RU2267795C1 - Method of measurement of complex excitement amplitude of channels of phased array - Google Patents

Abstract

FIELD: aerial measurement.

SUBSTANCE: method can be used for measurement of directional pattern of phased arrays and detection of damaged channels of phase array when using standard equipment of radar installations which have phased arrays as a part of equipment. Super-high frequency control signal is generated and irradiated by motionless probe at the direction of phased array. Control signal is received by phased array at alternate modulation of signal phase of each channel of phased array and signal is subject to coherent conversion to video frequency followed by selection of quadrature-phase component. Channel is switched to any phase state only once and at any phase state of the channel the analog signal is transformed into digital one K times (value of K varies from 100 to 10000 and higher depending on number of channels). Digital indications are rearranged in such a way that phase of signal of commutated channel in adjacent readings would differ for discrete value of switching of channel's phase. Signal spectrum is calculated. All the spectral components are reduced excluding components of useful signal which number equals to number of phase states of channel. Spectral component corresponding to zero frequency is reduced to zero. Fourier anti-transform is calculated. The sequence calculated has to represent complex excitement amplitudes for any phase state of channel.

EFFECT: improved precision of measurement; reduced time of measurement.

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Description

The invention relates to techniques for antenna measurements and can be used to measure the radiation pattern of a phased antenna array (PAR) and detect faulty channels of the PAR when using standard radar equipment, which includes the PAR.

There is a method of measuring the antenna pattern directly in the far zone, which consists in measuring the signal emitted (or received) by the test antenna located on the rotary support device at various angular positions of the tested antenna [1].

The disadvantage of this method is the need to use large antenna ranges and complex, expensive slewing devices.

A known aperture probe method for measuring the antenna radiation pattern, which consists in calculating it from the measured amplitude-phase distribution of the PAR field, moreover, the field strength at various points of the surface selected for measurement is recorded by a moving field meter [2].

The disadvantage of this method is the low accuracy of measurements when it is necessary to record a field within a large area (for large HEADLIGHTS) using moving mechanical devices.

There is a known switching method for measuring the complex amplitudes of excitation of PAR channels, by which the beam pattern is calculated, which consists in measuring the total PAR signal for various phasing of the PAR and extracting the signals of individual channels from the total received signal by solving a system of linear algebraic equations relating the amplitudes and phases of the signals of individual channels with measured amplitudes and phases of the total received signal [3].

The disadvantage of this method is the presence of errors in determining the complex amplitudes of the excitation due to changes in the phase of the total signal in the measuring equipment due to large changes in the level of the total signal with different phasing phased array.

Also known is a modulation method for measuring the radiation pattern of a PAR, based on spectral processing of signals.

To measure the complex amplitudes of excitation of the PAR channels, the super-high frequency control signal is generated, emitted by a stationary probe, the control signal is received by a phased array during phase modulation of the signal phase of each PAR channel, the signal is transferred to the video frequency, the phase-modulated signal of each PAR channel is isolated and measure the complex amplitudes of the extracted signal. In accordance with the reciprocity of the antennas, the signal can be emitted by the headlamp, and received by the probe.

A device for measuring the characteristics of phased arrays in a modulation manner contains an ultra-high frequency generator connected to the investigated PAR, a fixed probe connected to two phase bridges, to which reference signals with a phase shift of 90 ° are connected from the generator through a phase shifter, the outputs of the phase bridges through integrators are connected to the indicator input [4].

The modulator alternately modulates the phase of each channel with a frequency Q ranging from 0 to 2π. The modulation frequency is selected from the condition Q≪ω (where ω is the frequency of the signal generated by the generator), and the time constant τ of the integrators from the condition τ≫2π / Q. The signal at the input of the probe has the form

where M is the number of PAR channels;

And _m is the excitation amplitude of the m-th channel;

r _m is the distance from the m-th channel to the probe;

φ _m - phase of the excitation of the m-th channel;

k ₀ = 2π / λ;

λ is the wavelength;

A _j , φ _j - respectively, the amplitude and phase of the excitation of the j-th channel;

r _j is the distance from the j-th channel to the probe;

j is the number of the modulated channel;

φ _j (t) is the variable phase component of the modulated channel.

The law of phase change φ _j (t) is continuous or discrete (depending on the specific type of PAR).

The phase of the reference signal is also modulated synchronously with the phase modulation of the channel.

Signals are generated at the outputs of the phasometric bridges.

where B is the amplitude of the reference signal;

ψ ₀ (t) is the phase of the reference signal.

