RU2439595C1 - Radiometric reflection coefficient meter - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: radiometric meter, having a receiver, an antenna, a first directional coupler, a current source and a noise generator connected in series, has, between the output of the first directional coupler and the input of the receiver, a second directional coupler, a microwave switch, the input of which is connected to the output of the noise generator, the first and second outputs are respectively connected to second inputs of the first and second directional couplers, and the third output is connected to a matched load; also includes a synchronous low-pass filter, a high-pass filter, a comparator and a control unit, all connected in series to the output of the receiver, wherein the second input of the comparator is connected to the common bus of a radiometer, the first output of the control unit is connected to the control input of the synchronous low-pass filter and merged with the control input of the microwave switch, and its second output is the output bus of the radiometer.

EFFECT: simple device and algorithm of processing signals without converting their form and high accuracy of measurements.

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Description

The invention relates to microwave radiometry and can be used in industry, medicine to assess the structure of the object under study, the dynamics of its change in time according to the results of measuring the power reflection coefficient in the microwave range by measuring the reflectivity of electromagnetic noise signals.

Known radiometer selected as an analogue [A.S. USSR 1105832, MKI ³ G01R 29/08. Microwave Modulation Radiometer / BCAblyazov, K.T. Murzabulatov. - Publ. in B.I. No. 28, 1984], consisting of (Fig. 1) antenna 1, directional coupler 2, modulator 3, noise generator 4, receiver 5, first 6 and second 7 synchronous detectors, subtractor 8, first 9 and second 10 reference reference oscillators frequencies, the first 11 and second 12 registrars.

и

где G - коэффициент передачи приемника; Т - термодинамическая температура изучаемого объекта; R - коэффициент отражения по мощности на границе "антенна-объект"; Т₀ - термодинамическая температура модулятора; Т_гш - сигнал генератора шума, поступающий в тракт антенны через направленный ответвитель. В синхронных детекторах происходит выделение постоянных уровней напряжения из огибающих модулированных во входном устройстве радиометра сигналов. Выходной сигнал генератора шума 4 модулируется сигналом меандра с одинаковой частотой работы второго синхронного детектора 7. На выходе второго синхронного детектора напряжение равно:

Напряжение U₂ пропорционально коэффициенту отражения R и поступает на второй регистратор 12. Градуировка шкалы данного регистратора выполнена в единицах коэффициента отражения:

. Таким образом, второй регистратор отображает показания коэффициента отражения на границе антенны и объекта.In the radiometer, the directional coupler 2 is turned on so that the signal power of the noise generator 4 is directed towards the antenna and reaches the input of the radiometer only after reflection from the object of study. The control signals of the operation of the modulator 3 and the noise generator 4 are in the form of a meander and are generated by the reference pulse generators 9 and 10. The repetition rate of the control signal of the noise generator is selected less than the frequency of the signal for controlling the modulator. As a result, two periodic sequences of modulated signals take place at the output of the receiver:

and

where G is the gain of the receiver; T is the thermodynamic temperature of the studied object; R is the power reflection coefficient at the antenna-object interface; T ₀ - thermodynamic temperature of the modulator; T _gsh - the signal of the noise generator entering the antenna path through a directional coupler. In synchronous detectors, constant voltage levels are extracted from the envelopes of the signals modulated in the input device of the radiometer. The output signal of the noise generator 4 is modulated by a meander signal with the same frequency of operation of the second synchronous detector 7. At the output of the second synchronous detector, the voltage is:

The voltage U _{2 is} proportional to the reflection coefficient R and is supplied to the second recorder 12. The graduation of the scale of this registrar is made in units of reflection coefficient:

. Thus, the second recorder displays the readings of the reflection coefficient at the boundary of the antenna and the object.

The disadvantage of this circuit analog radiometer is the dependence of the readings on the transmission coefficient G of the measuring path. The drift and fluctuation of the transmission coefficient reduce the accuracy of the measurements.

A known radiometer for measuring the electromagnetic radiation of objects close to the antenna, selected as a prototype [A.S. USSR 1626210, MKI ⁵ G01R 29/26. Modulation radiometer / BCGaevsky, S.V. Marechek, Yu.V. Meshkov, Yu.N. Muskin, V.M. Polyakov. - Publ. in B.I. No. 5, 1991], comprising an antenna 1, a modulator 2 operating according to the principle of signal reflection, a directional coupler 3, a reference noise generator 4, a current source 5, a supplying noise generator, receiver 6, a switch 7 with a configuration of 1 × 2, the first 8, second 9 and third 10 synchronous detectors, first 11, second 12 and third 13 low-pass filters, adder 14, subtraction unit 15, first 16 and second 17 analog signal dividers, recorders 18 and 19, first 20 and second 21 reference frequency generators modulation.

