Abstract:
A mixer oscillator stage, which can be used both with a switched 2-band concept and a 3-band concept without changing the architecture of the oscillators which exceeds the number of mixers and by coupling the mixer(s) and oscillators via a switching circuit so as to switch between the different oscillators in dependence upon the part of the band converging the signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a mixer oscillator stage. The invention further relates to a mixer oscillator integrated circuit and to a tuner. The invention also relates to a receiver, and more particularly, but not exclusively, a television receiver. 
     2. Description of the Related Art 
     Such a mixer oscillator stage is used in, for example, television receivers. Different solutions are known for such a mixer oscillator stage. As is known, television signals comprise signals in the bands historically referred to as UHF and VHF bands. In most cases, the VHF band is split into at least two ranges (VHFL and VHFH). 
     For 3-band tuners, the different bands are often referred to as A, B and C, or Low, Mid and High. For 2-band tuners, it is customary to refer to the bands as VHF-Low, VHF-High and UHF, although the UHF band may be capable of receiving frequencies transmitted below the UHF band. 
     The most straightforward solution is to use the 3-band concept with a separate mixer and oscillator for each band. 
     The above solution is quite complex. To reduce the costs of three different bands, a switching between the low and high VHF signals is performed. The range switching is normally performed with a switching diode which selects a different coil inductance for VHF-Low and VHF-High. Especially, if the mixer oscillator stage is implemented as an integrated circuit which also has a PLL function, referred to as MO-PLL Combi IC, a 3-band tuner is not possible without exceeding the 28-pin structure that the 2-band switched version has. The 28-pin package is one of the largest cost-effective packages available today. 
     To reduce the number of necessary pins, it is proposed to pursue asymmetrical input/output functions. To achieve a good performance, a good isolation between the mixer/oscillator functions, like oscillators, mixers, and the IF amplifier, is necessary. It is hardly possible to achieve this good isolation by using asymmetrical input /output functions. 
     Balanced signal pairs and balanced circuitry have so far provided the best performance. The performance must meet the more stringent requirements recently imposed by the European CENELEC directive EN55020. EN55020 is a European directive on EMC requirements for television and sound broadcast receivers and associated equipment. Special attention must be paid during tuner design to ensure that the requirements are met. 
     One of the disadvantages of the known mixer oscillator stages (topologies) is that different mixer oscillator topologies are necessary for the 3-band concept and for the switched 2-band concept, making the development of at least two different topologies expensive and time-consuming. A further disadvantage is that the 3-band concept (especially MOPLL Combi) requires more input and oscillator pins necessary for the mixer oscillator integrated circuit. 
     SUMMARY OF THE INVENTION 
     It is object of the invention to overcome the disadvantages of the prior art and further to provide a mixer oscillator stage, a mixer oscillator integrated circuit, a tuner and a television receiver having an improved performance. 
     To this end, a first aspect of the invention provides a mixer oscillator stage comprising mixing means with at least one mixer, oscillator means with at least two oscillators and switching means coupled between the at least one mixer and the at least two oscillators, the number of oscillators exceeding the number of mixers 
     A second aspect of the invention provides a mixer oscillator integrated circuit incorporating the above mixer oscillator stage 
     A third aspect of the invention provides a tuner comprising the above mixer oscillator stage. 
     A fourth aspect of the invention provides a receiver comprising such a tuner. 
     By choosing number of oscillators to be larger than the number of mixers and by using switching means in between, the same mixer oscillator stage can be used for different applications. The cost of development are thereby largely decreased. 
     An embodiment of a mixer oscillator stage according to the invention has the features that the oscillator means comprises a first, a second and a third oscillator, the mixing means comprises a first and a second mixer, and the switching means are coupled between the mixers and the oscillators. 
     By using three oscillators and switching means, it is possible to use the same mixer oscillator stage for a 2-band switched concept and for a genuine 3-band concept. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and additional features which may optionally be used to implement the invention to advantage, will be apparent from and elucidated with reference to the examples described hereinafter and shown in the drawings. In these drawings: 
     FIG. 1 shows a first embodiment of a tuner according to the inventions; 
     FIG. 2 shows a second embodiment of a tuner according to the invention, and 
     FIG. 3 shows an embodiment of an input circuit for use in a tuner according to the invention. 
     Identical elements are denoted by the same reference numerals throughout the drawings. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of a tuner TUN 1  according to the invention, having an input I 1  for receiving an antenna signal. The input I 1  is coupled to a VHF-tuned input circuit VIC 1  and to a UHF-tuned input circuit UIC 1 . 
     The tuned input circuits VIC 1  and UIC 1  are coupled to amplifier stages VAM 1  and UAM 1 , respectively. Each amplifier stage is coupled to a bandpass filter VF 1  and UF 1 , respectively. 
     The bandpass filter VF 1  is coupled, via a varicap diode vd 1 , to a first input i 10  of a mixer oscillator integrated circuit MOIC 1 . 
     The bandpass filter UF 1  is coupled via a first and a second inductance ind 11  and ind 12 , to a second and a third input i 11  and i 12  of the mixer oscillator integrated circuit MOIC 1 . 
     The first input i 10  is coupled to a first mixer MIX 11  of the mixer oscillator integrated circuit, and the second and third inputs i 11  and i 12  are coupled to a second mixer MIX 12  of the mixer oscillator integrated circuit. 
     The tuner TUN 1  further comprises a first, a second and a third oscillator OSC 11 , OSC 12  and OSC 13 , these oscillators are being a part of a mixer oscillator stage MOS 1 , and being coupled to one of the mixers MIX 11  and MIX 12  via first and second switching means SW 11  and SW 12 , respectively. 
