RU2642475C2 - Zero radiometer - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: zero radiometer comprises an antenna, a matched load connected to the first input of a modulator, a radiometric receiver, series-connected pulse amplifier, a high-pass filter, a synchronous low-pass filter, a comparator, a control unit. The first and second outputs of the control unit are connected to the control inputs of the modulator and the synchronous low-pass filter, respectively, and the third output is the output bus of the radiometer. The common bus of the radiometer is connected to the second input of the comparator. The matched load and the modulator are mounted on the thermostatically controlled plate. A high-frequency key, the first switch, connected to the output of the radiometric receiver with its input, are additionally introduced. Its first and second outputs are connected to the same inputs of the second switch, respectively. The first output is connected directly, and the second - through the voltage divider. The output of the second switch is connected to the input of the pulse amplifier and control inputs of the first, second switches and the high-frequency key are grouped together and connected to the fourth output of the control unit. The antenna is connected to the second input of the modulator, whose output is connected to the input of the radiometric receiver through the high-frequency key.

EFFECT: simplifying the radiometer circuit and increasing the accuracy of measurements.

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Description

The invention relates to microwave radiometry and can be used to measure electromagnetic signals of the intrinsic thermal radiation of material media in Earth remote sensing systems, various natural objects, and industry.

Known radiometer [Hardy W.N., Gray K.W., Love A.W. An S-band Radiometer Design with High Absolute Precision // IEEE Transactions on Microwave Theory and Techniques. - 1974. - MTT-22, No. 4. - P. 382-391], selected as an analogue and consisting of (Fig. 1) modulator 3, which, depending on the control signal of the reference oscillator 10, alternates switching to the input of the receiver 7 of the matched load 14 and the antenna 1 connected to the modulator through a directional coupler 2. At the other input of this coupler, through the attenuator 4 and the high-frequency switch 5, controlled by the pulse shaper 12, the signal from the noise generator 6 is received. In the low-frequency part, the radiometer contains a synchronous detector 8, integrally connected in series a radiator 9, a voltage-frequency converter 11. The input device of the radiometer (modulator 3, directional coupler 2, attenuator 4, controlled high-frequency switch 5, noise generator 6, matched load 14) is installed on the thermostated board 15. The output signal of integrator 9 is fed to radiometer output (bus 13).

In the radiometer, the input of the noise generator signal into the antenna path is carried out by pulses of 40 μs duration, the repetition frequency of which depends on the output signal of the voltage-frequency converter. In the synchronous detector, a constant voltage level is extracted from the envelope of the matched load signals modulated in the input device of the radiometer and the sum of the signals of the antenna and the noise generator. During operation of this radiometer, the zero reception condition is fulfilled - the output signal is invariant to changes in the gain of the receiver and, therefore, its drift and slow fluctuations, the main frequency of which is less than the modulation frequency in the radiometer, do not affect the measurement accuracy.

Since the radiometer performs two types of pulse modulation (amplitude and frequency), therefore, the modulated signal sequence is a complex in form and time-varying pulse sequence (short pulses of the noise generator signal are superimposed on the antenna signal). The use of such classical operations as synchronous detection and integration in the radiometer to convert this form of modulated signals to the output signal leads to errors. This refers to the disadvantage of the described radiometer - analogue.

Known microwave zero radiometer [A.S. No. 1704107 of the USSR. MKI ³ G01R 29/08. Zero radiometer / A.V. Filatov, G.S. Bordon (USSR) - 4708980/09; declared 06/22/1989; publ. 01/07/1992. - Bull. No. 1. - S. 182], which is selected as a prototype. Its structural diagram is depicted in FIG. 2 and contains a series-connected antenna 1, a directional coupler 2, a modulator 3, a receiver 7, a pulse amplifier 16, a high-pass filter 17, a synchronous low-pass filter 18, a comparator 19 operating in the zero-organ mode, a control unit 20, at the output of which a digital code of the measured antenna signal is generated, which arrives on bus 13. The radiometer provides automatic input of the reference signal of the noise generator 6 into the directional coupler 2 through the attenuator 4, the setting of which is for the corresponding attenuation coefficient Nala noise generator occurs during the calibration. The pulse signal from the output 4 of the control unit 20 includes a direct current source 21. The controlled source 21 supplies the noise generator 6. The noise signal generated by the generator is the first reference signal. The second reference noise signal is generated by the coordinated load 14, which is located at the temperature of the thermostated board 15. To increase the stability of the radiometer, a modulator 3, a directional coupler 2, an attenuator 4, a noise generator 6, a controlled constant current source 21 are installed on the same board.

