FI89429C - Duplex filters - Google Patents

Description

1 89429

Duplex filters

The invention relates to a method according to the preamble of appended claim 1.

5 Radio communication equipment mainly uses two different modes of transport, ie simplex or duplex. In simplex traffic, traffic takes place in parallel between stations communicating with each other using either one frequency (single-frequency simplex traffic) or two frequencies (dual-frequency simplex traffic). A characteristic of simplex traffic is that when a station transmits, it cannot simultaneously listen to the counter station or stations. In duplex traffic, on the other hand, transmission and reception take place simultaneously using different frequencies. This is made possible by selecting the frequencies used to deviate sufficiently from each other to make it possible to construct a filter to distinguish signals of different frequencies. Especially in fixed installations (so-called base stations), different antennas for transmission and reception are used to improve the difference. In vehicles and handsets, the difference always has to be handled by the so-called with a duplex filter using one antenna.

As is known, a duplex filter is a crossover filter consisting of two parts, namely transmitter and receiver filters, which are arranged together at the antenna port or separately if the transmission and. different antennas are used for reception. The fit of the receive filter should be as good as possible and the emission attenuation as low as possible at the receive frequencies, with typical emission attenuation values of the order of 3 ... 5 dB. At transmitter frequencies, the attenuation of the receive filter should be high, with a reference value of> 65 dB to avoid spurious repetitions caused by the receiver's high-frequency amplifier and mixer. The receive filter-35 is generally implemented as a bandpass filter. Transmitter 2 89429 The filter must attenuate the power to the antenna as little as possible. High attenuation means not only heating of the filter, but also a higher need for transmitter power and thus also higher power consumption and a higher battery life of 5 or less. In addition to the useful signal, the transmitter also generates noise at the receiving frequencies. If this noise enters the antenna port and through it to the receiver, it "masks" the received signal, i.e. the sensitivity of the receiver deteriorates when the transmitter is turned on. Therefore, the reception frequencies of the attenuation of the transmitter filter should be 40 ... 70 dB depending on the quality and power of the transmitter. A large attenuation difference implemented with small resonators causes an attenuation variation in the passband that can be up to 1.5 dB. It must be omitted by adjusting the output power of the transmitter according to the frequency used, which complicates the transmitter and increases costs.

In practice, the implementation of a duplex filter becomes more difficult the higher its feasibility number S, 20 for which the formula applies:

f o B

(1) S = where D Q0 25 f0 is the operating frequency, B is the frequency difference between the extreme channels, i.e. the bandwidth (i.e. the tuning range), D is the difference between the transmission and reception frequencies, i.e. the so-called 30 duplex interval, and Q0 is the Q value of the resonators.

Operating frequency, bandwidth, or tuning range, and duplex interval are system constants, the choice of which is influenced by, for example: 35 - available frequencies, 3 89429 - traffic volume, especially when designing public networks based on estimation (several public networks have had to increase channels, ie bandwidth B is higher than initially planned), and - the perception at the time of establishment of the technical feasibility.

Usage changes and user requirements over the life of the system are usually unpredictable. Equipment manufacturers strive to compete for user popularity by developing their products to be more user-friendly. In the case of public networks, the pressure to develop is in particular: - smaller size, 15 - lightness and longer operating time in portable devices with a single battery charge, and - lower cost.

The feasibility number S of the duplex filter shown above is substantially affected by the resonator losses 20, i.e. the Q-value of the resonator, which depends on the volume of the resonator for all types of resonators. This means that the feasibility deteriorates as the space used decreases.

In addition to the Q value of the resonators, the properties of the filters are also affected by their topology and pole number (number of resonators). The emission attenuation of bandpass filters is typically greater than the emission attenuation of low or high pass filters. In the transmitter branch of duplex filters, the 30 special applications of the latter are commonly used, the so-called a trap or notch filter that provides low pass attenuation, high blocking attenuation in a narrow frequency band, and high edge steepness, but only either above or below the pass band. At other frequencies, the attenuation remains modest and special solutions have to be used.

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Looking at the feasibility number S shown in formula (1) above, it is observed that the system constants seem to leave little room for the designer / manufacturer of the filter: the Q value. However, in accordance with the development pressures described above, the device manufacturer 5 wants an even smaller space, which means an even lower Q value, i.e. (for the filter designer / manufacturer) practically fewer margins and thus higher manufacturing costs. However, as stated above, the device manufacturer wants an even cheaper price for his device.

