CN101915908A - Multi-coupling loop internal calibration method for satellite-borne microwave scatterometer - Google Patents

Abstract

The invention relates to a multi-coupling loop internal calibration method for a satellite-borne microwave scatterometer. In the calibration method, an internal calibration process is finished by using a calibration loop and a gain difference measurement loop, wherein the gain difference measurement loop is introduced to measure the gain difference G1LN of a low-noise amplifier when the power of the amplifier is switched on and switched off; meanwhile, calibration signal power Poc and echo signal power Por are obtained by switching off and switching on the power of the low-noise amplifier in the calibration loop respectively; signal power P0'' and signal power P0' are obtained at an output end of a receiver by switching off and switching on the power of the low-noise amplifier in the gain difference measurement loop so as to obtain the gain difference G1LN; and then Pr/Pt is obtained by calculation so as to finish the internal calibration. By using the method for controlling the power of the low-noise amplifier, the method has the advantages of greatly improving the power of a calibration signal, greatly reducing the requirement on isolation of the system and reducing the development difficulty of microwave front-end equipment in practical application, along with easy engineering realization.

Description

Calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer

Technical field

The invention belongs to the microwave remote sensing technique field, particularly relate to calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer.

Background technology

Calibration is a requisite link in the microwave scatterometer scatterometry.What interior calibration mainly solved is the uncertainty of transmitter emissive power, the uncertain factor influence that measurement brings to microwave scattering of receiver gain.In radar equation

${\mathrm{\σ}}^0\∝\frac{P_r}{P_t}$

Therefore, if measure P _r/ P _t, then except antenna gain, need not measure other characteristics of system and just can determine σ ⁰Value.The sampling value that transmits is come to the receiver calibration, also just do not need emissive power and receiver identity are done to measure respectively, and it is just passable only to need directly to measure its ratio.Microwave scatterometer is exactly to adopt this method to eliminate because the error that the gain instability of transmitter and receiver is brought.At present, the satellite-borne microwave scatterometer often adopts calibrating method in the single loop, and the theory diagram of calibrating method as shown in Figure 1 in the single loop.Rate-aided signal is coupled to receiving cable by the loop, the influence of gain instability to measuring of comparing with echoed signal and eliminating transmitter and receiver.The deficiency of calibrating method is exactly to require system that higher isolation is arranged in the single loop.

The principle of calculating isolation is to guarantee to import more than the little 20dB of signal of (being rate-aided signal) from LNA from the signal ratio that spuious approach such as antenna or circulator leaks.

Suppose that the signal power that the self-emission machine comes is 0dBm, the power that arrives antenna behind power amplifier is 50dBm, the gain of LNA is 30dB, the size of rate-aided signal is taken as-60dBm (principle chosen of rate-aided signal be receiver is not produced saturated), here supposition, make rate-aided signal not contaminated, just must guarantee rate-aided signal than more than the high 20dB of leakage signal, so just require at least isolation be 50-(60-20)=130dB.This shows that scaling method is very high to the requirement of isolation in the single loop.The front end development difficulty of high-isolation is very big.

Summary of the invention

The objective of the invention is to overcome the above-mentioned deficiency of prior art, calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer is provided, this method is introduced a loop of surveying gain inequality, and improve the power of rate-aided signal, and then reduce requirement to system's isolation by the method for control low noise amplifier power supply.

Above-mentioned purpose of the present invention is achieved by following technical solution:

Calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer, finish interior calibration process by calibration loop and survey gain inequality loop, wherein calibrate the loop and comprise coupling mechanism 2 and fixed attenuator 2, survey the gain inequality loop and comprise coupling mechanism 1 and fixed attenuator 1, concrete implementation procedure is as follows:

(1) connects the calibration loop, at first turn-off the power supply of low noise amplifier, the signal that signal source produces amplifies after overcoupling device 2 is coupled out a part of signal through travelling-wave tube amplifier (TWTA), the input end that enters receiver behind the process fixed attenuator 2 obtains the power P of rate-aided signal as rate-aided signal at the output terminal of receiver again _Oc:

$P_{\mathrm{oc}}=\frac{P_t}{L_f{L}_N{L}_c}\·G$

P wherein _tBe the emissive power that transmits, the signal that produces for signal source that transmits passes through the signal that obtains after travelling-wave tube amplifier (TWTA) amplifies, L _NBe the decay of low noise amplifier to signal, L _fBe the total losses of coupling mechanism 2 and fixed attenuator 2, L _cBe the damping capacity of numerical-control attenuator, G is the full gain of the receiver except low noise amplifier and numerical-control attenuator;

Connect the power supply of low noise amplifier afterwards, obtain the power P of echoed signal at the output terminal of receiver _Or:

$P_{\mathrm{or}}=\frac{P_r\·G_1\·G}{L_r}$

P wherein _rBe the power of echoed signal, G ₁Be the gain of low noise amplifier, L _rDamping capacity for numerical-control attenuator;

(2) power P of the rate-aided signal that step (1) is obtained _OcPower P with echoed signal _OrCompare and obtain following formula:

$\frac{P_{\mathrm{oc}}}{P_{\mathrm{or}}}=\frac{\frac{P_t}{L_f{L}_N{L}_c}\·G}{\frac{P_r\·G_1\·G}{L_r}}$

Further obtain:

$\frac{P_r}{P_t}=\frac{P_{\mathrm{or}}{L}_r}{P_{\mathrm{oc}}{L}_f{L}_c}\frac{1}{L_N{G}_1};$

(3) connect survey gain inequality loop, at first connect the power supply of low noise amplifier, the signal that signal source produces is coupled out a part of signal through coupling mechanism 1, again through the input end that enters receiver behind the fixed attenuator 1 as surveying the gain inequality signal, obtain signal power at the output terminal of receiver

$P_o^{\′}=\frac{P_{\mathrm{ot}}^{\′}{G}_{1}G}{L}$

Wherein

Be the fixed signal through fixed attenuator 1, L is the pad value of numerical-control attenuator;

Turn-off the power supply of low noise amplifier afterwards, obtain signal power at the output terminal of receiver

$P_o^{\&prime;\&prime;}=\frac{P_{\mathrm{ot}}^{\&prime;}G}{{\mathrm{<figure-callout\; id="0"\; label="L\; N\; L"\; filenames="HSA00000180449000031.png"\; state="\{\{state\}\}">L</figure-callout>}}_N{L}_{}{}_0}$

L wherein ₀Pad value for numerical-control attenuator;

(4) with the signal power that obtains in the step (3) With signal power

Compare and obtain following formula:

$\frac{P_o^{\&prime;}}{P_o^{\&prime;\&prime;}}=\frac{P_{\mathrm{ot}}^{\&prime;}{G}_{1}G/L}{\frac{P_{\mathrm{ot}}^{\&prime;}G}{{\mathrm{<figure-callout\; id="0"\; label="L\; N\; L"\; filenames="HSA00000180449000031.png"\; state="\{\{state\}\}">L</figure-callout>}}_N{L}_{}{}_0}}=G_1{L}_N\left(\frac{L_0}{L}\right)$

Further obtain:

$G_1{L}_N=\frac{P_o^{\&prime;}L}{P_o^{\&prime;\&prime;}{\mathrm{<figure-callout\; id="0"\; label="L"\; filenames="HSA00000180449000031.png"\; state="\{\{state\}\}">L</figure-callout>}}_0};$

(5) step (4) is obtained

Substitution step (2) obtains

In, try to achieve

Finish calibration in the multiple coupled loop of satellite-borne microwave scatterometer,

Wherein receiver comprise low noise amplifier, frequency mixer, in put, numerical-control attenuator and A/D.

In the calibrating method, receiver is connected with fixed attenuator 2 with fixed attenuator 1 by selector switch in the multiple coupled loop of above-mentioned satellite-borne microwave scatterometer, and selects to connect the calibration loop or connect survey gain inequality loop by selector switch.

In the multiple coupled loop of above-mentioned satellite-borne microwave scatterometer in the calibrating method, receiver also is connected with circulator by selector switch, when connecting the power supply of low noise amplifier in the step (1), selector switch is communicated with circulator, the input end of receiver receives the echoed signal from circulator, and wherein the power of echoed signal is Pr.