In the radiometer, the thermal radiation power of the investigated object is received at two different specified levels of the noise signal generated by the reference generator. The master oscillator 20 generates pulses that follow with a frequency f ₁ and duty cycle 2. The operation of the master oscillator 21 is synchronized by the pulses of the generator 20, and a rectangular pulse sequence is generated at the output of the generator 21, which also has a duty cycle equal to two, but the next with a frequency half as much , f ₂ = f _1/2 . The noise generator 4 produces two different reference signals T _gsh1 and T _gsh2 , and T _gsh2 / T _gsh1 . Modulation of the GSh noise generator is performed by pulses of the master oscillator 20. A directional coupler is included in the measuring path so that the signal of the noise generator is directed towards the output of the modulator.

, где k - постоянная Больцмана, df - полоса усиливаемых частот, Т_ш - эффективная температура собственных шумов приемника, приведенная к его входу. В следующем временном промежутке модулятор также закрыт, но генератор 4 вырабатывает сигнал величиной Т_гш2>Т_гш1. Поэтому мощность сигнала на входе приемника возрастает и становится равной:

.The master oscillators control the modulation of the signals in the receiving path. Since the oscillation phases of the oscillators are strictly synchronized, there are four repeating time intervals due to the different states of the modulator 2 and the noise generator 4. Thus, the full modulation period consists of four time intervals of equal duration. During the first modulation interval, the modulator 2 is turned off. The signal of the noise generator, equal to T _gsh1 at this beat, is completely reflected from the output of the open modulator, and is fed to the input of the receiver, and the signal power at its input is equal to:

where k is the Boltzmann constant, df is the band of amplified frequencies, T _W is the effective temperature of the receiver's own noise, reduced to its input. In the next time period, the modulator is also closed, but the generator 4 generates a signal of magnitude T _gsh2 > T _gsh1 . Therefore, the signal power at the input of the receiver increases and becomes equal to:

.

(считается, что антенна имеет высокий коэффициент полезного действия и ее собственное излучение пренебрежимо мало).In the next two time intervals, the modulator is open. The thermal radiation power of the object is supplied to the "antenna-object" interface. A part of the power proportional to TR, where R is the power reflection coefficient at the antenna-object boundary, is reflected from the interface and damped in the medium of the object. The remaining part of the power, proportional to T (1-R), is received by the antenna and fed through the open modulator to the receiver input. The signal of the noise generator, equal at this moment T _gsh1 , passes through an open modulator, is partially reflected from the “antenna-object” interface with a coefficient R, and also enters the receiver input. Thus, the total power of the signals at the input of the receiver is equal to:

(it is believed that the antenna has a high efficiency and its own radiation is negligible).

.Similarly, for the fourth, last modulation interval, the receiver input power is:

.

;

.The output voltage of the receiver is a repeating sequence of voltages U ₁ , U ₂ , U ₃ , U ₄ proportional to the powers of the input signals W ₁ , W ₂ , W ₃ , W ₄ with a proportionality factor G. The output voltages of the second 12 and third 13 filters are constant the time of which is significantly longer than the modulation period, after synchronous detection in the detectors 9 and 10, respectively, are equal to:

;

.

. Откуда

. Коэффициент отражения на границе раздела "антенна-объект" не зависит от коэффициента передачи радиометра.To study the structure of an object with a radiometer, the reflection coefficient at the antenna-object interface is measured. The signal U _f2 from the filter 12 is supplied to the division circuit 17 as a dividend, and the output voltage U _{f3 of the} filter 13 is the divider. Thus, the voltage at the input of the second recorder 19 is equal to:

. Where from

. The reflection coefficient at the antenna-object interface does not depend on the transmission coefficient of the radiometer.

In this radiometer, in order to isolate informative voltage levels from a complexly modulated signal sequence, a large number of types of signal conversion are used by various functional analog blocks (synchronous detectors, low-pass filters, dividers). Numerous sequential analog signal conversion procedures (both waveforms and amplitudes) lead to additional measurement errors. The noise generator produces two levels of the output signal, the switching between which is pulsed and which must be precisely tuned and calibrated.

The present invention solves the problem of simplifying the device for measuring the reflection coefficient and the signal processing algorithm without converting their shape and improving the accuracy of measurements.

To achieve this technical result, a second directional coupler, a microwave switch, the input of which is connected to the output of the generator, is inserted between the output of the first directional coupler and the input of the receiver, a second directional coupler, a serially connected current source and a noise generator, to a radiometric meter containing a receiver, an antenna, a first directional coupler, a serially connected current source and a noise generator noise, the first and second outputs are connected respectively to the second inputs of the first and second directional couplers, and the third output is connected to at the same time, a synchronous low-pass filter, a high-pass filter, a comparator, and a control unit are connected connected to the output of the receiver, the second input of the comparator connected to a common bus of the radiometer, the first output of the control unit connected to the control input of the synchronous low-pass filter and combined with the control input of the microwave switch, and its second output is the output bus of the radiometer.

