WO2010061047A1

WO2010061047A1 - Tunable antenna and tuning method

Info

Publication number: WO2010061047A1
Application number: PCT/FI2009/050873
Authority: WO
Inventors: Prasadh Ramachandran; Muhammad Nazrul Islam; Tuomas JÄÄSKÖ; Petteri Annamaa; Ville Majava; Zlatoljub D. Milosavljevic
Original assignee: Pulse Finland Oy
Priority date: 2008-11-03
Filing date: 2009-10-30
Publication date: 2010-06-03
Also published as: FI20086035A0

Abstract

A tunable antenna of small-sized radio devices and a tuning method. The radiator (610) of an antenna of a radio device is of conductive coating of a dielectric substrate(605). Fuses (F61, F62, F63) connect to the radiating conductor so that the physical shape of the radiator and its electrical size at the same time depend on which fuses are conductive and which are not. The radiating conductor functions as a common current path to the fuses, and the fuse to be blown is selected on grounds of the magnitude or duration of a direct current. The fuses are surface mounted components. Also so called anti-fuses, which can be permanently changed from non-conductive to conductive, may be used. The tuning of an antenna mounted in the end product hardly changes the appearance of the antenna, and no vapours hazardous to health are formed in the tuning.

Description

Tunable antenna and tuning method

The invention relates especially to a tunable antenna of small-sized radio devices and a method, by which the tuning is implemented.

The tuning of an antenna means here an one-time operation by nature, by which its resonance frequency or frequencies are precisely arranged to the points, which correspond to the operating bands of the antenna. This kind of tuning can be considered the last stage in the antenna manufacture, and it can take place also after mounting the antenna to a radio device. Namely, even a minor variation in the place of the antenna and the surrounding materials can cause shifts of the resonance frequencies that require tuning.

There are tuning methods known before, in which a part of the radiating element is removed through mechanical working or by means of a laser beam. As the element size thus is reduced, the resonance frequency of the corresponding part of the antenna structure increases. Naturally the element originally is large enough so as to have a safe tuning margin . The mechanical working is relatively inaccurate, and as a result small conductive chips may be left in the structure, risking a short-circuit as relatively strong electric fields occur in the antenna. A disadvantage of the method using a laser beam is that a protection arrangement is required for the worker, because as metal is removed by laser, also plastic material, on which the radiator is located, is vaporized at the same time. This means that the tuning of an antenna, especially one which has been mounted in the end product, becomes difficult. In addition, such a processing changes the appearance of the antenna, which may result in that the type approval made for the device is no longer valid.

An object of the invention is to is to reduce said disadvantages in the tuning of the antenna of small-sized radio devices related to prior art. An antenna according to the invention is characterized in that what is set forth in the independent claim 1. A method according to the invention is characterized by that what is set forth in the independent claim 9. Some advantageous embodiments of the invention are disclosed in the other claims.

The basic idea of the invention is as follows: The radiator of an antenna of a radio device is of conductive coating of a dielectric substrate. Fuses connect to the radiating conductor so that the physical shape of the radiator and at the same time its electrical size depend on which fuses are conductive and which are not. The radiating conductor functions as a common current path to the fuses, and the fuse to be melted broken, or 'blown', is selected on the grounds of the magnitude or duration of a direct current. The fuses are surface mounted components. Also so called anti-fuses, which can be permanently changed from non-conductive to conductive, may be used.

An advantage of the invention is that the tuning of an antenna mounted in the end product hardly changes the appearance of the antenna. This is due to the fact that the fuses are small in respect of the radiator size, and the change, when blowing a fuse, takes place only in a part of the area of the fuse. Another advantage of the invention is that the tuning is simple, because it normally does not require a separate connection to each fuse component for changing their conductive state. A further advantage of the invention is that no vapours hazardous to health are formed in the tuning, in which case a protection arrangement is not required for the worker.

The invention is described in closer detail in the following. In the description, reference is made to the accompanying drawings in which

Fig. 1 shows as a principled drawing an example of the antenna according to the invention and its tuning,

Figs. 2a, b show examples of the tuning process in an antenna according to Fig. 1 ,

Fig. 3 shows another example of the antenna according to the invention and its tuning,

Fig. 4 shows a third example of the antenna according to the invention and its tuning,

Fig. 5 shows a fourth example of the antenna according to the invention and its tuning,

Figs. 6a, b show an example of the practical implementation of an antenna according to the invention,

Fig. 7 shows an example of the shift of an operating band of an antenna based on the antenna component according to Fig. 6b, and Fig. 8 shows as a flow diagram an example of the method according to the invention.