The principle of operation of this radiometer is as follows. Two types of modulation are carried out in the radiometer: amplitude-pulse signals of the antenna and the matched load and pulse-width signal of the noise generator. Processing the envelope of noise signals modulated at the input (an antenna and two reference signals) in the low-frequency part of the measuring path includes two operations such as eliminating the DC component as a result of signal transmission through a high-pass filter and analyzing the voltage polarity at the comparator input at a time interval when to the receiver input matched load connected. Based on the result of the analysis, a control action on the current source is issued. If the polarity is positive, then the width of the pulse-width signal decreases, if negative, it increases. The increase or decrease in duration is performed by the control unit in each period of the amplitude-pulse modulation by an amount corresponding to the minimum discrete and proportional to the least significant bit of the output digital code of the radiometer. Thus, the monitoring mode of the radiometer is carried out. By changing the width of the pulse-width signal, the radiometer is maintained in a state of zero balance and the antenna signal is indirectly determined from this duration. Since there are no transformations of modulated signals (transformations of the waveform) in the low-frequency part of the radiometer in order to isolate informative voltage levels for regulating the zero balance, there are no errors associated with these transformations. The zero balance indicator in this radiometer is the voltage equal to zero when a matched load is connected to the receiver input, which fixes the comparator. In this radiometer, as in the analog radiometer, the principle of zero measurements is fulfilled, as a result of which the radiometer becomes insensitive to changes in the transmission coefficient of the measuring path.

The considered radiometer, selected as a prototype, has two reference noise sources installed at the input: a matched load and a noise generator. A current source is used to power the noise generator, and an attenuator and a directional coupler are installed to input this signal into the antenna path. In order for the antenna signal with minimal losses to be input to the modulator, to reduce its branching into the signal path of the noise generator, the directional coupler is selected with a low coupling coefficient. Therefore, the noise generator must have a sufficiently high excess power. To do this, in the prototype as a noise generator, semiconductor avalanche-span diodes are used, which have a number of advantages, which primarily include small dimensions, low values of the diode supply current. The disadvantage is the low stability of the generation of the noise signal, which, however, is the case for all semiconductor structures. This leads to the need for careful temperature control of the active zone of the diode and high stability of the power supply source.

The present invention solves the problem of simplifying the radiometer circuit and improving the accuracy of measurements when only one reference noise generator is used in the input node of the radiometer — a matched load whose stability is much greater than the stability of the semiconductor structures. The absence of a semiconductor noise generator greatly simplifies the input high-frequency part of the radiometer. It becomes the same as that of modulation radiometers, which are most often used in practice due to its simplicity. But in the proposed radiometer, in contrast to the modulation one, the mode of zero measurements is carried out, the main advantage of which is the absence of an influence on the measurement accuracy of changes in the transmission coefficient of the entire measuring path. This minimizes the measurement error.

To achieve this technical result, a zero radiometer containing an antenna, a matched load connected to the first input of the modulator, a radiometric receiver, a serially connected pulse amplifier, a high-pass filter, a synchronous low-pass filter, a comparator, a control unit, the first and second outputs of which are connected respectively with the control inputs of the modulator and synchronous low-pass filter, and the third output is the output bus of the radiometer, the common bus of which is connected to the second input m of the comparator, and the coordinated load and the modulator are installed on the thermostatically controlled board and are in thermal contact with it, a high-frequency key, a first switch, connected to the output of the radiometric receiver, are introduced, and its first and second outputs are connected respectively to the inputs of the second switch of the same name, the first the output is direct, and the second through the voltage divider, the output of the second switch is connected to the input of the pulse amplifier, and the control inputs of the first, second switch lei and the high-frequency key are combined together and connected to the fourth output of the control unit, the antenna is connected to the second input of the modulator, the output of which through the high-frequency key is connected to the input of the radiometric receiver.

In FIG. 1 is a structural diagram of an analog radiometer.

In FIG. 2 shows a structural diagram of a prototype radiometer.

