JPH02261232A - Calibration device for fading simulator - Google Patents

Abstract

PURPOSE:To set an object value accurately and to remarkably improve the performance as a fading simulator by accurately calculating each notch frequency and its depth to each propagation constant of a propagation line with a calibration signal and storing the result as a data. CONSTITUTION:At first a changeover switch 22 is switched to the position of a calibration signal while the propagation constant of a propagation line is set, the signal frequency is changed at a prescribed interval to read the attenuation at each frequency with a level detector 34. On the other hand, a prescribed relation exists between the amplitude characteristic and the frequency of a synthetic signal in the fading characteristic. Thus, a correct notch frequency and notch depth are obtained by calculation from plural data in the vicinity of the notch frequency and the result is stored as a calibration data into a memory 32e. At the actual test, the propagation constant to the required notch frequency and depth is read from a calibration data memory to set the constant accurately.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention allows one signal sent out from a transmitting device to pass through a plurality of propagation paths with different propagation constants, so that when received by a receiving device, two The present invention relates to a fading simulator that pseudo-generates a fading phenomenon caused by interference between two signals, and particularly relates to a fading simulator calibration device that calibrates a pseudo-fading phenomenon so that it has a set characteristic.

[Prior Art] When transmitting various types of data over long distances using microwaves of, for example, 1 to 20 GHz, a pair of antennas la and lb are arranged facing each other as shown in FIG. The signal output from the transmitter is wirelessly outputted from the transmitting side antenna 1a, and is received by the receiving device 3 via the receiving side antenna 1b. In this case, since antennas la and lb are visually placed opposite to each other, the radio waves radiated from antenna 1a are directly input to antenna 1b, but due to some conditions they may be reflected on the ground or diffracted. In some cases, the signal may be input to the antenna 1b on the receiving side.

In this case, two propagation paths are generated: a direct wave propagation path 4a and a reflected wave propagation path 4b. Each propagation path 4a.

Since the propagation path lengths in 4b are different, a delay time τ and a phase difference φ occur between the signals Ao and Ar propagating through the respective propagation paths 4a and 4b when the receiving device 3 receives each radio wave. Therefore, the signals Ao and Ar passing through each propagation path 4g and 4b can be expressed by equations (1) and (21).

Ao −aQ exp[Jωl ] −(
1) Ar−aRexp[jfω(t+r)+φl 1
-(2) Therefore, the signal At obtained by combining each signal Ao and Ar is At −Ao +Ar −a o [exp(jωt) + (a R/a o )expljω(t+ r )+
jφl] - (3) Note that the state where a□>aH is called minimum fading, and the state where a□<aR is called non-minimum fading. When the amplitude characteristic ^BS(A t) of this composite signal At is standardized with the direct wave signal AO, it becomes 8BS(A t> = ABS(A o+A r)-
Person BS [l+ρ expljωτ+jφ)ko=l+
p cos +p2-(4) However, ρ-a R/
a.

or ao/aR, (ρ≦1) θ−ωτ+φ Further, when the above amplitude characteristics are expressed in dB as Δω−2πΔf and Δf−f−Fp, the formula (5) is obtained.

A t(dB)=101og[I+ρ2−2ρcos
(Δωτ)] ... (5) When formula (5) is expressed with the horizontal axis as frequency f (-ω/2π), as shown in Figure 7, the amplitude decreases rapidly at a certain frequency Fp. . Further, the phase shifts by 180'' at this frequency Fp. That is, when the receiving device 3 combines the signals Ao and Ar, a fading phenomenon occurs.

Therefore, the receiving device 3 is required to have a function of normally extracting a signal even if the above-mentioned fading phenomenon occurs.

A fading simulator is used as a device for quantitatively testing the function of the receiving device 3 to remove the fading phenomenon and extract a normal signal.

This fading simulator creates a pseudo propagation path having the frequency characteristics shown in the seventh factor, and processes the signal input to the receiving device 3. That is, the specific frequency in FIG. 7 is called the notch frequency Fp, and the amount of attenuation from the reference level of the reception level at the notch frequency Fp is called the notch depth D.
It is sufficient to faithfully reproduce the notch frequency Fp and notch depth Dp.

Specifically, as shown in FIG. 8, the test signal input manually from the input terminal 5 is passed through the separation circuit 6 to each propagation path 4a,
Two propagation paths 7a corresponding to 4b.

