FR2673780A1 - METHOD FOR TUNING A FILTER AND FILTER FOR IMPLEMENTING THE METHOD - Google Patents

Abstract

According to this method, a tuning range is divided into elementary areas which together cover the entire tuning range, and the filter is tuned separately in each elementary area. So that the equipment remains as simple as possible and so that its manufacturing cost is also reduced, the tuning range is divided into a maximum of three basic zones. To this end, the tuning device incorporated in the filter comprises a converter (1) which controls the resonators (10; 20), the converter being controlled between three positions at most. Application to emission filters of transceiver equipment.

Description

The present invention relates to a method of tuning a filter and

  In this method, the tuning range is divided into elementary zones which together cover the entire tuning range, and the filter is tuned separately in each elementary zone. The filter according to the invention itself comprises a device. for tuning the filter in a plurality of elementary zones which together cover the entire tuning range The method according to the invention essentially applies to a dual filter comprising a transmission and reception filter for separating the transmitter signals and Receiver

one another.

  Radio communication installations

  basically use two different modes of operation

  Simple operation and dual or duplex operation In simple operation, transmission is performed between stations that communicate in one direction at a given time, using a single frequency (simple single frequency operation) or dual frequency (single dual frequency operation) A characteristic feature of simple operation is that a station can not

  simultaneously transmit data and listen to one or more

  other stations communicating with it

  milk, transmission and reception are simultaneous at different frequencies during double or duplex operation This is possible because the frequencies used are selected so that they differ sufficiently from each other to allow the construction of a filter allowing the separation of signals at different frequencies In a particular fixed installation (called a "base station"), separate antennas are used for transmission and reception, so that the separation is better In vehicles and mobile telephones, the separation must be always be performed by a double filter, using a single antenna. In known manner, the double filter is a separator filter comprising two parts, that is to say a transmission filter and a reception filter which are combined with an antenna input or which are separated when different antennas are connected. used for transmission and reception The adaptation of the receiver filter should be as good as possible and the attenuation in the bandwidth should be as low as possible at reception frequencies, examples of attenuation values in the bandwidth being of the order of 3 to 5 d B At the transmission frequencies, the attenuation of the receiver filter must be high, the nominal value being greater than 65 d B so that parasitic responses caused by the amplifier and the mixer to high frequencies of the receiver are avoided The receiver filter is usually a bandpass filter The emission filter must attenuate as little as possible the power applied to the antenna ne High attenuation not only causes the filter to overheat but also requires greater power

  for the transmitter and therefore causes greater consumption.

  energy consumption and the use of a larger battery.

  aunt or a shorter operating time In addition

  of the desired signal, the transmitter also creates noise

  If this noise reaches the antenna input and thus the receiver, it "masks" the received signal, ie the sensitivity of the receiver is impaired when the transmitter is turned on. , the attenuation of the transmission filter must be between 40 and 70 dB at reception frequencies, depending on the quality and capacity of the transmitter.

  high difference in attenuation achieved by resonating

  of small size causes a variation of attenuation

  up to 1.5 dB in the bandwidth. It must be compensated for by adjusting the output power provided by the transmitter according to the frequency used, so that

  the transmitter is complicated and the cost is increased.

  In practice, the difficulty of producing the double filter increases with its feasibility index S which corresponds to the following formula: S = f OB / DQQ (1) where f O is the operating frequency, B being the frequency difference between the channels extreme, that is to say

  the bandwidth (or tuning range), where D is the difference

  between the transmit and receive frequencies,

  that is, the duplex interval, and Q O being the coeffi-

Q resonators.

  The operating frequency, the bandwidth or tuning range and the duplex processing interval are system constants whose selection depends in particular on: the available frequencies, the traffic load, which is based on an estimate, especially in the realization of public networks (it was necessary to add channels to many networks intended for public use, that is to say that the bandwidth B is greater than that which was initially planned ), and technical possibilities considered as

  usable at the time of establishment.