In FIG. 3 presents a structural diagram of the proposed zero radiometer.

In FIG. 4 are timing diagrams explaining the principle of operation of the proposed zero radiometer.

The radiometer (Fig. 3) includes an antenna 1. The input unit includes a modulator 3 mounted on a thermostated board 15 and a matched load 14. The signal from the modulator output through a high-frequency switch 5 is fed to a radiometric receiver 7, the output of which is the envelope of the modulation signals. The measuring low-frequency channel consists of a pulse amplifier 16, a high-pass filter 17, a synchronous low-pass filter 18, and a comparator 19. To implement the zero method of operation in the radiometer, switches 22, 23 and a voltage divider 24 are installed between the output of the receiver and the input of the pulse amplifier. The radiometer operates under the control of block 20, from the third output of which the signal enters the output bus 13.

In the input unit of the radiometer, pulse-amplitude modulation of the antenna signals and matched load occurs with effective noise temperatures T _A and T _CH, respectively. The coordinated load is mounted on a thermostated board and acts as a reference noise generator in the radiometer.

The measuring channel is a radiometric receiver 7 with a linear transfer characteristic and a band of received frequencies dƒ. The receiver is designed according to the direct amplification scheme and includes high-frequency amplifiers, a bandpass filter, and a quadratic detector that selects the envelope of the modulation signals. The full receiver gain includes high-frequency signal amplification and signal conversion in a quadratic detector with coefficients G and β, respectively. The intrinsic noise temperature brought to the input of the receiver is T _W. The high-pass filter 17 is assembled according to the scheme of a single-link first-order filter (represents a dividing CR-circuit) with a boundary frequency much lower than the modulation frequency in the radiometer, and is intended to eliminate the DC component in the signals. As a result, the variable component of the signals with minimal distortion of the pulse shape is allocated at the filter output. The synchronous low-pass filter 18 pre-filters the signals, reduces the fluctuation component in the detected signals and thereby eliminates the overload of the next comparator 19. The filter consists of three single-link RC integrating circuits, in which the resistor is common to all circuits, and the constant components of the signals they are accumulated on three capacitors by their synchronous connection to the common bus of the radiometer via electronic keys. The keys have control inputs, which receive signals from output 2 of the control unit via a three-wire bus.

The principle of operation of the radiometer is illustrated by timing diagrams in FIG. 4 and is as follows. The pulse signals for amplitude modulation, received at the control input of the modulator 3 from the first output of the control unit 20, are a periodic sequence of rectangular pulses with a duty cycle equal to two. One period of this modulation consists of two equal half-periods with equal durations t _AIM (Fig. 4). In the first half-cycle, when the control signal is at a high level (the presence of a pulse at the output of the control unit 1), the antenna signal T _A is received at the input of the receiver 7. In the second half-cycle, at a low level of the control signal (lack of a pulse), a matched load with a noise temperature T _{CH is} switched through the modulator to the receiver input. Thus, amplitude-pulse modulation is carried out in the radiometer.

To implement pulse-width modulation, a high-frequency switch 5 is installed at the input of the receiver and two switches 22, 23 and a voltage divider 24 are installed at the output of the receiver. The first switch 22 operates in the selector mode, depending on the signal at the control input, the input signal is switched or on the first or second outputs. The second switch 23, in contrast, operates in multiplexer mode. One of its two inputs is connected to the output by a control signal.

The high-frequency key is opened by the control signal from the fourth output of the control unit. Switch is opened at a time interval of the second half-cycle of a pulse amplitude modulation, pulse width when the signal is not valid, i.e. at time t ^_ t _{PAM PWM} (see. FIG. 4). At the same time, the outputs and inputs of the switches 22 and 23 from position 1 go into position 2. This means that in the above time interval, the signal passes through a voltage divider 24 with a division coefficient α.

This signal consists of the sum of three signals. The first of them characterizes the intrinsic noise of a radiometric receiver with an effective temperature T _w . The second one is the same signal, but reflected from open key 5 with a power reflection coefficient R. The third signal T _{w, in,} arises as a result of interference of intrinsic noises and the same noises reflected from the key. Thus, the total signal will be equal to:

where the coefficient R for the open key is close to unity.

