Oscillator drive circuit and oscillator drive method

Abstract

To provide an oscillator drive circuit and an oscillator drive method that eliminates the need to manually set the tracking frequency range every time the oscillator is changed.SOLUTION: An oscillator driving circuit 11 follows the change in the oscillator's resonant frequency, and performs phase-locked loop (PLL) operation in a predetermined search frequency range (fVCOL to fVCOH) to check whether phase lock is possible, and performs PLL operation in a new search frequency range obtained by slightly shifting the search frequency range until the phase lock is possible.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a vibrator drive circuit and a vibrator drive method. [Background technology]

[0002] Ultrasonic transducers used for processing or cleaning are often connected to a driving oscillator that outputs a frequency close to or equal to the transducer's specific resonant frequency. This is because a frequency that is significantly different from the resonant frequency does not transmit power from the oscillator to the transducer, preventing it from achieving significant processing or cleaning capabilities.

[0003] On the other hand, the resonant frequency of ultrasonic transducers varies from one to another even with the same specifications and model number. In addition, the resonant frequency varies over time due to variations in the load conditions during use, such as during processing or cleaning. Since a driving oscillator with a fixed frequency cannot handle the variations in the resonant frequency of the transducer that accompany load variations, it is common to use a resonance-tracking driving oscillator (hereafter referred to as a resonance-tracking oscillator) that constantly monitors the resonant frequency of the transducer and adjusts the oscillation frequency to the resonance frequency. The resonance-tracking oscillator is the driving circuit for the transducer. This resonance-tracking oscillator is composed of a PLL (phase-locked loop) that performs negative feedback control of the oscillation frequency so that the phase difference between the transducer's voltage waveform and current waveform is 0°.

[0004] FIG. 1 is a block diagram of a typical conventional resonance point tracking oscillator 1.

[0005] The resonance point tracking oscillator 1 includes a PC (phase comparator) 2, a loop filter 3, a VCO (voltage controlled oscillator) 4, an oscillator 5, a probe 6 for detecting the voltage of the oscillator 5, and a CT 7 for detecting the current of the oscillator 5, which constitute a PLL.

[0006] PC2 outputs a signal proportional to the phase difference between two input signals, the positive input and the negative input. The negative input of PC2 is connected to the voltage signal of the oscillator 5, and the positive input is connected to the current signal of the oscillator 5. The output signal of PC2 contains a ripple component with the same frequency as the two input signals. The loop filter 3 smoothes the output of PC2 containing the ripple component to DC, and also appropriately sets the loop response for stable operation of the negative feedback of the PLL. The VCO 4 oscillates at a frequency proportional to the input voltage. The VCO 4 is assumed to have the ability to drive the oscillator 5, and if an external driving amplifier is used, for example, it is assumed to include this. The probe 6 detects the voltage waveform of the oscillator 5 and outputs it after adjusting it to a level suitable for the input level of PC2. The CT 7 converts the current of the oscillator 5 to a voltage and outputs it after adjusting it to a level suitable for the input level of PC2.

[0007] FIG. 2 is a diagram showing an example of typical impedance |Z| (=|V÷I|) and phase θ (=∠V-∠I, the phase of the voltage with the current as the reference phase) characteristics of the vibrator 5. In FIG.

[0008] There are two resonance frequencies based on the zero phase: a resonance frequency fr and an anti-resonance frequency fa. The vibrator 5 used for processing or cleaning purposes is often used at the resonance frequency fr.

[0009] Furthermore, the vibrator 5 often has multiple vibration modes (resonant frequencies). As an example, FIG. 2 shows an example in which different resonant modes exist on the lower and higher frequency sides than the resonant frequency fr. The series resonant frequency fr' exists at a frequency lower than the resonant frequency fr, and the series resonant frequency fr" exists at a frequency higher than the resonant frequency fr. However, these vibration modes are different from the intended use, and driving at these frequencies not only prevents the vibrator from exhibiting its intended performance (processing, cleaning, etc.), but may even destroy the vibrator 5, so it cannot be used at fr' or fr".

