Patent Abstract:
In a receiver, provision is made of a tuneable switchable bandpass filter comprising a primary resonant circuit and a first secondary resonant circuit. A second secondary resonant circuit can be switched into a part of the first primary resonant circuit, thereby producing a second primary resonant circuit. By virtue of the embodiment of the primary resonant circuit, the bandpass filter is switchable in terms of frequency range with little outlay on circuitry and, on account of the individual embodiment of a first and second secondary resonant circuit with individual coupling elements, can be optimized with regard to a constant amplitude response.

Full Description:
This application claims the benefit under 35 U.S.C. § 119 (a) of German patent application No. 10300892.6 filed Jan. 13, 2003. 
   FIELD OF THE INVENTION 
   The invention relates to a switchable tuneable bandpass filter in accordance with the preamble of the first claim. 
   BACKGROUND OF THE INVENTION 
   In receivers which have to be tuned over a wide frequency range, such as e.g. the radio-frequency receivers in radio or television tuners, the reception range is divided into a plurality of subranges. 
   Present-day customary television receivers have to be able to receive signals in a range from 48 MHz to 860 MHz. The properties of components of the kind usually used in receivers lead to a limitation of the tuning range that can be achieved. The entire frequency range that can be achieved by a television receiver is therefore divided e.g. into three ranges. In this case, a first range extends from 48 to 150 MHz, a second range extends from 150 to 430 MHz, and a third range extends from 430 to 860 MHz. The tuning over the three ranges is carried out by means of a three-band or a switched two-band concept in modern receivers. 
   In the case of the three-band concept shown in  FIG. 1 , the signal received by an antenna is conducted onto three parallel branches B 1 , B 2  and B 3 . Each branch is designed for a frequency range and comprises an input filter EF 1 , EF 2  and EF 3 , an input amplifier A 1 , A 2  and A 3 , as well as a bandpass filter BF 1 , BF 2  and BF 3 . The output signals of each of these branches are passed to a respective mixer MI 1 , MI 2 , and MI 3 , which is assigned a respective oscillator O 1 , O 2  and O 3 . An intermediate frequency can be tapped off in each case at the output of the mixers. 
   In order to reduce the circuitry outlay, a three-band concept in which there are only two mixers and oscillators is also employed. In the case of this concept shown in  FIG. 2 , one frequency range, e.g. B 3 , is connected to a mixer MI 3  and an oscillator O 3  as in the three-band concept described above. The two remaining frequency ranges B 1  and B 2  are firstly conducted, as in the three-band concept described above, via a respective individual branch with input filters EF 1  and EF 2 , input amplifiers A 1  and A 2  and bandpass filters BF 1  and BF 2 , wherein at the output of the bandpass filters a switch S feeds in each case one of the two branches to a common mixer MI 1  and oscillator O 1 . 
   A further reduction of the circuitry outlay can be achieved by using a switched two-band concept as shown in a diagrammatic illustration in  FIG. 3 . As in the two concepts described above, the switched two-band concept has an individual branch comprising input filter EF 3 , input amplifier A 3  and bandpass filter BF 3  for one frequency range, e.g. B 3 , which is passed to a dedicated mixer MI 3  and oscillator O 3 . The two remaining frequency ranges B 1  and B 2  are passed via a common branch comprising input filter EF 1 / 2 , input amplifier A 1  and bandpass filter BF 1 / 2  and fed to a dedicated mixer MI 1  and oscillator O 1 . The properties of the input filter EF 1 / 2  and the bandpass filter BF 1 / 2  are switchable in the case of the switched two-band concept. As a result, the filters can be adapted to the frequency range respectively selected. 