7b, one propagation path 7a is directly input to the synthesis circuit 8, and the other propagation path 7b has delay circuits 9. A phase shift circuit 10 and an attenuation circuit 11 are inserted. and,
The synthesis circuit 8 combines the direct wave signal Ao with the delay circuit 9. The signal A of the reflected wave created in a pseudo manner via the phase shift circuit 10 and the attenuation circuit 11 is combined and outputted from the output terminal 12. That is, the delay circuit 9, the phase shift circuit 10, and the attenuation circuit 11 are combined and output from the output terminal 12. By changing the propagation constant consisting of each delay time τ1 phase difference φ, attenuation amount ρ, etc. of the circuit 11, an arbitrary notch frequency Fp and notch depth Dp can be obtained.

[Problems to be Solved by the Invention] However, the fading simulator shown in FIG. 8 still has the following problems. That is,
The performance of the fading simulator is determined by how accurately the notch frequency Fp and notch depth Dp can be given.

The normally required value of notch depth Dp is 0 to 40 dB.
Since the range is very wide, for example, when Dp is set to -40 dB, the attenuation amount ρ in equation (4) becomes ρ-0.99. Therefore, providing this attenuation amount ρ accurately requires delay circuit 9 and phase shift circuit 1 inserted in this propagation path 7b.
Since the amount of attenuation at 0 is taken into consideration, even if the above-mentioned accuracy is maintained only by the attenuation circuit 11, it is useless and is extremely difficult. Therefore, there is a certain limit to the performance of the fading simulator.

In order to eliminate such inconveniences, an interference simulation test device has been proposed that controls the amount of attenuation using a pilot signal as shown in FIG. 9 (Japanese Unexamined Patent Publication No. 60-236529). That is, the test signal inputted from the input terminal 5 is combined with the pilot signal having the notch frequency Fp to be set which is outputted from the pilot signal generator 14 using the combining circuit 13, and the combined signal is sent to the separating circuit 6. To separate. One side is directly input to the synthesis circuit 8, and the other is inputted to the synthesis circuit 8 via the delay circuit 9 and the complex attenuator 15.
input to. Then, the output signal of the combining circuit 8 is sent to the output terminal 12 after detecting only the pilot signal component in the separating circuit 16 . And this separation circuit 16
The pilot signal component detected in is applied to the multiple search correlation circuit 17.

The complex correlation circuit 17 controls the complex attenuation value of the complex attenuator 15 so that the pilot signal component detected by the separation circuit 16 becomes zero. That is, complex attenuator 151 synthesis circuit 81 separation circuit 16. By forming a kind of feedback circuit with the complex correlation circuit 17 and canceling the pilot signal component outputted to the output terminal 12, as a result, a large notch depth Dp is obtained at the notch frequency Fp.

However, in a device as shown in FIG. 9, which constantly inputs a pilot signal while inputting a test signal, there is a problem that the pilot signal component cannot be completely removed by the feedback circuit. Therefore, there is a concern that the pilot signal may come out from the output terminal 12.

The present invention was made in view of these circumstances, and
By accurately calculating each notch frequency and notch depth for each propagation constant of the propagation path using the calibration signal, and measuring and storing them in advance as calibration data, the stored calibration data can be used to calculate the target. It is an object of the present invention to provide a calibration device for a fading simulator that can accurately set the notch frequency and notch depth and can greatly improve the performance of the fading simulator.

