RU2235340C1 - Null radiometer - Google Patents

Abstract

FIELD: passive radiolocation technologies.

SUBSTANCE: device has antenna, first and second coordinated loads, first and second band-pass filters, first and second modulators, receiver, upper frequencies filter, synchronous filter, comparator, controlling block, first, second and third outputs of which are connected to control inputs if first, second modulators and synchronous filter, respectively, while input of control block is connected to comparator output, second input of which is connected to common bus of radiometer, first coordinated load through first band-pass filter is connected to first input of first modulator, output of which is connected to first input of second modulator, while radiometer output is fourth output of control block, while first and second coordinated loads, first and second modulator are mounted on thermostatic plate, while into it third band-pass filter, prior amplifier of low frequency are inserted, output of second modulator is connected through connected in series receiver, prior low frequency amplifier, synchronous filter, low frequency amplifier, high frequency filter is connected to first input of comparator, second coordinated load is connected through second band-pass filter to second input of first modulator, and antenna is connected to second input of second modulator through third band-pass filter.

EFFECT: broader functional capabilities of radiometer for selecting measurements range.

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Description

The invention relates to passive radar and can be used to receive electromagnetic signals of a noise nature in a wide frequency range.

A known radiometer (A.S. No. 1337832, MKI ⁶ G 01 R 29/08 - analogue), a block diagram of which is shown in figure 1. The radiometer consists of an antenna 1, three band-pass filters 2, 5, 6, a receiver 3, a power divider in half 4, an attenuator 7, two quadratic detectors 8 and 9, a compensation unit 10, a low-frequency amplifier 11 and a recorder 12. Two band-pass filters: input filter 2 and the filter 5 connected to the first output of the power divider 4 are the same, have a frequency bandwidth δ f ₁ . The third band-pass filter 6 connected to the second output of the power divider has a band δ f ₂ . These frequency bands do not overlap. The amplitude-frequency characteristic of the amplifiers of the receiver 3 is uniform and the same in both bands δ f ₁ and δ f ₂ . The gain and noise of the receiver are changed by the same amount within these bands. Thus, at the input of the first square-law detector 8 receives the sum signal consisting of the input antennas and its own receiver noise, the total power is equal to (G / 2) k (T _a + T _δf1) δf ₁ wherein G - power gain signals in the receiver 3; 1/2 - division factor of the divider 4; k is the Boltzmann constant; T _a - effective noise temperature of the radiation resistance of the antenna; T _δf1 is the effective noise temperature of the receiver's own noise in the band δf ₁ reduced to its input.

The input of the second quadratic detector 9 receives only the intrinsic noise of the receiver, the power of which is determined from the expression (G / 2) kT _δf2 δf ₂ η, where T _δf2 is the effective temperature of the receiver noise in the band δf ₂ brought to the input _, η is the attenuator gain 7 Tuning the attenuator to the transfer coefficient η ₁ ensures equal noise powers at the inputs of quadratic detectors in the absence of a signal. That is, the equality (G / 2) k (T _a1 + T _δf1 ) δf ₁ = (G / 2) kT _δf2 δf ₂ η ₁ holds, where T _a1 is the noise temperature of the antenna when the signal does not fall within its radiation pattern .

When a signal appears at the input of the antenna, due to the selective properties of the filters, it will pass only to the input of the first quadratic detector 8 and thereby upset the balance of zero balance at the output of the radiometer. To restore the zero balance, the signal value at the input of the detector 9 is changed, changing the transfer coefficient of the attenuator 7. The balance will be achieved even with a different value of the attenuator absorption coefficient η ₂ . So, (G / 2) k (T _a2 + T _δf1 ) δf ₁ = (G / 2) kT _δf2 δf ₂ η ₂ , where T _a2 is the effective noise temperature of the antenna at the input of which the measured signal arrives. Then

The attenuator scale, calibrated in degrees Kelvin, is a measuring scale. In expression (1) in parentheses is a constant term, which is a reference value for a given radiometer when measuring. This reference coefficient depends not only on the ratio of the filter bands, but also on the intrinsic noise temperature of the receiver, which must remain constant. Thus, by adjusting the magnitude of the reference signal generated from the intrinsic noise of the radiometer, a state of zero reception is achieved when the gain of the receiver does not affect the accuracy of the measurements.

