JP2002164597A - Specific frequency component extracting system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a specific frequency component extracting system which surely removes a noise frequency component for stable frequency control. SOLUTION: An actuator 4 frequency-modulates the state quantity of an object 5 so that it is actuated around the prescribed frequency necessary to be kept. An amplitude value F of a specific frequency component is extracted from the detected state quantity detected from the state quantity output of the object 5. Thus, based on the error signal between the amplitude value F and a command value R from outside, the state quantity of the object 5 is kept at a prescribed frequency. To extract the specific frequency component from the detected state quantity, a synthetic filter 7 removing the noise frequency component except for the specific frequency is used which comprises an all-pass filter that allows an input signal to pass across the entire frequency range with a constant transmission gain.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a specific frequency component extraction system useful for frequency control of a standard light source or the like of a laser interferometer.

[0002]

2. Description of the Related Art An iodine-stabilized He-Ne laser is known as a standard light source of a laser interferometer. In an iodine-stabilized laser, wavelength stabilization control is performed using an inverted ram depression obtained in output light when an iodine molecule absorption cell is inserted into a resonator as a reference element. The inverted Lamb depression is a phenomenon in which when the standing wave frequency in the laser resonator matches the absorption line frequency of the iodine molecule, a peak occurs in the laser output as a result of a decrease in absorption loss.

In order to perform wavelength stabilization control using the inverted ram depression as a reference element, it is necessary to detect the inverted ram depression. For this purpose, a laser output light is frequency-modulated by adding a correction signal to a PZT element for controlling the laser cavity length. Then, the inverted ram depression is detected from the laser output light by the synchronous detection method, and feedback control is performed so that the inverted ram depression is locked at the absorption center (peak position frequency).

[0004] Specifically, in the inverted ram characteristic, the relationship between the light quantity and the frequency is approximated by a quartic equation. That is, by adding the correction signal of the fundamental frequency 1f to the PZT element, when the optical frequency of the laser output is near the absorption center of the inverted dent, the optical frequency includes the fundamental wave and the second to fourth harmonics 1f and 2f. , 3
The frequency components f and 4f are generated. From such laser output light, for example, frequency 3f is extracted as a specific frequency. Here, in the vicinity of the inverted ram depression, there is a correlation between the frequency and the amplitude of the frequency component. Therefore, by performing feedback control of generating an error signal by comparing the amplitude information with the command value, the laser output can be locked at the absorption center of the inverted ram depression. Here, the reason why 3f is used as the specific frequency is that the 3f component becomes linear on the coordinates of the light quantity-frequency characteristic when the light quantity-frequency characteristic approximated by the quartic equation is third-order differentiated, which is convenient for feedback control. It is because it becomes.

[0005]

However, when 3f is selected as the specific frequency in the above-described feedback control system of the wavelength-stabilized laser, the other frequency components 1f, 2f, and 4f obtained by frequency modulation of the laser light are not changed. And noise components. If these noise components are not reliably separated from the specific frequency components, they may cause deterioration in control performance in a feedback control system in the case of configuring a standard light source that requires extremely high stability, for example, 10 ^−11. .

In particular, in the above-described example of the iodine stabilized laser, the noise component frequencies 1f, 2f, and 4f are close to the specific frequency component 3f. Therefore, a noise compensation component cannot be removed by a characteristic compensation element used in a normal feedback control system. Further, a band-elimination filter having a higher-order pass frequency (for example, a Butterworth-type filter having an order of transfer characteristics of about 10) is applied to a feedback control system because the amount of phase fluctuation between input and output is too large. It is difficult to satisfy the stability condition (phase margin). The above problem is not limited to the wavelength-stabilized standard light source, but also exists in a feedback frequency control system of an optical or electrical reference signal source and a calibration signal source that require high-precision frequency stability.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a specific frequency component extraction system capable of performing high-stability frequency control by reliably removing noise frequency components.

