JPH07225136A - Method and apparatus for measuring electric signal - Google Patents

Abstract

PURPOSE:To easily measure a second order partial differential coefficient regarding a plurality of variables by adding modulation signals having different frequencies to dependent variables of a signal to be measured and by multiplying obtained signals by an alternate signal of frequencies equivalent to a difference or sum of two modulation signals. CONSTITUTION:A signal to be measured entering from an input terminal 1 enters an adder 6 after extra oscillating components are filtered off. A rectangular wave having a frequency equivalent to a difference or sum of frequencies of two modulation signals created by a rectangular wave generating circuit 5 enters the adder 6 and multiplied by a signal to be measured that is the output of the filter 4. As a result of multiplication, a signal outputted from the adder 6 can be regarded as the sum of a dc signal and an ac signal. The do signal is proportional to a second order partial differential coefficient and is therefore passed through a low-frequency pass filter 7 and amplified by time average, thereby selectively amplifying the second order partial differential coefficient.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric signal measuring method and an electric signal measuring instrument for measuring a differential spectrum of an electric signal.

[0002]

2. Description of the Related Art The following lock-in detection method is known as a conventional electric signal measuring method of this type.

That is, when a certain signal S is expressed as S (x) as a function of a certain variable x, in order to obtain the first derivative spectrum dS / dx of this signal S, the variable x is periodically changed. The signal S is measured by adding a minute modulation signal Δx (for example, a sine wave, a triangular wave, a rectangular wave, etc.). The signal obtained as a result of the measurement can be expressed as S (x + Δx), but when the amplitude of the modulation signal Δx is small, the signal can be expressed as the following equation.

[0004]

[Equation 1] By multiplying the obtained signal by a rectangular reference signal that changes in the same cycle as the minute modulation signal Δx, a signal component having the same frequency as the above-mentioned modulation signal becomes a DC signal, and other signals The number component and the component having no specific periodicity (in particular, noise) are converted into AC signals. After that, a direct current component, that is, the modulation signal Δx is passed through a low frequency pass filter having an appropriate time constant.
Only the first derivative dS / dx having the same frequency as is selectively amplified.

Similarly, by multiplying a rectangular wave having a frequency corresponding to twice the amplitude number of the modulation signal, only the ^second derivative d ² S / dx ² is selectively amplified. .

When this lock-in detection method is used, the signal S
Even if (x) is small and buried in noise, the differential signal can be selectively picked up, and by utilizing this feature, it is widely applied to spectroscopic analysis, Auger electron spectroscopy, Raman spectroscopy and the like.

[0007]

In the lock-in detection method as the conventional electric signal measuring method as described above, 1
The differential spectrum relating to a variable can be amplified even if it is second-order or higher, but it is not possible to selectively amplify the second-order or higher partial differential coefficients relating to different variables. Therefore, the measurement accuracy cannot be improved for a plurality of physical quantities.

For example, in measuring the surface distribution of an element using Auger electron spectroscopy, it is necessary to accurately measure the Auger electron dose and its change depending on the position. However, the lock-in detection method was used only to amplify a small Auger electronic signal,
There is a problem that it is difficult to measure fine distribution changes clearly with high spatial resolution.

An object of the present invention is to obtain an electric signal measuring method and an electric signal measuring device which solve the above problems.

[0010]

According to a first aspect of the present invention, there is provided an electric signal measuring method, wherein modulation signals having different frequencies are added to dependent variables of a signal to be measured, and the electric signal is measured. By multiplying the obtained signal by an AC signal having a frequency corresponding to the difference or sum of the frequencies of the two modulation signals, the second-order partial differential coefficient relating to a plurality of variables of the signal under measurement can be obtained. It can be measured easily and with high sensitivity.