The disadvantages of this radiometer include the low accuracy of measurements, which will depend on the stability of the intrinsic noise of the radiometer, from which the reference signal is formed. The presence of two quadratic detectors will also introduce an error in the measurement results caused by the uneven variation in their transmission coefficients as a function of the ambient temperature. An attenuator is used to adjust the reference signal. Therefore, stringent requirements must be imposed on the linearity of its transfer characteristic. Precision attenuators are expensive.

A known radiometer (RF Patent No. 2093845, MKI ⁶ G 01 R 29/08, G 01 S 13/95, 1997), selected as a prototype and consisting of (figure 2) antenna 1, two identical modulators 2 and 3 , two identical matched loads 4 and 5, performing the role of noise generators, two band-pass filters 6 and 7, a thermostatic board 8, a signal amplification path consisting of a receiver 9, a broadband amplifier 10, a high-pass filter 11, a synchronous filter 12, a comparator 13 ( zero-organ), as well as the control unit 14 and the digital output bus 15. A feature of the radiometer is ostroenie its inlet portion.

In this radiometer, three noise signals are exposed at the input: the measured antenna signal T _a and two reference signals. The reference signals are formed by the same coordinated loads 4 and 5, located on the thermostated board 8 at the same physical temperature T _about . The loads are connected to the inputs of the first modulator 2. The first load 4 is connected to the first input of the modulator 2 through a filter 6 with a passband δf ₁ , and the second load 5 is connected directly to the second input of the same modulator. The second bandpass filter 7 is installed at the output of the second modulator 3 in front of the receiver 9 and has a frequency bandwidth δf ₂ . Band-pass filters satisfy two conditions. Firstly, the band of the second filter 7 is wider than the band of the first filter 6 and the equality

where n is a positive number greater than one.

Secondly, the working strip δf ₁ is inside the strip δf ₂ . The receiving-amplifying path of the radiometer has a uniform amplitude-frequency characteristic in the band δf ₂ , and when the external conditions change, the gain of the receiver changes by the same amount in this band.

Thus, if relation (2) is satisfied, the power of the noise signals of the antenna, the first and second matched loads, at the input of the receiver 9 are respectively equal:

For signals, the inequality W _cn1 <W _a <W _cn2 must be _satisfied in the entire range of the antenna signal.

The signals of the coordinated loads in the modulator 2 are subjected to pulse width modulation. In modulator 3, the antenna signal and the output signal of the first modulator 2 are connected to the input of the receiver at regular intervals. The modulation law for the second modulator 3 is symmetrical pulses with a duty cycle of 2. Control of both modulators occurs from outputs 1 and 2 of the control unit.

The principle of operation of the prototype radiometer is as follows. The condition for zero balance in the radiometer is that the voltage at the input of the comparator 13 is equal to zero at half the modulation period of the modulator 3, when an antenna is connected to the input of the receiver 9. To implement the principle, a high-pass filter 11 is introduced into the measuring path of the radiometer, the cutoff frequency of which is lower than the modulation frequency of the modulator 3. The main purpose of the filter is to eliminate the DC component in the signal without noticeable distortion of the pulse shape. If the voltage at the input of the comparator 13 is equal to zero in the half-period of connecting the antenna, therefore, the volt-second areas of positive (signal of the second matched load 5) and negative (signal of the first matched load 4 passing through the filter 6) pulses in the other half-cycle are equal at its input when the output of the modulator 2 is connected to the receiver 9. Then the antenna signal is determined from the linear relation

where t _chis is the duration of the pulse-width signal arriving at the control input of modulator 2, t _mode is the duration of the symmetric pulse signal at the control input of modulator 3.

The equality of the volt-second area of the pulses is _adjusted by the control unit by changing the width of the pulse-width signal _tw without changing the main modulation period specified by the duration of t _mod .

As follows from expression (4), the antenna signal is directly determined through the duration t _chis , which is connected according to a linear law. The formula does not include the gain of the measuring path, as well as the intrinsic noise of the receiver, since the constant component of the signal is filtered out by the high-pass filter 11.