[0008]

SUMMARY OF THE INVENTION According to the present invention, a detected state detected from an output of a state quantity of the controlled object is frequency-modulated so that the state quantity of the controlled object fluctuates around a predetermined frequency to be maintained. A specific frequency for extracting the amplitude value of the selected frequency component from the amount and performing feedback control for maintaining the state quantity of the controlled object at the predetermined frequency based on an error signal between the amplitude value and an external command value In the component extraction system, in order to extract a specific frequency component from the detection state quantity, a frequency component other than the specific frequency is configured by combining an all-pass filter that passes an input signal with a constant transmission gain over the entire frequency range. It is characterized by using a synthesis filter for removal.

According to the present invention, by using a synthesis filter combining an all-pass filter, even if a specific frequency component and another frequency component are close to each other, the other frequency component can be surely suppressed without suppressing the specific frequency component. This makes it possible to perform highly stable feedback frequency control on a control target such as a laser to be wavelength-stabilized.

In the present invention, specifically, the synthesizing filter is set so that the phase shift amount changes in the range of 0 to 180 ° over the entire frequency range, and the other frequency components to be removed are shifted by 90 °. First and second cascaded
And an adder for adding the input of the first all-pass filter and the output of the second all-pass filter.

When there are a plurality of other frequency components to be removed, the synthesizing filter changes the phase shift in the range of 0 to 180 ° over the entire frequency range, and removes the other frequency components to be removed. A cascade-connected first and second all-pass filter set to shift by 90 °, and an adder that adds an input of the first all-pass filter and an output of the second all-pass filter. As the filter unit, a plurality of synthesis filter units may be connected in cascade corresponding to other frequency components to be removed.

Preferably, in the present invention, the synthesis filter is further provided with a band-pass filter for passing a component of a specific frequency. Thus, for a frequency component other than the required specific frequency component, an increase in gain due to the use of the synthesis filter can be suppressed, and control stability can be further improved.

In the present invention, an amplitude extraction circuit for extracting an amplitude value of a specific frequency component is, for example, a pair of all-pass filter circuits for generating a two-phase sine wave signal having a phase difference of 90 ° from an output signal of a synthesis filter. And a squaring circuit for squaring the two-phase sinusoidal signals obtained from the pair of all-pass filter circuits, and an adder for adding the outputs of the squaring circuits.

Alternatively, an amplitude extracting circuit for extracting an amplitude value of a specific frequency component generates a polyphase AC signal whose phase is slightly shifted from the output signal of the synthesis filter, and generates a peak of the amplitude of the polyphase AC signal. The sampling may be performed in synchronization with the value.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a feedback frequency control system according to an embodiment of the present invention. The control target 5 outputs a state quantity Y at a predetermined frequency in response to an external command value R. The actuator 4 controls the frequency of the control target 5. The operation amount U given to the control target by the actuator 4 is controlled based on an error correction signal C obtained by the error correction means 2. At this time, a correction signal P is given to the actuator 4 by the adder 3 separately from the error correction signal C. The correction signal P is, for example, an AC signal that satisfies P = msin (2πft). By adding the correction signal P, the state quantity Y of the control target 5 is frequency-modulated.

The state quantity Y of the control target 5 is frequency-modulated based on the operation amount U of the actuator 4, and at the same time, a disturbance D coming from temperature, mechanical stress, and other environmental factors is given. As a result, the state quantity Y includes a noise frequency component in addition to a frequency component (specific frequency) required for feedback control.

The state quantity Y is detected by the detector 6 as a detection signal S. A synthesis filter 7 is used to select from the detection signal S and extract only a specific frequency component. A signal T of a specific frequency component obtained by removing noise by the synthesis filter 7 is converted into an AC-DC converter 8
Is converted into a DC signal F, which is a feedback signal,
It enters the adder 1 together with the command value R. Specifically, AC-DC
The converter 8 is an amplitude extraction circuit that extracts an amplitude value of a signal of a specific frequency component. Then, the command value R and the feedback value F
Is supplied to the error correction means 2, a predetermined error correction signal C is generated, and control is performed such that the command value R = the feedback value F.

Specifically, assuming that this system is a wavelength stabilizing standard light source, the controlled object 5 is a gas laser to be stabilized. The actuator 4 is, for example, a PZT element that controls the resonator length of the gas laser. By applying the external correction signal P and the error correction signal C to the PZT element to control the laser cavity length, the state quantity Y, that is, the laser output light is frequency-modulated. At this time, the detection signal S obtained by the detector 6 includes a noise frequency component in addition to a specific frequency component to be used for feedback control due to frequency modulation and disturbance. The synthesis filter 7 removes this noise frequency component.