[0010] Here, the operation of tracking the resonance point for searching for the resonance frequency fr will be described with reference to FIG.

[0011] When the current VCO oscillation frequency is f4, the phase θ<0°, so the output of the loop filter 13 increases. As a result, the VCO input voltage increases, and the VCO oscillation frequency increases.

[0012] When the current VCO oscillation frequency is f6, the phase θ>0°, so the output of the loop filter 13 becomes small. As a result, the VCO input voltage becomes small, and the VCO oscillation frequency becomes low.

[0013] By repeating these two operations, the VCO oscillation frequency converges to the resonance frequency fr.

[0014] For the conventional resonance point tracking oscillator 1 in FIG. 1 to operate normally, the tracking frequency range must be limited to the range greater than f3 and less than f7, the resonance frequency fr of the vibrator 5 used must be in the range greater than f3 and less than f7, and no other resonance mode (fr' or fr") must exist in the range from f3 to f7.

[0015] Next, a case where the tracking frequency range is wider than necessary will be described.

[0016] When the VCO initial frequency is at f8, the phase θ<0°, so the output of the loop filter 13 becomes large, and the VCO input voltage also becomes large, so the VCO oscillation frequency becomes high. Therefore, the VCO oscillation frequency shifts in the opposite direction to the target frequency fr, and converges toward f9 (=fr"), which is a different resonant mode.

[0017] Similarly, when the VCO initial frequency is at f2, the phase θ>0°, so the output of the loop filter 13 becomes small, and the VCO input voltage also becomes small, so the VCO oscillation frequency becomes low. The VCO oscillation frequency shifts in the opposite direction to the target frequency fr, and converges toward f1 (=fr'), which is a different resonant mode.

[0018] That is, when the VCO initial frequency is f8 or f2, the resonance point tracking operation cannot be performed. The same applies when the VCO initial frequency is higher than f8 or lower than f2.

[0019] In order to stably perform the resonance point tracking operation using this oscillator 5, the tracking frequency range must be limited to between above f3 and less than f7 (= fa). Outside this range, the VCO oscillation frequency changes in the direction opposite to the target frequency fr.

[0020] FIG. 3 shows an example of the phase characteristics of another oscillator whose resonance frequency is slightly shifted higher than that of the oscillator 5 in FIG. 2. Since the resonance point tracking operation is determined by phase, the diagram of the magnitude of impedance (|Z|) is omitted.

[0021] When the VCO initial frequency is at f3, since the phase θ > 0°, the output of the loop filter 13 becomes small, and as a result, the VCO oscillation frequency becomes low. The VCO oscillation frequency changes in the direction opposite to the target frequency fr and converges to fr'.

[0022] FIG. 4 is a diagram showing an example of the phase characteristics of an oscillator with f7 < fr. When the tracking frequency range is limited to f3 to f7, naturally, oscillation cannot occur at frequencies higher than f7, so the target frequency fr cannot be reached.

[0023] On the other hand, as an improvement to the PLL configuration described above, there has been proposed a driving device for an ultrasonic oscillator that starts the ultrasonic oscillator with a reference frequency signal at or near its fundamental resonance frequency, and then drives the oscillator with a feedback output that is very close to the feedback signal obtained during fundamental resonance point driving (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0024]

Patent Document 1

[0025] As mentioned above, even for transducers of the same specifications, there is variation in the resonant frequency for each transducer, and there are other resonant modes that exist near the target resonance, so the frequency range to be tracked cannot be broadened indiscriminately.

[0026] Therefore, in conventional resonance point tracking oscillators, the user must manually set the tracking frequency range (lower limit frequency, upper limit frequency) for tracking the fluctuations in the resonance frequency depending on the vibrator used, and further, as described in detail, there are restrictions on the tracking frequency range.