With this radiometer, as follows from (4), it is possible to measure signals from T _o / n (t _chis = 0) to T _o (t _chis = t _modes ). Since the temperature of the temperature-controlled board T ₀ can only be changed within small limits, therefore, the upper limit of the measured signals can also be changed within small limits. The impossibility of changing the upper boundary of the measured signals within large limits is the disadvantage of the prototype radiometer. The lower boundary, as follows from equality (4), can vary over a wide range by changing the parameter n, which is determined by the ratio of the band pass filters 6 and 7.

The aim of the invention is to expand the functionality of the radiometer for the selection of the measurement range. The advantages of the proposed radiometer compared to the prototype are that this radiometer can measure signals in a wide dynamic range, and the restrictions on the choice of measurement range are removed. For the proposed radiometer, there is no rigid reference of the boundary of the measurement range to the specific thermodynamic temperature of the thermostated board, at which the agreed loads are located in the input part of the radiometer.

This goal is achieved by the fact that in the zero radiometer containing the antenna, the first and second matched loads, the first and second band-pass filters, the first and second modulators, the receiver, a high-pass filter, a synchronous filter, a comparator, a control unit, the first, second and third outputs which are connected to the control inputs of the first, second modulators and a synchronous filter, respectively, and the input of the control unit is connected to the output of the comparator, the second input of which is connected to the common bus of the radiometer, the first through the first band-pass filter, it is connected to the first input of the first modulator, the output of which is connected to the first input of the second modulator, and the radiometer output is the fourth output of the control unit, the first and second matched loads, the first and second modulator installed on the thermostated board, the third band-pass filter introduced , a low-frequency pre-amplifier and a low-frequency amplifier, the output of the second modulator is connected through a series-connected receiver, a low-hour pre-amplifier frequencies, a synchronous filter, a low-frequency amplifier, a high-pass filter to the first input of the comparator, the second matched load is connected through the second bandpass filter to the second input of the first modulator, and the antenna is connected to the second input of the second modulator through the third bandpass filter.

Figure 1 presents the structural diagram of the radiometer with the formation of the reference signal from the noise of the receiver using bandpass filters (analog).

Figure 2 presents the structural diagram of a zero radiometer with additional pulse-width modulation of the reference signals generated using the frequency properties of bandpass filters (prototype).

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

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

Figure 5 presents a map of variations in the measurement ranges for all possible cases.

According to the structural diagram of figure 3, the radiometer conventionally consists of three parts: the input unit, a linear measuring path and the control unit. The input unit in its composition has two noise generators, the role of which is played by the first 4 and second 5 matched loads, the first 2 and second 3 identical modulators having a 2 × 1 configuration, the first 6, second 7 and third 17 bandpass filters. Modulators are switched by control signals from the outputs of the control unit. Antenna 1 is connected to the input of the unit, and the output is connected to the measuring path of the radiometer. In order to minimize the errors of the radiometer, the coordinated loads, like generators of exemplary noise signals, are thermostatically controlled. Both modulators are also thermostatically controlled, since they contain semiconductor structures (diodes), the parameters of which depend on temperature. For this, the modulators, together with the agreed loads, are fixed on the thermostated board 8 and are with it at the same temperature T _o .

The main node of the measuring path is the receiver 9, where the amplified signals at high frequency are amplified and their quadratic detection is performed. Further, the signals are pre-amplified at a low frequency in a low-frequency pre-amplifier 10. Since the signals after the detector have high-frequency components of random nature, therefore, after the pre-amplification, low-frequency synchronous filtering of the signals is performed in the synchronous filter 11. The time constant of the synchronous filter has a small value and eliminates high-frequency noise in the signals. This eliminates the overload of the low-frequency amplifier 12. In the next measurement path circuit, the high-pass filter 13 eliminates the constant component and the modulated signal sequence is fed to the input of the comparator 14. Since the second input of the comparator is connected to a common point in the circuit, therefore, it determines the polarity of the input signal .

The output signal of the comparator in the levels of logical zero and one is fed to the input of the control unit 15, made on digital elements.