The interference term T _{w, in} arises due to the coherence of the intrinsic noise signals at the input and the same noises reflected from the high-frequency key. If the distance s between the key output and the receiver input is increased, the effective autocorrelation time interval of these signals decreases and when the condition (2s√ε / C ₀ ) ≥ (1 / dƒ) is satisfied, where ε is the dielectric constant of the substrate on which the high-frequency elements of the input node, C ₀ is the speed of light in vacuum, dƒ is the frequency band of the signals received by the radiometer, the two noise components will be uncorrelated. Hence, to avoid interference, the length of the segment s must be at least s _min = C ₀ / 2√ ε dƒ.

.If the interference term can be neglected when the above conditions are met, then the signal on the modulation time interval t _AIM- A _PWM will be equal to

.

Thus, at the first input of the comparator 19 at different time intervals there will be three levels of periodically repeated signals equal to:

where G is the gain of signals in terms of power in the receiver, β is the coefficient of conversion of power to voltage in the quadratic detector of the receiver, K _u is the gain of voltage in the pulse amplifier, dƒ is the frequency band received by the receiver, and k is the Boltzmann constant.

In FIG. Figure 4 shows the timing diagrams for the case of a set zero balance in the radiometer. Zero balance is considered to be established if in the first half-period of amplitude-pulse modulation, when the antenna signal is input to the receiver, the output voltage of the measuring path is zero and this equality fixes the comparator 19 operating in the zero-organ mode. Zero balance when the power of the radiometer is turned on is established and then, when the antenna signal changes, it is regulated by the corresponding change in the width of the pulse-width signal t _PWM . Since the signal at the input of the comparator 19 as a result of modulation is periodic and the constant component is excluded in these signals by the high-pass filter 17, then for one period the volt-second areas of the positive and negative pulses are equal:

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

Substituting (4) and (5) into equality (3) we obtain:

In the calibration process, which is described below, the gear ratio α of the voltage divider 24 (Fig. 3) is adjusted until the equality:

согласно (1) равна

.Where

according to (1) is equal

.

Substituting (7) into (6) we finally have:

From where, solving equality (8) with respect to the time of a pulse-width signal, we obtain:

From the last formula (9), a linear dependence of the _PWM duration t and the antenna input signal T _A follows. Therefore, through this duration, the antenna signal is indirectly determined. It also follows from formula (9) that the duration of the pulse-width signal is not affected by changes in the intrinsic noise T _W and the transmission coefficient of the measuring path of the radiometer (coefficients G, β, K _u ). Elimination of the influence of these two main destabilizing factors indicates that the radiometer works on the principle of zero measurements. Formula (9) includes only one reference signal — the noise temperature of the matched load. Therefore, the claimed requirements of the present invention for the use in the input high-frequency unit of one reference noise signal are satisfied instead of the two used in the prototype.

The measured antenna signal is determined from (9):

The boundaries of the range of changes are found from (10) by substituting the _PWM durations t equal to zero and the _AIM duration t. We get:

Thus, the range of variation of the noise signal of the antenna is from 0 degrees Kelvin to the effective noise temperature of the matched load T _CH equal to its thermodynamic temperature.

The measuring range is adjusted during the calibration of the radiometer. Calibration is carried out in two stages. At the first stage, instead of the antenna, a reference with a noise temperature T _{et, max} , which defines the upper limit of the measurement range T _{A, max} . The duration of the pulse-width modulation signal at this stage is set equal to the duration of the half-period of amplitude-pulse modulation, that is, t _PWM = t _{AIM. By} changing the temperature of the thermostated board and, therefore, the matched load, the signal T _CH is tuned until the equality T _CH + T _w = T _et , _max + T _w , which also fixes the comparator. When the equality is fulfilled, the voltage at the input of the comparator in the first half-cycle of the modulation becomes equal to zero. At this point, the first calibration step, as a result of which the noise signal of the matched load T _CH becomes equal to the signal of the connected standard, is considered complete.

The second calibration stage begins by connecting a noise standard to the radiometer input, which determines the lower limit of the measurement range. Since, according to (11), the lower boundary of the range corresponds to a signal with zero noise temperature, which in practice cannot be created, therefore this stage (unlike the prototype) is carried out in a modified form and consists in the following. The width of the pulse-width signal is set not equal to zero, as required by condition (11), but equal to half the half-period of the amplitude-pulse modulation, i.e., t _PWM = t _AIM / 2. In this case, a standard is connected to the radiometer input, the output noise signal of which T _et is equal to the middle of the measurement range, i.e., to the value of T _CH / 2. For example, for the range 0-300 K (which, as a rule, is used in radiometers for remote sensing of the Earth), the signal T _CH determines the upper limit and is equal to 300 K. Then the signal of this standard should be equal to 150 K.

Further, by changing the value of the attenuation coefficient α of the voltage divider, the equality of the volt-second areas of the negative and positive pulses is adjusted according to the output signal of the comparator, as in (6):

where T _et = T _CH / 2 and t _PWM '= t _AIM / 2. After substituting the last equalities in (12) and abbreviations, we obtain expression (7). Thus, the dividing coefficient a of the voltage divider is the second reference value that determines the steepness of the transfer characteristic of the radiometer, its slope. Since the voltage divider can be performed using precision resistors, and, accordingly, a stable attenuation coefficient can be obtained. Therefore, this coefficient, as a reference parameter, will have more stable characteristics than the second noise generator used for this purpose used in the prototype.

It should be noted that the second calibration step can also be carried out with a standard generating an arbitrary signal, not necessarily equal to the middle of the measurement range. For this, after connecting the standard, the pulse width of the pulse-width signal is set equivalent in digital representation to the noise signal generated by the standard. Otherwise, this calibration step is the same as described.

The digital control unit generates all the necessary signals for the functioning of the radiometer (Fig. 4), can be performed on digital integrated circuits of CMOS logic (hard logic), a microcontroller of small and medium performance, a programmable matrix. The digital code for the width of the pulse width signal is the digital equivalent of the measured antenna signal. The literature describes with sufficient completeness the design of microwave devices, such as modulators, bandpass filters and methods for their calculation [Bakharev S.I., Volman V.I., Lib Yu.N., et al. Ed. Volman V.I. Handbook of the calculation and design of microwave strip devices. - M.: Radio and communications, 1982. - 328 p.]. In this radiometer, these nodes are made on microstrip waveguide structures. The receiver uses transistor high-frequency amplifiers. Synchronous low-pass filters are described in [Frater. Synchronous integrator and demodulator // Devices for scientific research. - 1965. - T. 36, No. 5. - S. 53]. The 2 × 1 configuration switches and voltage divider circuits used in the radiometer are also well illuminated in the literature [152. Gutnikov B.C. Integrated electronics in measuring devices. - L .: Energoatomizdat, Leningrad, department, 1988. - 278 p.].

Unlike the prototype, the proposed radiometer uses a single reference noise source, and this greatly simplifies the input high-frequency path. It becomes similar to the input path of the classical modulation radiometer, where the differential measurement method is used as the basis of operation. Modulation radiometer due to the simple design of the input node is often used in the construction of various systems. In such a radiometer, the influence of the constant component of the intrinsic noise over the measurement interval is reduced to zero. But another destabilizing factor, drift and gain fluctuations, significantly affect the accuracy of measurements, since the steepness of the transfer characteristic depends on this parameter in a modulation radiometer. The proposed radiometer implements the zero method, the influence of the described destabilizing factor is reduced to zero, and the slope of the conversion characteristic is determined by the voltage divider assembled on precision resistors. Thus, the result is a simplification of the device while improving accuracy.

Claims (1)

A zero radiometer containing an antenna, a matched load connected to the first input of the modulator, a radiometric receiver, a serially connected pulse amplifier, a high-pass filter, a synchronous low-pass filter, a comparator, a control unit, the first and second outputs of which are connected respectively to the control inputs of the modulator and synchronous low-pass filter, and the third output is the output bus of the radiometer, the common bus of which is connected to the second input of the comparator, moreover, the coordinated load and the modulator is mounted on a thermostatically controlled board and is in thermal contact with it, characterized in that a high-frequency key, a first switch, connected to the output of the radiometric receiver, are inserted into it, and its first and second outputs are connected respectively to the inputs of the second switch of the same name, the first output directly, and the second through a voltage divider, the output of the second switch is connected to the input of the pulse amplifier, and the control inputs of the first, second switches and high-frequency of the key are combined together and connected to the fourth output of the control unit, the antenna is connected to the second input of the modulator, the output of which through the high-frequency key is connected to the input of the radiometric receiver.

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