     The output of each mixer MIX 11  and MIX 12  is coupled to an output O 1  of the tuner TUN 1  for supplying an IF signal. 
     The mixer oscillator integrated circuit MOIC 1  and the oscillators OSC 11 , OSC 12  and OSC 13  are part of the mixer oscillator stage. 
     This embodiment of the tuner TUN 1  operates as follows. 
     After receiving the antenna signal at the input I 1 , the signal in this embodiment is split into a VHF band and a UHF band, in which the tuned input circuit VIC 1 , the amplifier stage VAM 1  and the filter VF 1  will handle the VHF signals (as is known to those skilled in the art). The tuned input circuit UIC 1 , the amplifier stage UAM 1  and the filter UF 1  will handle the UHF signals (as is also known to those skilled in the art). 
     In this embodiment, the filtered signals are supplied to the mixer oscillator integrated circuit MOIC 1  at inputs i 10 , input i 11  and i 12 , respectively. The VHF signals are supplied to the mixer MIX 11 , the other input of which receives, via the switching means SW 11 , a signal from either the first oscillator OSC 11  or the second oscillator OSC 12 , depending on the part of the VHF band to which the required signal belongs to. After mixing, the mixer supplies the IF signal at the output O 1 . 
     Nowadays, UHF signals are handled symmetrically. The UHF filter UF 1  supplies the UHF signal symmetrically via the inductances ind 11  and ind 12  to the mixer MIX 12 , the other input of which receives a signal from either the second oscillator OSC 12  or the third oscillator OSC 13 , depending on the part of the UHF band the required signal belongs to. 
     In this way, the mixer oscillator stage can handle all the signals of a switched 2-band concept. Instead of an implementation as an integrated circuit, it is also possible to obtain a mixer oscillator stage with discrete components only. 
     FIG. 2 shows a second embodiment of a tuner TUN 2  according to the invention, having an input  12  for receiving an antenna signal. The input is coupled to tuned input circuits IC 21 , IC 22  and IC 23 , respectively. 
     Each tuned input circuit is coupled to an amplifying stage AM 21 , AM 22  and AM 23 , respectively. 
     Each amplifying stage is coupled to a filter F 21 , F 22  and F 23 , respectively. 
     The filter F 21  is coupled, via a varicap diode vd 2 , to a first input i 20  of a mixer oscillator integrated circuit MOIC 2 . 
     The filter F 22  is coupled, via a first and a second inductance ind 21 , ind 22  to a second and a third input i 21 , i 22 , respectively, of the mixer oscillator integrated circuit. 
     The filter F 23  is coupled, via a third and a fourth inductance ind 23  and ind 24 , to the second and the third input i 21 , i 22  respectively, of the mixer oscillator integrated circuit. 
     The tuner TUN 2  further comprises a first, a second and a third oscillator OSC 21 , OSC 22  and OSC 23 , these oscillators being a part of a mixer oscillator stage MOS 2 , and being coupled to one of the mixers MIX 21  and MIX 22  via first and second switching means SW 21  and SW 22 , respectively. 
     The output of each mixer MIX 21 , MIX 22  is coupled to an output O 2  of the tuner TUN 2  for supplying an IF signal. 
     This embodiment of the tuner TUN 2  operates as follows. 
     As will be noted in this embodiment of the tuner TUN 2 , this tuner is built up as a 3-band concept. Similarly as in FIG. 1, the mixer oscillator integrated circuit MOIC 2  receives the different filtered signals at either input i 20  or, as symmetrical signal, at the inputs i 21  and i 22 . 
     Also in this embodiment, the mixer oscillator stage MOS 2  comprises two mixers MIX 21  and MIX 22  and three oscillators OSC 21 , OSC 22  and OSC 23 . 
     The relevant mixer MIX 21  (MIX 22 ) receives the filtered signal at one input (symmetrical input) and the signal from either oscillator OSC 21  or OSC 22  (OSC 22  or OSC 23 ) at the other input. 
     The mixer concerned supplies the obtained IF signal to the output of the tuner TUN 2 . 
     As is shown in relation to FIGS. 1 and 2, the same mixer oscillator stage MOS 1 , MOS 2  can be used for the switched 2-band concept (FIG. 1) and the 3-band concept (FIG.  2 ). 
     As mentioned above, instead of an implementation as a mixer oscillator integrated circuit, it is also possible to implement the mixer oscillator stage with discrete components only. 
     FIG. 3 shows an embodiment of filter means for use in a tuner according to the invention. 
     The input circuit IC 3  receives an antenna signal at an input  13 . A first tuned circuit T 1 , comprising diode D 3  and inductance L 3 , is coupled to the input  13  via an inductance L 6  and a capacitance C 1 . A second tuned circuit T 2 , comprising diode D 1  and inductance L 1 , is coupled to the input I 3  via inductance L 4 . A third tuned circuit T 3 , comprising diode D 2  and inductance L 2 , is coupled to the input  13  via an inductance L 5  and the capacitance C 1 . The inductances L 4 , L 5  and L 6  are coupling coils which perform an impedance transformation from the antenna impedance to the varying frequency-dependent impedance of the tuned circuits T 1 , T 2  and T 3 . 
     For signals intended for tuned circuit T 1 , the filter can be considered as a high-pass filter comprising L 4 +L 1 , C 1  and L 5 +L 2 . 
     This input circuit has been described extensively in the U.S. Pat. No. 4,851,796 which patent is herein incorporated by reference. 
     In the above description, the idea of the invention has been described on the basis of some examples. It is to be noted that those skilled in the art will be well aware of many different solutions within the scope of the invention concerned. 
     The invention provides a mixer oscillator stage which can be used both with the switched 2-band concept and the 3-band concept without changing the architecture of the mixer oscillator stage.