JPH0154671B2 - - Google Patents

Description

[Detailed Description of the Invention] (Technical Field) This invention relates to an acousto-optic spectrum analyzer that detects frequency components contained in an electrical signal using an acousto-optic correlator and displays the detected frequency components in the form of a spectrum. It is.

(Prior art) Conventionally, there is a method in which a correlator is used to perform a cross-correlation operation between an unknown input signal and a sine wave signal, and the frequency components contained in the unknown input signal are detected based on the value of the correlation coefficient. is being attempted. In a cross-correlation calculation that calculates the similarity between two signals, if a sine wave signal with a known frequency is used for one signal, it is possible to obtain an output signal that looks as if the other signal was filtered by this sine wave frequency. It is possible. In other words, if the unknown input signal contains a frequency component equal to that of the sine wave signal, the similarity, which is the correlation output, will change in proportion to the amount of the component, and if the sine wave signal and the unknown signal have the same frequency, the maximum Correlation output is obtained. Since this maximum correlation output changes in proportion to the amplitude value of the unknown signal, by normalizing it by the amplitude of the unknown signal, the similarity ratio can be determined.
Now, if we use the correlator as a filter and repeatedly perform cross-correlation with the unknown signal while changing the sine wave signal, the correlation output will change over time to the frequency components contained in the unknown signal. It will represent the value. Therefore, by displaying the correlation outputs in a temporal order, a frequency spectrum can be obtained, and the resolution of this spectrum is determined by the number of times the correlation calculation is repeated. However, correlation calculations include complex and time-consuming calculations such as delay, multiplication, and integration, and conventional mechanical or electrical correlators can perform a single correlation calculation of several milliseconds to several tens of milliseconds. The calculation time was approximately 1.5 seconds. For this reason, it is difficult to process a large number of repeated cross-correlation calculations in a short period of time, and if we dared to increase the speed by performing parallel correlation processing, we would need complicated equipment and a large amount of expense.

(Objective of the present invention) The present invention uses an acousto-optic correlator capable of performing correlation calculations in real time, repeatedly performs correlation calculations at a period of several tens of microseconds, and obtains a frequency spectrum in several milliseconds. The purpose of this invention is to construct a device that is simpler and cheaper than conventional digital correlators and the like.

(Basic technology) Before describing embodiments of the present invention, the configuration and operation of an acousto-optic correlator will be explained. FIG. 1 is a schematic diagram of the configuration of a conventional acousto-optic correlator. In the figure, two electrical signals to be subjected to correlation calculation are applied as external modulation signals to a pair of amplitude modulators 1a and 1b. The modulated signal subjected to amplitude modulation within the amplitude modulator is transmitted to the ultrasonic optical modulator 2.
A pair of ultrasonic transducers 3a, 3 disposed within
It is a sine wave vibrating at a resonant frequency of b, and is generated by a pair of sine wave oscillators 4a and 4b.
The sine waves amplitude-modulated by the pair of amplitude modulators are suitably amplified in power by a pair of signal amplifiers 5a and 5b, and applied to the pair of ultrasonic transducers. Forming a spatial ultrasound signal. These two ultrasonic signals propagate at the speed of sound within the ultrasonic light modulator, reach a pair of ultrasonic absorption members 6a and 6b provided oppositely, are absorbed, and disappear. Next, the operating principle in which these two ultrasonic signals are subjected to a correlation calculation in real time by the Fourier transform optical system 100 will be described. The light source of the conversion optical system is a laser 7, and the beam width of the laser beam emitted from this is expanded by a lens.
The ultrasonic light is incident on the ultrasonic light modulator. The light beam within the ultrasonic light modulator crosses the two ultrasonic signals at right angles and undergoes phase modulation. When this phase-modulated light is converged again by a lens, several diffracted light spots are generated on the focal plane of the lens. Among these diffracted light bright spots, only the bright spots that are the result of being diffracted twice by crossing the two ultrasound signals in the ultrasonic modulator are detected by the spatial light intensity filter 8, and the amount of light is measured by the photoelectron. A converter 9 performs photoelectric conversion, an amplifier 10 amplifies and detects a current value I(t). In this case, if the envelope of the spatial phase pattern due to the two ultrasonic signals in the ultrasonic optical modulator is U ₁ (x), U ₂ (-x), then the following equation holds according to Rayleigh's theorem. It has been known. However, the (-) sign of U ₂ (-x) indicates that the envelope time waveforms U ₁ (t) and U ₂ (t) of the ultrasound signals applied to the ultrasound transducer are expressed in the space within the ultrasound optical modulator. When converting into a waveform, the ultrasonic transducers are arranged facing each other, which means that if the same spatial axis x is used, the spatial positions are opposite.