   The switchable bandpass filter is embodied as an inductively reference-point-coupled two-circuit bandpass filter and is illustrated in a simplified embodiment in  FIG. 4 . A signal S passes from an antenna (not illustrated in  FIG. 4 ) and an input filter (likewise not illustrated) via the input amplifier A 1  to a first primary resonant circuit. The first primary resonant circuit comprises the variable capacitance C 1  and the series circuit formed by the inductances L 1  and L 2 . The switches S 1  and S 2  are open in this case. One connection of the inductance L 2  is connected to a first connection of an inductance L 3  and completes the series circuit of the first primary resonant circuit. The second connection of the inductance L 3  is at a fixed potential. A first secondary resonant circuit comprising the series circuit formed by the inductances L 4  and L 6  as well as a variable capacitance C 2  is likewise connected to the first connection of the inductance L 3 , so that a T-shaped arrangement is produced overall. The coupling elements RK and CK are connected to the output-side end of the bandpass filter, which coupling elements connect the bandpass filter BF 1 / 2  to the mixer MI 1 . In order to switch the frequency range of the bandpass filter BF 1 / 2 , the two switches S 1  and S 2  may be closed. The inductances L 2  and L 4  are thus short-circuited and the circuit has a new common reference point inductance L 5 . In this case, the inductance L 5  is expediently chosen to be smaller than the inductances L 2  and L 4 , so that the components connected to these elements can subsequently be disregarded. In one exemplary embodiment, the values of the inductances differ by the factor three. The second primary resonant circuit thus produced then comprises the variable capacitance C 1  and also the series circuit formed by the inductances L 1  and L 5 . The second secondary resonant circuit comprises the series circuit formed by the inductances L 6  and L 5  and also the variable capacitance C 2 . The bandpass filter that is frequency-switched in this way furthermore remains connected to the coupling elements RK and CK and consequently connected to the mixer. The advantage of the lowest outlay compared with the first two concepts is opposed by the disadvantage that the coupling elements CK and RK can be set optimally only for one range. As a result, relatively large level differences can occur over the two frequency ranges and impair the properties of a tuner. 
   In order to avoid this effect, although it is possible for the coupling elements to be configured in switchable fashion, this nevertheless results in an increase in the circuitry outlay again. 
   For these reasons there is a need for a switchable tuneable bandpass filter which has a uniform level profile over the two frequency ranges with a low circuitry outlay. 
   SUMMARY OF THE INVENTION 
   The invention proposes for this purpose the switchable tuneable bandpass filter described below. Advantageous refinements of the invention are specified in the subclaims. 
   In the case of the construction of a tuner with a switchable tuneable bandpass filter according to the invention, the bandpass filter has a primary resonant circuit and two secondary resonant circuits. An input signal from an antenna passes to an input amplifier, from the output of which it is conducted to the primary resonant circuit. The primary resonant circuit is embodied in switchable fashion in such a way that in each case a first or second secondary resonant circuit can be selected. In a first switch position of the switch, a first primary resonant circuit comprises a first variable capacitance and also the series circuit formed by a first and a second inductance. The second inductance is connected to a first reference point inductance. The first secondary resonant circuit comprises a third inductance, which is likewise connected to the common first reference point inductance, and a second variable capacitance. The output of the first secondary resonant circuit is connected to a mixer by means of first coupling elements. The series circuit formed by the first and second inductances in the first primary circuit expediently has the same inductance as the third inductance in the first secondary circuit. This advantageously results in the desired T-shaped filter configuration. 
   In a second switch position, a second primary resonant circuit comprises the first variable capacitance and the first inductance of the first primary resonant circuit described above. The now closed switch connects the first inductance to a second common reference point inductance. A second secondary resonant circuit, comprising a fourth inductance and a third variable capacitance, is connected to the second common reference point inductance. The output of the second secondary resonant circuit is connected to the mixer by means of second coupling elements in a known manner. Since the second common reference point inductance, which takes effect when the switch is closed, is lower in value than the remaining second inductance of the first primary resonant circuit in the first switch position, the elements connected to the circuit via the remaining second inductance can be disregarded. The values of the inductances differ by the factor three, for example, other expedient ratios also being possible. A second switch for disconnecting the unrequired part of the circuit is thus superfluous. 