[Means for Solving the Problems] In order to solve the above problems, in the present invention, a test signal inputted through an input terminal is divided by a separation circuit, and a plurality of test signals having different propagation constants such as propagation time and attenuation amount are divided. In a fading simulator, the output frequency of the test signal changes at predetermined frequency intervals over the entire frequency range of the test signal. A calibration signal generation circuit that outputs a calibration signal, a changeover switch that is inserted between the input terminal and the separation circuit, and that switches the input signal to the separation circuit to a test signal or a calibration signal, and attenuation of the output signal of the synthesis circuit. a level detector that detects the amount of attenuation at each frequency, a propagation constant setting means that variably sets the propagation constant in each propagation path, and a changeover switch to the calibration signal side to apply the calibration signal to the separation circuit. This propagation constant setting is determined from the attenuation measurement control means that determines n1 using a level detector, the attenuation at each frequency obtained by the attenuation measurement control means, and the propagation constant set by the propagation constant setting means. Approximate notch frequency reading means for reading the approximate notch frequency of the fading phenomenon at the time, and fading characteristic calculation for calculating the notch frequency and notch depth of the fading phenomenon at the time of propagation constant setting from each attenuation amount at multiple frequencies near the approximate notch frequency. means, a calibration data memory that stores each notch frequency and each notch depth for each propagation constant obtained by this fading characteristic calculation means as calibration data, and a changeover switch to the test signal side to select the required notch. The apparatus is equipped with a test execution control means for reading a propagation constant corresponding to the frequency and notch depth from a calibration data memory and setting it for each propagation path by a propagation constant setting means to generate a pseudo-fading phenomenon by a test signal. .

[Operation] According to the fading simulator calibration device configured as described above, the changeover switch is switched to the calibration signal side before actually inputting a test signal and executing a fading simulation test on the receiving device. Then, with the propagation constant of the propagation path set, the frequency of the calibration signal is changed at regular frequency intervals, and the amount of attenuation at each frequency is obtained from the level detector. Therefore, the approximate notch frequency at the corresponding propagation constant can be read from the attenuation characteristics at each frequency. Note that the accuracy of the approximate notch frequency in this case is the accuracy of the frequency interval in the calibration signal.

On the other hand, as shown in equation (4), in the fading characteristics shown in FIG. 7, a certain relationship exists between the amplitude characteristics and frequency of the composite signal. Therefore, by selecting a plurality of frequencies near the notch frequency and calculating the correct notch frequency and notch depth from the relationship between each frequency and the attenuation amount.

The notch frequencies and notch depths for each propagation constant thus determined are stored in a calibration data memory as calibration data.

By reading the propagation constant corresponding to the required notch frequency and notch depth from the calibration data memory and setting it during the actual test, there is no need to apply a calibration signal during the actual test, and the notch frequency can be accurately measured. and notch depth can be set.

Further, since the notch frequency is calculated, the calculated notch frequency can take a frequency value between the frequency intervals of the calibration signal, and thus the frequency accuracy is greatly improved.

[Example] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a fading simulator incorporating a calibration device according to an embodiment. The test signal inputted through the input terminal 21 is divided into the signal AO corresponding to the direct wave and the signal Ar corresponding to the reflected wave by the separation circuit 24 via the changeover switch 22, and the signal Ao is transmitted via the propagation path 25a. is input to the synthesis circuit 26, and the signal A' is input to the propagation path 25.
It is also input to the synthesis circuit 26 via b. Propagation path 2
5a, only an attenuation circuit 27 for setting the attenuation jl (notch depth) in a non-minimum fading state is inserted, while a phase shift circuit 28.5a is inserted in the propagation path 25b. Delay circuit 29. Attenuation circuit 30. and attenuation circuit 3 for setting the amount of attenuation (notch depth) in the minimum fading state.
1 is inserted. The phase shift circuit 28. Delay circuit 29
．． The phase difference φ, delay time τ, and attenuation amount ρ of the attenuation circuit 30 are set by the control section 32, which is composed of, for example, a microcombiner, via the D/A converters 28a, 29a, and 30a, respectively. Further, the amount of attenuation of each attenuation circuit 27.degree. 31 is also set by the control section 32 via each D/A converter 27a, 31a. Note that the attenuation amount of each attenuation circuit 27, 31 is controlled to a constant value during the calibration operation for fading characteristics.

The output signal At of the synthesis circuit 26 is output to an output terminal 3 connected to the receiving device 3 shown in FIG. 6, for example, as a test object.
3 and input to the level detector 34. The level detector 34 detects the level of the output signal At, and the detected level value is converted into a digital value by the A/D converter 34a, and then the control unit 32. is manually powered.

Further, a calibration signal output from a calibration signal generation circuit 35 is applied to the other terminal of the changeover switch 22. This calibration signal generation circuit 35 is composed of a circuit that generates a reference frequency such as a crystal oscillator, and a synthesizer that obtains various frequencies from this reference frequency.
For example, 4M over the entire frequency range of the test signal applied to the
The output frequency f changes at a predetermined frequency interval ΔF (-4 MHz) of IIz. The calibration signal generation circuit 35 and changeover switch 22 are controlled by the control section 32.