In Fig. 1 there is as a principled drawing an example of the antenna according to the invention and its tuning. The antenna is of monopole type and its radiator 110 comprises a straight first portion starting from the feed point FP of the radiator, an U-shaped bend portion and a straight tail portion. The first and tail portions are substantially parallel. The radiator could also be a loop so that its tail end would extend beside the feed point FP and be connected to the other conductor of the feed line of the antenna. Relatively close to the bend portion there are three fuse branches, one end of each branch being connected to the first portion of the radiator and the other end to the tail portion. The third fuse branch 123 is located closest to the bend portion, the second fuse branch 122 is a little farther and the first fuse branch 121 is farthest from the bend portion. When all the fuses are undamaged and the fuse branches then conductive, both the physical and electrical length of the radiator is at its minimum; it depends on the location of the first fuse branch. In the example of the figure the first fuse has been blown broken at the point BK, in which case the radiator length slightly increases; it depends now on the location of the second fuse branch 122. Correspondingly, if also the fuse of the second branch would be blown, the radiator length would again increase, and if finally the fuse of the third branch would be blown, the radiator length would further increase and would then be at its maximum. When the radiator length increases, the resonance frequency of the antenna decreases and the operating band shifts downwards. The antenna is designed so that when the fuses are undamaged, the variation range of its resonance frequency is above the right resonance frequency.

A fuse/fuses is/are blown by connecting a suitable tuning voltage between the first and tail end of the radiator. In order for the tuning voltage not to be shorted via the bend portion of the radiator, this has been cut by a blocking capacitor C1 1. The capacitance of the blocking capacitor is high enough so that its impedance at the radio frequencies is practically zero. Thus the bend portion is continuous at the frequencies of the operating band of the antenna.

In Fig. 2a there is an example of the tuning process in an antenna according to

Fig. 1. It is decided in the process, whether fuses are blown and if so, which fuse or fuses. Each possible fuse blowing is preceded by finding out the location of the operating band of the antenna e.g. by measuring the reflection coefficient as a function of frequency. If the measurement indicates that the resonance frequency of the antenna has to be lowered, a fuse will be blown. A first resistor R1 and first fuse F1 are in series in the branch corresponding to the first fuse branch 121 , a second resistor R2 and second fuse F2 are in series in the branch corresponding to the second fuse branch 122 and a third resistor R3 and third fuse F3 are in series in the branch corresponding to the third fuse branch 123. Parallel with these branches there is an adjustable voltage source V. One terminal of the source is connected for example to the feed point of the radiator and the other terminal to the opposite, open end of the radiator. The symbol V also means the source voltage, or tuning voltage, and the symbol R also means the resistance of the resistor in question.

The fuses are identical with each other, but the resistors are not. The resistance R1 is the lowest (may be also zero), the resistance R2 is higher and the resistance R3 is the highest. The voltage, which is needed for blowing a fuse, is set by the serial resistor. When the tuning voltage V is raised from an initial value to a certain level, the current l-i of the first resistor reaches the value, by which the fuse blows, or the fuse conductor breaks. Let us call this current value IB- The other fuses remain undamaged, because their current is lower. If the resonance frequency of the antenna still has to be lowered, the voltage V is raised more until the current I₂ of the second resistor reaches the value IB, and if the resonance frequency of the antenna further has to be lowered, the voltage V is further raised until the current b of the third resistor reaches the value Iβ.

In Fig. 2b there is another example of the tuning process in an antenna according to Fig. 1 . Only the fourth fuse F4 is in the branch corresponding to the first fuse branch 121 , only the fifth fuse F5 is in the branch corresponding to the second fuse branch 122 and only the sixth fuse F6 is in the branch closest to the bend portion of the radiator. Parallel with these branches there is a voltage source V, which includes or is connected to a timer T. The timer can be set to switch a certain source voltage on for a desired time. When the voltage is switched on, a current begins to flow in the fuses, current U in the fourth fuse, current I₅ in the fifth fuse and current \Q in the sixth fuse. In practice these currents are at least nearly equal.