In the multiple coupled loop of above-mentioned satellite-borne microwave scatterometer in the calibrating method, low noise amplifier is two rank amplifiers, when turn-offing the power supply of low noise amplifier in step (1) and the step (3), for satisfying certain signal to noise ratio (S/N ratio), the first rank low noise amplifier is set does not cut off the power supply, the second rank low noise amplifier power cutoff.

In the multiple coupled loop of above-mentioned satellite-borne microwave scatterometer in the calibrating method, the pad value L and the L of numerical-control attenuator in the step (3) ₀Be numerical-control attenuator maximum attenuation value L _MAXWith minimal attenuation value L _MINBetween value.

The present invention compared with prior art has following advantage:

(1) calibrating method is finished interior calibration process by adopting the calibration loop with surveying two loops, gain inequality loop in the present invention, wherein introduces the gain inequality G when surveying the gain inequality loop and being used for measuring low noise amplifier and turning on and off power supply ₁L _N, in the calibration loop, obtain the rate-aided signal power P respectively simultaneously by the power supply that turn-offs and connect low noise amplifier _OcWith the echoed signal power P _Or, in surveying the gain inequality loop, obtain signal power respectively by the power supply that turn-offs and connect low noise amplifier

And signal power

By calculating P _r/ P _t, finishing interior calibration, the present invention has improved the power of rate-aided signal greatly, thereby has reduced the requirement to system's isolation significantly by adopting the method for control low noise amplifier power supply;

(2) calibrating method thereby has overcome the defective that there is big difficulty in the front end development of high-isolation in the prior art owing to greatly reduce requirement to system's isolation in the present invention, has reduced the development difficulty of front-end equipment in the practical application, is easy to Project Realization;

(3) be equivalent to an attenuator behind the low noise amplifier power cutoff among the present invention, the receiving system noise is bigger, in order to satisfy certain signal to noise ratio (S/N ratio) requirement, can adopt the low noise amplifier on two rank, wherein the first rank low noise amplifier does not cut off the power supply, the second rank low noise amplifier power cutoff can obtain the less decay of 20～30dB.

Description of drawings

Fig. 1 is the interior calibrating method of single loop in the prior art;

Fig. 2 realizes schematic diagram for calibrating method in the multiple coupled loop of the present invention;

Fig. 3 is a low noise amplifier composition frame chart of the present invention;

Fig. 4 is low noise amplifier test result figure of the present invention;

Fig. 5 is receiver structure and each level point apportioning cost figure among the calibrating method embodiment in the present invention.

Embodiment

By specific embodiment the present invention is carried out further detailed description below in conjunction with accompanying drawing:

Be illustrated in figure 2 as calibrating method realization schematic diagram in the multiple coupled loop of the present invention, calibrate the loop as seen from the figure and comprise coupling mechanism 2 and fixed attenuator 2, transmitting of signal source output amplified after overcoupling device 2 is coupled out a part of signal through travelling-wave tube amplifier (TWTA) TWTA, enter the input end of receiver as rate-aided signal through behind the fixed attenuator 2 again, the power supply of low noise amplifier LNA disconnects when receiving rate-aided signal, this moment, LNA just lost original enlarging function, on the contrary signal there is certain decay, supposes the L that decays to signal _N, the rate-aided signal power of receiver output is so

L wherein _fBe the total losses of coupling mechanism 2 and fixed attenuator 2, L _cDamping capacity for numerical-control attenuator, G is the full gain of the receiver except low noise amplifier and numerical-control attenuator, wherein receiver comprise low noise amplifier, frequency mixer, in put, numerical-control attenuator and A/D, only show the part assembly of receiver among Fig. 1.

When receiving echoed signal, the signal that receives from antenna enters the input end of receiver through circulator, because echoed signal power is less, so will connect the power supply of LNA, this moment, the gain of LNA was G ₁, the echoed signal power of receiver output terminal is so

P wherein _rBe the received power of echoed signal, L _rDamping capacity for numerical-control attenuator.