In the control section 32, as shown in the figure, a propagation constant setting means 32a and an attenuation measurement control means 32b.

Approximate notch frequency reading means 32C, fading characteristic calculating means 32d1, calibration data memory 32e.

The test execution control means 32f etc. may be configured using, for example, a control program or R.
It is formed of a memory element such as AM.

The propagation constant setting means 32a is connected to each D/A converter 28a, 2
Phase shift circuit 28.9a, 30a, 31a, 27a.
Delay circuit 29. A propagation constant consisting of a phase difference φ, a delay time τ, an attenuation amount ρ, etc. for the attenuation circuit 30, and each attenuation circuit 3
Set a fixed attenuation to 1.27. Further, the attenuation amount lP1 constant control means 32b sets the changeover switch 22 to the calibration signal side and controls the calibration signal generation circuit 35 to sequentially change the frequency f of the calibration signal every 4 MHz, for example, when the frequency f of the calibration signal is changed from 4 MHz to The signal level of the output signal At at each frequency f is read via the level detector 34.

Next, as shown in FIG. 3, the approximate notch frequency reading means 32c reads the signal level of the output signal At at each frequency f at each predetermined frequency interval ΔF (=4MIIz) measured by the attenuation measurement control means 32b. The frequency corresponding to the minimum signal level is read as the approximate notch frequency FO. Then, the approximate notch frequency FO that has been read is sent to the next fading characteristic calculation means 32d.

The fading characteristic calculation means 32d selects two frequencies fl and f2 approximately near the notch frequency FO from among the frequencies f of the calibration signal, and controls the attenuation-p1 constant control means 32.
The corresponding propagation constants φ, τ, ρ are determined using the level values A ll and A t2 at the corresponding wave numbers fl and f2 of each layer obtained from b and the propagation constants set in the propagation constant setting means 32a.
The final notch frequency Fp and notch depth Dp at
Calculate.

As shown in FIG. 2, the calibration data memory 32e has an area for storing characteristic values consisting of the notch frequency Fp and notch depth Dp calculated for each propagation constant obtained by the fading characteristic calculation means 32d. has been done.

The test execution control means 32f includes the calibration data memory 32.
With each characteristic value for each propagation constant stored in e,
Indicating that a fading test is to be actually performed on the receiving device 3, the selector switch 22 is set to the test signal side, and the phase angle φ corresponding to the required notch frequency Fp and notch depth Dp is set.

The propagation constants of the delay time τ and the attenuation amount ρ are read out and set in the phase shift circuit 28. Delay circuit 29.
The attenuation circuit 30 is set. Then, the specified notch 1. is applied to the output signal At from the synthesis circuit 26. +! A pseudo fading phenomenon having a wave number Fp and a notch depth Dp is generated.

Next, a method for determining the notch frequency Fp and notch depth Dp from each level value All, At2 will be explained.

In the above equation (5), Δωτ< 1 (rad)
When , coscΔωτ)−1−(Δωτ)2/2 and α−
(1-ρ) (1, At(dB)=101ogl
α2+<Δωr ) 21 -(6) This equation (6) holds true when the notch depth Dp is large and near the notch frequency Fp. Therefore, from equation (6), the loss from Od[I, that is, the notch depth A t (ratio), can be found from equation (7). The above-mentioned propagation constants may be inserted into this equation (7).

A t(ratio)=[a 2+(Δ(IJ r )
2]-α2+(2πτΔf)2...(7) However, Δf-f-Fp At(ratio)=1 0-'^+ (481,
Therefore, as shown in FIG. 3, two appropriate frequencies fl near the approximate notch frequency FO are calculated from equation (7).

Attenuation amount A tl(rarlo) at f2, A
By measuring t2(ratio) with the level detector 34, it becomes possible to calculate the correct notch frequency Fp. That is, if the deviation amounts of each frequency fl and f2 from the notch frequency Fp are Δfl and Δf2, (
7) By substituting each part into the equation and solving the simultaneous equations, Δf 1
-1(A tl-A t2)/(2πτ)2411/2
The α tone becomes −πτ 2 (8).

Therefore, the notch frequency Fp and notch depth D can be calculated from Δf1 and α (actual notch depth expressed in ratio).
p(dB) = −201og(1/a) is obtained.