The synthesis filter 7 is constituted by using a plurality of all-pass filters. 2A and 2B show an equivalent circuit and a functional block of a known primary all-pass filter 10 used. FIG. 3 shows the transfer characteristics of the all-pass filter 10, where the gain is zero over the entire frequency range. FIG. 4 shows the phase shift characteristics. That is, when the all-pass filter 10 is normalized by the noise frequency component 1f of interest, the phase shift amount (phase delay amount) becomes 90 ° at this frequency 1f,
A 90 ° phase shifter is configured such that the phase shift amount changes in the range of 0 to 180 ° over the entire frequency range. This phase shift characteristic is obtained by selecting the time constant of the resistor R1 and the capacitor C1 in FIG.

The synthesis filter 7 is configured by combining two such all-pass filters 10. That is, as shown in FIG. 5, the all-pass filters 10a and 10b are connected in cascade, and an adder 11 for adding the input of the first-stage filter 10a and the output of the second-stage filter 10b is provided, thereby forming the synthesis filter 7.

With such a configuration, the combined phase shift characteristics are as shown in FIG. That is, the two-stage all-pass filter 10
a, 10b, the frequency component 1f is 180
° phase shifted. Therefore, the input of the first stage and the output of the second stage are out of phase with each other for the frequency component 1f, and are canceled by the addition. In the frequency range lower than 1f, the phase shift amount is 0 °, and in the high frequency range, the phase shift amount is 360 °.
Becomes Accordingly, the transfer characteristic of the synthesis filter 7 is shown in FIG.
The gain of the frequency component 1f becomes zero in principle, and the noise frequency component is removed. Frequency 1f
Except for the vicinity, the gain is constant over a wide frequency range and the phase shift amount is zero.

Therefore, assuming that the signal S detected by the detector 6 includes the noise frequency component 1f and the specific frequency nf (n ≠ 1), the specific frequency component nf
Only can be extracted. By feeding back the output of the specific frequency component, high-performance frequency control can be performed.

FIG. 8 shows an example in which there are two noise frequency components to be removed, for example, 1f and 2f, and two-stage synthesis filter units 7a and 7b are used to remove both of them. The first stage synthesis filter unit 7a is the same as that in FIG. Second stage synthesis filter unit 7b
Is composed of two-stage all-pass filters 10c and 10d, which are 90 ° phase shifters having a phase shift amount of 90 ° at a frequency 2f, and an adder 12.

In this case, the combined phase shift characteristics are as shown in FIG. 10, and the combined transfer characteristics are as shown in FIG. Thus, the frequency components 1f and 2f can be selectively removed. When it is desired to remove the frequency component 4f, a synthesis filter unit corresponding to the frequency 4f may be provided. That is, in general, when there are a plurality of noise frequency components to be removed, a plurality of synthesis filter units may be connected in cascade corresponding to each noise frequency.

FIG. 11 shows the synthesis filter unit 7 of FIG.
It is a specific circuit configuration example of a and 7b. Buffers 13 and 14 are provided at the output section of each of the synthesis filter units 7a and 7b. Specifically, for example, the noise frequency components to be removed are 1f = 1.3 kHz and 2f = 2.6 kHz. At this time, the resistor R1 and the capacitor C1 of the all-pass filters 10a and 10b of the synthesis filter unit 7a
Is T1 = C1R1 = 1 / 2π (1.3 × 10 ³ ), and the all-pass filter 1 of the synthesis filter unit 7b
0c and 10d, the resistor R2 and the capacitor C2 are expressed by T2 = C
It is assumed that 2R2 = 1 / 2π (2.6 × 10 ³ ).

FIG. 12 shows a detection signal S including a noise component.
This is an example in which the frequency characteristic of the state variable Y of the frequency-modulated control target 5 is approximated by a quartic curve. The noise components are 1f, 2f, 4f,
3f is a specific frequency. In this example, the component of 4f is 1
f and 2f are sufficiently small.