  Variations of the operating modes during the service life of the installation and the conditions set by the users are usually impossible to predict Equipment manufacturers tend to compete by producing more and more products

  easy to use In public networks, the properties

  following subjects were considered particularly

desirable:

  a small size, a low weight and a long operating time before replacement of the battery in a portable apparatus, and

a reduced price.

  The aforementioned feasibility index S of the double filter is significantly affected by the losses in the resonator, ie the Q coefficient of the resonator which depends on the

  resonator volume for all types of resonators.

  This means that the feasibility decreases

  look for a smaller dimension.

  In addition to the Q coefficient of the resonators,

  properties of the filter are also affected by their topo-

  and their number of poles (number of resonators).

  The attenuation in the bandwidth of the pass filters

  band is usually greater than that of the low-pass or high-pass filters The branch of the double-filter thus often uses a particular modification of the aforementioned filters, called the "band-eliminating filter", which gives a low attenuation in the bandwidth , high attenuation in a narrow band of frequencies, and a steep slope, but

  only above and below the bandwidth.

  At other frequencies, the attenuation remains low and it is

  necessary to use special provisions.

  Like the feasibility index S of formula (1), the constants of the installation do not seem to leave much freedom for the designer and the manufacturer of the filter, apart from the Q coefficient. As described above, the manufacturer of the apparatus however, wants to reduce the size of the dual filter, so that the Q coefficient is reduced In practice, this means that the margins available for the designer and the manufacturer are

  reduced so that manufacturing costs are increased.

  As mentioned above, the equipment manufacturer

  may, however, want to reduce the cost of fitting.

  When looking more closely at the index of

  It appears that only the operating frequency

  This is due to the fact that the tuning range, ie the bandwidth B, can be reduced by division of the working band. in many

  elementary zones B to B so that the elemen-

  1 together cover the frequency band B to be used (B = Bl + B 2 + B 3 + Bn) In this way,

  the feasibility index is increased in each area

  The feasibility index S also determines the number of resonators necessary for producing a filter having predetermined attenuation properties in a bandwidth and in a stopping band, ie a "gradient". The number of resonators contained in the filter is an integer, and the filter can thus be made with N or N + 1 poles in the limit areas. However, the attenuation in the bandwidth increases exponentially with increasing number of poles (number of resonators), and this means that, to achieve the same attenuation in the bandwidth, the

  coefficient Q must be increased, ie the resonances

  If the number of poles increases For the smallest possible number of poles, i.e. the smallest possible number of resonators, the filter has a smaller dimension and is simpler and

cheaper.

  The principle of agreement based on division

  above mentioned in elementary zones has been used for the improvement

  performance of the filter, especially in

  military regrets in which the mechanical agreement and often the automatic tuning are realized by an engine

  step-by-step or servo control with an electronic

  In addition, a dual filter is commercially available which is intended for the apparatus of a mobile telephone network, such as "NMT-450" and in which the transmit and receive filters may be tuned by a motor not to not and the chord range is divided into

  eight elementary zones Thus, the stepper motor

  carries eight steps, and the rotation device is a notched shaft disposed throughout the filter, the notches acting as adjustment capacitors The shaft is grounded by a number of springs, and the shaft and the springs must be coated with gold so that the contacts do not

do not pose any problems.

  As indicated in the foregoing presentation, the apparatus of the prior art, implementing the division of the tuning range, are very complicated and therefore have a difficult and costly fabrication. As a result, they have been used essentially in applications in which the price of the equipment

  is not essential (such as military equipment

  The known filter described above and intended for civil applications is not only expensive in these particular applications but also sensitive to

  disturbances during operation.

  The present invention relates to the use of the

  principle of an agreement based on the division into

  so that the equipment remains as simple as

  possible and advantageous from the point of view of manufacturing costs

  cation, that is to say that an optimum solution is obtained between the advantages derived from the agreement principle and the manufacturing costs. This compromise is obtained by implementing a method according to the invention which is characterized in that the tuning range is divided at most into three elementary zones (1 <N <3) The filter according to the invention is itself characterized in that the tuning device comprises a converter controlling the resonators, the converter can be controlled between three positions

maximum output.