According to target calibration formula P decided at the higher level but not officially announced _r/ P _t, the power P of the rate-aided signal that receiver is received _OcPower P with echoed signal _OrObtain with comparing

$\frac{P_{\mathrm{oc}}}{P_{\mathrm{or}}}=\frac{\frac{P_t}{L_f{L}_N{L}_c}\&CenterDot;G}{\frac{P_r\&CenterDot;G_1\&CenterDot;G}{L_r}}$

$\frac{P_r}{P_t}=\frac{P_{\mathrm{or}}{L}_r}{P_{\mathrm{oc}}{L}_f{L}_c}\frac{1}{L_N{G}_1}---\left(1\right)$

By formula (1) as can be known, require P _r/ P _t, L _NG ₁(gain inequality when being LNA connection, power cutoff) is necessary for known quantity, therefore draw a loop (loop as shown in phantom in Figure 2) of measuring gain inequality, survey the gain inequality loop and comprise coupling mechanism 1 and fixed attenuator 1, the signal that signal source produces is coupled out a part of signal through coupling mechanism 1, enter the input end of receiver again behind the process fixed attenuator 1 as surveying the gain inequality signal, connect and survey the gain inequality loop, connect, turn-off the power supply of low noise amplifier LNA then respectively, obtain connecting signal power under the LNA power supply status respectively at the output terminal of receiver With the signal power of turn-offing under the LNA power supply status

When connecting the LNA power supply, for preventing the saturation of receiver, the pad value of numerical-control attenuator is changed to L; When disconnecting the LNA power supply, the pad value of numerical-control attenuator is changed to L ₀, wherein the pad value L and the L of numerical-control attenuator ₀Be numerical-control attenuator maximum attenuation value L _MAXWith minimal attenuation value L _MINBetween value.

$P_o^{\&prime;}=\frac{P_{\mathrm{ot}}^{\&prime;}{G}_{1}G}{L}---\left(2\right)$

$P_o^{\&prime;\&prime;}=\frac{P_{\mathrm{ot}}^{\&prime;}G}{L_N{L}_0}---\left(3\right)$

Wherein

Be fixed signal through fixed attenuator 1.

Formula (2) compared with formula (3) obtains:

$\frac{P_o^{\&prime;}}{P_o^{\&prime;\&prime;}}=\frac{P_{\mathrm{ot}}^{\&prime;}{G}_{1}G/L}{\frac{P_{\mathrm{ot}}^{\&prime;}G}{{\mathrm{<figure-callout\; id="0"\; label="L\; N\; L"\; filenames="HSA00000180449000031.png"\; state="\{\{state\}\}">L</figure-callout>}}_N{L}_{}{}_0}}=G_1{L}_N\left(\frac{L_0}{L}\right)$

$G_1{L}_N=\frac{P_o^{\&prime;}L}{P_o^{\&prime;\&prime;}{L}_0}---\left(4\right)$

Formula (4) substitution formula (1) is obtained formula (5) P _r/ P _t:

$\frac{P_r}{P_t}=\frac{P_{\mathrm{or}}\&CenterDot;P_o^{\&prime;\&prime;}}{P_{\mathrm{oc}}\&CenterDot;P_o^{\&prime;}}\frac{L_r\&CenterDot;L_0}{L_c\&CenterDot;L}---\left(5\right)$

Finished target process decided at the higher level but not officially announced by above-mentioned calibration loop and survey gain inequality loop, wherein selector switch is used for selecting to connect the calibration loop or connecting and survey the gain inequality loop, selector switch is connected between receiver and fixed attenuator 1, fixed attenuator 2 and the circulator, select to connect the calibration loop when selecting switch, and when connecting the LNA power supply, selector switch is communicated with circulator, the input end of receiver receives from the echoed signal of circulator output, when turn-offing the LNA power supply, selector switch and fixed attenuator 2 are connected, and the input end of receiver receives rate-aided signal; When selecting switch to select to connect survey gain inequality loop, selector switch and fixed attenuator 1 are connected, and the input end of receiver receives surveys the gain inequality signal.

The present invention is by turn-offing the power supply of low noise amplifier, it is become an attenuator, and then improve the power of rate-aided signal, because the microwave scatterometer signal bandwidth is narrower, so behind the low noise amplifier power cutoff, frequency characteristic does not have bigger variation in narrower bandwidth, can guarantee that amplitude versus frequency characte is smooth.Signal is by behind the low noise amplifier of power cutoff, and signal can not be subjected to big influence, so the present invention program is feasible.