However, looking more closely at the feasibility number S, it is stated that only the operating frequency f0 and the duplex interval D are absolute constants. Namely, the tuning area, i.e. the bandwidth 15, can be reduced by dividing the operating band into several sub-areas Β ^ ,. Β ,, so that the sub-areas together cover the used frequency band B (8 = 6 ^ 82 + 83 +. · · + Βη) · higher feasibility rate in each area. From the feasibility number S also depends on how many resonators are needed to implement a filter with certain blocking and emission attenuation characteristics, i.e. how "steep" the filter is. The number of resonators in the filter grain is an integer, so in the boundary regions the filter can be implemented as an n- or n + 1-pole. However, the emission attenuation increases exponentially as the pole number (number of resonators) increases exponentially, which means that a higher Q value, i.e. larger resonators, is required for the same emission attenuation when a higher pole number is used. With the smallest possible number of 30 poles, i.e. the smallest possible number of resonators, the filter becomes smaller in size and simpler and cheaper.

The tuning principle based on the subdivision described above has been used to improve the performance of the filter, especially in military devices where mechanical and often automatic tuning is performed by a servo or stepper motor equipped with electronic control. In addition, there is a duplex filter for devices in a car telephone network, such as the NMT-450, in which both the transmitter and receiver filters are tunable by a stepper motor and in which the tuning range is divided into eight sub-areas. The stepper motor thus has eight stages, and a notched shaft running through the entire filter is used as the tuning member, where the notches act as weather capacitors. The shaft is grounded with multiple springs, and both the shaft and springs have had to be gilded to avoid contact problems.

As can be seen from the above, the known devices based on the division of the tuning range are very complex and thus also difficult and expensive to manufacture. Thus, they have mainly been used only in applications where the price of the device is not a determining factor (e.g. military equipment). The known filter for civil use described above is not only purpose expensive, also 20 prone to malfunctions.

It is therefore an object of the present invention to take advantage of the subdivision-based tuning principle so that the apparatus can be considered as simple and cost-effective as possible, i.e. an optimal solution between the benefit of the tuning principle and the manufacturing cost is achieved. This is achieved by a filter according to the invention, which is characterized by what is described in the characterizing part of the appended claim 1.

The basic idea according to the invention is to divide the tuning area into only two, or at most three, sub-areas, in which case the the greatest relative benefit is obtained from the transition to the principle and in which case the device structure, namely the tuning mechanism, can still be considered simple.

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The solution according to the invention provides an economically viable and reliable solution, especially for a duplex filter, also for general cellular networks in which the development pressures are in accordance with the above.

According to a preferred embodiment of the invention, the sub-area-based tuning principle is used in a duplex filter only in the transmitter filter where it is most useful. This simplifies the duplex filter because the tuning mechanism is not required in the 10 receiver filters.

According to another preferred embodiment, the excitation means comprise an electromagnet. By using an electromagnet, the tuning mechanism itself is made very simple.

The invention will now be described in more detail with reference to the example of Figure 1 below, which shows a perspective view of a magnetically tunable transmitter filter of a duplex filter according to the invention.

The magnetically tunable transmitter filter of the duplex filter shown in Fig. 1 comprises two parallel transmission line resonators 10 and 20, each of which is formed in a manner known per se from a transmission line 11 formed by a straight metal wire, respectively. 21, surrounded by a metal or metal-coated housing 30 (cover 25 not shown). There is a housing partition 31 between the resonators.

Attached to the outside of the housing is an electromagnet 1 which is controllable between two or three positions, i.e. between two extreme positions and possibly a central position between them. Connected to the electromagnet is a transfer member 30 which moves under the control of an electromagnet. The transfer member in this case consists of a rod 2 connected at one end to an electromagnet and passing through a transmitter filter (both resonators). The electromagnet to move the rod 1 2 of the arrow A-35 in accordance with a back and forth two or three indirect excitation position 7 b 9 429 method. For tuning, each resonator is provided with a substantially U-shaped, flat-type spring 12 and 22, respectively, in the longitudinal direction of the rod 2. Attached at one end to the housing 30 and at the other end to the rod 2. As the rod 2 moves under electromagnetic control, each spring moves closer or farther. the corresponding transmission line, whereby the excitation capacitance between the open end of the transmission line and the spring changes. This change, in turn, causes the resonant-10 frequency of the resonator to change as desired, tuning the filter to either sub-area Bx or B2.

The electromagnet 1 is controlled to different positions (extreme positions and possibly the middle position) in its simplest way by switching the control voltage of the magnet between the ON and 15 OFF states under the control of the channel control of the device. Since the use of channel control in tuning control is known per se and does not fall within the scope of the actual inventive idea, it is not shown in more detail.

The resonators 10 and 20 are connected to the structure surrounding the transmitter filter in a manner known per se, which is why the connections are not shown separately.