[0027] On the other hand, since the resonant frequency of the vibrator changes depending on the load condition and the magnitude of the driving power, the preset tracking frequency range must be widened, taking into consideration the change in the resonant frequency caused by these changes. Widening the tracking frequency range is undesirable because it increases the possibility that another resonant mode of the vibrator used will fall within the range.

[0028] In order to widen the preset tracking frequency range to some extent, it is necessary to select a vibrator whose resonant frequency range is within a certain range or select a vibrator whose difference between the desired resonant frequency and a nearby resonant mode is equal to or greater than a certain level, which reduces yields and results in increased costs.

[0029] Even in Patent Document 1, in which the PLL configuration is improved, it is necessary to start the ultrasonic transducer with a reference frequency signal at or near its fundamental resonance frequency.

[0030] The present invention has been made in consideration of the above circumstances, and provides a vibrator driving circuit and a vibrator driving method that eliminate the need to manually set the tracking frequency range every time the vibrator is changed. [Means for solving the problem]

[0031] In order to solve such problems, the present invention provides an oscillator driving circuit that tracks changes in the resonant frequency of the oscillator, performing PLL (phase-locked loop) operation in a predetermined search frequency range to check whether phase lock is possible, and then performing the PLL operation in a new search frequency range obtained by slightly shifting the search frequency range until the phase lock is possible.

[0032] In the drive circuit for the vibrator, the circuit configuration of the drive circuit for the vibrator may be configured to include a phase comparator that receives as input a voltage signal applied to the vibrator and a current signal flowing through the vibrator, a loop filter that smoothes the output of the phase comparator, a switching means for switching between the output of the loop filter and a VCO initial setting value, a VCO whose oscillation frequency changes according to the input voltage of the signal selected by the switching means, a frequency detection means that counts the oscillation frequency of the output of the VCO, a frequency comparison means that compares the magnitude relationship between the output of the frequency detection means and a lower limit frequency and an upper limit frequency of the search frequency range, a logical AND that generates a signal to switch the switching means to the output side of the loop filter when the output of the frequency comparison means is equal to or greater than the lower limit frequency and equal to or less than the upper limit frequency and when an initialization signal is false, and a control unit that monitors the output of the frequency comparison means, outputs the lower limit frequency, the upper limit frequency, the initialization signal, and the VCO initial setting value, and when it is confirmed that the phase lock is not possible within the search frequency range, sets the new search frequency range by slightly moving the search frequency range.

[0033] In the drive circuit for the oscillator, the circuit configuration of the drive circuit for the oscillator may be configured to include a phase comparator that receives as input a voltage signal applied to the oscillator and a current signal flowing through the oscillator, a loop filter that smoothes the output of the phase comparator, a switching means for switching between the output of the loop filter and an NCO initial setting value, an NCO whose oscillation frequency changes according to the input signal of the signal selected by the switching means, a frequency comparison means for comparing the magnitude relationship between the output of the loop filter and a lower limit frequency and an upper limit frequency of the search frequency range, a logical AND that generates a signal for switching the switching means to the output side of the loop filter when the output of the frequency comparison means is equal to or greater than the lower limit frequency and equal to or less than the upper limit frequency and when an initialization signal is false, and a control unit that monitors the output of the frequency comparison means, outputs the lower limit frequency, the upper limit frequency, the initialization signal, and the NCO initial setting value, and when it is confirmed that the phase lock is not possible within the search frequency range, sets the new search frequency range by slightly moving the search frequency range.

[0034] The present invention also provides a method for driving an oscillator, which performs a PLL (phase-locked loop) operation in a predetermined search frequency range to check whether phase lock is possible, and then performs the PLL operation in a new search frequency range obtained by slightly shifting the search frequency range until the phase lock is possible.