Figure 1 presents the structural diagram of the analogue.

Figure 2 shows the structural diagram of the prototype.

Figure 3 presents the structural diagram of the proposed meter reflectance radiometric type.

Figure 4 shows a diagram of a synchronous low-pass filter.

Figure 5 shows the structural diagram of the control unit.

Figure 6 presents timing diagrams explaining the principle of operation of the radiometric meter.

The composition of the radiometric reflection coefficient meter (Fig. 3) includes antenna 1, first 2 and second 4 directional couplers, microwave switch 5, noise generator 6, current source 7. The measuring channel consists of a receiver 8, a synchronous low-pass filter 9, a filter high frequencies 10, comparator 11, control unit 12, from the second output of which the signal enters the output bus 13.

The reference signal of the noise generator 6 through the microwave switch 5 passes to the second inputs of the identical directional couplers. The first 2 and second 4 directional couplers are included in the input circuit so that for the first coupler the signal of the noise generator goes to the output of the antenna 1, for the second to the input of the receiver 8. The reference signal is generated by a semiconductor noise generator using an avalanche-span diode, through the active zone of which the current of source 7 flows. The microwave switch 5 consists of three high-frequency keys, some of the terminals of which are connected together and connected to the noise generator 6 (they are the input of the switch), while others you ode keys connected to the three outputs of the switch. The switch operates in selector mode. Depending on the control signals, only one key is switched, and thus the signal of the noise generator is supplied to only one of the selected outputs.

The measuring channel is a radiometric receiver with a linear transfer characteristic and a band of received frequencies df. The receiver 8 includes high-frequency amplifiers, a band-pass filter, a quadratic detector, emitting an envelope of modulation signals, linear low-frequency pulse amplifiers. A synchronous low-pass filter 9 pre-filters the signal, reduces the fluctuation component in the detected signals and thereby eliminates the overload of the comparator 11. The filter circuit is known [for example, RF Patent No. 2093845, IPC G01R 29/08, G01S 13/95. Zero radiometer / Filatov A.V. - No. 5057798/09; declared 08/05/92; publ. 10.20.97. Bull. No. 29. - 7 S.] and is shown in figure 4. It consists of three single-link integrating RC circuits in which the resistor is common to all circuits, and the constant components of the three levels of modulated input signals are accumulated on three capacitors by synchronously connecting them to the radiometer common bus via controlled electronic keys. The keys have control inputs, which receive signals from the control unit via a three-wire bus. The high-pass filter 10 is assembled according to the scheme of a single-link first-order filter (represents a dividing CR-circuit) with a cutoff frequency much lower than the modulation frequency in the radiometer (f _cp << 1 / 2t _modes ), and is designed to eliminate the DC component in the signals. As a result, the variable component of the signal with minimal distortion of the pulse shape is allocated at the filter output.

The control unit 12 is completely similar to that described in [A.S. USSR No. 1704107, IPC G01R 29/08, G01S 13/95. Zero radiometer / Filatov A.V., Bordonsky G.S. - No. 4708980/09; declared 06/22/89; publ. 01/07/92. Bull. No. 1. - 5 s], produces all the necessary signals for the operation of the device. The block diagram of this block is shown in Fig. 5 and includes two digital counters (reverse 14 and direct count counter 15), a digital comparator 16, triggers 17 and 18 for generating a pulse-width signal t _chis and a signal of a symmetrical meander t _mode, respectively, a clock generator 19, the decoder of the triggers 20. Depending on the levels of logical signals t _chis and t _modes at the input of the decoder, one of three lines is activated at its output (code combinations of signals are described below when considering the principle of operation). These lines control the switch 5 in the input device (Fig. 3) and the synchronous filter 9 in the low-frequency part of the measuring path.

The principle of operation of the radiometric reflectance meter is illustrated by the timing diagrams in Fig.6 and is as follows. Two types of pulse modulation are synchronously performed in the meter: amplitude and latitudinal. For this, in the digital control unit 12, two control logic signals t _mod and t _{sch are generated} . The signal t _mode has a symmetrical impulse shape of the meander type, the signal t _chis varies in duration. These signals from the output of the decoder 20 of the control unit via a three-wire bus are fed to the control inputs of both a synchronous filter 9 and a microwave switch 5. Depending on the logic levels of the signals t _modes and t _bus , the following three periodically repeating noise signals _arrive at the input of receiver 8.