∫｜U ₁ (x)｜ ² ｜U ₂ (−x)｜ ^{2 dx} =∫｜F _{ U ₁ (x) ₁ (x) U ₂ (-x)} indicates the Fourier transform of the ultrasound signal envelopes U ₁ (x), U ₂ (-x), and α is the spatial axis parallel to the spatial axis x on the Fourier transform plane. represents. The right side of equation (1) is U ₁ (x) and U ₂ (−x)
The output current I of the photoelectric converter indicates the total light amount of the optical image obtained by Fourier transforming the product of
(t). Also, the ultrasound signal envelope U ₁
Considering that (x) and U ₂ (-x) move in opposite directions within the ultrasonic optical modulator at an ultrasonic velocity v, the following equation can be derived from equation (1).

In equation (2), d represents the width of the luminous flux. From equation (2), it can be seen that the photoelectric converter output current C(t) represents the calculation result of the square value superposition of each of the ultrasound signal envelopes U ₁ (t) and U ₂ (t). However, the square value superposition integral shown in equation (2) is different from the general form, and is a form in which both U ₁ (t) and U ₂ (t) move (delay). In addition, if U ₂ (t) is periodic like a continuous sine wave and has symmetry, consider that U ₂ (t) = U ₂ (-t) ...(3), and (2 )ceremony , which represents the square value cross-correlation calculation of U ₁ (t) and U ₂ (t).

(Summary of the Invention) The present invention uses the acousto-optic correlator described above, and uses a linear FM signal whose frequency changes linearly with time as one of the two input signals, and uses the other as an unknown signal to mutually interact. The configuration is such that a correlation calculation is performed and the correlation output is displayed so as to temporally represent frequency components.

(Configuration) FIG. 2 is a configuration diagram of an embodiment of the acousto-optic spectrum analyzer of the present invention. The configuration and operating principle of the acousto-optic correlator 101 are as described above. The linear FM signal generator 102 consists of a sine wave signal generation circuit 11, a sweep signal generation circuit 12, and a frequency modulation circuit 13, in which the sine wave signal is linearly frequency modulated by the sweep signal in the modulation circuit. A linear FM signal is generated. This linear FM signal is input to the correlator and cross-correlated with the unknown signal input from the signal-under-measurement input terminal 14. Since the frequency of the linear FM signal changes in proportion to time, the correlation output represents the degree of similarity between the frequency components contained in the unknown signal and the frequency components of the linear FM signal over time. That is, as in the case where an unknown signal is input to a frequency filter whose pass band changes continuously, frequency components are outputted over time. Therefore, if this correlation output is displayed in a sweep manner using the sweep signal on an output display 15 such as a cathode ray tube or a chart recorder, a spectrum in which the sweep direction represents the frequency can be obtained.

FIG. 3 shows an example of a linear FM signal used in this correlator. This signal is transmitted to the ultrasonic transducers 3a and 3b.
It is created by 100% amplitude modulating a sine wave with a resonance frequency of , using a linear FM signal, and
Corresponds to (t). It also represents an unknown signal corresponding to U(t) in FIG. 4, which is also obtained by sub-modulating the sine wave signal with the unknown input signal. FIG. 5 shows the cross-correlation output obtained by applying the modulation signals shown in FIGS. 3 and 4 to the vibrator in the correlator and performing correlation calculations. FIG. 5 shows the concept of the entire output waveform, and the detailed numerical precision is omitted. The envelope of the waveform in Figure 5, that is, the magnitude of the correlation value, indicates the frequency component contained in the unknown signal, and as is clear from the figure, it is vertically symmetrical, so only one side of the envelope is detected. As a result, a spectrum waveform 19 as shown in FIG. 6 can be obtained. Currently, the values that can be put into practical use include a resonant frequency of an ultrasonic transducer of 10 to 15 [MHz], a detectable spectral band of 20 [KHz] to 1.2 [MHz],
An operating speed of about 1 [ms] is considered.