Be illustrated in figure 3 as the composition structural drawing of low noise amplifier of the present invention, low noise amplifier comprises biasing circuit, input coupling, transistor amplifies and the output coupling, wherein input coupling and output coupling are finished the impedance conversion between signal source and load and the transistor, what wherein play amplification is the field effect transistor amplifier, turn-off the power supply of LNA, in fact just equaled to make field effect transistor amplifier inefficacy among the LNA.

Chosen a frequency of operation in the Ku frequency range, the branch two-stage is amplified, the low noise amplifier of enlargement factor 25dB is tested, and test result records on arrow net analyser.Test result as shown in Figure 4, as seen from Figure 4, at centre frequency 13.256GHz, bandwidth is in the frequency band of 10MHz, changes in amplitude is stably substantially, sees low noise amplifier in narrower bandwidth by the experiment test result, amplitude versus frequency characte is smooth.The low noise amplifier gain that experiment is adopted is 25dB, divide two-stage to amplify, decay to behind the power cutoff-51dB about, do not need so big decay in the actual design, for obtain less decay (20～30dB), can adopt and only turn-off the one-level or the method for two-stage wherein.

Adopt the situation of calibrating method reduction isolation in the present invention below by case-study:

Because be equivalent to an attenuator behind the low noise amplifier power cutoff, the receiving system noise is bigger.In order to satisfy certain signal to noise ratio (S/N ratio) requirement, the method that adopts is that the first rank low noise amplifier does not cut off the power supply, the second rank low noise amplifier power cutoff, be illustrated in figure 5 as receiver structure and each level point apportioning cost figure among the interior calibrating method embodiment of the present invention, (LNA1 is 15dB to receiver except that LNA among Fig. 5, LNA2 is 15/-35dB), also comprise two frequency mixer (frequency mixer 1 and frequency mixer 2), numerical-control attenuator, put and A/D in three, wherein the gain of putting in three is respectively 25dB, 25dB and 60dB, the loss of two frequency mixer is-10dB, the maximum attenuation value L of numerical-control attenuator _MAX=64dB, minimal attenuation value L _MIN=0dB.

The calculating principle of isolation is to guarantee to import more than the little 20dB of signal of (being rate-aided signal) from LNA from the signal ratio that spuious approach such as antenna or circulator leaks.By top reception structure, the rate-aided signal maximum can be taken as-15dBm, if the value of rate-aided signal is big again, then the linear characteristic behind the LNA amplifier power cutoff can not guarantee, here the maximum input signal of supposing A/D is 4dBm, maximum input signal behind the LNA power cutoff is chosen as 0dBm, the selection principle of rate-aided signal is to guarantee LNA, A/D is unsaturated, the isolation that calculates like this is: and 50-(15-20)=85dB, the emissive power of wherein supposing transmitter is 50dBm, in the interior calibrating method of prior art single loop, the size of rate-aided signal is taken as-60dBm (principle chosen of rate-aided signal be receiver is not produced saturated), make rate-aided signal not contaminated, just must guarantee that rate-aided signal is than more than the high 20dB of leakage signal, so require at least isolation be 50-(60-20)=130dB, the insulated degree requirement of the isolation that this shows calibrating method in the present invention 130dB of calibrating method in the single loop is much smaller.

The above; only be the embodiment of the best of the present invention, but protection scope of the present invention is not limited thereto, anyly is familiar with those skilled in the art in the technical scope that the present invention discloses; the variation that can expect easily or replacement all should be encompassed within protection scope of the present invention.

The content that is not described in detail in the instructions of the present invention belongs to this area professional and technical personnel's known technology.

Claims (5)

1. calibrating method in the multiple coupled loop of satellite-borne microwave scatterometer, it is characterized in that: finish interior calibration process by calibration loop and survey gain inequality loop, wherein calibrate the loop and comprise coupling mechanism 2 and fixed attenuator 2, survey the gain inequality loop and comprise coupling mechanism 1 and fixed attenuator 1, concrete implementation procedure is as follows:

(1) connects the calibration loop, at first turn-off the power supply of low noise amplifier, the signal that signal source produces amplifies after overcoupling device 2 is coupled out a part of signal through travelling-wave tube amplifier (TWTA), the input end that enters receiver behind the process fixed attenuator 2 obtains the power P of rate-aided signal as rate-aided signal at the output terminal of receiver again _Oc:

$P_{\mathrm{oc}}=\frac{P_t}{L_f{L}_N{L}_c}\&CenterDot;G$

P wherein _tBe the emissive power that transmits, L _NBe the decay of low noise amplifier to signal, L _fBe the total losses of coupling mechanism 2 and fixed attenuator 2, L _cBe the damping capacity of numerical-control attenuator, G is the full gain of the receiver except low noise amplifier and numerical-control attenuator;

Connect the power supply of low noise amplifier afterwards, obtain the power P of echoed signal at the output terminal of receiver _Or:

$P_{\mathrm{or}}=\frac{P_r\&CenterDot;G_1\&CenterDot;G}{L_r}$

P wherein _rBe the power of echoed signal, G ₁Be the gain of low noise amplifier, L _rDamping capacity for numerical-control attenuator;

(2) power P of the rate-aided signal that step (1) is obtained _OcPower P with echoed signal _OrCompare and obtain following formula:

$\frac{P_{\mathrm{oc}}}{P_{\mathrm{or}}}=\frac{\frac{P_t}{L_f{L}_N{L}_c}\&CenterDot;G}{\frac{P_r\&CenterDot;G_1\&CenterDot;G}{L_r}}$

Further obtain:

$\frac{P_r}{P_t}=\frac{P_{\mathrm{or}}{L}_r}{P_{\mathrm{oc}}{L}_f{L}_c}\frac{1}{L_N{G}_1};$

(3) connect survey gain inequality loop, at first connect the power supply of low noise amplifier, the signal that signal source produces is coupled out a part of signal through coupling mechanism 1, again through the input end that enters receiver behind the fixed attenuator 1 as surveying the gain inequality signal, obtain signal power at the output terminal of receiver

$P_o^{\&prime;}=\frac{P_{\mathrm{ot}}^{\&prime;}{G}_{1}G}{L}$

Wherein

Be the fixed signal through fixed attenuator 1, L is the pad value of numerical-control attenuator; Turn-off the power supply of low noise amplifier afterwards, obtain signal power at the output terminal of receiver

$P_o^{\&prime;\&prime;}=\frac{P_{\mathrm{ot}}^{\&prime;}G}{L_N{L}_0}$

L wherein ₀Pad value for numerical-control attenuator;

(4) with the signal power that obtains in the step (3)

With signal power

Compare and obtain following formula:

$\frac{P_o^{\&prime;}}{P_o^{\&prime;\&prime;}}=\frac{P_{\mathrm{ot}}^{\&prime;}{G}_{1}G/L}{\frac{P_{\mathrm{ot}}^{\&prime;}G}{L_N{L}_0}}=G_1{L}_N\left(\frac{L_0}{L}\right)$

Further obtain:

$G_1{L}_N=\frac{P_o^{\&prime;}L}{P_o^{\&prime;\&prime;}{L}_0};$

(5) step (4) is obtained

Substitution step (2) obtains

In, try to achieve

Finish calibration in the multiple coupled loop of satellite-borne microwave scatterometer,

Wherein receiver comprise low noise amplifier, frequency mixer, in put, numerical-control attenuator and A/D.

2. calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer according to claim 1, it is characterized in that: described receiver is connected with fixed attenuator 2 with fixed attenuator 1 by selector switch, and selects to connect the calibration loop or connect survey gain inequality loop by selector switch.

3. calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer according to claim 1, it is characterized in that: described receiver also is connected with circulator by selector switch, when connecting the power supply of low noise amplifier in the step (1), selector switch is communicated with circulator, the input end of receiver receives the echoed signal from circulator, and wherein the power of echoed signal is P _r

4. calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer according to claim 1, it is characterized in that: described low noise amplifier is two rank amplifiers, when turn-offing the power supply of low noise amplifier in step (1) and the step (3), for satisfying certain signal to noise ratio (S/N ratio), the first rank low noise amplifier is set does not cut off the power supply, the second rank low noise amplifier power cutoff.

5. calibrating method in the multiple coupled loop of a kind of satellite-borne microwave scatterometer according to claim 1 is characterized in that: the pad value L and the L of numerical-control attenuator in the described step (3) ₀Be numerical-control attenuator maximum attenuation value L _MAXWith minimal attenuation value L _MINBetween value.

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