Abstract:
Ordinarily, at least two types of mixer oscillator integrated circuits for tuners have to be used, i.e., one for the symmetrical output mode and one for the asymmetrical output mode. In order to handle both output modes, a tuner includes a switching circuit, whereby, in the asymmetrical mode, one output of the tuner is coupled to ground.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of The Invention 
     The invention relates to a tuner. The invention further relates to an IF amplifier for use in such a tuner. The invention further relates to a mixer oscillator integrated circuit comprising such an IF amplifier. 
     2. Description of The Related Art 
     Such a tuner, IF amplifier, and mixer oscillator integrated circuit are known in the art, and can be used in television receivers, radio receivers, and other applications. 
     A disadvantage of the known tuner, IF amplifier and mixer oscillator integrated circuit, is that there has to be two kinds of tuners, IF amplifiers and mixer oscillator integrated circuits, namely, depending on the output of the tuner, a symmetrical and an asymmetrical version. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to obtain a tuner, an IF amplifier and a mixer oscillator integrated circuit that can be used for both symmetrical and asymmetrical outputs. To this end a first aspect of the invention provides a tuner comprising an input circuit and a mixer oscillator stage, said mixer oscillator stage comprising an IF amplifier coupled to an output of the tuner, characterized in that the tuner further comprises switching means for switching between a symmetrical and an asymmetrical output mode, whereby, in the asymmetrical mode, one output of the tuner is coupled to a reference voltage by the switching means, and the tuner comprises control means for controlling the switching operation. 
     A second aspect of the invention provides an IF amplifier for use in such a tuner. 
     A third aspect of the invention provides a mixer oscillator integrated circuit comprising such an IF amplifier. 
     In this way, it is possible to use the same tuner, IF amplifier or mixer oscillator integrated circuit, respectively, for both kind of output signals, by switching the IF amplifier between the two modes, and by coupling one output of the tuner to ground in the asymmetrical mode. 
     In the symmetrical output mode, a signal is present on both outputs of the tuner (IF amplifier, mixer oscillator integrated circuit, respectively) to be processed further, while, in the asymmetrical output mode, an output signal is present on only one output, and the other output is coupled to ground. 
     In another embodiment of the invention, by using a switchable IF amplifier, the amplifier itself is switched between the two modes. 
     In a further embodiment of the invention, the IF amplifier comprises two amplifying means, one for the symmetrical mode and one for the asymmetrical mode, and the control means controls which amplifying means operative. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention and additional features, which may optionally be used to advantageously implement the invention, will be apparent from and elucidated with reference to the examples described below hereinafter and shown in the drawing, in which: 
     FIG. 1 is a block diagram of a tuner according to the invention; and 
     FIG. 2 is a schematic diagram of an IF amplifier for use in the tuner. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a block diagram of a tuner according to the invention. The tuner comprises an input  1  for receiving an input signal, for example, from an antenna, the input  1  being coupled to an input circuit  3  for processing the input signal. The input circuit  3  is coupled to a mixer oscillator stage  5  for obtaining and supplying an IF signal. The mixer oscillator stage  5  supplies the IF signal (asymmetrical or symmetrical), via outputs O 1  and O 2 , to a surface acoustic wave (SAW) filter  7 . 
     A general description of the operation of the tuner is not necessary as this is generally known to a person skilled in the art. 
     The mixer oscillator stage  5  includes, in this embodiment, a mixer oscillator integrated circuit  51 . The mixer oscillator integrated circuit comprises an IF amplifier  511  and switching means  513 , while the mixer oscillator stage  5  further includes control means  53  for controlling the switching means. It should be noted that the switching means can (partly) be combined with the IF amplifier (as shown in FIG. 1 by the dashed overlap between IF amplifier  511  and switching means  513 ). 
     The switching means  513  operates, under control of the control means  53 , to switch the IF amplifier between the two modes, that is, the symmetrical mode and the asymmetrical mode. Further, the switching means comprise a switch SW for coupling one output (O 2  in this example) to ground in the asymmetrical mode. In this way, it is possible to use the same tuner T, IF amplifier  511  or mixer oscillator integrated circuit  51 , respectively, for both output signals. 
     In the symmetrical output mode, a signal is present on both outputs O 1  and O 2  of the tuner (IF amplifier, mixer oscillator integrated circuit respectively) to be processed further, while, in the asymmetrical output mode an output signal is present on only one output O 1 , and the other output O 2  is coupled to ground. 
     FIG. 2 shows an embodiment of part of an mixer oscillator integrated circuit  251 , according to the invention, comprising the IF amplifier  2511  and the switching means  2513 . The IF amplifier comprises amplifying means AMP for receiving the IF input signal(s) IF-IN at inputs IA 21 , IA 22 . Outputs OA 21  and  0 A 22  of the amplifying means AMP are coupled, via a transistor Q 1  and a resistor R 5 , on the one hand, and via a transistor Q 2  and a resistor R 6 , on the other hand, to outputs  021  and  022 . The transistors Q 1  and Q 2  are both emitter followers at the output. 
     Feedback resistors R 1 , R 2 , R 3  and R 4  determine the gain of the IF amplifier  2511 , whereby, in this embodiment, the sum of the values of resistors R 1  and R 2  corresponds with the value of resistor R 4 . 
     In the symmetrical mode, switch SW 22  of the switching means  2513  is open. In the asymmetrical mode the switch SW 22  is closed and the gain is increased in the upper branch, including the resistors R 1  and R 2 . In this way, it is possible to obtain a substantial equal overall gain in both the asymmetrical and the symmetrical mode. 
     The DC current for the output emitter followers is set by current sources which, in this embodiment, are implemented by transistors Q 3 , Q 4  and Q 5 . 
     In the symmetrical mode, Q 3  and Q 5  provide equal DC currents for Q 1  and Q 2 , while Q 4  is cut off. 
     In the asymmetrical mode, Q 5  is switched off, and Q 4  is switched on, doubling the DC current of Q 1 , compared to the symmetrical mode. This doubling is necessary, as is well known to the person skilled in the art, if a minimal dissipation is required. In the symmetrical mode, both outputs contain information, and the DC current is approximately one-half the value required when the whole voltage swing is present at only output in the asymmetrical mode. 
     Instead of one amplifying means as described above, the invention can also be implemented with two amplifying means, one for the symmetrical mode and one for the asymmetrical mode. Also, in this case, one output (O 22  in FIG. 2) is coupled by switching means to ground in the asymmetrical mode. 
     The person skilled in the art will be well aware of many variations of the above-described embodiments using the same idea of invention. 
     For example, it is possible, not necessary, to incorporate the control means within the mixer oscillator integrated circuit. 
     FIG. 2 shows a very detailed embodiment of part of the mixer oscillator integrated circuit, which, of course, is not the only way to implement the IF amplifier for this invention. Further, the switching means may be arranged totally inside the mixer oscillator integrated circuit, partly inside the mixer oscillator integrated circuit, partly outside the mixer oscillator integrated circuit, or even totally outside the integrated circuit. 
     Ground may be any suitable reference voltage.