RU1840949C - Radio pulse detector - Google Patents

Abstract

FIELD: physics, navigation.

SUBSTANCE: invention relates to radar engineering and can be used in receiving channels of radar stations. The result is achieved due to that the device comprises series connected detector, base clipper, signal maximum selection unit, threshold unit, pulse former, first delay unit, series-connected second delay unit, switch, peak clipper, first dispersive delay line, second detector, subtractor unit, modulus computing unit, first adder, ratio computing unit, low-pass filter, second threshold unit and coincidence unit, and series-connected second dispersive delay line, third detector, second adder and voltage divider, the output of which is connected to the second input of the first adder, wherein the output of the pulse former is connected to the control input of the switch and the input of the first delay unit, the output of which is connected to the second input of the coincidence unit, the input of the second delay unit is connected to the input of the first detector, the output of the peak clipper is connected to the input of the second dispersive delay line, the output of the third detector is connected to the second input of the subtractor unit, the output of the second detector is connected to the second input of the second adder, the output of which is connected to the second input of the ratio computing unit.

EFFECT: high noise-immunity.

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Description

The proposed detector relates to radio engineering and can be used in the receiving channels of the radar.

Known detectors of radio pulses that solve the problem of detecting signals against interference (noise).

So, described in the book of M. Skolnik “Introduction to the Technique of Radar Systems”, “Mir”, M., 1965, UDC 621.396.96, p. 509, the detector continuously compares the current value of the mixture of signals and noise with a certain threshold level.

The disadvantage of such a detector, especially when it is implemented in channels for automatic processing of information, is that one long pulse (by the level of the detection threshold) gives information about the presence of several targets in adjacent range resolution elements, since it is believed that the target is in all elements ranges where the threshold level is exceeded at the corresponding time points.

A detector is also known (Pungin N.A. On the question of losses due to accumulation due to temporary quantization, Bulletin of the Leningrad Electrotechnical Institute named after V.I. Ulyanov (Lenin), vol. 84, L., 1969, p. 109, paragraph 4) , in which not all current values of the input mixture are compared with the threshold level, but only the values corresponding to the maxima of the envelope of the mixture. This detector is accepted by us as a prototype.

It contains a detector connected in series, a limiter from below, a maximum isolator, a threshold node, and a video pulse shaper.

When the radar detectors are used in the digital signal processing equipment described on page 509 of M. Skolnik’s book, the signals are temporarily sampled, the range is divided into sections approximately equal to the radar resolution in range, and the signals are independently detected in each range section by comparing the level of the input mixture with the threshold. If the signal enters simultaneously at two adjacent sections of the range and has a sufficiently high level, then it can exceed the threshold in both sections, the processing equipment will record the presence of two targets, which is a disadvantage.

The prototype device was created specifically to eliminate this drawback, to eliminate the occurrence of an interference pulse. Since even a powerful signal that partially fell into two sections of the range has one envelope maximum, then finding this maximum and comparing it with the threshold level, we get only one target, and the appearance of interfering pulses, “multiplication” of targets, is excluded. The lower limiter here serves to unload the chains of finding the maxima from the operations of finding the maximums of pulses of known small amplitude.

In the arsenal of the United States aviation (see Models of electronic countermeasures of aviation of a probable enemy, there are many types of impulse jamming stations: AN / ALQ-19, AN / ALQ-32, Granger, AN / ALQ-41, AN / ALQ-51, AN / ALQ-15, AN / ALQ-35, etc.

Stations can create imitating interference for auto-tracking channels (leading in range and angular coordinates), and masking interference for detection channels. In the form of masking interference using synchronous multiple response interference (MOS) and chaotic impulse noise.

Interference both in stations that interfere with auto-tracking channels (AN / ALQ-41, AN / ALQ-51, etc.) and in stations that create MOS (AN / ALQ-15, AN / ALQ-35, etc.) has carrier frequency root mean square error of 2 MHz. Error distribution is normal, with a zero mean value.

To assess the noise immunity of a known detector, we take the useful signal duration τ _c = 0.2 μs, 4 MHz bandwidth of the IF amplifier, and a rectangular shape of the amplitude-frequency characteristic of the IF amplifier. Rectangular pulses take interference with the duration τ _p = 0.4 psec, with neflyuktuiruyuschim level exceeding the detection threshold is 6; 12; 18 dB

The dependence of noise attenuation under such conditions is determined by curve 3 of the graph in Fig. 10.15 of the book by V.I. Tikhonov "Statistical Radio Engineering", "Sov. Radio", M., 1966, p. 435. For the indicated interference levels, the detector threshold corresponds to ρ = 0.4; 0.2; 0.1. From the graph it follows that the interference pulses will exceed the detection threshold and give a false alarm in cases where the mismatch of their carrier frequency relative to the carrier frequency of the signal is less than 0.8 / τ _p ; 1.8 / τ _p ; 3 / τ _p , i.e. 2; 4,5; 7.5 MHz respectively. We will find the probability of these events using the table of the probability integral (ibid., P. 666).