[0035] In the method for driving the transducer, when the phase lock is enabled, a tracking frequency range may be expanded or narrowed around the search frequency range in which the phase lock is enabled. Effect of the Invention

[0036] According to the vibrator driving circuit and vibrator driving method of the present invention, the setting of the tracking frequency range of the resonance point tracking oscillator is automated, thereby simplifying the operation, and the restriction on the resonance frequency range is relaxed, making it possible to use vibrators having a wider range of resonance frequencies. [Brief description of the drawings]

[0037] [Figure 1] It is a block diagram of a typical conventional resonance point tracking oscillator. [Diagram 2] It is a diagram showing characteristic examples of the representative impedance |Z| (=|V÷I|) and phase θ (=∠V - ∠I, the phase of the voltage with the current as the reference phase) of the oscillator. [Diagram 3] It is a diagram showing a characteristic example of the phase of another oscillator whose resonance frequency is slightly shifted to the higher side of the oscillator in FIG. 2. [Figure 4] It is a diagram showing a characteristic example of the phase of an oscillator with f7 < fr. [Diagram 5] It is a block diagram of a resonance point tracking oscillator according to a preferred first embodiment of the present invention. [Figure 6] It is a diagram showing a representative phase characteristic example of the oscillator and an example of the exploration frequency range in this embodiment. [Figure 7] It is a block diagram of a resonance point tracking oscillator according to a preferred second embodiment of the present invention.

Mode for Carrying Out the Invention

[0038] Hereinafter, with reference to the drawings, preferred embodiments of the present invention will be described. However, the present invention is not limited to the following description, and various modifications and changes can be made by those skilled in the art based on the gist of the invention described in the claims or disclosed in the mode for carrying out the invention. Such modifications and changes are also included in the scope of the present invention.

[0039] The resonance point tracking oscillators 11, 11' (see FIGS. 5 and 7) of the present invention are obtained by adding an exploration function for the resonance frequency fr to the configuration of the conventional resonance point tracking oscillator 1. The resonance point tracking oscillators 11, 11' are drive circuits of the oscillator that follow the change in the resonance frequency of the oscillator, perform PLL (Phase Locked Loop) operation in a predetermined exploration frequency range to confirm the feasibility of phase lock, and perform PLL operation in a new exploration frequency range where the exploration frequency range is slightly shifted until phase lock becomes possible.

[0040] In the conventional resonance point tracking oscillator 1, the resonant frequency of the oscillator had to be within the range of f3 to f7 in FIG. 2, but in the resonance point tracking oscillators 11, 11' of the present invention, the resonant frequency range of the oscillator is permitted to be between f1 and f9, and the resonant frequency range of the oscillator is significantly expanded compared to the conventional range. Therefore, the resonant frequency range of the oscillator that can be used is relaxed, and the yield is improved. In addition, the device can automatically search for the resonant frequency fr and set the tracking frequency range without the user having to manually set the tracking frequency range, so operability is also significantly improved.

[0041] The resonance point search operation may be performed, for example, when a vibrator is attached or replaced, when the power is turned on, when the PLL phase lock is released, or at any other timing determined by the user.

[0042] Preferred embodiments of the resonance point tracking oscillators 11 and 11' will be described below with reference to specific circuit configurations.

[0043] First Embodiment

[0044] Fig. 5 is a block diagram of the resonance point tracking oscillator 11 according to the first preferred embodiment of the present invention. Fig. 6 is a diagram showing examples 1) to 4) of typical phase characteristics of the oscillator 15 and examples of the search frequency range.

[0045] Similar to the conventional resonance point tracking oscillator 1, the resonance point tracking oscillator 11 includes a PC (phase comparator) 12, a loop filter 13, a VCO (voltage controlled oscillator) 14, and an oscillator 15.

