RU2321003C1 - Method of determining frequency and type of modulation of received signals - Google Patents

Abstract

FIELD: radio electronics.

SUBSTANCE: method can be used for determination of carrier frequency and type of modulation of signals received within specified frequency range. Method of determination of frequency and type of modulation of received signals is based upon search of signals within specified frequency range due to re-tune of super-heterodyne receiver, upon formation of frequency roll-out onto screen of cathode-ray tube, transformation of frequency of received signal and amplification of its voltage, detection and application of signal to vertical-deflecting plates of cathode-ray tube. As a result, pulse is formed onto screen, position of which pulse is used to determine carrier frequency of received signal onto frequency roll-out; effective factor of amplitude modulation, efficiency frequency deviation, spectrum width of received signal, spectrum width depending on efficient frequency deviation are determined. Efficient factor of amplitude modulation and relation of spectrum width to efficient frequency deviation is compared with specified numerical values and amplitude or angular modulation of received signal is determined. Detected video signal is subject to integration and comparison with threshold voltage. In case threshold voltage exceeds, frequency of heterodyne is measured at given moment of time. Code of heterodyne frequency, measured at given moment of time, is compared with codes of heterodyne frequencies measured before. In case the codes to be compared are not equal, specific types of modulation of received signal are provided, video signal is applied to vertical-deflecting plates of cathode-ray tube and code heterodyne frequency, measured at that specific moment of time, is memorized. Excess information can be removed due to exclusion of repeat measurement and registration of carrier frequency in form of modulation of continuous signal to be received.

EFFECT: improved reliability of operation.

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Description

The proposed method relates to the field of electronics and can be used to determine the carrier frequency and the type of modulation of signals received in a given frequency range.

Known methods for determining the frequency and type of modulation of received signals and devices for their implementation (USSR copyright certificates No. 5224138, 620907, 868614, 1000930, 1012152, 1180804, 1187095, 1272266, 1290192, 1354124, RF patents No. 2124216, 2230330, 2276375, US patent No. 4443801. Vakin SA, Shustov LN Fundamentals of radio countermeasures and electronic intelligence. M: Sov. radio, 1968, p. 386-396, Fig. 10.3 and others).

Of the known methods closest to the proposed is the "Method for determining the frequency" (RF patent No. 2230330, G01R 23/00. 2002), which is selected as a prototype.

The specified method provides a determination of the carrier frequency and the type of modulation of the received signal, which are important characteristics of radio emissions, providing a solution to a number of tasks of electronic reconnaissance of electronic equipment (RES).

However, for a panoramic receiver that implements the known method of determining the frequency, the presence of known information about the carrier frequency and the type of modulation of the received signals is characteristic. This is due to the fact that any continuous signal falls into the passband of the panoramic receiver in each cycle of its tuning. Therefore, the redundancy of the received information about the carrier frequency and the type of modulation of the received continuous signal is determined by the number of local oscillator tuning cycles.

An object of the invention is to eliminate redundant information by eliminating the repeated measurement and registration of the carrier frequency and the modulation of the received continuous signal.

The solution of this problem is achieved by the fact that according to the method for determining the frequency and type of modulation of the received signals, based, in accordance with the closest analogue, on the search for a signal in a given frequency range by tuning the superheterodyne receiver, forming a frequency sweep on the screen of a cathode ray tube, frequency conversion the received signal, amplifying it by voltage, detecting and applying to the vertically-deflecting plates of the cathode ray tube, resulting in the formation of an impulse is generated that determines the carrier frequency of the received signal on the frequency sweep, determines the effective amplitude modulation coefficient, effective frequency deviation, spectrum width of the received signal, the ratio of the spectrum width to the effective frequency deviation, compares the effective amplitude modulation coefficient and the ratio of the spectrum width to effective deviation frequencies with certain numerical values and determining the amplitude or angular modulation of the received signal according to the following crit Riyam:

- for amplitude modulation

where m _eff is the effective coefficient of amplitude modulation;

- отношение ширины спектра к эффективной девиации частоты;

- the ratio of the width of the spectrum to the effective frequency deviation;