The indicated boundaries of the detunings correspond to Z = 1; 2.25; 3.75. The probability that the interference exceeds the threshold is 0.68; 0.976; 0.9998.

From this it follows that the known detector of signals from impulse noise is not protected, the probability of interference passing to the output of the unit at real interference levels is close to unity. The frequency selectivity of the preceding intermediate-frequency amplifier and the matched filter, as follows from the obtained results, gives some effect only at low interference levels. At high levels of interference, even weakened due to a guidance error in frequency, the interference creates a signal in the receiver passband that is sufficient to overcome the threshold. Therefore, to ensure noise immunity of the equipment, additional measures are necessary to suppress interference.

If the interference is chaotic (HIP), then it can be suppressed with certain success in the future by known methods of inter-period multiplication, etc. (Theoretical Foundations of Radar, under the editorship of YD Shirman, Sov. Radio, Moscow, 1970, Fig. 7.53).

However, all these methods of additional processing require for joint processing of signals of several periods of repetition of the radar, which is not always desirable, lead to some deterioration of the signal / own noise of the receiver. They fundamentally cannot provide suppression of synchronous impulse noise (MOS, etc.).

The introduction of amplitude limiters, for example, in the form of a SHOW block (IS Gonorovsky, Radio-technical targets and signals, Sov. Radio, Moscow, 1971, §16.12), in some cases allows suppressing impulse noise. However, they create non-linearity of the amplitude characteristic, which is unacceptable in the modes of measuring angular coordinates, etc. SHOW units work effectively only in cases where the duration of the interference is much shorter than the duration of the useful signal, if their durations differ little, the interference passes through the device in the same way as the useful signal, there is no gain in noise immunity (ibid., P. 623, 624).

Thus, the known detector of radio pulses is not resistant to impulse noise. Introduction to the receiving channel of known additional interference suppression units does not solve the problem completely, but at the same time as increasing noise immunity it worsens some other characteristics of the receiver (linearity of the amplitude characteristic, sensitivity, etc.). But the main thing here is that synchronous interference is not suppressed (the repetition frequency of which is equal to or a multiple of the repetition frequency of radar probe pulses), the pulse duration of which is close to the duration of the radar probe pulses.

The aim of the present invention is to increase the noise immunity of the detector of radio pulses.

This goal is achieved by the fact that in the detector of radio pulses, which contains a series-connected detector, a bottom limiter, a maximum isolator, a threshold node and a video pulse shaper, a coincidence node and a series-connected gating node, the signal input of which is connected to the detector input and the control input to the shaper output, are introduced video pulses, an upper limiter, an envelope bandpass filter and a second threshold node, the output of which is connected to one of the inputs of the match node I, whose second input is connected to the output of the video pulse shaper.

Introduced a gating unit, a top limiter, a band-pass filter with envelope separation and coupling the signal input of the gating unit with the input of the RF signal of the device, and the control one with the output of the video pulse shaper, and the use of the indicated limiting features of the device made it possible to isolate a signal characterizing the instantaneous frequency (single pulse) at the moments of maximum values of the envelope of the input signal.

Together with an additional threshold node that sets the limits of possible changes in the instantaneous frequency of the useful signal, and with a coincidence node, these design features made it possible to use the amplitude sign to match the tolerance of the instantaneous frequency of single radio pulses at moments corresponding to the envelopes of the input signal mixture with interference.

Figure 1 presents the functional diagram of the proposed detector.

Figure 2 presents the amplitude-frequency characteristics of the IF (a) and a band-pass filter (b).

Figure 3 presents the timing diagrams of the processes of passage of signals and interference through the blocks of the proposed device.

The proposed detector comprises interconnected detector 1, a limiter from the bottom 2, a maximum isolator 3, a threshold node 4, a video driver 5, a delay unit 6, a match node 7, and also a delay unit 8, a gate unit 9, a limiter interconnected in series from above 10, a band-pass filter 11 with an envelope separator, a threshold node 12. The input of the delay node 8 is connected to the input of the detector 1, the output of the threshold node 12 is connected to the second input of the coincidence node 7.