FIG. 13 is a frequency spectrum of a signal T obtained from the detection signal shown in the spectrum of FIG. 12 through the synthesis filter shown in FIG. Specific frequency component 3
It can be seen that the noise frequency components 1f and 2f are clearly removed without suppressing f. As a result, the extraction of the specific frequency component is performed with good S / N, and high-performance feedback frequency control becomes possible.

Up to this point, as the synthesis filter, an all-pass filter which is a phase shift circuit having a phase delay of 90 ° with a noise frequency component of interest has been used, but a phase shift circuit having a phase lead of 90 ° is used. It goes without saying that an all-pass filter may be used.

In FIG. 1, the performance of the AC-DC converter 8 is also important for the performance of the feedback control. Conventionally, a synchronous detection method for extracting a specific frequency component nf from the input signal T using the same reference signal as the specific frequency nf has been used for this part. In this method, however, a synchronous detection method such as 1f, 2f, 4f, or the like is used. It is difficult to remove noise components.
Preferably, in the present invention, an amplitude extraction circuit configured using an all-pass filter for extracting the amplitude of a specific frequency component is used for the AC-DC converter.

FIG. 14 shows a configuration example of such an amplitude extraction circuit (see Japanese Patent Application Laid-Open No. 10-9848). The pair of all-pass filters 81 and 82 receive the signal T, and the sine wave signals S1 = Asin (2πnft) and S2 = Ac which are out of phase by 90 ° with respect to the specific frequency nf.
os (2πnft). Specifically, the all-pass filters 81 and 82 have a center frequency of 90 ° phase shift of nf−
Δf, nf + Δf. As a result, the above-described two sine wave signals S1 and S2 having a phase difference of 90 ° near the specific frequency nf are obtained. These sine wave signals are squared by squaring circuits 83 and 84, respectively,
Add by 5. That is, {Asin (2πnft)}
^The amplitude value A can be detected by the calculation of 2+ {Acos (2πnft)} ² = A ²

By using such an amplitude extracting circuit for the AC-DC converter 8, even if there is a noise component of a frequency deviated from the specific frequency nf which cannot be completely removed by the synthesizing filter 7 in the signal T, these components can be used. The noise component is removed, and the amplitude value of the specific frequency nf can be extracted. Therefore, the performance of the feedback control is improved.

As another method of the AC-DC converter 8, a sampling method of a polyphase AC signal can be applied. This is a method in which polyphase AC signals whose phases are slightly shifted from the signal T are generated, and sampling is performed in synchronization with the peak value of the amplitude of these polyphase AC signals. For example, as a polyphase AC signal, 9
A four-phase AC signal whose phase is shifted by 0 ° is generated, and the amplitude of the AC signal is extracted at a sampling cycle of 1/4 that of a single-phase AC signal. When this method is used, the influence of the phase delay of the zero-order hold caused by the sampling operation is reduced, and high-accuracy amplitude extraction becomes possible.

In the synthesis filter 7, 180
In order to cancel the frequency component of the phase difference, the two cascaded all-pass filter inputs and outputs are added as described above. Therefore, the gain is doubled for frequency components other than the noise frequency component to be removed. Therefore, in order to suppress a component in a frequency band other than the specific frequency nf, it is effective to further arrange a band-pass filter that allows the specific frequency component to pass through, for example, an output unit of the synthesis filter 7.

Further, in the embodiment, when there are a plurality of frequency components to be removed, a multi-stage synthesis filter is used as shown in FIG. 8, but this is performed by using a single-stage synthesis filter and a subtractor. It is also possible to carry out. In this case, at the specific frequency 3f to be selected, the phase amount (phase delay amount) of each of the all-pass filters 10 of the synthesis filter.
Is set to 90 °, the detection signal S is input to the synthesis filter 7, and the output signal S ′ is output from the synthesis filter 7.
Get. The gain of this output signal S ′ is 3 f
In other cases, the detection signal is twice as large as the detection signal. Therefore, the detection signal is attenuated by 1/2 to obtain a correction signal S ″. Only the components can be extracted. here,
In the subtraction of the correction signal S ″ from the output signal S, after each frequency versus intensity data is obtained, a subtraction may be performed between the frequency versus intensity data.