The detection state of the frequency spectrum is determined by the duration of the unknown signal (signal length), the duration of the linear FM signal, and the beam width d in the ultrasonic optical modulators 2a and 2b.
It can be divided into two types depending on the relationship. In the first state, as shown in FIG.
This is a case where the length of the unknown signal within b is shorter than the length of the linear FM signal and the beam width d. In this case, the frequency spectrum of the unknown signal, ie, the correlation output, is output until the unknown signal finishes passing through the ultrasonic light modulator. Since the two signals are moving in opposite directions within the modulator,
The maximum length of the linear FM signal to be used in the correlation calculation of the unknown signal is 2d. The frequency resolution of the output spectrum is inversely proportional to the degree of frequency shift, which indicates how much the linear FM signal changes in frequency over time. Therefore, when the length of the linear FM signal is limited as in this state, if the degree of frequency shift is increased for the purpose of expanding the frequency band of the output spectrum, the frequency resolution will deteriorate.
Conversely, if the degree of frequency shift is reduced for the purpose of improving frequency resolution, the detectable spectral band becomes narrower. Furthermore, in order to improve both, it is conceivable to widen the beam width d, but this cannot be increased unnecessarily due to the physical limits of the optical device. Now, in the second state, as shown in FIG. 8, the length of the unknown signal is approximately equal to the beam width d. In this case, increase the length of the linear FM signal. At this time, the unknown signal and the linear signal calculated in correlation within the ultrasonic optical modulator are
The FM signal is only a small part of the total signal.
Therefore, the unknown signal must be stored in a suitable storage device installed outside the device of the present invention, read out repeatedly, and input into the device.

In this state, the length of the linear FM signal can be made sufficiently long, and compared to the first state, desired values of frequency resolution and frequency band can be secured. However, if the resolution and bandwidth are to be improved at the same time, the number of repetitions of the unknown signal must be increased, and the spectrum output time is several [ms].
It becomes longer and longer. Furthermore, an external storage device is required, which complicates the overall configuration of the device.

(Effects) The present invention has the above-described configuration, and has the function of detecting and displaying the frequency spectrum of an electrical signal at high speed by using an acousto-optic correlator capable of real-time operation as a variable frequency filter. be. As mentioned above, there are two types of spectrum detection methods: one in which the entire spectrum can be detected even with a single signal under test, and one in which the signal under test is repeatedly generated to obtain a spectrum with sufficient accuracy. ,
Both methods can be realized with the same device configuration, so if they are used appropriately, the effects can be further increased. Furthermore, if we use a sine wave signal whose frequency changes logarithmically over time instead of a linear FM signal,
A logarithmically compressed frequency spectrum is obtained. Such a function is one of the major features of this spectrum analyzer that uses a correlator as a programmable filter, and has the feature that settings such as frequency axis band and scale interval can be easily realized.

[Brief explanation of drawings]

Fig. 1 shows the configuration of a conventional acousto-optic correlator, Fig. 2 shows an embodiment of the present invention, Fig. 3 shows a linear FM signal, and Fig. 4 shows an unknown signal. , FIG. 5 is a diagram showing the correlation output, FIG. 6 is a diagram showing the spectrum display, FIG. 7 is a diagram showing the state of the correlation calculation, and FIG. 8 is a diagram showing the repeated correlation calculation. In the figure, 1a and 1b are amplitude modulators, 2 is an ultrasonic optical modulator, 4a and 4b are sine wave oscillators, and 5a and 5b
is a signal amplifier, 7 is a laser, 8 is a spatial light intensity filter, 9 is a photoelectric converter, 10 is an amplifier, 11 is a sine wave signal generation circuit, 12 is a sweep signal generation circuit,
13 is a frequency modulation circuit, 15 is an output indicator, 10
0 is a Fourier transform optical system, 101 is an acousto-optic correlator, and 102 is a linear FM signal generator.

Claims (1)

[Claims] 1. An acousto-optic spectrum analyzer for detecting the frequency spectrum of an input electrical signal, comprising: a linear FM signal generator 102 that generates a linear FM signal with a single shot or a constant repetition period; an acousto-optic correlator 101 that receives an electrical signal and the linear FM signal and performs a cross-correlation calculation;
An acousto-optic spectrum analyzer comprising an output display 15 that receives the correlator output and the sweep signal of the linear FM signal generator and displays a frequency spectrum. 2. The acousto-optic correlator 101 includes a pair of sine wave oscillators 4a, 4b; a pair of amplitude modulators 1a, 1b for amplitude modulating the output of the oscillators by the linear FM signal and the input electric signal. and;
The output signals of the pair of amplitude modulators are input signals,
A Fourier transform optical system 100 having an ultrasonic light modulator 2 whose ultrasonic propagation directions are opposite to each other; a laser 7 that is a light source of the optical system; and a spatial light intensity filter that detects output light of the optical system. 8; and a photoelectric converter 9 for photoelectrically converting the output light. 3. The linear FM signal generation device 102 includes a sine wave signal generation circuit 11; a sweep signal generation circuit 12;
The acoustic device according to claim 1 or 2, further comprising a frequency modulation circuit 13 that frequency-modulates the output signal of the sine wave signal generation circuit using the output signal of the sweep signal generation circuit. Optical spectrum analyzer.