Frequency adjusting bench and method for a coupled monolithic resonator.

p0001The present invention relates to a method and a setting bench to the frequency of a monolithic coupled resonator including a quartz.

p0002Coupled monolithic quartz consists essentially of a quartz plate coated on one side of two separate electrodes and on the other side, or a common electrode, or two separate electrodes interconnected. This type of quartz is used in particular in filters adapted to filter circuits medium frequency radio receivers.

p0003Currently, adjustment to the frequency of a coupled monolithic quartz is performed after metallization of the electrodes on the quartz plate and attaching the quartz to its support.

p0004Different methods can be used to make this adjustment. Among these methods, there may be mentioned electroplating gold salt to the metallizations of quartz. This method schematized in the flowchart of Figure 1 is first measured, for example by phase control, the different frequencies of the quartz, namely the high frequency, the low frequency and balancing. then control balancing. To do this, according to the values ​​obtained either one goes to either separate electrodes to the electrolytic bath, then we start measurement operations or, balancing being obtained, is carried out the control of the low frequency. In this case, if the low frequency is lower than the nominal frequency, the common electrode is happening or both electrodes coupled in a bath of cyanide then is repeated measurement operations. If the low frequency is greater than or equal to the nominal frequency, is carried out the control of bandwidth given by the difference between the high and low frequency. If bandwidth is greater than the desired bandwidth, we pass two separate electrodes in the electrolytic bath; if less than all the electrodes are passed to the electrolytic bath. The adjusting is finished when the bandwidth is equal to the desired bandwidth.

p0005The main disadvantages of this fully manual method are being very long and to appeal mainly to the experience of the operator. In this case, in fact the production of medium is twenty-five quartz per day for a working period of seven hours.

p0006To overcome these drawbacks, the US company Transat Corporation has developed a method and device for adjusting the frequency of a coupled monolithic quartz which are to make additional deposits on parts of the electrodes. Thus, the bandwidth is adjusted by metallizing a game between the two electrodes of the common side or by metallizing the center of the two electrodes on the opposite side and the nominal frequency is adjusted by metallizing the center electrodes of the common side. This method, although fast, requires prior sorting of quartz after machining, because the adjustment frequency coupled monolithic quartz is limited, in this system, 0.15% of the nominal frequency and 30% of bandwidth . It is therefore necessary to perform a more precise adjustment, by etching, before metallization. On the other hand, the two adjustment methods described above are carried out separately from the base metallisation. Therefore, the adjustment frequency may change other characteristics of quartz.

p0007The present invention aims to remedy these drawbacks by a method and setting the frequency of a monolithic resonator coupled device which consists in a vacuum evaporation metallization total slave to adjust the nominal frequency and the band bandwidth quartz.

p0008Accordingly, the present invention relates to a last frequency process for coupled monolithic quartz having two faces vis-a-vis two separate electrodes and a common electrode is coupled to two electrodes, characterized in that the is carried out, all at once, a base metallization, then detecting the transmission frequency curve of quartz mounted in short circuit; using this curve is measured at a given instant the high and low frequencies of the quartz and checked balancing; the high and low frequencies are compared with the frequencies and obtain the result of the comparison is sent as a control metallization caches, the measurement and comparison of frequencies being recommenced until the desired values ​​with, in each time verification of balancing.

p0009According to a preferred embodiment of the present invention, the method consists:<ul><li>a) providing a base metallization,</li><li>b) for the duration of the following, cyclically detecting for each separate transmission curve electrode quartz frequency connected in short-circuit,</li><li>c) to realize the balancing of the quartz by metallizing one or other of the two electrodes separated to bring into line the transmission zones of two successive curves corresponding to the electrodes shorted,</li><li>d) using the above curves, to determine the high and low frequencies of the quartz at a given instant,</li><li>e) calculating the bandwidth,</li><li>f) comparing the bandwidth to a bandwidth reference</li><li>g) metallizing the two electrodes separated for causing the bandwidth to the calculated value of the reference bandwidth,</li><li>h) simultaneously comparing the low frequency at a nominal frequency of reference,</li><li>i) metallizing the common electrode or both electrodes coupled to cause the low frequency detected value of the nominal reference frequency,</li><li>j) balance quartz as described in c),</li><li>k) then, to resume operations in d) until the desired values ​​of the bandwidth and the low frequency is obtained and that the balancing is achieved and, at that time, stop the metallization.</li></ul>

p0010According to another feature of the present invention, to avoid exceeding the reference values, which then requires the demetallized quartz and repeat the last frequency of operation, it also compares the low frequency at a frequency of threshold and when this threshold frequency is reached, it reduces the metallization rate. SEVERAL threshold frequency may be determined.