Next, the procedure of a calibration process in which the control unit 32 determines each notch frequency Fp and notch depth Dp for each propagation constant and sets them in the calibration data memory 32e will be explained using the flowchart of FIG. First, when a calibration command is input from an operation panel or the like, the selector switch 22 is switched to the calibration signal side in step S1. Then, in S2, the propagation constant setting means 23a sets one propagation constant φ, τ, and ρ for each circuit 28, 29, and 30. Then, for example, the frequency f of the calibration signal is set to 4Ml1z over the entire measurement range.
(Then, the level value At at each frequency f is set to n1 and then stored. Then, each stored Δp) The frequency indicating the maximum attenuation at the constant frequency f is roughly set as the notch frequency FO. Find it as. Two frequencies fl and f2 approximately in the vicinity of the notch frequency FO are determined. Then, fip is added to the end corresponding to each frequency fl, f2.
1. Read the determined level values A ll and A t2 and attenuate m A tl (ratio).

Convert to At2 (ratio). Then, Δf1 is calculated from equation (8) to calculate the correct notch frequency Fp. Also, α is calculated using the same formula (8) to find the correct notch depth Dp.

Since the notch frequency Fp and notch depth Dp corresponding to one propagation constant φ, τ, and ρ have been calculated above, these calibration data are stored in the empty area of the calibration data memory 32e shown in FIG. Then, the process returns to S2 and starts calculating the notch frequency Fp and notch depth Dp for the next propagation constant.

When the notch frequency Fp and notch depth Dp for all propagation constants have been stored in the calibration data memory 32e in S4, all the calibration processes have been completed.
Switch the switch 22 to the test signal side and wait for the test to be executed.

According to the fading simulator calibration device configured in this way, when a configuration command is input from the operation panel,
Notch frequency Fp and notch depth Dp indicating fading characteristics for each propagation constant are automatically calculated and stored in the calibration data memory 32e. Therefore, in an actual test, when the notch frequency Fp and notch depth Dp of the required fading characteristics are specified, φ and τ corresponding to the notch frequency Fp and notch depth Dp.

The propagation constant of ρ is read from the calibration data memory 32e and automatically set in each circuit 28, 29, 30. Therefore, in this fading simulator, it is possible to obtain fading characteristics that accurately reproduce the specified notch frequency Fp and notch depth Dp. Therefore, accurate testing of the receiving device 3 can be performed.

In addition, the notch frequency Fp of the required fading characteristic
and notch depth Dp are not stored in the calibration data memory 32e, the notch depth Dp in the calibration data memory 32e is
Propagation constants φ and τ corresponding to the notch frequency Fp and notch depth Dp near the required notch frequency Fp and notch depth Dp.

The propagation constants φ' and τρ' with slightly different values of ρ are set in each circuit 28, 29.30, and the notch frequency Fp'' and notch depth Dp'' under these conditions are calculated using the method described above. Then, the calculated notch frequency Fp +
, Find the difference between the notch depth Dp', the notch frequency Fp to be set, and the notch depth Dp, and adjust each propagation constant φ to be set in each circuit 28, 29, and 30 until this difference falls within the allowable limit. By changing the values of τ and ρ, ΔP1 of the notch frequency Fp and notch depth Dp is determined again.

By calibrating the fading characteristics using such a procedure, it becomes possible to generate an accurate pseudo-fading phenomenon to be set.

Next, the characteristics when calculating the notch frequency Fp will be explained using FIG. 5.

As mentioned above, since the calibration signal is outputted from the calibration signal generation circuit 35 at a predetermined frequency interval ΔF (-4MHz),
When the minimum value of the measured value At at each frequency f is read and the frequency f corresponding to this minimum value is set as the notch frequency, as shown in the figure, it often does not match the correct notch frequency Fp.

Therefore, the obtained notch frequency will include an error of ΔF. In order to reduce this error, the frequency interval ΔF may be set small, but if the frequency interval ΔF is set small, the number of measurement frequencies increases and each level value At
This increases the time required to measure the calibration process, significantly reducing the overall calibration processing efficiency.

Therefore, as shown in the present application, the frequency Fn corresponding to the level value indicating the minimum value is once set as the approximate notch frequency FO, and the frequency fl in the vicinity thereof is set.