- for angular modulation

when angularly modulating the frequency and phase detection of the received signal, determining the ratio of the amplitudes of the spectral components taken at the beginning of the frequency axis to the amplitudes of the spectral components taken at a certain distance from the beginning of the axis, comparing the resulting relationship with unity, if this ratio is more than one, we conclude on the frequency modulation of the received signal, if less - on the phase, differs from the closest analogue in that the detected video signal is integrated, compared with thresholds with voltage and when it is exceeded, the local oscillator frequency is measured at a given time, the local oscillator frequency code measured at a given time is compared with the previously measured local oscillator frequency codes, if the compared codes are not equal, they determine the type of modulation of the received signal, supply a video signal to vertically-deflecting plates a cathode ray tube and storing the code of the local oscillator frequency measured at a given time.

The structural diagram of a device that implements the proposed method is presented in figure 1. Frequency-time diagrams explaining the principle of eliminating redundant information about the carrier frequency and the type of modulation of the received signal are depicted in figure 2.

The device contains a series-connected receiving antenna 1, input circuit 2, high-frequency amplifier 4, mixer 6, the second input of which is connected to the local oscillator 5 output, intermediate frequency amplifier 7, first amplitude detector 8, video amplifier 9, fifth key 41, and vertically-deflecting plates a cathode ray tube (CRT) 11, horizontally deflecting plates that are connected to the output of the CRT frequency generating device 10 11. The control inputs of the input circuit 2, high-frequency amplifier 4, local oscillator 5 and devices VA 10 forming a frequency sweep connected to the respective outputs of the search unit 3, which can be used as a sawtooth voltage generator or electric motor. To the output of the high-frequency amplifier 4, a first key 12, a second amplitude detector 13, a first high-pass filter 14, a first quadrator 16, a first voltage divider 17, the second input of which is connected through the first low-pass filter 15 to the output of the second amplitude detector 13, are connected in series, and a first comparison unit 23, the two outputs of which are the first and second outputs of the device. To the output of the first key 12, a frequency detector 18, a second low-pass filter 19, a second quadrator 20 and a second voltage divider 22 are connected in series, the second input of which is connected through the spectrum analyzer 21 to the output of the first key 12, and the output is connected to the second input of the first comparison block 23 . To the output of the first key 12, a second key 24 is connected in series, the second input of which is connected to the second output of the first comparison unit 23, a phase detector 25, a third low-pass filter 26, a third amplitude detector 28 and a second comparison unit 30, the second input of which is connected through a second a high-pass filter 27 and a fourth amplitude detector 29 are connected to the output of the phase detector 25, and two outputs are the third and fourth outputs of the device. A third high-pass filter 32, a sixth amplitude detector 33 and a third comparison unit 34 are connected to the output of the frequency detector 18 in series, the second input of which is connected through the fifth amplitude detector 31 to the output of the second low-pass filter 19, and the two outputs are the fifth and sixth outputs of the device. An integrator 35, a threshold block 36, a third key 37, the second input of which is connected to the output of the local oscillator 5, a frequency meter 38, a fourth comparison unit 39 and a fourth key 40, the second input of which is connected to the output of the video amplifier 9, are connected in series to the output of the video amplifier 9, and the output connected to the second input of the first key 12. The first output of the fourth comparison unit 39 is connected to the second input of the fifth key 41. The sixth key 42 is connected in series to the output of the frequency meter 38, the second input of which is connected to the second output of the fourth block 39 comparison, and block 43 memory, the output of which is connected to the second input of the fourth block 39 comparison.

The proposed method is implemented as follows.

The search for signals in a given frequency range is carried out using a search unit 3, which, according to a sawtooth law, consistently changes the settings of the input circuit 2, high-frequency amplifier 4 and local oscillator 5. At the same time, the search unit 3 controls the frequency sweep generation device 10 on the screen of the cathode ray tube 11.

The received signal after frequency conversion in the mixer 6 and amplification in the intermediate-frequency amplifier 7, detection in the detector 8 and additional amplification in the video amplifier 9 is supplied through a public key 41 to the vertical-deflecting plates of the CRT 11, as a result of which a pulse is generated on the screen (frequency mark ), whose position on the frequency sweep determines the carrier frequency of the received signal.