The band-pass filter 11 contains two dispersive ultrasonic delay lines (DLS) 13, 14, the inputs of which are interconnected and are the input of the band-pass filter 11; two detectors 15, 16 of the output signals DULZ 13, 14; a subtractor 17 and an adder 18 of the output signals of the detectors 15, 16; a computer module 19, the input of which is connected to the output of the subtractor 17; a voltage divider 20, the input of which is connected to the output of the adder 18; an adder 21, one input of which is connected to the output of the computer module 19, and the other to the output of the voltage divider 20; ratio calculator 22, the dividend input of which is connected to the output of the adder 18, and the divider input is connected to the output of the adder 21, and a low-pass filter (LPF) 23, the input of which is connected to the output of the ratio calculator 22, and the output is the output of the band-pass filter 11.

Detector 1 is a conventional amplitude detector. The limiter from the bottom 2 is a diode, the direction of switching of which coincides with the polarity of the output signal of the detector 1, but a blocking voltage is applied to it, equal to twice the rms voltage of the noise.

Highlighter 3 is a differentiating element that determines the moment of changing the sign of the derivative of the signal envelope and only at that moment feeds its current input envelope value to threshold node 4. Threshold node 4 has a threshold level equal to twice the rms noise value at the detector input.

Shaper of video pulses 5 - kipp relay. At each start from the side of threshold node 4, it generates at the output one pulse of a rectangular shape with a duration of 0.5 μs and an amplitude of 3 volts.

Delay node 6 - 0.5 µsec low-frequency delay line. Match Node 7 is a standard microcircuit that performs the logical AND operation.

Delay node 8 is a delay line of intermediate frequency signals with an average bandwidth of 30 MHz, a bandwidth of 5 MHz, and a delay of 0.25 μs. The gating unit 9 is a diode bridge switch of intermediate frequency signals, transmits signals only during the existence of the gating pulse coming from the video pulse shaper 5.

Top limiter 10 - two-way symmetrical diode limiter of intermediate frequency signals.

Dispersion delay lines 13, 14 have an average frequency bandwidth of 30 MHz, a bandwidth of 8 MHz, and slope dispersion characteristics of +16 μs / MHz and -16 μs / MHz, respectively. The maximum delay in each of these lines is 0.5 μs. Detectors 15, 16 are amplitude, diode. Adders 18, 21 and subtractor 17 are balanced. The voltage divider 20 is a resistive divider with a ratio of A = 10. The low pass filter 23 has a bandwidth of 2 MHz. The calculator of the module 19 is made in the form of a differential amplifier, the inputs of which are connected to the output of the subtractor 17 through diodes connected in a different direction. The delay node 16 is a low-frequency multi-tap delay line, the delay value is 0.5 μs.

The proposed detector operates as follows.

The detector 1 selects the envelope of the signal mixture with noise entering the input of the device. The maximum separator 3 selects the maximum values of the envelope and only feeds them to the threshold node 4. When this threshold is exceeded, a rectangular video pulse with a duration of 0.5 μs corresponding to the duration of the useful signal is formed in node 5. The limiter from the bottom 2 cuts off signals whose levels are obviously small compared to the threshold in the node 4, and thereby unloads the maximum 3 isolator from unnecessary operations. The video pulse from the output of node 5 generates a radio pulse in the gating node 9, the filling frequency of which is the filling frequency of the input signal mixture with interference. The delay unit 8 delays the input mixture so that the gating video pulse is located symmetrically with respect to the maximum point of the envelope, while the filling frequency of the radio pulse generated in node 9 will correspond exactly to the instantaneous frequency of the mixture at the maximum point. The limiter from above 10 eliminates the influence of the amplitude of the signals on the accuracy of the subsequent determination of the instantaneous frequency.

In the band-pass filter 11 is a narrow-band filtering of radio pulses with the allocation of the envelope of the signal. The principle of narrow-band filtering of radio pulses is as follows. The radio pulse arriving at the input of the bandpass filter (without intrapulse modulation) is fed to the inputs of two dispersion delay lines that differ from each other only in the sign of dispersion. The maximum delay in the line is approximately equal to the duration of the input radio pulse. At the output of these dispersion delay lines, when exposed to an input radio pulse, radio pulses will also appear, moreover, at the middle (intermediate) frequency, the envelopes of these output radio pulses are exactly the same, when a detuning appears, differences in envelopes appear (basically, shifting them in different directions).