p0011According to a particular embodiment of the present invention, the metallizations of two electrodes separated from the common electrode or electrodes and coupled to one or other of the two electrodes separated are carried out by acting on three mattes. In fact, the caches used for the metallization of the common electrode and to the metallization of the two separate electrodes are automatically controlled according to the result of the various comparisons while the cache for the metallization of either separate electrodes is controlled manually.

p0012The present invention also relates to a bench for carrying out the method above. This bench essentially comprises a bell provided with metallization metallization and mattes means, a device for measuring at each instant the frequency transmission curve of the quartz mounted in short-circuit, a device for cyclically switching the separately mounted electrode short circuit, means to achieve from the transmission curves balancing quartz, means to realize every moment, from the transmission curves, detection and tracking of high-frequency, low nominal and quartz , means for using different frequencies to control the metallization means and at least some of the mattes.

p0013The present invention will be better understood on reading the description of a last bank the frequency of a monolithic coupled resonator according to the present invention, this description being made hereinafter with reference to the accompanying drawings in which:<ul><li>- Figure 1 is a flowchart of a method of adjusting the frequency of the prior art,</li><li>- Figures 2A and 2B are respectively a diagram of a quartz mounted in short circuit for measuring the frequency transmission characteristic and the representation of the transmission frequency curve,</li><li>- Figure 3 shows different curves and diagrams explaining the principle used in the method of the present invention,</li><li>- Figure 4 is a block diagram of a frequency update bench used in the present invention,</li><li>- Figure 5 is a block diagram of the wobble circuit and quartz frequency control used in the bench of figure 4,</li><li>- Figure 6 is a block diagram of the control logic circuit of Figure 5,</li><li>- Figures 7A, 7B and 7C are respectively a diagram of a peak detector used in the circuit of Figure 6 and the curves of the input and output voltages of said circuit,</li><li>- Figures 8A, 8B, 8C and 8D are respectively a diagram of the servo circuit used in the circuit of Figure 6 and the voltages at various points of this circuit,</li><li>- Figures 9A to 9I show the signals obtained at various points of the circuit of Figure 6,</li><li>- Figures 10A to 10E show the position of the caches in different moments of the process.</li></ul>

p0014The principle implemented in the method of the present invention consists in detecting the transmission frequency curve of a monolithic coupled resonator connected in short-circuit and to use the frequencies given by this curve to control the metallization of the electrodes of the resonator.

p0015Thus, when the quartz, symbolized by separate electrodes A and B and the common electrode C, is mounted such that a separate electrodes, namely the electrode B is coupled with the common electrode C, if applying a voltage E of variable frequency at the input of the quartz, the output is a voltage U which varies as shown in the curve of Figure 2B. On this curve, two transmission poles are observed in frequency and two zeros. However, the poles are at low and high frequencies of the quartz. It is therefore possible by detecting the two maxima of the transmission frequency curve to obtain the low frequency and high frequency quartz.

p0016On the other hand, as shown in Figure 3, if one switches cyclically separate electrode set A or B short-circuit with the electrode C, is obtained in two frequency transmission curves. However, the balancing of quartz is formed when these two curves are superimposed. In the present invention therefore achieves the balancing of the quartz causing the two curves shown in Figure 3 to overlap, as will be explained in more detail below.

p0017Thus, according to the method of the present invention, is firstly directs base metallization on the quartz plate which allows the blade to vibrate and accordingly to detect the frequency transmission characteristic. Then, for the duration of the following, one cyclically detects, for each separate electrode put in short circuit, the transmission curve of the quartz frequency. The two curves thus obtained are used to perform balancing quartz. Thus, metallizing one or other of the two electrodes separated so as to bring into line the transmission zeros of two successive curves. Then, using the above two curves, one determines the high and low frequencies of the quartz at a given time. Using the frequencies so determined, we calculate the bandwidth and compares this bandwidth with a bandwidth reference which actually corresponds to the bandwidth to get to the quartz that is metal. Depending on the result of the comparison, metallizing the two electrodes separated so as to bring the calculated bandwidth value of the bandwidth reference. Simultaneously, the low frequency comparing determined from the transmission frequency curve at a nominal frequency of reference and the common electrode is metallized or both electrodes coupled for feeding the frequency detected based on the value of the reference nominal frequency , namely the nominal frequency selected for quartz. then rechecked balancing quartz as described above and is repeated operations for measuring high and low frequencies of the quartz at a given time until the desired values ​​of the bandwidth and the low frequency is obtained and that the balancing is carried out, then stopped the metallization.