[0046] The PC 12 outputs a signal proportional to the phase difference between two input signals, the positive input and the negative input. The negative input of the PC 12 is connected to the voltage signal of the oscillator 15, and the positive input is connected to the current signal of the oscillator 15. That is, the PC 12 receives the voltage signal applied to the oscillator 15 and the current signal flowing through the oscillator 15 as inputs. The output signal of the PC 12 contains a pulsating component (ripple component) of the same frequency as the two input signals. The loop filter 13 smoothes the output of the PC 12 containing the pulsating component to make it a direct current, and also appropriately sets a loop response for stable operation of the negative feedback of the PLL. The VCO 14 oscillates at a frequency proportional to the input voltage. Note that the VCO 14 has the ability to drive the oscillator 15, and includes an external driving amplifier, for example, when used. The resonance point tracking oscillator 11 may include a probe 16 as a voltage detection means for detecting the voltage of the oscillator 15, and a CT 17 as a current detection means for detecting the current of the oscillator 15. The CT 17 may be a shunt resistor or a current sensor. The probe 16 detects the voltage waveform of the vibrator 15, adjusts it to a level suitable for the input level of the PC 12, and outputs it. The CT 17 converts the current of the vibrator 15 to a voltage, adjusts it to a level suitable for the input level of the PC 12, and outputs it.

[0047] In order to have the function of resonance point searching operation, the resonance point tracking oscillator 11 of this embodiment further includes a switch 18 as a switching means, a frequency counter 19 as a frequency detection means, a comparator 20 as a frequency comparison means, a logical AND 21, and a control unit 22.

[0048] The switch 18 switches between the output of the loop filter 13 and the VCO initial setting value (VCOini).

[0049] The VCO 14 is connected to a switch 18 , and the oscillation frequency changes according to the input voltage of the signal selected by the switch 18 .

[0050] The frequency counter 19 is connected to the output of the VCO 14 and counts the oscillation frequency of the output of the VCO 14 .

[0051] Comparator 20 is connected to the output of frequency counter 19, and compares the output (fVCO) of frequency counter 19 with the lower limit frequency (fVCOL) and upper limit frequency (fVCOH) of the search frequency range (fVCOL to fVCOH). Comparator 20 receives the output of frequency counter 19 and fVCOL and fVCOH from control unit 22 as inputs, outputs a comparator output (3) to be described later to AND 21, and outputs comparator outputs (1) and (2) to control unit 22.

[0052] Here, fVCO, fVCOL, and fVCOH are all shown as digital values, but they can also all be analog values.

[0053] The logical product 21 is connected to the output (3) of the comparator 20, and generates a signal for switching the switch 18 to the output side of the loop filter 13 when the output of the comparator 20 is equal to or higher than the lower limit frequency and equal to or lower than the upper limit frequency and when the initialization signal is false. The logical product 21 receives the comparator output (3) from the comparator 20 and an inverted initialization signal from the control unit 22 as inputs, and outputs a switching signal to the switch 18.

[0054] The control unit 22 controls the operation of the resonance point tracking oscillator 11, monitors the output of the comparator 20, and outputs the lower limit frequency, the upper limit frequency, the initialization signal, and the VCO initial setting value. If it is confirmed that phase lock is not possible within the search frequency range, it sets a new search frequency range by slightly shifting the search frequency range.

[0055] The VCO oscillation frequency measured by frequency counter 19 is set as fVCO. The magnitude relationship between fVCO, fVCOL, and fVCOH is compared by comparator 20. The search frequency range is set to fVCOL to fVCOH. The search frequency range is shifted slightly, and PLL operation is performed in a new search frequency range until phase lock is possible, that is, until fVCOL≦fVCO≦fVCOH, and the resonant frequency fr is searched for. The comparator has the following three outputs. (1) If fVCOL>fVCO, it outputs true, otherwise it outputs false. (2) Output true when fVCOH < fVCO, and false otherwise. (3) Output true when neither (1) nor (2) is true, and false otherwise. As a result, output true only when fVCOL ≤ fVCO ≤ fVCOH.