  The invention is based on the discovery that, in dual filters, when using the subdivision principle, the greatest relative advantage is usually obtained when the number of elementary zones is 2 or at most 3, so that the apparatus, especially the tuning mechanism, still has a simple structure. The invention relates to a cost-effective and operationally reliable solution, in particular for dual filter which is also incorporated into the public cellular network which tends to develop, in the manner

previously described.

  In the preferred embodiment of the invention, the tuning principle using elementary zones is used in the dual filter only in the transmission filter in which it gives the greatest advantage. double filter is simpler than before because the tuning mechanism is not

  not necessary in the reception filter.

  In another preferred embodiment, the

  tuning device comprises an electromagnet

  An electromagnet makes the tuning mechanism very simple.

  Other features and advantages of the invention

  will be better understood by reading the description

  which will follow of an exemplary embodiment of the invention, with reference to the accompanying drawing in which the single figure is a perspective view of a magnetically tunable emitter filter belonging to a double filter

according to the invention.

  The magnetically tunable transmission filter belonging to the double filter shown in FIG. 1 comprises two parallel resonators 10 and 20 with a transmission line respectively, formed of a straight wire and surrounded by a housing 30 which is formed of metal. or which is coated with metal (the lid is not shown) A partition wall 31 of the housing is

placed between the resonators.

  An electromagnet 1, which can be controlled between two or three positions, that is to say two extreme positions and an optional intermediate position, is fixed to the outside of the housing. A displacement member that moves while being guided. In this particular case, the displacement member comprises a rod 2 connected at one end to the electromagnet and arranged in the entire emission filter (its two resonators ) The electromagnet moves the rod 1 alternately between the two or three tuning positions as indicated by the arrow A. When tuned, the two resonators have a leaf spring 12 and 22 respectively

  which has a practically U-shape in the longitudinal direction

  tudinal of the rod 2 and which is attached to a first

  end to the housing 30 and at the other end to the rod 2.

  When the rod 2 moves while being guided by the electro-

  magnet, the two springs move toward or away from the corresponding transmission line respectively so that the tuning capacitance formed between the open end of the transmission line and the spring varies. This change causes the desired change in frequency of resonance of the resonator so that the filter is tuned to the elementary zone Bl or B 2 * The electromagnet, in the simplest case, goes through the different positions (the extreme positions and possibly the intermediate position) by connection of the control voltage of the electromagnet between the states of presence and absence under the control of the channel control member of the apparatus As the use of a channel control for the tuning is already known and does not fall within the scope of the invention, it is not described

not in more detail.

  The resonators 10 and 20 are connected in known manner to a structure surrounding the emission filter, and

  the connections are not represented separately.

  In an illustrative performance test obtained in practice, prototypes were made by computer simulation for the emitter branch of an NMT-450 dual filter, both using a conventional approach (n = 1, c ie the whole chord range without subdivision) and by use of the above-mentioned structure

  whose tuning range is divided into two elemen-

  (n = 2, that is to say that the electromagnet has two positions) The comparison was as follows: n = 1 N = 2 Number of resonators 3 2 Attenuation of the bandwidth 1.8 d B 1.1 d B * Variation of attenuation 1 d B 0.2 d B Attenuation in the stopband 58 d B 63 d B ** * Frequency of transmission ** Reception frequency In this comparison, the difference in the number of resonators is due to the transition to the principle of subdivision; the necessary properties of slope can be obtained by omission of a resonator The resonators have the same dimension and the space required in the filter according to the invention is therefore equal to both

  third of the space needed with a filter without

  The space required for the electromagnet 1 is much smaller than that of the third resonator. In the NMT network for example, when the power transmitted to the antenna must be 15 W, the difference in attenuation of

  bandwidth of 0.7 d B appearing from the comparison

  its indicates that the use of the apparatus according to the invention reduces the amount of energy transformed into heat of 8.45 W (with a yield of 40%) When the size of the apparatus is reduced, the provision of cooling can become decisive so that the

  reduction of power transformed into heat has an important

vital importance.