p0018The above method has been developed using the bandwidth and the low frequency of quartz. However, it is obvious to the skilled person that one can simply use the low frequency and high frequency quartz to control the metallization or select other parameters that can be obtained from the transmission curve frequency. On the other hand, to avoid exceeding the reference values, according to the method of the present invention, the low frequency is compared to a threshold frequency, and when this threshold frequency is reached, it reduces the metallization rate to stop the metallization at the right time.

p0019will now be described with reference to Figures 4 to 10, a bench for the implementation of the method of the present invention.

p0020As shown in Figure 4, the tester essentially comprises a metallization of bell 10, a switching and measuring circuit of the quartz 20, a machine control relay housing 30, a machine control panel 40, a processor 50, a frequency meter 60, a viewer 70 and a wobble control circuit and the frequencies of the quartz 80.

p0021More specifically, the metallization of bell 10 includes a frame 11 on which is placed the quartz plate 12 to be metallized. This quartz plate is surrounded by three mattes CH1, CH3, CH2 which can be moved to close one or the other part of quartz according to a control command from the relay box 30 or by manual movement as will be explained in more detail below. On the other hand, the metallization bell comprises heating the filaments to achieve the metallization by vacuum evaporation. These resistance wires are also connected to the housing 30 so as to be controlled manually or automatically as will be explained below.

p0022The machine control relay box is connected to the control panel Machine 40. The role of the machine control panel is to initiate a number of transactions, including the base metallization process, and serve as an interface between the computer and the machine control relay housing 30 when the automatic operations have been initiated.

p0023The ECU 50 is mainly used to perform the calculation of the bandwidth from the high and low frequencies from the frequency and comparing the bandwidth and the different frequency from the frequency to the reference values ​​that have been initialized to the inside the computer.

p0024The frequency counter 60 is used primarily as a means to scan the high and low frequencies detected by the circuit 80. It further allows better display of these frequencies. However, it is obvious to those skilled in the art that other scanning means may be used instead of the frequency meter 60.

p0025The viewer 70 can simultaneously display, by superposition, the two frequency transmission curves obtained when either of the separated electrodes is short-circuiting. Using the viewer, one can control the balance quartz. Equilibrium is reached when the transmission zeros of the two curves displayed on the viewer are in agreement.

p0026The wobble circuit 80 and control the crystal frequency will be described in detail with reference to Figures 5 to 8. As shown in Figure 5, the circuit 80 essentially comprises a nominal frequency of generator 81, a frequency generator 82 high and a low frequency generator 83. the nominal frequency of generator 81 sends a nominal frequency wobbled at the input of the measurement circuit of quartz. The measuring circuit used in this case is a well known conventional circuit skilled in the art and will not be described in detail here. However, this measuring circuit has an associated switching circuit to short-circuit with the common electrode, one or other of two separate electrodes. As will be explained below, the wobbled nominal frequency corresponds to the nominal frequency of the quartz at a given instant wobbled about a few tens of kilohertz in order to obtain at the output of the measuring circuit a voltage varying as a function of the crystal frequency, which allows for the transmission frequency curve of the quartz at a given time. The nominal frequency wobbled from the generator 81 is also sent to the input of a mixer 84 and a mixer 85 that receive a high frequency and the other a low frequency, respectively from the generators 82 and 83. The frequency high and low frequency generators from 82 and 83 are also sent to the frequency to be scanned and displayed. For a nominal frequency wobbled, the generator 81 receives an sawtooth of the sawtooth generator 86. This sawtooth has been chosen with a low frequency, ie 50 Hz in the illustrated embodiment, to avoid problems in viewing frequency transmission curves. Indeed, the saw tooth from the generator 86 is also sent to the input X of the viewer 70. On the other hand, the sawtooth is sent to the control logic circuit 87 which also receives the signal SM_FH from the mixer 84, the signal SM_FB from the mixer 85 and the voltage shaping via the matching circuit and shaping end 88 of the measuring circuit 20.