[0056] The comparator outputs (1) and (2) are sent to the control unit 22, and the control unit 22 monitors these and outputs fVCOL, fVCOH, the initialization signal, and VCOini. The comparator output (3) takes the logical product with the inverted initialization signal from the control unit 22, and switches the switch 18 with the result.

[0057] Referring to 1) in FIG. 6, three states (a, b, c) will be described.

[0058] 1)-a The control unit 22 outputs the initialization signal = true, fVCOL = a frequency slightly higher than f1, fVCOH = fVCOL + BW, and VCOini = a DC value between fVCOL and fVCOH respectively. BW is the bandwidth that determines the exploration frequency range, which can be set to a predetermined range, for example, a frequency value less than f1 - f3. When the initialization signal = true, the switch 18 is on the false side, so the VCO oscillation frequency is a frequency (between fVCOL and fVCOH) corresponding to VCOini.

[0059] 1)-b In the state of 1)-a, the control unit 22 outputs the initialization signal = false. The switch 18 becomes on the true side, and the resonance point tracking operation starts. The control unit 22 monitors the comparator outputs (1) and (2) for a preset fixed time.

[0060] 1)-c When the comparator output (1) or (2) becomes true, the control unit 22 sets the initialization signal to true. Since the switch 18 becomes the false side, the VCO oscillation frequency returns to the frequency corresponding to VCOini and does not exceed the range of fVCOL~fVCOH. In this case, since the phase θ>0°, in order for fVCOL>fVCO, the comparator output (1) becomes true. If the resonance frequency fr exists in this frequency range, the comparator output is (3) true, (1) and (2) false, so in this case, it can be seen that the resonance frequency fr does not exist between fVCOL and fVCOH. This state is referred to as the state of "phase lock impossible".

[0061] Referring to 2) in FIG. 6, three states (a, b, c) will be described.

[0062] 2)-a Next, from the control unit 22, fVCOL=(previous fVCOL)+α, fVCOH=fVCOL+BW, and VCOini is set to a DC value between (new) fVCOL and fVCOH. The exploration frequency range is shifted to the higher frequency side by overlapping it with the previous fVCOL and fVCOH. Here, α can be arbitrarily set where 0<α<BW. If α is large, it can be expected that the number of repetitions of 2) will decrease, and if α is small, the risk of missing the resonance frequency fr will be lower.

[0063] 2)-b Similar to 1)-b, the control unit 22 sets the initialization signal = false. The switch 18 becomes the true side to start the resonance point tracking operation, and the comparator outputs (1) and (2) are monitored by the control unit 22.

[0064] 2)-c Depending on the value of VCOini, the comparator output may be such that (1) becomes true (when VCOini corresponds to a frequency lower than f3) and (2) becomes true (when VCOini corresponds to a frequency higher than f3), but the comparator output (3) does not become true, so the target resonance frequency fr does not exist. This state is also referred to as the state of "phase lock impossible".

[0065] When the above operation 2) is repeated, fVCOL and fVCOH increase by the frequency α each time.

[0066] Next, a description will be given of the operation when fVCOL or fVCOH increases, resulting in fVCOL becoming a frequency lower than the resonant frequency fr and fVCOH becoming a frequency higher than the resonant frequency fr.

[0067] Referring to 3) in FIG. 6, three states (a, b, c) will be explained.

[0068] 3)-a The control unit 22 sets fVCOL=(previous fVCOL)+α (lower than the resonance frequency fr, as in 3 in FIG. 6), fVCOH=fVCOL+BW (higher than the resonance frequency fr, as in 3 in FIG. 6), and VCOini=a DC value between (new) fVCOL and fVCOH. The search frequency range is shifted to a higher frequency side by overlapping with the previous fVCOL and fVCOH.

[0069] 3)-b As in 1)-b and 2)-b, the control unit 22 sets the initialization signal to false. The switch 18 switches to the true side to start the resonance point tracking operation, and the control unit 22 monitors the comparator outputs (1) and (2).