Since the phase of the reference signal is synchronous with the phase of the channel signal, then

φ _j (t) -ψ ₀ (t) = α ₀ = const

Therefore, the output of the integrators will be signals

Further, the signals x _j and y _j enter the indicator, in particular a computer, where, after a simple conversion according to the formula

x _j a _j -y _j b _j = A _j cos (φ _j + α ₀ )

y _j a _j + x _j b _j = A _j sin (φ _j + α ₀ )

in turn, the real and imaginary parts of the complex amplitudes of the excitation of the PAR channels are obtained. The numbers a _j = r _j cos (k ₀ r _j ) and b _j = r _j sin (k ₀ r _j ) for each channel are known and stored in the computer memory.

The radiation pattern of the PAR is found as a discrete Fourier transform of the complex excitation amplitudes of individual channels of the PAR.

The disadvantage of this device is the presence of errors in measuring the complex amplitudes of the excitation of the PAR channels due to signal distortion at the time of switching the phase of the controlled channel (phase shifter) and a long measurement time.

The purpose of the invention is to improve the measurement accuracy of the complex amplitudes of the excitation channels of the HEADLIGHTS and reduce the measurement time.

In order to achieve the goal in a method for measuring complex amplitudes of excitation of channels of a phased array, including generating a super-high frequency control signal, emitting it with a stationary probe, receiving a control signal with a phased antenna array by alternately modulating the phase of the signal of each phased array channel, converting the signal to a video frequency and generating quadrature signal components, the channel is switched to each phase state once; in each phase state of the channel, they are carried out sequentially a series of conversions of the analog signal into a digital form and a series of digital samples of the signal is stored, one by one from each series, all digital samples of the signal are read, the signal of the switched channel is extracted from the obtained sequence, and the complex amplitudes of the channel excitation are calculated.

Figure 1 shows the structural diagram of a device for implementing the method, where the following notation

one - probe; 2 - HEADLIGHTS; 3, 4 - phase detector; 5 - generator; 6 - modulator; 7 - phase shifter 90 °; 8 - synchronizer; 9, 10 - ADC; eleven - a computer.

The device contains a stationary probe 1 installed in the aperture of the investigated PAR 2, the probe 1 is connected to a generator 5, to which the reference inputs of the first phase detector 3 are also connected, and through the 90 ° phase shifter 7 of the second phase detector 4, the output of the PAR is connected to the signal inputs phase detectors 3 and 4, the outputs of which through the ADCs 9 and 10 are connected to the computer 11, the control inputs of the PAR 2 channels are connected to a modulator 6, which is connected to the first output of the synchronizer 8, the second output of which is connected to the computer 11.

The method is as follows.

The generator 5 generates a control continuous microwave signal with a frequency of ω ₀ , which is supplied to the probe 1, emitting a signal in the direction of the HEADLIGHT 2.

The signal at the output of PAR 2, consisting of M channels, is determined by the expression

where t is time;

U _m is the module of the amplitude of the excitation of the m-th channel;

ω ₀ is the circular frequency.

φ _m - the initial phase of the signal of the m-th channel.

When switching the phase state of the phase shifter of the r-th channel at the command of modulator 6, the signal can be represented as

where U _r is the amplitude module of the r-th channel,

φ _r is the initial phase of the signal of the r-th channel,

Δφ _r - discrete phase switching of the r-th channel.

A microwave signal with a frequency of ω ₀ from the generator 5 also arrives at the reference inputs of the phase detector 3 and through the phase shifter 90 ° - phase detector 4.

Phase detectors 3 and 4 convert the output signal of the PAR 2 to the video frequency and extract the in-phase and quadrature components of the signal, respectively

In complex form, the signal at the outputs of phase detectors can be represented as

The right side of the above expression does not depend on time.

According to the commands of the modulator 6, the phase state of the discrete p-bit phase shifter of the channel with a step (discrete) equal to 2π / L (L = 2 ^p ) is successively changed. The signal at the ADC inputs in this case can be represented as

where l is the number of the phase state of the phase shifter.

модуль и аргумент которого зависят от комплексных амплитуд некоммутируемых каналов ФАР. Второе слагаемое (полезный сигнал) - вектор, аргумент которого периодически изменяется на величину 2π/L.The first term (interference signal) in the above expression does not change in time, and it can be represented as a vector

the module and argument of which depend on the complex amplitudes of the non-switched channels of the PAR. The second term (useful signal) is a vector whose argument periodically changes by 2π / L.

Thus, the selection of the spectral components of the useful signal in the modulation method is similar to the task of separating signals reflected from moving targets against the background of reflections from single structures.

Increasing the phase of the r-channel phase shifter by one discrete interval

T ₀ ≥T _g · J ₀ ,

where T _g is the sampling period of the ADC;

J ₀ is the number of ADC samples for one phase state of the channel, and discarding the first J _p = T _p / T _g signal samples obtained during switching T _{p of the} phase shifter from one phase state to another, at the input of computer 11 we get the matrix || U _g || dimension L × K. The value of K = J ₀ -J _p .

Each row of this matrix is samples (digital samples) of the signal with a fixed state of the phase shifter of the r-th channel.