By calculating the correct notch frequency Fp from the level values A tl and A t2 of f2 using the method described above, even if the frequency interval ΔF is a somewhat large value,
The measurement accuracy of the notch frequency Fp can be greatly improved. Furthermore, since it is not necessary to set the frequency interval ΔF as small as the required frequency accuracy, the efficiency of the calibration work can be greatly improved.

By the way, in the example device, when the frequency interval ΔF of the calibration signal is 4 MHz, the calculated notch frequency F
It was possible to improve the frequency accuracy of p to about 1/40, about 100 ktlz.

In addition, since the calibration signal is cut off by the changeover switch 22 during actual test operation, the calibration signal is not transmitted to the receiving device 3.
It is possible to prevent adverse effects on the test results.

[Effects of the Invention] As explained above, according to the fading simulator calibration device of the present invention, each propagation in the propagation path is measured using a calibration signal whose frequency changes sequentially at predetermined frequency intervals prior to an actual test. The notch frequency and notch depth for each constant are calculated and stored as calibration data. In an actual test, the target notch frequency and notch depth can be set accurately by using the stored calibration data. Therefore, the performance of the fading simulator can be greatly improved.

In addition, since the notch frequency and notch depth are determined by calculation, even if the frequency interval of the calibration signal is not small, accurate notch frequency and notch depth can be obtained. By increasing the size, the calibration processing efficiency can be greatly improved.

[Brief explanation of drawings]

1 to 5 show a calibration device for a fading simulator according to an embodiment of the present invention;
Figure 2 is a block diagram showing the general configuration, Figure 2 is a diagram showing the calibration data memory, Figure 3 is a diagram explaining the notch frequency calculation procedure, Figure 4 is a flowchart showing the operation, and Figure 5 is the effect. FIG. 6 is a diagram showing a general data transmission system, FIG. 7 is a diagram showing fading characteristics, and FIGS. 8 and 9 are block diagrams showing conventional fading simulators. be. 21... Input terminal, 22... Selector switch, 24...
...Separation circuit, 25a, 25b...Propagation path, 26...
- Synthesizing circuit, 28... Phase shift circuit, 29... Delay circuit, 30... Attenuation circuit, 32... Control section, 32a...
・Propagation constant setting means, 32b... Attenuation amount all setting means, 32c... Approximate notch frequency reading means, 32d
...Fading characteristic calculation means, 32e...Calibration data memory, 32f...Test execution control means, 33...
- Output terminal, 34... Level detector, 35... Calibration signal generation circuit.

Claims (1)

[Claims] A separation circuit (24
), and after passing through multiple propagation paths (25a, 25b) with different propagation constants such as propagation time and attenuation amount,
In a fading simulator that generates a pseudo-fading phenomenon in the output signal of the combining circuit (26) by combining it with a combining circuit (26), outputting a calibration signal whose output frequency changes at predetermined frequency intervals over the entire frequency range of the test signal. a calibration signal generation circuit (35); a changeover switch (22) inserted between the input terminal and the separation circuit to switch the input signal to the separation circuit to the test signal or the calibration signal; a level detector (34) for detecting the amount of attenuation of the output signal; a propagation constant setting means (32a) for variably setting the propagation constant in each of the propagation paths; attenuation measurement control means (32b) for measuring the attenuation at each frequency with the level detector in a state where the voltage is applied to the separation circuit; and attenuation at each frequency obtained by the attenuation measurement control means and the propagation Approximate notch frequency reading means (32c) reads the approximate notch frequency of the fading phenomenon at the time of setting the propagation constant from the propagation constant set by the constant setting means, and from each attenuation amount at a plurality of frequencies near the approximate notch frequency. Fading characteristic calculating means (32d) for calculating the notch frequency and notch depth of the fading phenomenon when setting the propagation constant, and each notch frequency and each notch depth for each propagation constant obtained by this fading characteristic calculating means. A calibration data memory (32e) is stored as calibration data, and by switching the changeover switch to the test signal side, reading a propagation constant corresponding to the required notch frequency and notch depth from the calibration data memory, and setting the propagation constant. A calibration device for a fading simulator, comprising test execution control means (32f) set in each propagation path by a means to generate a pseudo fading phenomenon by the test signal.