In this example the fuses are different by sensitivity. The fourth fuse F4 is the most sensitive so that it blows first when an equal current flows in all fuses. In Fig. 2b time T₄ is the blowing time of the fourth fuse at a certain suitable current. Correspondingly the blowing time of the fifth fuse F5 at the current in question is T₅ and the blowing time of the sixth fuse F6 at the current in question is TQ, time T₅ being longer than T₄ and time TQ being longer than T₅. If the resonance frequency of the antenna has to be lowered, the tuning voltage V is first kept on as long as time T₄, after which the fourth fuse blows, but the other fuses remain undamaged, because the time required for their blowing is longer. If the resonance frequency of the antenna still has to be lowered, the tuning voltage is kept on as long as time T₅, after which the fifth fuse blows, and if the resonance frequency of the antenna further has to be lowered, the tuning voltage is kept on as long as time TQ, after which the sixth fuse blows. In this case, in each stage, the connecting time of the current is based on the specified values for the fuses.

Alternatively, the voltage source may include a measuring circuit of its total current lti and a monitoring circuit of the changes in the total current. In this case the tuning voltage V is switched on without time setting. When the most sensitive fuse blows, the total current l_tι drops a certain amount. The monitoring circuit of the current changes detects the drop and switches the tuning voltage V off. If the resonance frequency of the antenna still has to be lowered, the tuning voltage is again switched on, and the above-mentioned action repeats, the second sensitive fuse blowing this time. The rounds are continued until the operating band of the antenna is at the right place.

Fig. 3 shows another example of the antenna according to the invention and its tuning. The radiator 310 of the antenna forms in this case a meander pattern. For tuning the antenna it comprises two fuse branches. When the first fuse branch 321 is undamaged, it takes a small shortcut across a meander bend, and the second fuse branch 322 takes a shortcut across the following meander bend when going from the feed point FP towards the open end of the radiator. One straight portion of the meander pattern is then between the fuse branches. In this portion there is the blowing point BP for connecting the tuning voltage.

When both fuses are undamaged, both the physical and electrical length of the radiator is at its minimum. When either fuse is blown, the radiator length increases, in which case the resonance frequency of the antenna decreases and the operating band shifts downwards. In the example of the figure the fuse of the second fuse branch 322 has been blown, which decreases the resonance frequency a little more than breaking the first fuse branch, because the second fuse branch is slightly farther from the meander bend than the first fuse branch. If the fuses of both branches are blown, the radiator length is naturally at its maximum. Thus there are four alternatives for the radiator length and the location of the antenna operating band in the example of Fig. 3.

The first fuse branch can be blown by connecting the tuning voltage V between the above-mentioned blowing point BP and the first end of the radiator, or the end on the side of the feed point FP. The second fuse branch can be blown by connecting the tuning voltage V between the blowing point BP and the tail end of the radiator.

In order for the tuning voltage not to be shorted, the bend of the radiator parallel with the first fuse branch has been cut by a blocking capacitor C31 and the bend of the radiator parallel with the second fuse branch has been cut by a blocking capacitor C32. At the radio frequencies the impedance of the blocking capacitors is practically zero.

Naturally, the amount of the fuse branches can also be more than two, and they need not to be located at the consecutive bends of the meander. Fig. 4 presents a third example of the antenna according to the invention and its tuning . The basic antenna is similar to the one in Fig .1 . In th is case the components to be used for tuning are in series with the radiator conductor 410, near to its tail end, and are of anti-fuse type. In them the amorphous silicon between two metal conductors can be crystallized to a permanently conductive mode because of the effect of an electric field. They can also be based on MIT (Metal Insulator Transition) material, such as VO₂ based composition, which kind of material can be changed to be permanently conductive by an electric field, heating or mechanical stress.

In the example there are two anti-fuses; the second anti-fuse 422 is closer to the tail end of the radiator 410 than the first anti-fuse 421 . Fig. 4 shows the initial situation, in which both anti-fuses are non-conductive. This means that both the physical and electrical length of the radiator is at its minimum. When a sufficient tuning voltage V is directed to the first anti-fuse 421 , it becomes conductive, in which case the radiator length slightly increases and the resonance frequency of the antenna correspondingly decreases. If the resonance frequency still has to be lowered, the tuning voltage is connected over the second anti-fuse 422, in which case also it becomes conductive.

Also fuses that are blown can be connected in series with the radiating conductor. In this case one starts from the fuse closest to the tail end of the radiator, the blowing of which increases a little the resonance frequency of the antenna. If the change is not enough, the next fuse starting from the tail end of the radiator is blown etc.