The principle of operation of the radiometer is as follows. The control unit generates three control pulse signals. The signals from the first and second outputs thereof enter the input part of the radiometer and modulate the measured and reference signals. The control signal t _chis from the first output (figa) of the control unit is pulse-width, that is, varies in duration. This signal in the modulator 2 is a pulse-width modulation of the signals of the matched loads, which act as reference noise generators with effective noise temperatures equal to the temperature of the thermostated board T ₀ . The control signal t _mod from the output 2 of the control unit (figb) produces a symmetric modulation of the antenna signal and already pre-modulated reference signals of the matched loads. The control signal of the modulator 3 has a duty cycle equal to two, therefore, includes two equal half-periods in duration. In the first half-cycle, as follows from fig.4b, the output of the modulator 2 is connected to the input of the receiver; in the second half-cycle - antenna 1 through the second input of modulator 3, When the output of modulator 2 is connected to the input of the receiver through modulator 3, in the latter a pulse-width modulation of the signals of the matched loads is carried out.

So, at the input of the receiver 9 at different points in time, one of the following signals acts:

- matched load signal 4, whose noise power is

where k is the Boltzmann constant, δ f ₁ is the passband of the band-pass filter 6, T ₀ is the thermodynamic temperature of the thermostated board on which load 4 is installed. Since it is matched to the transmission line and all other connections are matched, therefore its effective noise temperature is also T _about ;

- signal matched load 5, whose power is equal

- an antenna signal whose power at the receiver input is

where T _a is the effective temperature of the noise of the antenna.

In the receiver 9, these signals are amplified by power with a coefficient G, then they are quadratically detected with the conversion coefficient of the detector β. Thus, an envelope is emitted at the output of the receiver, which is amplified at a low frequency in amplifiers 10 and 12 with a common gain K _u .

Therefore, at the input of the high-pass filter 13 at different times of the modulation time there are signals whose voltage amplitudes are equal to:

The purpose of the high-pass filter 13 in the radiometer is the exclusion of the DC component in the periodic signal. To do this, it must satisfy the following requirements - the signal should pass through it with minimal distortion of the pulse shape, which is ensured by the proper choice of the lower half-filter frequency.

At the input of the high-pass filter, in addition to the signals U _sn1 , U _cn2 , U _a , a continuously operating unmodulated signal is received, caused by the intrinsic noise temperature of the receiving measuring path, which is equal to

where δ f is the band of frequencies received by the receiver; T _W - the effective temperature of the noise floor of the receiver, reduced to its input.

Since the constant component is excluded from the high-pass filter, therefore, the voltage U _mp will also not be present at its output, since this voltage does not change at modulation intervals.

Thus, at the output of the filter 13 for an arbitrary width of the pulse-width pulse signal _tw , generated at the first output of the control unit 15, a signal with a periodic structure, the shape of which is shown in Fig. 4c, will act.

For one signal period in the time diagram of FIG. 4c with the constant component excluded, the equality of the volt-second areas of the pulses having positive and negative signs is observed. That is, S ₁ = S ₂ + S ₃ . This case corresponds to an unconfigured zero balance in the radiometer. The zero balance in the radiometer is considered adjusted if the volt-second pulse area S ₃ is equal to zero. The setting for the condition S ₃ = 0 is carried out under the control of the control unit 15, which controls the width of the pulse-width signal t _chis at the output 1. The control signal for the control unit, according to which it controls the duration t _chis , is the signal from the output of the comparator 14, which is the terminal node measuring path. The control unit analyzes the comparator signal synchronously, that is, during the second modulation half-cycle, when the antenna signal passes through the measuring path of the radiometer.

The control algorithm for adjusting the duration t _{chis by} the control unit is as follows: if the impulse S _{3 is} negative (located below the zero time axis), then the control unit reduces the duration t _chis , if the impulse S _{3 is} positive, the control unit increases the duration t _chis .

Then, for an unchanged antenna signal T _a with a constant modulation half-cycle duration equal to t _modes , the time diagram of signals to change t _chis will shift upward (decrease t _chis ) or down (increase t _chis ) relative to the zero time axis. For FIG. 4c, a change in the duration t _{sys by} the control unit occurs to decrease it and continues until the momentum S ₃ disappears. For this case, the timing diagram is shown in FIG. 4d, when the condition S ₃ = 0 was fulfilled as a result of adjusting the duration t _chis .