By sequentially reading the columns of the matrix (initially the first column, then the second, etc.), we obtain a sequence x (n) of length N = L × K, for which the phase of the phase shifter of the rth channel in adjacent samples of the signal differs by a discrete of 2π / L.

The obtained sequence is the initial one for estimating the complex amplitudes of the excitation of the rth channel by spectral processing of the signal.

This formation of the sequence reduces the time of its formation, because requires all L switchings of the phase shifter and eliminates the measurement errors of the complex amplitudes of the excitation of the PAR channels due to signal distortion at the moment of switching the phase of the controlled channel (phase shifter).

Multiplying the sequence x (n) by the weight function w (n) and performing the fast Fourier transform (FFT), we obtain the signal spectrum [5]

The choice of the weight function is determined by the required suppression of the interference signal [5]. It is known that for maximum suppression of a signal with a zero frequency in all FFT filters, it is advisable to use a weight function that provides dips in the amplitude-phase characteristic at points that are multiples of 2π / N. Figure 2 shows as an example the amplitude-phase characteristic of the fourth filter of a 16-point FFT.

To isolate the spectral components of the switched channel signal from the spectrum Z (k), it is necessary to choose components with an index k that is a multiple of N / L, i.e.

Z _s (k) = Z (kN / L), k = 0 ... L-1

and zero out the zero spectral component Zs (0) = 0.

Calculating the inverse Fourier transform

where C is the normalization factor, we obtain the complex channel excitation amplitudes s ₀ (l) for each phase state of the phase shifter.

The normalization factor is determined by the expression

The modules and arguments of complex amplitudes s ₀ (l) characterize the amplitudes and phases of the channel excitation in the l state of the phase shifter, respectively.

The synchronizer 8 generates control signals to the modulator 6 and the computer 11, which provides synchronization of the switching time of the phase shifters of the phased array and the recording process of the digitized signals in the computer.

Modeling of the process of measuring complex excitation amplitudes at different ratios of the useful signal to noise and noise, as well as various errors in the installation of the phase shifter phase, confirmed the high accuracy of measuring complex amplitudes.

Table 1, as an example, shows the results of modeling estimates of the complex excitation amplitudes for a three-bit phase shifter (p = 3) for the ratio of the interference amplitude U _c to the amplitude of the useful signal U _r equal to 100 (40 dB) for N = 1024. As the weight function, the Hamming weight function [6] was used.

Table 1. Phase state (l) 0 one 2 3 four 5 6 7 Phase shifter, degrees four 20,2 107.5 121.7 192 213.2 299.5 314.7 The measured phase, hail 5.1 22.3 110.5 124.1 190.5 210.7 296.7 312.7 Phase measurement error, deg -1.1 -2.1 -3 -2.4 1,5 2,5 2,8 2 Amplitude measurement error,% -0.6 -0.5 0.3 0.5 0.6 0.4 -0.4 -0.5

Figures 3 and 4 show the spectra of the signals Z (k) and Zs (k), respectively.

Experimental studies conducted with a phased array consisting of several thousand channels confirmed the possibility of measuring complex channel excitation amplitudes using standard radar equipment with N values greater than 8192.

Thus, from the results of modeling and experimental studies, it follows that the proposed method allows the measurement of complex amplitudes of the excitation channels of a phased array, consisting of several thousand channels.

Literature

1. Fradin A.Z., Ryzhkov E.V. Measurement of the parameters of antenna-feeder devices - M .: Communication, 1972 - 236 p.

2. Methods for measuring the characteristics of microwave antennas. Zakharyev L.N., Lemansky A.A., Turchin V.I. et al. ed. N.M. Tseytlina - M .: Radio and communications, 1985 - 368 p.

3. Copyright certificate of the USSR No. 13225382, cl. G 01 R 29/10, 1987.

4. Copyright certificate of the USSR No. 476522, cl. G 01 R 29/10, 1975 (prototype).

5. Rabiner L., Gould B. Theory and application of digital signal processing. Per. from English under the editorship of Yu.N. Aleksandrov - Moscow: Mir, 1978 - 848 p.

6. Borton D., Ward G. Handbook of radar measurements. Per. from English under the editorship of M.M. Weisbane - M .: Sov. Radio, 1976 - 392 p.

Claims (1)

A method for measuring the complex amplitudes of the excitation of channels of a phased array antenna (PAR), including generating a super-high frequency control signal, emitting it with a stationary probe, receiving a PAR signal with alternating phase modulation of the signal of each PAR channel, converting the signal to a video frequency and generating quadrature components, characterized in that the channel is switched to each phase state once, in each phase state of the channel, a series of s analog signal into digital form and storing digital samples of the signal series, alternately, one from each series, all of the digital read signal samples from the received sequence is isolated dial-channel signal and calculating a complex amplitude of the excitation channel.

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