p0027The control logic 87 develops frequency correction signals εFN, εFH, εFB that are sent respectively to the generators 81, 82, 83 and whose function will be explained below. It is also developing a marker signal which is sent to the Z input of the viewer, this marker signal to track directly on curves, low and high frequencies. In addition, the voltage U detected after the measuring circuit 20 is sent after adjustment and shaping on the Y input of the viewer so as to obtain the frequency-transmission characteristic.

p002887 the control logic circuit shown in Figure 6 essentially comprises two quartz 90 and 90' which the resonance frequencies were selected so as to be slightly offset with respect to one another to avoid beats. Thus, in the embodiment shown, the resonant frequency of the quartz 90 is 3.5 MHz while the resonance frequency of the quartz 90' is 3 MHz. Both quartz and 90 90' respectively receive the signals SM_FH and SM_FB from the mixers 84 and 85. They are connected in each output to a peak detector 91', 91". The voltage U setting end form of the circuit 88 is also supplied to a peak detector 91.

p0029The three peak detectors 91, 91', 91" are undertaken, for example, as shown in Figure 7A. Namely, they contain two operational amplifiers 910, 911 connected together such that the input signal E are applied to the direct input of the amplifier 910 so that the direct input of the amplifier 911 receives a signal reset. Indirect inputs of the two amplifiers 910 and 911 are interconnected and connected to the output of the amplifier 911; the signal of the peak detector outputs is obtained on the amplifier 910. On the other hand, between the two direct inputs of amplifiers 910 and 911 are connected two diodes 912 and 913 in parallel, the diodes being mounted so as to be bandwidths , one in one direction, the other in the other. Between the input of the amplifier 911 and ground, mounted a capacitor 914. It will explain the operation of the peak detector using figures 7B and 7C. Thus, after reset, the input voltage E charges the capacitor 914 through the diode 913 whose meaning is passing. The output voltage of the amplifier 911 mounted in follower is always less than the voltage E. As a result, the output S is unchanged. After passage at the top of the first frequency transmission curve of the quartz su shown in FIG 7B, the diode 913 is blocked and the capacitor 914 being charged maintained at the output of follower amplifier 911 a voltage higher than the voltage E , which causes a sudden change of polarity of the level of the output S. When the voltage E drops below the conduction threshold of the other diode 912, the capacity 914 is discharged and this until the next peak of the curve in Figure 7B. One thus obtains the output voltage shown in FIG 7C. For a repetitive system can discharge the capacitor 914 using a signal reset reset to end up in the same starting position each cycle.

p0030The signals from the peak detectors 91, 91', 91" are sent on detection logic 92, 92', 92" which consist of JK circuits in the embodiment shown and give from a signal of the type shown in Figure 7C rectangular signals constituting the reference signal figure, the signal reference high frequency, the reference signal low frequency, the marker top and bottom marker signal. These detection logic receive as input a reset signal for the cyclic operation of the system.

p0031The MH and MB signals from the 92' 92" and detection logic are sent to a 95 made up of NAND circuit that, from both MH and MB signals, to obtain a single signal sent to the input Z of the viewer. On the other hand, the corresponding MH and MB signals are sent to the control circuit 93', 93". A similar servo circuit 93 receives the reference signal from the circuit FIG 92. FIG centering reference voltage is also applied to the servo circuits 93, 93" while the slaving circuit 93' receives a reference voltage high frequency FH. The servo circuit 93 is controlled by the action of a synchro-figure key while the control circuits 93', 93" are controlled by the action of a synchro-marker button.

p0032The constitution of the control circuits 93, 93', 93" and their operation will be better understood by reference to Figures 8A to 8D which gives a schematic representation of these circuits. Each servo circuit essentially comprises a comparator consisting in known manner of an operational amplifier 930 having a feedback loop formed of a resistor 931 and a capacitor 932 connected in parallel between the output of the operational amplifier 930 and the negative input of the amplifier. After the signal detection logic circuit, namely the reference signal pattern or the up and down marker marker signals is sent to the negative input of amplifier 930 through a filter circuit 938. The amplifier 930 receives at its positive input a reference voltage which, according to the case, is constituted either by a voltage reference to FIG centering circuits 93 and 93", either by the high-frequency reference signal from the logic circuit 92 to 93' circuit. an error signal of the comparator output is obtained which is applied via a resistor 936 to the positive input of an operational amplifier 933. This operational amplifier 933 is mounted in the ramp generator. It comprises between its output and its negative input against a feedback loop with a capacity 934. The capacity 934 is connected to a synchronization button 935 that, when closed, sends a positive voltage and charges the capacitor 934. On the Furthermore, the negative input of the amplifier 933 is connected via a resistor 936 to a potentiometer 937 which allows a centering adjustment as will be explained below.