If t _mod = 1 and t _sch = 0, the output of the noise generator 6 through the microwave switch 5 is connected to the matched load 3 and its signal does not enter the measuring path (no noise). In this case, an antenna signal equal to T (1-R) acts at the input of receiver 8, where T is the thermodynamic temperature of the object, R is the power reflection coefficient at the antenna-object boundary.

In the second case, when t _mod = 1 and t _sch = 1, the signal of the noise generator 6 through the switch 5 enters the second directional coupler 4. The antenna signal is summed with the signal of the noise generator: T (1-R) + T _gh .

In the third case, if t _mod = 0 and t _sch = 0, the signal of the noise generator 6 enters the first directional coupler 2 through switch 5. Thus, at the receiver input, there will be a sum of signals T (1-R) + T _rh R. One of these signals, equal to T (1-R), is an antenna signal; the second, T _rsh R, is the signal of the noise generator, which enters the antenna path through the directional coupler 2, passes towards the antenna and is reflected from the antenna-object boundary with a coefficient R.

Figure 6 shows the timing diagrams for the case of a set zero balance. Zero balance is considered to be established if in the half-period of the modulation, when the antenna signal and the noise generator signal reflected from the object are received at the receiver input (t _mod = 0 and _tw = 0), the output voltage of the measuring path is zero, and this equality fixes the comparator 11. when the power balance is automatically set to zero, and then changing the antenna signal is controlled by a corresponding change in the duration of the pulse width t of the signal _SIS in the control unit 12. Since the signal at the input of the comparator 11 has a perio the character is static and the constant component is excluded in these signals by a high-pass filter 10, then for one period the volt-second areas of the positive and negative pulses are equal. Since the voltage at the input of the comparator is equal to zero in one half-period of modulation, therefore, the equality of the volt-second areas of the positive and negative pulses is performed in another half-period of modulation:

where U ₊ and U _- are the amplitudes of the positive and negative impulses equal to:

. Откуда:where G is the transmission coefficient of the measuring path, equal to the product of the amplification factors of the signals at high and low frequencies and the transmission coefficient of the quadratic detector; k is the Boltzmann constant, df is the receiver frequency band, T _W is the effective temperature of the intrinsic noise of the radiometric meter, reduced to the input of the receiver. Substituting (2) and (3) into equality (1), we obtain:

. From:

From the last formula (4), there follows a linear dependence of the reflection coefficient under study on the duration t _chis. Therefore, through this duration, the reflection coefficient at the antenna-object interface can be indirectly determined. From the formula (4) it follows that the measurements are not affected by changes in the transmission coefficient G and the intrinsic noise T _{w of the} measuring path, the signal level T _{g r} the noise generator. From the latter it follows that the influence of drift and low-frequency fluctuations of the output signal of the noise generator 6 on the measurement accuracy is significantly reduced. It must maintain stability over the measurement period.

According to (4), with full reflection, _tw = t _modes and R = 1. If the object under study is a completely black body t _chis = 0, and accordingly R = 0.

In a radiometric meter, high-frequency passive devices are made on microstrip waveguide structures. The microwave switch was created using three-directional keys (SP3T) manufactured by NEC, such as the UPG2227T5F. The literature describes quite fully the designs of microwave nodes, directional couplers, switches, bandpass filters and methods for their calculation [for example, Mazepova OI, Meshchanov VP, Prokhorova NN et al. Ed. Feldstein A.A. Guide to the elements of strip technology. - M .: Communication, 1979. - 336 s]. The receiver uses transistor amplifiers.

In contrast to the prototype, the scheme of the proposed radiometric reflection coefficient meter has a simpler structure, the design is simplified. The proposed processing algorithm allows, without transforming the shape of the modulated signal sequence, to measure the reflection coefficient from the object under study by expressing it through the width of the latitudinal pulse and display it in digital form without using standard analog-to-digital converters. The measurement accuracy is not affected by changes in the transmission coefficient of the measuring path and inherent noise. Significantly reduced the influence of drift and fluctuations in the signal of the reference noise generator on the measurement accuracy.

Claims (1)

A radiometric meter comprising a receiver, an antenna, a first directional coupler, a serially connected current source and a noise generator, characterized in that a second directional coupler, a microwave switch, the input of which is connected to the output of the noise generator, is connected between the output of the first directional coupler and the input of the receiver, the first and second outputs are connected respectively to the second inputs of the first and second directional couplers, and the third output is connected to the matched load, also input s connected to the output of the receiver are serially connected a synchronous low-pass filter, a high-pass filter, a comparator, a control unit, the second input of the comparator connected to a common bus of the radiometer, the first output of the control unit connected to the control input of the synchronous low-pass filter and combined with the control input of the microwave switch, and its second output is the output bus of the radiometer.

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