Fig. 5 shows a fourth example of the antenna according to the invention. The antenna is of PIFA (Planar Inverted-F Antenna) type, wh ich means that it comprises a radiating plane 510 and ground plane GND, and the former is short- circuited from its certain point SP to the latter. The radiating plane is of conductive coating of a relatively thin dielectric substrate 505. Close to the short-circuit point SP there is the feed point FP of the radiating plane. The antenna is a dual-band one, which shows from that it has, viewed from the short-circuit point, two branches with different lengths: the longer branch B1 , on which the lower operating band is based, and the shorter branch B2, on which the upper operating band is based. In the example the tuning regards only the lower operating band.

The radiating plane 510 is divided by blocking capacitor C51 to two parts for the tuning according to the invention. The first part 511 comprises the shared starting portion of the longer B1 and shorter B2 branch, and the open end of the shorter branch. The second part 512 only comprises the most of the longer branch B1 . The blocking capacitor is located at an edge of the radiating plane so that the longer branch B1 is as long as possible measured via that capacitor. In this example, two fuses are connected between the first 51 1 and second 512 part, from which the first fuse 521 is located farther from the blocking capacitor than the second fuse 522. When the fuses are conductive also they, in addition to the blocking capacitor, form a part of the longer branch B1. For this reason both the physical and electrical length of the longer branch is in that case at its minimum, and the resonance frequency, which corresponds to the lower operating band of the antenna, is at its maximum. If the first fuse 521 is blown, the length of the longer branch increases, and the resonance frequency, which corresponds to the lower operating band of the antenna, decreases. If also the second fuse 522 is blown, the length of the longer branch further increases and the resonance frequency, which corresponds to the lower operating band of the antenna, further decreases being then at its minimum. The antenna is designed so that when the fuses are undamaged, its lower operating band is surely so high that there is no need to shift it upwards.

The fuse(s) is/are blown by connecting a suitable tuning voltage V between the first 511 and second 512 part of the radiating plane. The fuses may have a serial resistor in accordance with Fig. 2a, in which case it depends on the value of the tuning voltage, which fuse(s) is/are blown. Also fuses with different sensitivity can be used, in accordance with Fig. 2b.

Instead of using a blocking capacitor, the parts of the radiating plane can be at their coupling point so close to each other that an adequate capacitance springs up without a discrete capacitor. As a whole, the PIFA can naturally also have only one band or more than two bands.

Figs. 6a and 6b show as magnified an example of the practical antenna component according to the invention. The antenna component 600 comprises a relatively thin and elongated dielectric substrate 605 and a radiator 610 as its conductor coating. Only these parts are visible in Fig. 6a, and in Fig. 6b the antenna component is visible in its entirety. In the starting end of the radiator there are its feed point FP and short-circuit point SP. From the latter the radiator is connected to the ground plane, when the antenna component is mounted in a radio device. This means that an antenna implemented by the antenna component in the example is of IFA (Inverted-F Antenna) type. The ground plane of the ready antenna is located, seen from above, either beside the radiator or partly under it. The feed point FP and short-circuit point SP are contact pads, in which the conductor coating is reinforced by a precious metal to improve the reliability of the contact to the feed and short-circuit conductors.

From its starting end the radiator 610 continues, being narrower than the starting end, close to one longitudinal edge of the substrate to its second end and then in the transverse direction towards the other longitudinal edge. Halfway the head side of the substrate there is a gap in the radiator, after which it continues near to the other longitudinal edge and then in the direction of this edge a certain distance back towards the starting end. The gap divides the radiator to the first part 61 1 beginning from the starting end and the second part 612 comprising the tail end. The gap is there also in this case in order for the radiator conductor not to short the voltage source to be used in the tuning according to the invention. A blocking capacitor C61 is connected across the gap, for which reason the radiator is continuous at the operating frequencies of the antenna.

Close to the transverse portion of the radiator, where the blocking capacitor C61 is located, there are three parallel fuse branches between the first and second part of the radiator. Closest to the capacitor there is the third fuse branch, in which the third fuse F63 and third resistor R63 are in series. Next is the second fuse branch, in which the second fuse F62 and second resistor R62 are in series. Farthest from the capacitor there is the first fuse branch, in which there is only the first fuse F61. All the discrete components, that is the three fuses, two resistors and one capacitor, are surface mounting components.