p00338B to 8D represent the voltage at different points A, B and C of the control circuit. Thus, initially, the voltage at the point B shown in Figure 8C is equal to the reference voltage. Then, when you press the sync button 935 is started the circuit as explained to handle e more detail below. Accordingly, the load capacitor 934, which provides the pulse shown in Figure 8D. Releasing the synchronization key and the capacitor 934 discharges slowly creating a ramp at the point C. On the other hand, the point A receives a rectangular wave signal shown in Figure 8B obtained from the detection logic. This rectangular signal is integrated by the circuit 938 and compared with the reference voltage at point B to give an error signal. This error signal, from the implementation of the system, decreases slowly as shown in Figure 8C, is compared to the slow ramp and blocks its evolution when the frequencies of different generators correspond to the different frequencies of the quartz since the signal from the point C is sent as an error signal on these generators. System lock is thus obtained and a continuous monitoring of different frequencies. These functions will become more evident in the description of the operation of the wobble circuit and control quartz.

p0034Thus, as shown in Figure 5, the εFH signals εFB from the servo-control circuits 93' and 93" are sent as error signals on the low frequency generator 83 and high frequency 82. On the other hand, the signal from the servo circuit 93 is added to the sawtooth signal after the sawtooth generator 86 as shown in Figure 6, this signal is applied to a circuit 94 consisting of an operational amplifier connected as an inverter. circuit output is obtained from this 94 εFN an error signal over the saw tooth signal applied to the nominal frequency of generator 81.

p0035will now be described in more detail with reference to Figures 9A-9I operation of the wobble circuit 80 shown in Figures 5 to 8. After performed by manual control of the system a base metallization, is started the operation automatic system. To enable this automatic operation, the nominal frequency of generator 81 was set to a frequency value equal to the nominal frequency or stop frequency less .DELTA.F. In this case, .DELTA.F can be chosen equal to 200 kHz to a stop frequency of 21.4 MHz. Furthermore, the low frequency generator 83 is positioned at a value equal to the nominal frequency or stop frequency less the frequency of the quartz placed in the circuit 87 and chosen equal to 3 MHz in this case more ΔF1 and the generator high frequency 83 is positioned at a value equal to the stop frequency less the frequency of quartz mounted in the circuit 87 and selected to be 3.5 MHz more ΔF1. Indeed, to obtain a correct metallization quartz and not exceed the stop frequency and bandwidth chosen, it is necessary to have a sense of growing wobbling for the nominal frequency generator 81 and to have a sense of wobble descending to the low frequency generator and the high frequency generator.

p0036The system initialization is performed by pressing the sync button-figure. This creates, as mentioned above, a slow ramp which is applied to the nominal frequency of generator 81 and that is changing the frequency of the generator in the upward direction to a chosen value equal to the nominal frequency ΔF2. As shown in Figure 9B, this ramp is superimposed on a 50Hz frequency sawtooth in the embodiment chosen, from the generator 86. In Figure 9, the direction of the ramp is decreasing because the varicap diodes positioned input of the generator 81 are connected to a negative voltage, which corresponds to a reversal of the direction of the ramp.

p0037When the frequency of the generator 81 reaches the value of the nominal frequency of the quartz placed on the measuring circuit in the circuit 20, the frequency transmission curve as shown in Figure 9C is shown on the screen of the viewer 70 (see part C of Figure 9C). At this time, the peak detector circuit is in operation. A rectangular signal is obtained from this detection as explained above with reference to Figures 7. This signal is proportional to the position of the figure on the screen as shown in FIG 9D, part C. This signal is integrated and compared to a adjustable reference value called, in this case, centering FIG as explained with reference to figures 6 and 8. the error signal obtained at the comparator output is applied to the operational amplifier of the slow ramp generator and abruptly stops its evolution when the voltage of FIG centering is obtained, that is to say when the error signal is zero as shown in figures 9C, 9D and 9E, part E and following. At this time, the system is locked, that is to say that any change in the crystal frequency will not change the position of the figure, but that the quartz frequency change will be followed at the generator 81.