[0070] 3)-c Regardless of the value of VCOini, the comparator output (3) becomes true. In other words, the frequency range including the resonance frequency fr is reached, and the resonance point tracking operation is successful. This state is called the "phase lock possible" state.

[0071] Since the region including the resonance frequency fr has been clarified, the operation of transducer 15, such as processing and cleaning, can be continued with the settings of fVCOL and fVCOH for which phase locking is possible. In this case, fVCOL to fVCOH is the tracking frequency range.

[0072] The resonance frequency fr exists within the frequency width α between the previous fVCOH, when phase lock was not possible, and the current fVCOH, when phase lock is possible. It is preferable to increase the frequencies of fVCOL and fVCOH by about (BW-α) / 2 so that the center of the frequency width α is near the center frequency of fVCOL and fVCOH. (If α is small and close to zero, it is possible to increase the frequency by only BW / 2, and if α is large and close to BW, it is also possible not to increase the frequency.)

[0073] Alternatively, the tracking frequency range may be expanded by expanding fVCOL to a lower value and fVCOH to a higher value, centered on the search frequency range in which phase locking is possible, so as to cover the resonant frequency fluctuation range expected under the load conditions and drive power range of transducer 15. Conversely, the tracking frequency range may be narrowed by increasing fVCOL and narrowing fVCOH to a lower value. The tracking frequency range may be expanded or narrowed by narrowing one of fVCOL or fVCOH and expanding the other.

[0074] The following description will be given with reference to 4) in FIG.

[0075] Although this does not actually occur, the operation when fVCOL becomes a frequency higher than f7 will be described. In the example of 4) in Figure 6, when switch 18 is set to the true side and resonance point tracking operation is started, the comparator output (2) becomes true, so it is understood that the resonance frequency fr does not exist within that frequency range.

[0076] As described above, by limiting the search frequency range to a relatively narrow range (BW in the above explanation) and attempting to track the resonance point within that range, it becomes possible to search for a frequency range including the resonance frequency fr with a simple configuration. Then, an appropriate tracking frequency range including the resonance frequency fr for driving the transducer 15 can be automatically set.

[0077] Also, in the above, at the time of initialization, "VCOini = DC value between fVCOL and fVCOH" or "VCOini = (new) DC value between fVCOL and fVCOH" was set. As an example, this may be "a DC value slightly higher than fVCOL", "a DC value slightly lower than fVCOH", "a DC value approximately halfway between fVCOL and fVCOH", or any other "DC value between fVCOL and fVCOH".

[0078] In the above, an example was given of a case where the frequency is searched in a direction of increasing by α each time from near the lower limit frequency, but conversely, the frequency may be searched in a direction of decreasing from near the upper limit frequency.

[0079] <Second embodiment>

[0080] FIG. 7 is a block diagram of a resonance point tracking oscillator 11' according to the second preferred embodiment of the present invention.

[0081] In the first embodiment, the PC 12 and the VCO 14 are premised on analog operation. In this embodiment, the VCO 14 in the first embodiment is changed to an NCO (numerically controlled oscillator) 14' (DDS+sine wave ROM), fVCO to fNCO, fVCOL to fNCOL, fVCOH to fNCOH, and VCOini to NCOini. Most of the configurations and operations are the same as those in the first embodiment, except that the analog circuit has become a digital circuit and the analog quantity (voltage value, etc.) has become a digital numerical value, so a description of the similar configurations and operations will be omitted.

[0082] In the case of an analog-operating VCO 14, the relationship between the VCO input voltage and the VCO oscillation frequency is not stable due to variations in component constants and temperature changes, so it was necessary to monitor the output frequency of the VCO 14 and control its operation.

[0083] In this embodiment, since the NCO 14' operates digitally, a highly stable relationship can be maintained between the NCO input command value and the NCO oscillation frequency, and therefore operation control may be performed by monitoring the loop filter output value that becomes the NCO input.