   The invention advantageously makes it possible to achieve a uniform gain response over the two switchable frequency ranges without significantly increasing the overall outlay. 
   In no way the invention is restricted to the application in radio and television tuners; rather, it can be used wherever a large frequency range to be received is subdivided into a plurality of bands for the realization of a receiver. This may include, inter alia, satellite receivers or receivers for wireless data networks. 
   The invention is explained in more detail below with reference to the circuit sketches of an exemplary embodiment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the figures: 
       FIG. 1  shows the basic illustration of a three-band concept as is known from the prior art; 
       FIG. 2  shows the basic illustration of a three-band concept with two-band mixer/oscillator as is known from the prior art; 
       FIG. 3  shows the basic illustration of a switched two-band concept known from the prior art; 
       FIG. 4  shows a diagrammatic illustration of the practical embodiment of a bandpass filter according to the switched two-band concept from  FIG. 3 ; 
       FIG. 5  shows a diagrammatic illustration of an exemplary embodiment of the switched bandpass filter according to the invention; 
       FIG. 6  shows an exemplary circuit of the switchable bandpass filter according to the invention from  FIG. 5 . 
   

   In the figures, identical reference symbols designate identical or similar elements. 
     FIGS. 1 to 4  have been explained above in the description of the prior art. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 5  shows a diagrammatic illustration of a simplified practical embodiment of the switchable tuneable bandpass filter according to the invention. An input signal S passes from an antenna (not shown in the figure) via an input filter (likewise not illustrated in the figure) to an input amplifier A 1 . In a first switch position of a switch S 1 , the switch S 1  is open. The signal passes from the output of the input amplifier A 1  to a first primary resonant circuit. The first primary resonant circuit is formed from a variable capacitance C 1 , the series circuit formed by two inductances L 1  and L 2  and also an inductance L 3 . In this case, the inductance L 3  is a common reference point inductance for the first primary resonant circuit and a first secondary resonant circuit. In this switched position of the switch S 1 , the first secondary resonant circuit is formed by the inductance L 4  and the variable capacitance C 2 . In order to obtain the desired property of the bandpass filter, the inductance formed from the series circuit formed by the inductances L 1  and L 2  of the first primary circuit expediently has exactly the same value as the inductance L 4  of the first secondary resonant circuit. The inductance L 3  forms a so-called inductive reference point coupling for the two first resonant circuits. The first resonant circuits can be tuned by varying the capacitances C 1  and C 2 . The output of the first secondary resonant circuit is connected via coupling elements RK 1  and CK 1  to the input of a mixer MI, to which the signal is fed for further processing. 
   In a second switch position of the switch S 1 , the switch is closed. The second primary resonant circuit produced in this way is now formed by the variable capacitance C 1  and the inductance L 1 , which are connected to a new common reference point inductance L 5 . In this case, the inductance L 1  is a partial inductance of the first primary resonant circuit as exists when switch S 1  is open. The second secondary resonant circuit is formed by an inductance L 6  and a variable capacitance C 3 , which are connected to the common reference point inductance L 5 . The output of the second secondary resonant circuit is connected to the input of the mixer MI via coupling elements RK 2  and CK 2 . In order to obtain the desired property of the bandpass filter in this switch position as well, the inductance L 1  expediently has exactly the same value as the inductance L 6 . In the embodiment of the bandpass filter according to the invention, that part of the circuit which is not required when switch S 1  is closed is not disconnected. This is possible because the reference point inductance L 5  is very much smaller than the inductance L 2 . The components which are furthermore connected to the inductance L 2  and belong to the first primary and secondary resonant circuits can thus essentially be disregarded. The second resonant circuit can be tuned by varying the capacitances C 1  and C 3 . What has a particularly advantageous effect in this case is the fact that the coupling elements RK 1 , RK 2 , CK 1  and CK 2  can be optimized for the respective frequency range and, consequently, it is possible to achieve a constant amplitude response over both frequency ranges. 
