver-frequency meter is fed to the electrical input of the signal acousto-optical deflector and simultaneously to the input of the pulse shaper, the output of which is connected to the second input of the time interval meter, characterized in that a beam splitter, a line of photodetectors, are included in it after the optical matching system in the direction of the diffracted light beam, a set of video amplifiers, a set of threshold devices, a solver and an adder, and the outputs of the line of photodetectors are sequentially loaded onto the set video amplifiers, a set of threshold devices and a deciding device, the output of which, as well as the output of the time interval meter, are included at the input of the adder.

Images