p0038above is explained the operation of the capture of the frequency transmission curve of the quartz mounted on the measuring circuit in the circuit 20, will now be explained the operation of the capture of high and low frequencies of the quartz enabling control Automatic metallization and the creation of markers which is superimposed on the frequency-transmission curves.

p0039Capture high and low frequencies is done simultaneously for the two frequencies and a substantially identical manner to the catch of FIG. So when you press the Sync button-marker, the frequency of the low frequency generator 83 is a value in the illustrated embodiment, the stop frequency - 3 MHz + 800 KHz and frequency of the high frequency generator 82 is a value equal to the stop frequency - 3.5 MHz + 800 KHz. These frequencies were selected for low frequencies could range between 18.8 MHz and 18 MHz and high frequency could range between 18.3 MHz and 17.5 MHz. However, it is obvious to those skilled in the art that these frequencies have been given solely by way of example. The frequencies of generators 83 and 82 are mixed with the frequency of the nominal frequency of generator 81. This produces at the mixer output 85 a signal SM_FB having a frequency of 3 MHz + or - and a wobble signal SM_FH having a frequency of 3.5 MHz + or - the wobble.

p0040The change in frequency being in this still a growing way case, each signal is applied to a 3 MHz quartz for the low frequency and 3.5 MHz to the high frequency so as to avoid the beats between the two frequencies. The quartz from signals such as shown in Figures 9G and 9H are sent to a peak detecting circuit operating as mentioned above. At the output of the peak detection circuit a rectangular signal proportional to the position of the low frequency to the signal from the 3 MHz crystal and that of the high frequency to the signal from the 3.5 MHz crystal. After integration of these signals, they are each sent to a comparator and compared with either the high frequency reference signal corresponding to the high frequency quartz mounted on the measuring circuit 20 and from the logic detection circuit 92, or with the reference voltage centering figure used to capture the figure in the case of low frequency. Error signals from the two comparators are applied to operational amplifiers ramp generators and abruptly quit the evolution of the ramp thereby locking system respectively on the low frequency and high frequency quartz. Low and high frequencies thus obtained are sent to a computer via a frequency so as to obtain metallization control signals. On the other hand, signals from detection logic 92, 92' are sent to the circuit 95 to give the pulses shown in FIG 9I which are applied on channel Z of the viewer 70 so as to display the top marker and the marker low compared with the transmission curve of the quartz frequency. The position of the markers that have to be locked on the FB and FH spikes can be modified slightly by adjusting the centering voltage.

p0041now be explained with reference to Figures 10A-10E, the position of the caches at the different metallization phases depending on the control signals from the computer. first of all be recalled that, in the present invention, the balancing, that is to say the superposition of two frequency transmission curves obtained upon switching of the electrodes short-circuited, is done manually. However, it is obvious to those skilled in the art that automatic control of the cache to achieve this balancing may be considered. In the embodiment shown, the metallizations are produced by using three masks, namely CH1 cover acting on the common electrode, the CH2 cover acting on the separate electrodes and the CH3 cover acting on the half-electrodes. At the beginning of the metallization, the covers are positioned as shown in FIG 10A so as to create a base layer. When this base layer is created, the frequency transmission characteristic is detected as explained above, and from this curve, a first manual balance correction is carried out by action on the CH3 cache as shown in Figures 10B and 10C. Then, by automatic action CH1 cache that is closed, the cover being opened and CH2 CH3 cover being in a variable position along the balance is carried out adjusting the bandwidth. Once this adjustment obtained by automatic command from the computer, the CH1 cover is open and, the cache CH 2 being closed, is carried out by evaporation at full power so as to adjust the low frequency to the selected frequency, then when this low frequency is obtained, is carried out manually balance correction by acting on CH3 and it automatically starts adjusting the bandwidth and the adjustment of the low frequency to obtain the desired values. On the other hand, towards the end of the above operations, evaporation is performed at reduced power from a threshold frequency to the stop frequency in order not to exceed the specified values.

p0042It is obvious to those skilled in the art that various modifications may be made, or the method or the device, without departing from the scope of the claims below.