  We can show, by a similar simulation on

  computer, that a further reduction in the attenuation

  bandwidth occurs when three elementary zones are used, although with an amplitude that does not reach that of the transition of a zone with two

elementary areas.

  The principle of subdivision can be applied to the receiver filter As the main function of the receiver filter is however to prevent the propagation of unwanted signals to the receiver, the number of circuits can not be reduced and, thus, the division

  is not as advantageous as in the emission filter.

  For this reason, it is preferable to use the principle of the invention only in the transmission filter. As the reception filter can be realized as the transmission filter described above or in a known manner,

  is not shown separately.

  Although the invention has been described with reference to the example shown in the drawing, it is obvious that

  the invention is not limited to this embodiment.

  Thus, the invention can be applied to all high frequency filters, although the previous example relates to a

  dual filter The tuning mechanism including electro-

  magnet, the rod and the springs can be of very different type Firstly, the springs formed of metal or coated with metal and acting as tuning device can be replaced for example by a dielectric member formed of a material having a constant dielectric greater than

  that of air and which is moved between the resonators.

  Such a dielectric member affects the dielectric constant

  effective resistance of the resonator and thus

  frequency in the same way as the metal spring.

  Then, the electromagnet can be replaced for example by an electromagnetic converter suitable for this purpose, for example a piezoelectric or magnetostrictive converter In principle, it is possible to replace the electromagnet by a converter to which no mechanical member of displacement is fixed but which varies the dielectric constant of a fixed dielectric member in the resonator, by an electric or magnetic field, so that the resonance frequency varies in the same way as in the case of a dielectric member mobile device with a fixed dielectric constant.

  converter is indicated in the appended claims,

  it should be noted that it designates not only the electromagnet described in the example shown in the figure but also in the previous variants The transmission filter may itself comprise several converters; the simplest solution is obtained however so that a

  converter simultaneously grants all the resonators. In principle, the tuning can also be achieved for example with PIN or capacitive diodes, but a problem posed by these components is that they are not linear for high energy levels. The type of resonator incor-

  poré to the filter may also vary within the scope of the invention.

Claims (8)

  A method of tuning a filter, wherein a tuning range is divided into elementary zones which together cover the entire chord range, and the filter is tuned separately over each elementary zone, characterized in that the chord range is agreement is divided into three   elementary zones at most (1 <N 3).   The method of claim 1, wherein the filter is a dual filter comprising separate transmit and receive filters for separating the transmitter and receiver signals from each other, characterized in that the division into zones elementary and the agreement   are made only in the transmission filter.   3 Filter, comprising a device (1, 2, 12, 22)   to filter in several basic areas.   comprising the converter device (1) controlling resonators (10; maximum output.   Filter according to claim 3, characterized in   the converter is an electromagnet (1).   Filter according to Claim 3 or 4, characterized in that at least one displacement member (2) is connected to the converter so that it mechanically displaces a tuning device (12; 22) placed in each resonator (10; 20).   Filter according to claim 5, characterized in that the tuning member is a spring (12, 22) acting as an electrode of a capacitor, so that the capacitance   of agreement varies between the spring and a transmission line.   sion (11; 21) of the resonator (10; 20).   Filter according to claim 5, characterized in that   what the tuning device is a dielectric member.   Filter according to any one of the claims   at 7, characterized in that the displacement member comprises a rod (2) to which the tuning device (12; 22) of each resonator (10; 20) is fixed, the rod simultaneously moving the tuning devices (12; 22) of all the resonators (10; 20).   9 filter according to claim 3, characterized in that the converter is a converter creating an electric or magnetic field at least, and a fixed dielectric member is placed in each resonator, the electric or magnetic field at least varying the   dielectric constant of the dielectric member.