[0035]

As described above, according to the present invention, a specific frequency component can be suppressed by using a synthesis filter combining an all-pass filter, even if a specific frequency component and another frequency component are close to each other. It is possible to reliably remove other frequency components without using any of them, thereby enabling highly stable feedback frequency control.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a feedback frequency control system according to the present invention.

FIG. 2 is a diagram showing a configuration of an all-pass filter used in the present invention.

FIG. 3 is a diagram showing gain characteristics of the all-pass filter.

FIG. 4 is a diagram showing a phase shift characteristic of the all-pass filter.

FIG. 5 is a diagram showing a configuration of a synthesis filter used in the embodiment of the present invention.

FIG. 6 is a diagram showing gain characteristics of the synthesis filter.

FIG. 7 is a diagram showing phase shift characteristics of the synthesis filter.

FIG. 8 is a diagram illustrating a configuration of a synthesis filter used in another embodiment.

FIG. 9 is a diagram showing gain characteristics of the synthesis filter.

FIG. 10 is a diagram showing phase shift characteristics of the synthesis filter.

FIG. 11 is a diagram showing a specific configuration example of the synthesis filter.

FIG. 12 is a diagram illustrating an example of an output frequency spectrum of a control target.

13 is a diagram showing a frequency spectrum obtained by removing a noise frequency component from the output of FIG. 12 by a synthesis filter.

FIG. 14 is a diagram illustrating a configuration example of an amplitude extraction circuit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Adder, 2 ... Error correction means, 3 ... Adder, 4 ... Actuator, 5 ... Control target, 6 ... Detector, 7 ... Synthesis filter, 8 ... AC-DC converter.

Claims (9)

[Claims] 1. A state quantity of a controlled object is frequency-modulated so as to fluctuate around a predetermined frequency to be maintained, and a selected frequency component of a selected frequency component is detected from a detected state quantity detected from a state quantity output of the controlled object. A specific frequency component extraction system that extracts an amplitude value and maintains the state quantity of the control target at the predetermined frequency based on an error signal between the amplitude value and an external command value. To extract the ingredients,
A specific frequency component extraction system using a synthesis filter configured to combine an all-pass filter that passes an input signal with a constant transmission gain over the entire frequency range and that removes frequency components other than the specific frequency. 2. The synthesis filter according to claim 1, wherein a phase shift amount is changed in a range of 0 to 180 ° over the entire frequency range, and a phase component to be removed is shifted by 90 °.
The specific frequency according to claim 1, further comprising: first and second all-pass filters connected in cascade; and an adder that adds an input of the first all-pass filter and an output of the second all-pass filter. Component extraction system. 3. The cascade connection of the synthesis filter, wherein the phase shift amount is changed in a range of 0 to 180 ° over the entire frequency range, and the frequency component to be removed is set to be shifted by 90 °. A synthesis filter unit including first and second all-pass filters, and an adder for adding an input of the first all-pass filter and an output of the second all-pass filter, a plurality of filters corresponding to frequency components to be removed; 2. The specific frequency component extraction system according to claim 1, wherein the synthesis filter units are connected in cascade. 4. The specific frequency component extraction system according to claim 1, wherein the synthesis filter further includes a band-pass filter that passes the component of the specific frequency. 5. An amplitude extraction circuit for extracting an amplitude value of the specific frequency component, comprising: a pair of all-pass filter circuits for generating a two-phase sine wave signal having a phase difference of 90 ° from an output signal of the synthesis filter; 2. A squaring circuit for squaring two-phase sinusoidal signals obtained from the pair of all-pass filter circuits, and an adder for adding an output of the squaring circuit. Specific frequency component extraction system. 6. An amplitude extracting circuit for extracting an amplitude value of the specific frequency component, generates a polyphase AC signal whose phase is slightly shifted from an output signal of the synthesis filter, and calculates an amplitude of the polyphase AC signal. 2. The specific frequency component extraction system according to claim 1, wherein sampling is performed in synchronization with the peak value. 7. The specific frequency component extraction system according to claim 1, wherein the control target is a laser whose wavelength is to be stabilized. 8. The specific frequency component extraction system according to claim 1, wherein the state quantity of the control target is maintained at the predetermined frequency by feedback control. 9. The specific frequency component extraction system according to claim 1, wherein the specific frequency is a third harmonic of a fundamental wave for performing the frequency modulation.