RU2439594C1 - Zero radiometer - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: radiometer for generating two reference signals employs one noise generator. This enables to minimise the effect of changes in the output power of the generator on measurement accuracy. The zero radiometer comprises series-connected antenna, directional coupler, first modulator, receiver, pulsed amplifier, high-pass filter, synchronous low-pass filter, comparator, control unit; the second input of the comparator is connected to the common bus of the radiometer, the first and second outputs of the control unit are connected to corresponding control inputs of the synchronous low-pass filter and the first modulator, and the third is the output bus of the radiometer, the output of the current source is connected to the input of the noise generator, the output of the first attenuator is connected to the second input of the directional coupler. The directional coupler, first modulator, first attenuator, noise generator and current source are mounted on a temperature-controlled board and are in thermal contact with the board; a second attenuator and a second modulator are also mounted on a temperature-controlled board and are in thermal contact with the board, and their first and second outputs are respectively connected to the input of the first attenuator and the second input of the first modulator through the second attenuator, and the input of the second modulator and its control input are connected to the output of the noise generator and the fourth output of the control unit, respectively.

EFFECT: broader functionalities of the radiometer.

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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 [WNHardy, KWGray and AWLove // IEEE MTT-22, N4, 1974, pp382-391], consisting of (Fig. 1) modulator 3, depending on the control signal of the reference generator 10, alternating switching on 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 microwave key 5, controlled by the pulse shaper 11, the signal of the noise generator 6 is received. In the low-frequency part, the radiometer contains connected in series synchronous detector 8, integrator 9, voltage-frequency converter 10. The input device of the radiometer (modulator 3, directional coupler 2, attenuator 4, controlled microwave key 5, noise generator 6, matched load 14) are located in the thermostat 12. The output signal of the integrator 9 enters the output of the radiometer (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 a mixture of antenna and noise generator signals. When this radiometer is in operation, the condition of zero reception is fulfilled - the output signal does not depend on the gain of the receiver and, therefore, its changes and slow fluctuations (less than the modulation frequency in the radiometer) do not affect the measurement accuracy.

Since the modulated signal sequence is a time-varying pulse sequence (short pulses of the noise generator signal are superimposed on the antenna signal), the application of such classical operations as synchronous detection and integration to its conversion leads to errors. This relates to the disadvantage of the described radiometer, which is selected as an analog.

Known microwave RF radiometer [Filatov A.V., Bordonsky G.S. // Zero radiometer. A.S. USSR No. 1704107, G01R 29/08, G01S 13/95 - prototype], the structural diagram of which is shown in figure 2, containing a series-connected antenna 1, directional coupler 2, modulator 3, receiver 7, low-frequency amplifier 8, high-pass filter 9 , a synchronous low-pass filter 10, a comparator (zero-organ) 11, a control unit 12, at the output of which a digital code of the measured antenna signal is generated, arriving on bus 14. The radiometer provides automatic input of the reference signal of the noise generator 5 into the directional coupler 2 through the attenuator 4, p Adjustment of which occurs during the calibration process. The pulse signal from the output 4 of the control unit 12 includes a direct current source 6. The controlled source 6 supplies the noise generator 5. The noise signal generated by the generator is the first reference signal. The second reference noise signal is generated by the coordinated load 15, which is located at the temperature of the thermostated board 13. To increase the stability of the radiometer, a modulator 3, a directional coupler 2, an attenuator 4, a noise generator 5, and a controlled current source 6 are installed on the same board.

A feature of the operation of this radiometer is as follows. Processing the envelope of the signals at the output of the receiver (at a low frequency) includes the following operations: eliminating the DC component of the voltage by a high-pass filter and analyzing the polarity of the voltage at the input of the comparator for a period of time when a matched load is connected to the input of the receiver. 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, there are no errors associated with these transformations. 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 limitations on the range of measured signals. The upper limit of the measuring range is fixed and is determined by the equivalent temperature of the agreed load.

The present invention solves the problem of expanding the functionality of the radiometer over the range of measured antenna signals and improving the accuracy of measurements while maintaining the ability of the radiometer to operate in zero measurement mode. The radiometer uses one noise generator to form two reference signals. This minimizes the effect of changes in the generator output power on the measurement accuracy.

To achieve this technical result, a radiometer containing a series-connected antenna, a directional coupler, a first modulator, a receiver, a pulse amplifier, a high-pass filter, a synchronous low-pass filter, a comparator, a control unit, the second input of the comparator is connected to a common radiometer bus, the first and second the outputs of the control unit are connected to the corresponding control inputs of the synchronous low-pass filter and the first modulator, and the third is the output bus of the radiometer, the output of the current source is it is single with the input of the noise generator, the output of the first attenuator is connected to the second input of the directional coupler, and the directional coupler, first modulator, first attenuator, noise generator, current source are installed on the thermostated board and are in thermal contact with it, installed on the thermostated board and located with it, in thermal contact, the second attenuator and the second modulator, the first and second outputs of which are connected respectively to the input of the first attenuator and the second input of the first modulator and through the second attenuator, and the input of the second modulator and its control input are connected to the output of the noise generator and the fourth output of the control unit, respectively.

Figure 1 presents the structural diagram of a radiometer-analogue.

Figure 2 shows the structural diagram of the prototype radiometer.

Figure 3 presents the structural diagram of the proposed zero radiometer.

Figure 4 shows the timing diagrams explaining the principle of operation of the zero radiometer.

The composition of the radiometer includes (Fig. 3) antenna 1. The input unit includes first and second modulators 3 and 16 mounted on a thermostated circuit board 13, a noise generator 5, a direct current source 6, first and second attenuators 4 and 15, a directional coupler 2. Measuring the channel consists of a receiver 7, a pulse amplifier 8, a high-pass filter 9, a synchronous low-pass filter 10, a comparator 11, a control unit 12, from the third output of which the signal enters the output bus 14.

In the input unit of the radiometer, the signals are modulated. The first signal of the first modulator 3 receives the input signal of the antenna 1 through a directional coupler 2, in which the first reference signal T _{rsh1 is} added to the antenna signal. This reference signal is generated by a semiconductor noise generator 5 using an avalanche-span diode, through which the current of source 6 flows. Through the second modulator 16, the noise generator signal is fed to the input of the attenuator 4, where it is attenuated to the required value (the attenuator is adjusted during calibration, which is described below) equal to:

where T _gsh - noise temperature of the output signal of the generator 5; α ₁ - transmittance of the attenuator 4, equal to unity, if the signal passes through it without attenuation, and zero in the case of complete absorption of the signal; T ₀ - thermodynamic temperature of thermostated circuit board 13; β is the transfer coefficient of the directional coupler. The modulator 16 is controlled by a pulse-width pulse signal _tw , coming from the fourth output of the control unit 12.

The second reference signal in the radiometer is formed from the same signal of the noise generator 5 and is fed to the second input of the modulator 3 through the second modulator 16 and the second attenuator 15, in which it is attenuated to the value:

where α ₂ is the transmittance of the attenuator 15. The first modulator 3 is controlled by a symmetrical pulse signal t _modes of the meander type.

In order to minimize errors, the input node of the radiometer, namely the noise generator 5, attenuators 4 and 15, modulators 3 and 16, the directional coupler 2 are located on the thermostatically controlled circuit board 13 and are in thermal contact with it.

The measuring channel is a radiometric receiver with a linear transfer characteristic and a band of received frequencies df. The receiver 7 includes high-frequency amplifiers, a band-pass filter, a quadratic detector, which emits the envelope of the modulation signals. The high-pass filter 9 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 intended 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. A synchronous low-pass filter 10 pre-filters the signal, reduces the fluctuation component in the detected signals and thereby eliminates the overload of the next comparator 11. The filter consists of three single-link RC integrating circuits, in which the resistor is common to all circuits, and the constant components of three modulated input signals (antenna, antenna and first reference signal, second reference signal) are accumulated on three capacitors by their synchronous connection to a common radio bus Dr. controlled by electronic keys. The keys have control inputs, which receive a signal from the control unit via a three-wire bus.

The principle of operation of the radiometer is illustrated by timing diagrams in figure 4 and is as follows. The modulation signals supplied to the control input of the first modulator 3 from the second output of the control unit 12 are a periodic sequence of rectangular pulses with a duty cycle equal to two. One modulation period consists of two equal half-periods with equal durations of t _modes (Fig. 4). In the first half-cycle of the modulation, when the signal t _mode is low at the output of the control unit 2, the reference signal Т _{Гш2 is} received at the input of the receiver 7 through the second input of modulator 3. In the second half-cycle, at a high level of the signal t _mod , through the first input of modulator 3 the input of the receiver receives the signal of the antenna 1. In this half-cycle of modulation, a pulse-width signal with a duration of _{tw is} generated at the fourth output of the control unit.

Figure 4 shows the timing diagrams for the case of a set zero balance in the radiometer. The zero balance is considered to be established if, in the first half-period of the modulation, when the signal Т _{Гш2 arrives} at the input of the receiver, the output voltage of the measuring path is zero and this equality fixes the comparator 11. The zero balance is established when the radiometer is powered on and then adjusted by changing the antenna duration accordingly pulse-width signal t _chis . Since the signal at the input of the comparator 11 as a result of modulation is periodic and the constant component is excluded from these signals by the high-pass filter 9, 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 zero in the first half-period of modulation, therefore, the equality of the volt-second areas of the positive and negative pulses is performed in another, the second half-period of modulation when the antenna is connected to the input of the receiver:

wherein U ₊ and U _{- -} amplitude of positive and negative pulses equal _{to: U + = Gkdf [(T} a + T + T _{gsh1 w)} - (T _NH2 T _w)] = Gkdf (T _a + T + T _{NH2 gsh1} ); U _- = Gkdf _[(NH2 + T T _m) - (T _a + T _w)] = Gkdf (T _NH2 -T _a), where G - factor of proportionality between the input signals T _a, T _gsh1 T _NH2 and strains on the output of the high-pass filter, which includes high and low frequency gains, the gain of the quadratic detector; k is the Boltzmann constant, df is the band of received frequencies, T _W is the effective temperature of the noise floor of the radiometer, brought to the input of the receiver. Substituting U ₊ and U _- in Equation (3) we obtain: Gkdf [(T _a + T -T _{gsh1 NH2)} t _SIS = Gkdf (T _NH2 -T _a) (t -t _{mod SIS).} From:

From the last formula follows a linear dependence of the duration t _SHIS and the input signal of the antenna T _a . Therefore, through this duration, the antenna signal can be indirectly determined. Also, from the formula (4) it follows that the duration of the pulse width t of the signal _SIS is not affected by changes in the intrinsic noise of T _u and the measurement channel gain radiometer (G ratio). Elimination of the influence of these two main destabilizing factors indicates that the radiometer works on the principle of zero measurements.

The measurement range, the values of the maximum and minimum antenna signals can be found from (4):

Substituting in (5) the durations of _tsys , equal to zero, and the duration of t _modes, we obtain: Т _{а, max} = Т _гш2 ; T _{a, min} = T _gsh2 -T _gsh1 .

The measurement boundaries are adjusted by changing the absorption of the signals in the attenuators 4 and 15 during the calibration process. Calibration is carried out in two stages. At the first stage, a standard with a noise temperature T _{et, max} , which defines the upper limit of the measurement range, is connected to the radiometer input instead of an antenna. The signal T _{gsh1 is} not generated, t _chis = 0. The signal T _{rsh2 is regulated by} changing the transmittance of the attenuator 15. The calibration stage is completed when the voltage at the input of the comparator in the first half-period of the modulation becomes zero. When this condition is met, T _{e, max} = T _{a, max} = T _gsh2 .

The second stage of calibration begins with connecting to the input of the radiometer a standard with a low noise temperature T _{e, min} , which determines by analogy the lower limit of the measurement range. For this case, t _chis = t _mod . The signal T _{rsh1 is adjusted} using the attenuator 4. As in the first stage, the radiometer is tuned to the lower limit until the voltage at the input of the comparator is set to zero in the first modulation half-cycle. In this case, T _{e, min} = T _{a, min} = T _gsh2 -T _gsh1 .

The minimum value of the signal of the noise generator is subject to the limitation T _{gsh, min} ≥T _gsh1 / β.

In a radiometer, two reference signals are generated by a single noise source. Expression (4) for the duration of the pulse-width signal taking into account (1) and (2) has the form:

The main error in the measurements is made by changes in the output signal of the noise generator T _gh and temperature variations T _{0 of the} board 13, on which the input nodes of the radiometer are installed. Given these circumstances, the expression for calculating the error of duration t _chis will be of the form:

where δT _gsh and δT ₀ are the absolute deviations of the signals of the noise generator and thermostated board from the nominal values, respectively.

After substituting (6) in (7) and solving the equality, we obtain:

δt _chis = {[(α ₂ -α ₁ ) T ₀ + α ₁ T _a ] δT _gh + [(α ₁ -α ₂ ) T _gh + (1-α ₁ ) T _a ] t _mod } / β [T ₀ + α ₁ (T _gh -T ₀ )] ² .

In the course of numerical experiments, the following results were obtained. The measurement error when the antenna signal changes is linear. For minimum antenna signals, the error value is minimal. Further, there is an increase in the error in the direction of increasing the antenna signals. For example, when the signals T _gh and T ₀ change by 1% in the measurement range 0–400 K, the error is from zero for the minimum antenna signal to 1% for the maximum signal. Thus, if in modulation radiometers of differential measurements with an increase in the temperature difference between the antenna and the reference source, the error increases accordingly (due to the multiplicative component), then in this radiometer it decreases on the contrary.

The control unit 12 of the radiometer is completely similar to the prototype; it generates all the necessary signals for the functioning of the radiometer. The radiometer monitors the operating mode and automatically maintains zero balance. As follows from the above description of the principle of operation of the radiometer, a necessary condition for zero reception is to maintain at the input of the comparator 11 a zero voltage in the phase of passing through the measuring path of the noise generator signal T _gsh2 . After analyzing the output signal of the comparator, the control unit corrects the digital code of the pulse width of the pulse width signal _tw , which leads to its change in the next modulation half-cycle. The digital code for the width of the pulse-width signal is the digital equivalent of the measured antenna signal and is fed to the output bus 14 of the control unit.

In the radiometer, the control unit is made on digital CMOS logic integrated circuits. The literature describes quite fully the designs of microwave devices, such as a modulator, attenuator, directional coupler, and methods for calculating them [for example, Bakharev S.I., Volman V.I., Lib Yu.N. et al. Ed. Volman V.I. A guide to 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 amplifiers. Band filters are made on oncoming rods [Mazepova OI, Meshchanov VP, Prokhorova NN et al. Ed. Feldstein A.A. Guide to the elements of strip technology. - M .: Communication, 1979. - 336 p.]. Synchronous low-pass filters are described in [Frater. Synchronous integrator and demodulator // Devices for scientific research. - 1965. - T.36, No. 5. - S.53].

Unlike the prototype, the proposed radiometer after the necessary calibration operation can be configured to an arbitrary measurement range of the antenna signal. This is done by adjusting the reference signals in the attenuators formed by a single noise generator. The formation of reference signals using a single noise source increased the accuracy of the measurements. The minimum measurement error is achieved when measuring signals near the lower boundary of the measurement range.

Claims (1)

A zero radiometer containing a series-connected antenna, a directional coupler, a first modulator, a receiver, a pulse amplifier, a high-pass filter, a synchronous low-pass filter, a comparator, a control unit, the second input of the comparator is connected to a common radiometer bus, the first and second outputs of the control unit are connected to the corresponding control inputs of the synchronous low-pass filter and the first modulator, and the third is the output bus of the radiometer, the output of the current source is connected to the input of the noise generator, the output is the first attenuator is connected to the second input of the directional coupler, and the directional coupler, the first modulator, the first attenuator, the noise generator, the current source are installed on the thermostated board and are in thermal contact with it, characterized in that the installed on the thermostated board and located with it in thermal contact of the second attenuator and the second modulator, the first and second outputs of which are connected respectively to the input of the first attenuator and the second input of the first modulator through the second att an aenuator, and the input of the second modulator and its control input are connected to the output of the noise generator and the fourth output of the control unit, respectively.

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