Modulated oscillation in the most general form can be written:

Here ω _s , φ (t) is the carrier frequency and the initial phase of the oscillation;

φ (t) = ∫ω (t) dt + φ (t) + φ _c is the oscillation phase;

U (t) = U _c [1 + m · sin Ωt] - envelope of the oscillation;

where U _c is the amplitude of the carrier in the absence of modulation;

m is the coefficient of amplitude modulation;

 Ω is the frequency of the modulating function.

For a signal with amplitude modulation (AM), expression (1) will look like:

If the AM signal is fed to the input of the amplitude detector 13 from the output of the high-frequency amplifier 4 through the public key 12, then a voltage is generated at its output:

Therefore, at the output of the amplitude detector 13, when an AM signal is exposed to its input, a modulating function is allocated in which useful information is stored.

If the input of the amplitude detector 13 receives a signal with angular modulation (UM), then U (t) = U _c = const and expression (1) takes the form:

those.

It can be seen from the obtained expressions that in the absence of a parasitic PA with amplitude modulation of the oscillation and parasitic AM with angular modulation of the oscillation, it is possible to distinguish the amplitude-modulated signal from the signal with angular modulation by passing it through the amplitude detector 13.

The following parameters can be used as informative signs of recognition of signals with amplitude and angular modulations:

- effective coefficient of amplitude modulation

- среднеквадратическое значение переменного напряжения сигнала и шума на нагрузке амплитудного детектора 13;Where

- the rms value of the alternating voltage of the signal and noise at the load of the amplitude detector 13;

M (t) = ΔU (t) · sin Ωt is the modulating function;

- effective frequency deviation

where T is the signal duration;

- the spectrum width Δω _{c of the} received signal.

For the AM signal, these signs are equal to:

For the UM signal:

The effective amplitude modulation coefficient m _eff is determined using an amplitude detector 13, a high-pass filter 14, a first low-pass filter 15, a first quad 16 and a first voltage divider 17.

The effective frequency deviation Δω _DEF is determined using a frequency detector 18, a second low-pass filter 19, a second quadrator 20 and a second voltage divider 22.

The width of the amplitude spectrum Δω _{c of the} received signal is determined using the spectrum analyzer 21.

The ratio Δω _c / Δω _def is determined in the voltage divider 22. In the first comparison block 23, the measured values of m _eff and Δω _c / Δω _{def are} compared with certain numerical values of m ₀ and K ₀ . The comparison results determine the type of modulation (amplitude or angular) of the received signal.

If the received signal has angular modulation, then a constant voltage from the second output of the comparison unit 23 is supplied to the control input of the key 24, opening it. In the initial state, keys 12 and 24 are always closed. In this case, the received signal with angular modulation from the output of the high-frequency amplifier 4 through the public keys 12 and 24 is received for further processing.

It should be noted that the recognition of the type of angular (frequency or phase) modulation is a complex technical task. This is due to the difficulty of identifying informative features by which it is possible to distinguish a signal with frequency modulation (FM) from a signal with phase modulation (FM), since frequency and phase modulations, due to the integro-differential coupling between the frequency and phase, the oscillations have much in common with each other, which justifies the existence of the combined term "angular modulation".

Note that due to this connection, frequency modulation is always accompanied by a change in the phase of the modulated oscillation, and when phase modulation is performed, there is always a change in the frequency of the radio signal. These changes are inextricably linked with each other and the whole thing is which of them is primary, i.e. which one is proportional to the modulating function. With frequency modulation, obviously, the primary is the change in frequency, and with phase modulation is the change in the phase of high-frequency oscillations.

It should be noted that the recognition of FM and FM signals with a harmonic modulating function is generally impossible. However, real oscillations have a modulating function much more complex than harmonic. Therefore, there is a certain opportunity for the recognition of FM and FM signals using, as a sign of recognition, the deformation of the modulating function at the output of the frequency 18 and phase 25 detectors.

Let the expansion of the modulating function in a Fourier series on a certain time interval have the following form:

where U _i , Ω _i , φ _i - amplitude, frequency and initial phase of the i-th spectral component.