There is a similar contact pad TP as in the feed and short-circuit points in the tail end of the radiator for connecting a tuning voltage. One of the contacts, between which the tuning voltage exists, is pressed against the contact pad TP and the other either against the feed point FP or the short-circuit point SP. If the antenna requires shifting the operating band downwards, the tuning voltage is raised so that the first fuse F61 without a serial resistor blows. If the operating band still has to be shifted, the tuning voltage is raised so that the second fuse F62 blows. If the operating band has further to be shifted, the tuning voltage is further raised so that the third fuse F63 blows. The order is this, because the resistance of the second resistor is lower than the one of the third resistor.

Fig. 7 shows an example of the shift of a resonance frequency and corresponding operating band of an antenna based on the antenna component board according to Fig. 6b. The longitudinal dimension of the radiator is 30 mm and the transversal dimension is 5.5 mm. Curves 70-73 show in different tuning states the variation of the antenna's reflection coefficient as a function of frequency. Curve 70 corresponds to the state, in which all the three fuses are undamaged. The resonance frequency of the antenna is then about 1560 MHz (fθ). Curve 71 corresponds to the state, in which the first fuse F61 is blown and the others are undamaged. The resonance frequency of the antenna is then about 1520 MHz (f1 ). Curve 72 corresponds to the state, in which only the third fuse F63 is undamaged, the resonance frequency of the antenna being then about 1480 MHz (f2). Curve 73 corresponds to the state, in which all the three fuses are blown, the resonance frequency of the antenna being then about 1450 MHz (f3). Thus the resonance frequency and the operating band at the same time can be shifted in this example on average by steps of 35 MHz, which step is about 2.4% of the mid frequency of the adjusting range of the operating band.

In free space and range of the usable operating band, the efficiency of the above- described antenna varies between the values -2,5 dB and -0,5 dB depending on the frequency and tuning state. Thus the efficiency remains good in spite of the tuning arrangement. Namely, the serial resistors of the fuses unavoidably degrade the antenna efficiency a little. In this example the resistances of the resistors are 0.5 Ω and 1.0 Ω.

Fig. 8 shows as a flow diagram an example of the tuning method according to the invention. In the preparation stage a radio device, in which the antenna to be tuned has been mounted, is placed on the measuring bed of the testing equipment. The fuse components, which belong to the radiating structure, are conductive, in the case of anti-fuses non-conductive. When needed, the tuning process comprises more than one tuning turn. In the diagram the value of the index I indicates the current tuning turn so that the ordinal number of the tuning turn is the value of I added by one. The number of the possible tuning states is marked N, which then is the maximum number of the tuning turns. In step 801 the resonance frequency corresponding to the band of the antenna to be shifted is measured, and in step 802 whether the resonance frequency is right with adequate accuracy is checked. If not, whether a tuning possibility still exists is checked. This is carried out by incrementing (step 803) the value of the index I by one and checking (step 804) the new value. If this is less than N, the tuning possibility exists. In this case it is continued to step 805, in which the conductivity state of the next untreated fuse component in order is changed. In other words a usual fuse is blown or an anti-fuse is changed to be conductive. A new tuning turn is begun thereafter by returning to step 801 , or measuring the resonance frequency.

If in step 802 it is noticed that the resonance frequency is the right one with adequate accuracy, the tuning process is ended. The tuning process is also ended, if in step 804 it is noticed that all the tuning states already have taken place. The latter case means that the antenna tuning has not succeeded.

The testing equipment can be implemented so that the tuning process goes on automatically after the start. The result is naturally readable for example on the display of the equipment.

In th is description and in the claims the term 'fuse component' means a component, which can be either a fuse to be blown or an anti-fuse.

A tunable antenna and tuning method according to the invention has been described before. These may naturally differ from those described in detail. For example, the shape of the radiating element of the antenna as well as the number and location of the fuses may vary. In stead of the resonance frequency, the border frequencies of the operating band of the antenna can be measured in the tuning process, and decide on these grounds on the possible blowing of a fuse. The inventional idea can be applied in different ways within the limits defined in the independent claims 1 and 9.

Claims

1. A tunable antenna of a radio device, a radiator (1 10; 310; 410; 510; 610) of which is of conductor coating of a dielectric substrate (505; 605), characterized in that also at least one surface mounted fuse component (F1 , F2, F3; F4, F5, F6; 421 , 422; 521 , 522; F61 , F62, F63), connected galvanically to the radiator, is located on the substrate to change a physical shape and at the same time electric size of the antenna for tuning the antenna.