[0084] In this case, there is no need to use the frequency counter 19, and the comparator 20 serving as frequency comparison means is connected to the output of the loop filter 13 and compares the magnitude relationship between the output of the loop filter 13 and the lower and upper limit frequencies of the search frequency range.

[0085] Moreover, the PC 12 and the loop filter 13, which are based on analog circuits, may be a so-called "digital PLL" that realizes the same functions as the analog circuits. [Explanation of symbols]

[0086] 11,11' Resonance tracking oscillator (drive circuit for vibrator) 12 PC (phase comparator) 13 Loop Filter 14 VCO (Voltage Controlled Oscillator) 14' NCO (Numerically Controlled Oscillator) 15 Transducers 16 Probe (voltage detection means) 17 CT (current detection means) 18 Switch (switching means) 19 Frequency counter (frequency detection means) 20 Comparator (frequency comparison means) 21 Logical AND 22 Control section

Claims

1. A drive circuit for a vibrator that follows a change in the resonant frequency of the vibrator, A PLL (phase-locked loop) is operated within a predetermined search frequency range to check whether phase lock is possible, A drive circuit for an oscillator, which performs the PLL operation in a new search frequency range obtained by slightly shifting the search frequency range until the phase lock becomes possible.

2. The circuit configuration of the drive circuit for the transducer is a phase comparator to which a voltage signal applied to the vibrator and a current signal flowing through the vibrator are input; a loop filter for smoothing the output of the phase comparator; A switching means for switching between the output of the loop filter and a VCO initial set value; a VCO whose oscillation frequency changes in accordance with the input voltage of the signal selected by the switching means; a frequency detection means for counting the oscillation frequency of the output of the VCO; a frequency comparison means for comparing the output of the frequency detection means with a lower limit frequency and an upper limit frequency of the search frequency range; a logical AND for generating a signal for switching the switching means to the output side of the loop filter when the output of the frequency comparison means is equal to or higher than the lower limit frequency and equal to or lower than the upper limit frequency and when an initialization signal is false; and a control unit that monitors an output of the frequency comparison means, outputs the lower limit frequency, the upper limit frequency, the initialization signal, and the VCO initial setting value, and when it is confirmed that the phase lock is impossible within the search frequency range, sets the new search frequency range by slightly shifting the search frequency range; The transducer drive circuit of claim 1 , configured to include:

3. The circuit configuration of the drive circuit for the transducer is a phase comparator to which a voltage signal applied to the vibrator and a current signal flowing through the vibrator are input; a loop filter for smoothing the output of the phase comparator; A switching means for switching between the output of the loop filter and an NCO initial set value; an NCO whose oscillation frequency changes in accordance with the input signal selected by the switching means; a frequency comparison means for comparing the magnitude relationship between the output of the loop filter and a lower limit frequency and an upper limit frequency of the search frequency range; a logical AND for generating a signal for switching the switching means to the output side of the loop filter when the output of the frequency comparison means is equal to or higher than the lower limit frequency and equal to or lower than the upper limit frequency and when an initialization signal is false; and a control unit that monitors an output of the frequency comparison means, outputs the lower limit frequency, the upper limit frequency, the initialization signal, and the NCO initial setting value, and when it is confirmed that the phase lock is impossible within the search frequency range, sets the new search frequency range by slightly shifting the search frequency range; The transducer drive circuit of claim 1 , configured to include:

4. A PLL (phase-locked loop) is operated within a predetermined search frequency range to check whether phase lock is possible, A method for driving an oscillator, the method comprising: performing the PLL operation in a new search frequency range obtained by slightly shifting the search frequency range until the phase lock becomes possible.

5. 5. The method for driving a transducer according to claim 4, further comprising the step of: when the phase lock is enabled, expanding or contracting a tracking frequency range with the search frequency range in which the phase lock is enabled as a center.

Images