It is known that at the output of the phase detector 25, the oscillation phase will be distinguished:

and at the output of the frequency detector 18, the differential from the phase is obtained:

Consider the case when the type of detector corresponds to the type of angular modulation of the received signal.

For FM ω (t) = M (t), φ (t) = 0 and at the output of the frequency detector 18 we will have:

When FM ω (t) = 0, φ (t) = M (t) and at the output of the phase detector 25 we will have:

If the type of detector does not match the type of angular modulation, then the following situations are possible.

Let the FM signal be input to the phase detector 25. Moreover, ω (t) = M (t), φ (t) = 0 and at the output of the phase detector 25 we will have:

Analyzing formula (11), we see that the spectrum of FM vibrations after phase detector 25 undergoes deformation. With increasing number of the spectral component, its amplitude will decrease, i.e. the ratio of the amplitudes of the spectral components taken at the beginning of the frequency axis to the amplitude of the spectral components taken at a certain distance from the beginning of the axis will be more than 1.

Now we consider the passage of the FM oscillation through the frequency detector 18.

For FM, ω (t) = 0, φ (t) = M (t) and at the output of the frequency detector 18 we will have:

From formula (12) it is seen that the spectrum of the FM vibration at the output of the frequency detector 18 also undergoes deformation. With an increase in the number of the spectral component, its amplitude will increase, i.e. the ratio of the amplitudes of the spectral components taken at the beginning of the frequency axis to the amplitudes of the spectral components taken at a certain distance from the beginning of the axis will be less than 1.

The received UM signal from the output of the high-frequency amplifier 4 through the public keys 12 and 24 is fed to the inputs of the frequency 18 and phase 25 detectors. Low-pass filters 19 and 26 emit spectral components located at the beginning of the frequency axis. High-pass filters 27 and 32 emit spectral components located at some distance from the beginning of the axis. Amplitude detectors 28, 29, 31, and 33 extract the envelopes of the corresponding spectral components. Blocks 30 and 34 of the comparison determine the ratio of the amplitudes of the spectral components taken at the beginning of the frequency axis to the amplitudes of the spectral components taken at a certain distance from the beginning of the frequency axis, at the outputs of the phase 25 and frequency 18 detectors. Depending on the indicated relationship, a decision is made on the form of the angular (frequency or phase) modulation of the received signal.

If the specified ratio is greater than unity at the output of the phase detector 25, and the indicated ratio is approximately equal to unity at the output of the frequency detector 18, then the received signal has frequency modulation.

If at the output of the frequency detector 18 the ratio of the amplitudes of the spectral components taken at the beginning of the frequency axis to the amplitudes of the spectral components taken at some distance from the beginning of the frequency axis is less than unity, and at the output of the phase detector 25, this ratio is approximately equal to unity, then the received signal has phase modulation.

For a panoramic receiver, which is part of a device that implements the proposed method, the presence of redundant information about the carrier frequency and the type of modulation of the received signals is characteristic. This is due to the fact that any continuous signal falls into the passband Δf _{P of the} panoramic receiver in each cycle of its tuning (figure 2). Therefore, the redundancy of the information obtained is determined by the number of cycles of tuning the local oscillator 5.

To eliminate this drawback, a panoramic receiver with a device for eliminating the re-determination and registration of the carrier frequency and the type of modulation of the received signals can be used.

Keys 40, 41 and 42 in the initial state are always closed.

The received signal, for example, at the carrier frequency f ₁ (Fig. 2) after frequency conversion, detection and amplification is integrated (accumulated) in the integrator 35 and compared with the threshold voltage U _pores in the threshold unit 36. When the threshold level U _pores is exceeded in the threshold block 36 forms a constant voltage, which is supplied to the control input of the key 37, opening it. In this case, the frequency f _{Г1 of the} local oscillator 5 at a given time t ₁ through a public key 37 is measured by a frequency meter 38 in a digital code and is supplied to the first input of the comparison unit 39, the second input of which is supplied with the codes of the previously measured frequencies of the local oscillator 5 from the memory unit 43. In the initial state, there is no information in the memory of the memory unit 43.

If the compared codes are not equal, then the comparison unit 39 generates a constant voltage, which is supplied to the control inputs of the keys 40, 41 and 42, opening them.

In this case, the code of the measured frequency f _{G1 of the} local oscillator in the first cycle of its tuning through the public key 42 enters the memory unit 43, where it is recorded in its memory. The video signal (figure 2, b) from the output of the video amplifier 9 at time t ₁ through the public key 41 enters the vertical deflecting plates of the CRT 11, as a result of which a frequency mark is formed on its screen, the position of which on the horizontal scan uniquely determines the carrier frequency of the received signal. The same video signal through the public key 40 is fed to the control input of the key 12, opening it. In this case, the received signal from the output of the high-frequency amplifier 4 through the public key 12 is fed to the inputs of the second amplitude detector 13, the frequency detector 18, the spectrum analyzer 21, and the second key 24 for further processing in order to determine the type of its modulation.

In the second and subsequent cycles of tuning the local oscillator 5, the continuous signal at a frequency f ₁ will again fall into the passband Δf _{P of the} panoramic receiver (amplifier 7 of an intermediate frequency). In this case, the compared codes will be equal and there are no voltage comparisons at the outputs of block 39, the keys 40, 41, and 42 remain in the closed state.

Therefore, only in the first cycle of tuning the local oscillator 5 is a visual assessment of the carrier frequency f _{1 of the} received signal, determining the type of modulation and writing the frequency code f _{G1 of the} local oscillator at a given time in the memory unit 43.

If, for example, in the second local oscillator tuning cycle, another continuous signal at a frequency f ₂ falls into the passband Δf _{П of the} panoramic receiver, then the device operates in a similar way. In this case, the frequency code f _{Г2 of the} local oscillator 5 measured at time t ₂ is compared with the frequency code f _{G1 of the} local oscillator 5 measured at time t ₁ .

Since these codes are not equal to each other, the comparison unit 39 generates a constant voltage, which is supplied to the control inputs of the keys 40, 41 and 42, opening them. In this case, the value of the carrier frequency f _{2 of the} received signal is visually evaluated on the screen of the CRT 11, the type of modulation is determined by other units of the device, and the frequency code f _{Г2 of the} local oscillator 5, measured at time t ₂ , is recorded in the memory unit 43.

Thus, the proposed method for determining the frequency and type of modulation of the received signals in comparison with the prototype and other technical solutions for a similar purpose eliminates redundant information. This is achieved by eliminating the repeated measurement and registration of the carrier frequency and the type of modulation of the received signal.

Claims (1)

A method for determining the frequency and type of modulation of received signals, based on the search for signals in a given frequency range by tuning the superheterodyne receiver, forming a frequency sweep on the cathode ray tube screen, converting the frequency of the received signal, amplifying it by voltage, detecting and applying to vertically deflecting plates a cathode ray tube, as a result of which a pulse is formed on the screen, by the position of which the carrier frequency is determined at the frequency sweep signal, determining the effective amplitude modulation coefficient, effective frequency deviation, spectrum width of the received signal, the ratio of the spectrum width to the effective frequency deviation, comparing the effective amplitude modulation coefficient and the ratio of the spectrum width to the effective frequency deviation with certain numerical values and determining the amplitude or angular modulation of the received signal according to the following criteria: for amplitude modulation

where m _eff is the effective coefficient of amplitude modulation;

- the ratio of the width of the spectrum to the effective frequency deviation; for angular modulation

the implementation of the angular modulation of the frequency and phase detection of the received signal, determining the ratio of the amplitudes of the spectral components taken at the beginning of the frequency axis to the amplitudes of the spectral components taken at some distance from the beginning of the axis, comparing the resulting relationship with unity, if this ratio is greater than unity, conclude on the frequency modulation of the received signal, if less - on the phase, characterized in that the detected video signal is integrated, compared with the threshold voltage and at excess frequency, measure the local oscillator frequency at a given time, compare the code of the local oscillator frequency measured at that time with the codes of the previously measured local oscillator frequencies, if the compared codes are not equal, they determine the type of modulation of the received signal, supply the video signal to the vertically deflecting cathode ray tube plates, and storing the code of the local oscillator frequency measured at a given time.

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