2. An antenna according to claim 1 , characterized in that the radiator (1 10) comprises a bend portion, on one side of which there is a starting portion of the radiator and on other side is a tail portion substantially parallel to the starting portion, and relatively close to the bend portion being at least one fuse branch (121 , 122, 123) comprising a fuse component, one end of which is connected to the starting portion and the other end to the tail portion.

3. An antenna according to claim 2, characterized in that the number of the fuse branches is at least two, and each branch comprises a fuse (F1 ; F2; F3) and in series with it a resistor (R1 ; R2; R3) to set a voltage needed for blowing the fuse, the resistor (R1 ) with lowest resistance being in the branch which is located farthest from said bend portion.

4. An antenna according to claim 2, characterized in that the number of the fuse branches is at least two, and each branch comprises a fuse (F4; F5; F6) with sensitivity different than the sensitivity of other fuses, in which case the time needed for blowing a fuse with a certain current is different for each fuse, the most sensitive fuse (F4) being in the branch, which is located farthest from said bend portion.

5. An antenna according to claim 1 , characterized in that the radiator (310) comprises bend portions so that it forms a meander pattern, and close to at least one bend portion there is a fuse branch (321 ; 322) with a fuse component, which branch shortens in conductive state said bend portion.

6. An antenna according to claim 1 , characterized in that it is a PI FA, a radiating plane (510) of which comprises a first (511 ) and second (512) part, which are coupled capacitively to each other in a coupling point, and the radiating structure further comprises at least one fuse branch (521 , 522) with a fuse component, one end of which branch is connected to the first part (51 1 ) and the other end to the second part (512).

7. An antenna according to claim 6, characterized in that the PIFA is a dual- band one, because its radiating plane (510) comprises, viewed from its short- circuit point (SP), two branches with different lengths, and said first part (51 1 ) comprises a shared starting portion of the longer (B1 ) and shorter (B2) branch and open end of the shorter branch, and said second part (512) only comprises the most of the longer branch, said coupling point being located at an edge of the radiating plane so that the longer branch (B1 ) is as long as possible measured via the coupling point, and each fuse branch being located relatively close to the coupling point, in which case a tuning implemented by the fuse components regards only a lower operating band of the antenna.

8. An antenna according to claim 1 , characterized in that said at least one fuse component (421 , 422) is/are close to electrically outermost end of a monopole type radiator (410) in series with the radiator conductor.

9. A method for tuning an antenna of a radio device, in which method at least one resonance frequency of the antenna structure is measured and physical shape of a radiator of the antenna is changed in order to set said resonance frequency, characterized in that to change the physical shape of the radiator, a conductivity state of a fuse component between two parts of a conductor of the radiator is changed (805), the resonance frequency is again measured (801 ) and its correctness is checked (802), and these steps are repeated, if the resonance frequency is not right with an adequate accuracy and there are more than one fuse component between the parts of the radiator conductor.

10. A method according to claim 9, characterized in that each fuse component is a fuse, and changing its conductivity state means blowing the fuse, or melting fuse conductor broken, and when more than one fuse is needed to be blown, they are blown in a determined order.

11. A method according to claim 10, characterized in that to determine the blowing order of the fuses, each fuse (F1 , F2, F3) has a serial resistance different to other serial resistances, and in the method

- a tuning voltage connected across fuse circuits is first set so that only the current of the fuse with lowest serial resistance (R1 ) exceeds a limit, in which the fuse blows - when needed, the tuning voltage is raised so that the current of the fuse with the next lowest serial resistance (R2) exceeds a limit in which the fuse blows, and

- when needed, the previous step is repeated.

12. A method according to claim 10, characterized in that to determine the blowing order of the fuses they (F4, F5, F6) are different by sensitivity, or the time needed for blowing a fuse with a certain current is different for each fuse, and in the method

- a current is led to the fuses first for such a time that only the most sensitive fuse (F4) blows - when needed, a current is led to the remaining fuses for such a time that the next sensitive fuse (F5) blows

- when needed, the previous step is repeated.

13. A method according to claim 12, characterized in that in each blowing stage the time, for which the current is led, is based on values specified to the fuses.

14. A method according to claim 12, characterized in that in each blowing stage the magnitude of the current is monitored, and the tuning voltage is switched off when a drop is detected in the magnitude of the current.