Given that the amplitudes of the pulses S ' ₁ and S' ₂ in Fig. 4d are proportional to the corresponding signal differences δf ₂ T _o - δf ₃ T _a and δf ₃ T _a - δf ₁ T _o , for the equality of the volt-second areas of the pulses S ' ₁ = S ' ₂ with a new duration t', the _chis can be written

Having performed the abbreviations and solving equality (10) with respect to the t ' _shear , we obtain the transfer characteristic of the radiometer, which will be determined by the formula

From the last formula it follows that the duration t ' _{shear is} directly proportional to the antenna signal T _a . Therefore, through this duration, the antenna signal can be found indirectly, which is connected with it by linear relation (11). The measurement accuracy is not affected by the transmission coefficient of the entire measuring path, equal to the product Gβ K _u , as in radiometers operating according to the zero method. Formula (11) for finding the pulse width of a pulse-width signal does not include a signal associated with the intrinsic noise of the radiometer. As explained above, this signal is continuous and not modulated. Therefore, it does not pass through the high-pass filter to the comparator. However, in addition to the measured signal of the antenna, formula (11) for finding the t ' _chis includes the passbands of the filters 6, 7, 17, the temperature of the thermostatically controlled circuit board T _о and the modulation half-life t _mod . These values must be kept constant. Their changes will lead to measurement errors.

To determine the boundaries of the measurement range, we solve equality (11) with respect to the effective noise temperature of the antenna T _a .

Substituting in the formula (12) the extreme values of the duration t ' _n , equal to zero and the duration of the half-period t _modes , we obtain the minimum and maximum boundaries of the measuring range of the antenna signal:

Therefore, analyzing (13), we can conclude that by changing the passband of the filters, you can adjust the measurement range to the selected one, keeping the temperature of the thermostated board constant and equal to T _o .

The passband δ f _{2 of the} second filter 7 is larger than the passband δ f _{1 of the} first filter 6, since, by definition, the reference signal generated by the second matched load 5 is larger than the reference signal produced by the first matched load 4. This follows from the timing diagram of FIG. That is, δ f ₂ = nδ f ₁ and n> 1. This condition is always satisfied.

The conditions establishing the connection between the bands of the first 6 and second 7 filters with the band of the input filter 17 through which the measured antenna signal is received, we find as follows. Let be

δ f ₂ = pδ f ₃ , where p> 1 or p <1;

δ f ₁ = mδ f ₃ , where m> 1 or m <1. (14)

Then the frequency bands δ f ₁ , δ f ₂ , δ f ₃ are interconnected by the following relationships:

From relations (15) we can obtain the following important equality:

Since n> 1, therefore, p> m.

Given the replacement n = (δ f ₂ / δ f ₁ ), p = (δ f ₂ / δ f ₃ ), m = (δ f ₁ / δ f ₃ ), expression (12) for determining the antenna signal can be written

Since p = nm from (16), expression (17) can be written in the following form:

From (18), the boundaries of the measurement range at t ' _sis = 0 and t' _sis = t _modes taking into account (15) are then equal to:

Consider a numerical example. A typical case of the static temperature of the input units of radiometers is a temperature of 325 K (52 ° C). That is, T _o = 325 K. If you select a signal reception band with a radiometer equal to 100 MHz (δ f ₃ = 100 MHz), you can determine the bands of the other two filters if you need to measure the antenna signal in the ranges:

a. 100 K-200 K;

b. 100 K - 325 K;

in. 100 K - 450 K;

325 K - 450 K;

d. 400 K - 600 K.

These measurement ranges of the antenna signal are shown in figure 5 and characterize all possible combinations of measurement ranges relative to the reference temperature T _about .

Given (19), we can write

Then using relations (20), we can calculate the filter bands 6 and 7 for ranges a. - d:

a. δ f ₁ = 30.8 MHz; δ f ₂ = 61.5 MHz;

b. δ f ₁ = 30.8 MHz; δ f ₂ = 100 MHz;

in. δ f ₁ = 30.8 MHz; δ f ₂ = 138.5 MHz;

g. δ f ₁ = 100 MHz; δ f ₂ = 138.5 MHz;

d. δ f ₁ = 123.1 MHz; δ f ₂ = 184.6 MHz.