As a practical performance example, based on computer simulation, prototypes have been made of the transmitter branch of the NMT-450 duplex filter using both 25 conventional solutions (n = 1, i.e. the entire tuning range without subdivision) and using the structure described above by dividing the tuning range into two subdivisions (n = 2 i.e. electromagnet two positions). The comparison shows the following: 30 n = l n = 2 resonators: 3 2 emission attenuation: 1.8 dB 1.1 dB transmitter frequency.

attenuation variation: 1 dB 0.2 dB

blocking attenuation: 58 dB 63 dB receiver frequency

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In comparison, the different number of resonators is due to the transition to the subdivision principle; the required steepness characteristics can be achieved by omitting one resonator. The resonators are of the same size, so that the filter according to the invention 5 can accommodate 2/3 of the space required by the indivisible. The space required by the electromagnet 1 is substantially smaller than the space required by the third resonator. The 0.7 dB emission attenuation difference that appears in the comparison means, for example, in a NMT-450 network where the power to the antenna must be 15 W, that the heat-converting power is reduced by 8.45 W (with an efficiency of 40%) when using the device according to the invention. . As the size of the equipment decreases, the arrangement of cooling can become a decisive factor, so a reduction in the power converted into heat is very important.

Similar computer simulations show that switching to three components further reduces emission attenuation, but not as much as moving from one to two components.

20 There is no obstacle to applying the division principle to a receiver filter as well. However, since the main function of the receiver filter is to prevent irrelevant signals from entering the receiver, the number of circuits cannot be reduced, and the benefit of division 25 is not achieved as in a transmitter filter. For this reason, it is advantageous to use the principle according to the invention only in the transmitter filter. Since the receiver filter can be implemented either in the same way as the transmitter filter described above, or in a manner known per se, the receiver filter is not shown separately.

Although the invention has been described above with reference to the example according to the accompanying drawing, it is clear that the invention is not limited thereto, but can be modified in many ways within the scope of this inventive idea of the appended claims. Thus, although the accompanying example 9 89429 relates to a duplex filter, the solution according to the invention can be used for all high-frequency filters. The tuning mechanism formed by the electromagnet, the rod and the springs can also be of many types. First, the metal or metal-coated springs acting as excitation bodies can be replaced, for example, by a dielectric piece made of a material having a higher dielectric constant than air, which is moved in the resonators. Such a dielectric piece 10 affects the effective dielectric constant of the resonator and thus changes the frequency as well as the metal spring. Secondly, the electromagnet can be replaced, for example, by any suitable electromechanical transducer, such as a piezoelectric or magnetostrictive transducer. In principle, an electromagnet could be replaced by a transducer which is not connected to a mechanical transfer member, but which uses an electric or magnetic field to change the dielectric constant of a stationary dielectric piece inside the resonator and thus change the resonance-20 frequency as a moving but fixed dielectric constant. piece. Thus, when referring to a transducer in the appended claims, it is to be understood that it includes not only the electromagnet shown in the example of the figure, but also the other alternatives described above. In principle, the transmitter filter can also have several transducers, but the simplest solution is achieved in that one converter tunes each resonator simultaneously. In principle, tuning could also be performed, for example, with PIN or capacitance diodes, but the problem with these solutions is the nonlinearity of components at high power levels. The type of resonators in the filter can also be changed within the scope of the inventive idea.

Claims (7)

10 8 9 42 9 A duplex filter comprising separate transmitter and receiver filters for separating transmitter and receiver signals, and means (1, 2, 12, 22) for tuning the filter to a plurality of sub-areas (B1... Bn) which together covering the entire tuning area (B), characterized in that at least the tuning means of the transmission filter comprise a converter (1) controlling the resonators (10; 20), which is controllable between a maximum of three output positions. Duplex filter according to Claim 1, characterized in that the transducer consists of an electromagnet (1). Duplex filter according to Claim 1 or 2, characterized in that at least one transfer element (2) is connected to the transducer, which mechanically transfers the excitation body (12; 22) in each resonator (10; 20). A duplex filter according to claim 3, characterized in that the excitation body is a spring (12, 22) which acts as a second plate of the capacitor, changing the excitation capacitance between the spring and the transmission line (11; 21) of the resonator (10; 20). Duplex filter according to Claim 3, characterized in that the excitation body is a dielectric piece. A duplex filter according to claim 3, 4 or 5, characterized in that the transfer member consists of 30 rods (2) to which the tuning body (12; 22) of each resonator (10; 20) is attached, the rod displacing each resonator ( 10; 20) the tuning piece (12; 22) simultaneously. A duplex filter according to claim 1, characterized in that the transducer is a transducer providing an electric and / or magnetic 89429 magnetic field, and in that each resonator has a stationary dielectric piece whose dielectric constant is changed by said electric and / or magnetic field. . 12 8 9 42 9