Having selected the envelopes with detectors 15, 16 and subtracting the obtained video pulses, we get zero at the intermediate frequency and a monotonic increase in the difference with an increase in detuning in any direction. Dividing the total signal of the video pulses by the module of their difference, we obtain a selective amplitude-frequency characteristic with a maximum at the nominal intermediate frequency. The passband is set by a voltage divider 20, the output signals of which are summed with the signal difference module before calculating the ratio in block 22. The low-pass filter smooths the level ripples generated by the pulses during processing. The duration of the output pulse of the low-pass filter is 0.6 ÷ 1 μs.

The amplitude-frequency characteristics of the amplifier, from which the signals are sent to the detector, and the band-pass filter 11 are shown in figure 2, a and figure 2, b, respectively. The threshold level of the node 12 is shown in Fig.2, b dash-dotted line. The passband of the filter 11 at the threshold level in node 12 is set to 40 kHz.

The passage of signals and interference through the proposed detector is shown in Fig.3. Figure 3, a shows the envelope of the mixture of signals and noise at the input of the device (signal at the output of detector 1). The horizontal dashed line shows the level of restriction from below in the node 2, figure 3, b - signals at the output of this node. In Fig. 3, the dotted lines show the video pulses generated in the node 5. The leading edge of each pulse coincides with the maximum signal at the output of node 2. The delay of the input signal in the delay node 8 ensures that the maximum envelope is aligned with the center of the gate pulse. This is conventionally shown in figure 3, in the shift of a rectangular pulse (solid line).

Figure 3, g shows the radio pulses generated in the gating node 9 by the output video pulses of the node 5 from the input mixture of signals and interference delayed in the node 8.

Figure 3, a and 3, b show 3 pulses exceeding the threshold. The first of them is considered a signal from the target, the frequency of filling it lies within the bandwidth of the filter 11.

The second and third pulses are considered interference. When the pulse passes from the target through the band-pass filter 11 and the threshold node 12, a video pulse is formed (Fig. 3, e), the presence of which confirms that the filling frequency of the radio pulse is normal for useful signals. The delay unit 6 equalizes the delays of the signals corresponding to each other, received at the inputs of the coincidence node 7, which is necessary for its normal operation.

Thus, the signal at the detector output (Fig. 3, e) will be only in the case when the input signal has not only an amplitude exceeding the threshold at node 4, but also has an instantaneous frequency corresponding to normal Doppler frequency shifts of the useful signal.

Useful signals from the targets will pass to the detector output at the same levels as in the known detector.

The interference described above will go to the detector output only in those cases when they have not only a large amplitude, but also their filling frequency will fall into the passband of filter 11 (in the f band of _p.s. ± 20 kHz).

The probability of this is determined for the above interference characteristics also from the table of the probability integral (book by V.I. Tikhonov, p.666). With a standard error of frequency σ = 2 MHz and a filter band of 11 equal to 40 kHz (f _p.h ± 20 kHz), the value of Z will be 20 kHz: 2 MHz = 0.01. The probability of the instantaneous interference frequency getting into the passband is 0.008.

Since the probability of passing an interference pulse to the output in a known detector is 0.68 for the conditions considered above; 0.976; 0.9998, and in the proposed for all three cases 0.008, the gain in noise immunity, characterized by the probability of interference passing to the detector output, is estimated as the ratio of the probabilities and is, respectively, 85, 122 and 124.

It should be emphasized that in the proposed detector, such interference pulses are also suppressed that in conventional equipment cannot usually be suppressed, for example, sequences of synchronous pulses whose envelope shape and the law of intrapulse modulation are close to those of radar signals. It is only necessary that there be at least a small error in setting the carrier frequency of the interference, and such an error, as indicated in the literature, takes place in real interference stations.

Claims (1)

A radio pulse detector comprising a series-connected detector, a bottom limiter, a signal maximum extraction unit, a threshold block and a pulse shaper, characterized in that, in order to increase noise immunity, a first delay unit is introduced, connected in series with a second delay unit, a key, a limiter from above, a first dispersion delay line, second detector, subtraction unit, module calculation unit, first adder, ratio calculation unit, low-pass filter, second threshold block, and the block is the same I and the second dispersion delay line, the third detector, the second adder and the voltage divider connected in series, the output of which is connected to the second input of the first adder, the output of the pulse shaper connected to the control input of the key and the input of the first delay unit, the output of which is connected to the second input of the coincidence unit , the input of the second delay unit is connected to the input of the first detector, the output of the upper limiter is connected to the input of the second dispersion delay line, the output of the third detector is connected to the second input house subtracting unit, and the output of the second detector is connected to a second input of the second adder, the output of which is connected to the second input of the calculation unit in the relationship.

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