An electric signal measuring device according to claim 2 is
A band-pass filter for removing unnecessary frequency components from the signal under measurement, an AC signal generation circuit for generating an AC signal having a frequency corresponding to the difference or sum of the frequencies of the two input modulation signals, and A multiplier for multiplying the output of the band-pass filter and the output of the AC signal generating circuit, and a low-frequency pass filter for selectively amplifying the second-order partial differential coefficient by taking the time average through the output of the multiplier. By including the above, it is possible to realize an apparatus that implements the electrical signal measuring method according to claim 1 with a simple configuration.

[0012]

【Example】

Example 1. First Embodiment FIG. 1 is a block diagram showing a first embodiment of an electric signal measuring device of the present invention.
Reference numerals 3 and 3 are input terminals for the modulation signal, and 4 is a band-pass filter for removing unnecessary frequency components, which are not necessarily required when the measured signal as an input signal does not have unnecessary frequency components. Reference numeral 5 is a rectangular wave generation circuit for generating a rectangular wave having a frequency corresponding to the difference or sum of the respective frequencies from the two modulation signals input from the input terminals 2 and 3,
6 is a multiplier for multiplying the outputs of the band pass filter 4 and the rectangular wave generating circuit 5, 7 is a low frequency pass filter, 8
Is the final output terminal.

Next, the operation will be described. The signal under test S input from the input terminal 1 enters the multiplier 6 after the excess frequency component is removed by the bandpass filter 4. An example of the electric signal before entering the multiplier is shown in FIG. Further, a square wave having a frequency corresponding to the difference or the sum of the frequencies of the two modulation signals generated by the square wave generating circuit 5 enters the multiplier 6 and is the output of the band pass filter 4. The signal under measurement S is multiplied.
An example of this rectangular wave is shown in FIG.

As a result of the multiplication, the signal output from the multiplier 6 becomes, for example, as shown in FIG. This output signal can be regarded as the sum of the DC signal and the AC signal as shown in FIG. Among them, since the DC signal is proportional to the second-order partial differential coefficient, it is possible to selectively amplify the second-order partial differential coefficient by passing it through the low-frequency pass filter 7, taking the time average, and then amplifying it. You can

A logical explanation will be given below. When the signal S to be measured is expressed as S (x, y) as a function of two variables x and y, modulations Δx and Δy having different frequencies are added to the variables x and y to perform measurement. When done, the resulting signal is represented as S (x + Δx, y + Δy). When the modulation amplitude is small, this signal is expressed as

[0016]

[Equation 2] Here, for simplicity, it is assumed that the modulation signals Δx and Δy are sine waves having frequencies ω _x and ω _y , respectively.
In this case, the above equation is applied as follows.

[0017]

[Equation 3] Here, δ _x and δ _y are the amplitudes of the modulated signals Δx and Δy, respectively. Further, this equation can be rewritten as the following equation.

[0018]

[Equation 4] As a result, by multiplying ω _x or ω _y or a rectangular wave having a frequency that is an integral multiple of these, a multi-order differential coefficient for a certain variable can be obtained as in the usual lock-in detection method. it can.

On the other hand, the frequency difference ω _x of the modulated signals
If −ω _y or the sum ω _x + ω _y does not match with ω _x or ω _y , multiply the square wave with the difference or sum frequency to obtain the second-order partial differential coefficient ∂ ² S / ∂x ∂y can be selectively amplified.

Here, any method may be used to create the rectangular wave corresponding to the difference or sum of the frequencies, but when the modulation signal is a sine wave, for example, the following method is used. it can. That is, each modulated signal is sin (ω
When expressed as _x t) and sin (ω _y t), first, these modulation signals are passed through a multiplier to take the product of the modulation signals.

The resulting signal is

[0022]

[Equation 5] Is expressed as

In order to extract only the frequency difference of the modulation signal, that is, the component having the frequency of ω _x -ω _y from this signal, it is passed through a low frequency pass filter capable of removing the frequency of about ω _x + ω _y. . On the contrary, in order to take out the component in which the frequency of ω _x + ω _y is fixed, it is sufficient to pass it through a high-frequency pass filter capable of removing the frequency of about ω _x −ω _y . as a result,
The obtained signal becomes a sine wave having a frequency of ω-ω or ω _x + ω _y . By passing this sine wave through a comparator with a threshold value of 0 V, the frequency ω _x −ω
A square wave of _y or ω _x + ω _y can be created.