The frequency bands of all filters may not overlap, but these bands should be in the signal reception band of the radiometer. The center frequencies of these filters are determined by the receiver of the radiometer and the received signals.

In the radiometer, the control unit is fully consistent with the control unit of the prototype radiometer. The mode of his work is tracking. The control unit continuously monitors the signal at the output of the comparator of the measuring path in every second modulation half-cycle. Analyzing this signal, the control unit, according to the above-described algorithm, in each first half-period of modulation corrects the width of the pulse-width signal _tw . A change in the antenna signal will lead to the appearance of a voltage at the input of the comparator of negative or positive polarity in the second modulation half-cycle. This corresponds to the case that the path is unbalanced and the output is not reliable. The control unit in accordance with the signal of the comparator directionally changes the duration of the pulse-width signal. The modulated sequence of signals at the input of the comparator is shifted relative to the zero time axis and, when the voltage is set to zero in the half-switching period of the antenna, the control unit stops adjusting the pulse-width signal. The path is balanced again, which means that the width of the pulse-width signal that determines the antenna signal is valid.

In the control unit, the width of the pulse-width signal is represented by a digital code that is issued to the digital bus 16, which is the output of the radiometer.

The synchronous filter, arranged in the same way as the synchronous filter of the prototype, is a first-order filter and performs low-pass filtering of signals, smoothing their high-frequency emissions. The synchronous filter is an RC circuit with three switched capacitors. Each capacitor filters one of the three input signals, of which two reference and one signal, measured from the antenna.

The low frequency preamplifier and low frequency amplifier are linear voltage amplifiers.

The high-pass filter is a dividing CR circuit eliminating a constant component in a periodic signal.

Bandpass filters are passive microwave devices, do not include semiconductor elements and are assembled from quarter-wave transmission lines. That is, they have stable parameters and may not be thermostated. They are calculated according to well-known techniques widely described in the literature.

Thus, the advantages of the proposed radiometer are that it works by the zero method, when changes in the gain of the measuring path do not affect the accuracy of the measurements. Its second advantage is insensitivity to the constant component of the intrinsic noise of the radiometer. At the output of the control unit, the signal is presented in digital form, since the width of the pulse-width signal can be easily converted into code. This means that the control unit does not have standard analog-to-digital converters. The digital code may be subject to further processing by digital systems, including computer tools. In the radiometer, by a free choice of filters, you can change the reception band of the antenna signals (δ f ₃ ) and the boundaries of the measurement range (δ f ₁ , δ f ₂ ) without changing the reference signals of the noise generators, which are the matched loads. The measurement range can be selected within the required limits of the antenna signal, and this range will not be rigidly tied to the temperature of the thermostat. Since the loads are at the same temperature T _o , it is possible to measure this temperature with precision contact electronic thermometers and then calculate the antenna signal according to formula (12). It follows that the temperature control scheme and the accuracy of maintaining the temperature in the thermostat can be simplified. If you perform filters with electronic restructuring of the bands, therefore, it is possible to carry out a change in the measurement range in an operational way.

Claims (1)

A zero radiometer containing an antenna, first and second matched loads, first and second bandpass filters, first and second modulators, a receiver, a high-pass filter, a synchronous filter, a comparator, a control unit, the first, second and third outputs of which are connected to the control inputs of the first, the second modulators and synchronous filter, respectively, and the input of the control unit is connected to the output of the comparator, the second input of which is connected to the common bus of the radiometer, the first matched load through the first bandpass filter it is connected with the first input of the first modulator, the output of which is connected to the first input of the second modulator, and the radiometer output is the fourth output of the control unit, the first and second matched loads, the first and second modulators mounted on a thermostatically controlled circuit board, characterized in that a third strip filter, low-frequency pre-amplifier and low-frequency amplifier, the output of the second modulator is connected through a series-connected receiver, low-frequency pre-amplifier, sync filter, low-frequency amplifier, high-pass filter to the first input of the comparator, the second matched load is connected through the second band-pass filter to the second input of the first modulator, and the antenna is connected to the second input of the second modulator through the third band-pass filter.

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