   In an exemplary circuit of the switchable bandpass filter according to the invention in  FIG. 6 , a signal S passes from an antenna (not illustrated in the figure) via an input filter (likewise not illustrated in the figure) and via a coupling capacitor  5  to an input amplifier A 1 . The input amplifier A 1  is essentially formed by a transistor. At the output of the amplifier A 1 , the variable capacitance  C 1    from  FIG. 5  is formed by a tuning diode  1  and a capacitor  2 , which are connected in series from the output of the amplifier to earth. A tuning voltage U 1  is passed to the cathode of the tuning diode  1  via a resistor  3 . The signal furthermore passes to one connection of the inductance L 1 , to the second connection of which a switching diode  4  and an inductance L 2  are connected. The switching diode  4  together with a resistor  6  and a capacitor  7  forms the switch  S 1    from  FIG. 5 . In order to open or close the switch  S 1   , a switching voltage U 2  is applied to the cathode of the switching diode  4  via the resistor  6 . The switch is closed or open depending on whether the p-n junction of the switching diode  4  is forward- or reverse-biased. In this case, the capacitor  7  serves for decoupling the rest of the circuit from the switching voltage. 
   If the switching voltage U 2  is lower at least by the forward voltage of the diode than an operating voltage UB, which passes via an inductance L 3  to the inductance L 2  and thus to the switching diode  4 , the switch is closed and the second secondary resonant circuit is switched in. In this case, a capacitor  8  serves for avoiding a short circuit between the operating voltage UB and earth and at the same time represents an RF connection of the reference point inductance L 3  to earth. As mentioned in the description of  FIG. 5 , the inductance L 2  is very much greater than the inductance L 5 . Therefore, L 2  and the further components of the first secondary resonant circuit that are connected to the inductance L 2  can be disregarded for the further function. The capacitance  C 1    and the inductance L 1  form a part of the second primary resonant circuit from  FIG. 5 . Via the closed switch  S 1   , the signal passes to the reference point inductance L 5  and also to an inductance L 6  of the second secondary resonant circuit. From the inductance L 6 , the signal furthermore passes to a variable capacitance  C 3   , which, in the same way as the variable capacitance  C 1   , is formed from the series circuit formed by a tuning diode  11  and a capacitance  12 . A further tuning voltage U 3  is applied to the cathode of the tuning diode  11  via a resistor  13 . From the inductance  16 , the signal then passes to the input of a mixer MI via coupling elements RK 2  and CK 2 . 
   If the switching voltage U 2  of the switch  S 1    is greater than or equal to the operating voltage UB, the switching diode  4  turns off and the switch is open. In this case, the signal passes from the input amplifier A 1  via the series circuit formed by the inductances L 1  and L 2  to a first connection of the reference point inductance L 3 . The capacitor  C 1    and the inductances L 1  and L 2  now form a part of the first primary resonant circuit from  FIG. 5 . A second connection of the reference point inductance L 3  is at operating voltage potential and is connected to earth in terms of RF by means of the capacitor  8 . From the first connection of the reference point inductance L 3 , the signal passes via an inductance L 4  to a variable capacitance  C 2   . In the same way as the variable capacitances  C 1    and  C 3    described above, the variable capacitance  C 2    is formed from the series circuit formed by a tuning diode  16  and a capacitor  17 . A tuning voltage U 4  is applied to the cathode of the tuning diode  16  via a resistor  18 . From the inductance L 4 , the signal passes to the input of the mixer MI via coupling elements RK 1  and CK 1 . 
   The tuning voltages U 1 , U 3  and U 4  may expediently be combined to form a single tuning voltage, but this is not necessary for the function of the circuit.

Technology Classification (CPC): 7