Hereinafter, this method will be referred to as a double reference signal type lock-in detection method, in distinction from the normal lock-in detection method.

Example 2. Next, a second embodiment in which the above-mentioned multiple reference signal type lock-in detection method is applied to Auger electron spectroscopy will be described. When measuring the element distribution on the surface using Auger electron spectroscopy, generally, it is adjusted so that only the electrons having the energy specific to the element to be measured can pass through the electron energy analyzer, and the incident electron beam on the surface is adjusted. By scanning, the amount of electrons emitted from each location on the surface with specific energy is measured. Since the Auger electron dose is usually very weak, the electron energy passing through the electron energy analyzer is multiplied by a modulation signal, and a rectangular wave with the same period as this modulation is multiplied by the electron beam intensity signal to obtain an electron with a specific energy. A method of selectively amplifying the dose is taken.

Here, for the purpose of measuring the state of element distribution with higher accuracy, a modulation signal is also applied to the voltage applied to the deflection coil for scanning the incident electron beam, and the voltage is passed through the electron energy analyzer. The double reference signal type lock-in detection measurement was performed using a rectangular wave corresponding to the difference between the frequency of the modulated signal and the electron energy. As a result, a differential image of the element distribution is obtained, and the boundary of the surface element distribution can be clearly observed.

Example 3. Next, a third embodiment in which the multiple reference signal type lock-in detection method is applied to a scanning tunneling microscope and a scanning tunneling spectroscopy will be described.

By scanning the surface of the sample while applying the modulation signal to the voltage (bias voltage) applied between the sample and the probe, the voltage-current characteristic at each place on the surface of the sample can be obtained. Information on element distribution at each location can be obtained. Here, the modulation signal is also applied to the voltage applied to the piezoelectric element that controls the scanning of the probe, and the measurement is performed. From the obtained signal, using a rectangular wave having a frequency corresponding to the sum of the frequency of the modulation signal applied to the bias voltage and the frequency of the modulation signal applied to the voltage applied to the piezoelectric element, a double reference signal type lock is used. When the in-detection measurement is performed, a differential image of the scanning tunneling spectroscopic image can be obtained, and the element distribution on the sample surface can be observed more clearly than in the usual method.

[0029]

As described above, according to the first aspect of the present invention, the second-order partial differential coefficient relating to a plurality of variables of the signal under measurement that cannot be measured by the ordinary lock-in detection method can be easily and It was possible to obtain an electric signal measuring method capable of measuring with high sensitivity.

According to the second aspect of the invention, the electric signal measuring apparatus for carrying out the electric signal measuring method can be obtained with a simple structure.

[Brief description of drawings]

FIG. 1 is a block diagram showing an electric signal measuring device according to a first embodiment of the present invention.

FIG. 2 is a signal waveform diagram of each part in FIG.

[Explanation of symbols]

 4 Band pass filter 5 Square wave generation circuit 6 Multiplier 7 Low frequency pass filter

Claims (2)

[Claims] 1. The measurement is performed by adding modulation signals having different frequencies to the dependent variable of the signal under measurement, and the frequencies of two modulation signals are obtained for the obtained signals. A method for measuring an electric signal, characterized in that an AC signal having a frequency corresponding to the difference or the sum of is multiplied. 2. A bandpass filter for removing unnecessary frequency components from a signal under measurement, and an AC signal for producing an AC signal having a frequency corresponding to the difference or sum of the frequencies of two input modulation signals. A generation circuit, a multiplier that multiplies the output of the band-pass filter and the output of the AC signal generation circuit, and the output of the multiplier, take the time average, and selectively amplify the second-order partial differential coefficient. An electric signal measuring device, comprising: