Field of the invention The invention relates to a tuner oscillator stage, a tuner comprising such a stage, and an electronic apparatus comprising such a tuner.

Background of the invention Power supply voltage ripple rejection may be performed with the well-known RC low-pass filter. A resistor is provided in series with one of the power supply voltage lines and a capacitor is connected between both power supply lines to which the load is connected.

The capacitor smoothes the supply voltage supplied to the load. The filtering action of the RC circuit improves by increasing the value of both the resister and the capacitor. However, the amount of filtering is limited because a too high value of the resistor causes a too large voltage drop across the resistor, which leads to a too high dissipation and to a too low voltage across the load. A large value of the capacitor gives rise to a too large and/or a too expensive capacitor.

Summary of the invention It is an object of the invention to provide a tuner oscillator stage with an improved supply voltage ripple suppression.

To this end, a first aspect of the invention provides a tuner oscillator stage as defined in claim 1. A second aspect of the invention provides a tuner as defined in claim 4. A third aspect of the invention provides an electronic apparatus as defined in claim 5.

Advantageous embodiments are defined in the dependent claims.

The tuner oscillator stage comprises a transistor which has a control electrode, and two electrodes of a main current path. A power supply voltage is present between a first and a second terminal. A first impedance is coupled between the control electrode and the first terminal. A series arrangement of an element having a low impedance for AC current and a well defined voltage drop for DC current and a second impedance is coupled between

the control electrode and the second terminal. The element (which may be a circuit comprising several elements) allows a decreased value of the second impedance which causes the supply voltage ripple on the collector originating from the supply voltage ripple on the control terminal to decrease without disturbing the DC drive of the transistor. In this way, the supply ripple on the collector directly originating from the supply voltage is better compensated.

In a preferred embodiment, the element is a diode which is poled to be conductive.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Brief description of the drawings In the drawings: Fig. 1 shows a typical prior-art oscillator tank circuit, Fig. 2 shows a typical prior-art series mixer and oscillator design, Fig. 3 shows a simplified oscillator circuit according to an embodiment of the invention, and Figs. 4 shows waveforms of signals occurring in the simplified oscillator circuits of Fig. 2 and Fig. 3.

Detailed description of a preferred embodiment Fig. 1 shows a typical prior-art oscillator tank circuit.

Ripple susceptibility in a TV receiver is defined as the peak-to-peak value of a sine-wave signal from 20 Hz to 500 kHz superposed on the power supply voltage, causing an amplitude modulation with a modulation depth of 0.28% on the picture carrier after passing the Nyquist slope of the IF filter of the receiver.

Fig. 1 shows a typical oscillator circuit with a bipolar transistor BJT1 and an emitter resistor Re, and a resonance circuit comprising a coil L1, a capacitor C1, and the capacitors C2 and C3 together with a varicap diode Dl whose capacitance value depends on the voltage across it. The supply voltage ripple is introduced via two paths. The ripple on the power supply voltage Vs will be present on the collector of the transistor BJT1 via the collector resistor Rc. The ripple on the band-switch power supply voltage Vbsl will be attenuated with the low-pass filter comprising the resistor Rb and the capacitor Cb. The ripple originating from the supply voltage Vbsl can be easily suppressed by using a resistor

Rb with a high value which does not lead to a too large voltage drop across this resistor Rb because the current into the base of the transistor BJT is very small. The ripple on the power supply voltage Vs will be present on the collector directly and thus has to be filtered sufficiently, which give rise to a large and/or expensive capacitor (not shown) because a large value of the resister Rc is not possible due the relatively large current through it. In tuners, a large capacitor causes a problem because the space is not available, so that a very expensive tantalum capacitor is required.

Fig. 2 shows a typical prior-art series mixer and oscillator design. The oscillator circuit in Fig. 2 is almost identical to that shown in Fig. 1, and the same elements are denoted by the same references. The only difference is that the collector resistor Rc has been replaced by an impedance L2. The mixer comprises a transistor BJT2 with a base connected to a band-switch power supply voltage Vbs2 via the resistor R1. A filter capacitor C4 together with the resistor RI filters the band-switch voltage Vbs2 to suppress the power supply ripple on the base of the transistor BJT2. The collector of the transistor BJT2 is connected to the power supply voltage Vs via the resistor R2. A capacitor C5 together with the resistor R2 filters the power supply voltage Vs to suppress the power supply ripple on the collector of the transistor BJT2. The resistor R3 improves the DC behavior of the mixer stage M.

The mixer stage M has the functions of both mixer and ripple rejecter.

Fig. 3 shows a simplified oscillator circuit according to an embodiment of the invention.

In the oscillator circuit of Fig. 1 and the oscillator and mixer circuit of Fig. 2, the suppression of the power supply voltage Vs requires a large and/or expensive capacitor.

This problem becomes more severe in modern tuners which have a low power supply voltage Vs (for example, 5 volts). The low power supply voltage Vs requires the voltage drop across the filter resistor to be very small, requiring an excessively large or expensive smoothing capacitor. The series arrangement of the mixer and the oscillator shown in Fig. 2 has the further drawback that, due to the voltage drop across the mixer transistor BJT2, the voltage available for the oscillator decreases. This also becomes a severe problem at low power supply voltages Vs.

The invention is based on the recognition that the influence on the total ripple on the collector is a sum of the ripple caused via the base and the ripple on the collector

caused by the supply voltage coupled to the collector, wherein these two ripple contributions have opposite signs, and thus may compensate each other. The prior-art approach of suppressing both the ripple on the base and on the collector as much as possible will not give rise to an optimal compensation.

The transistor Tr has a base connected to a power supply voltage Vbs (which may be a band-switch voltage) via a resistor RIO. A series arrangement of a diode DIG and a resister Rl 1 is connected between base and ground G. The collector of the transistor Tr is connected to the power supply voltage Vs via the impedance Zc, and the emitter of the transistor is connected to earth via resistor R12. The voltage on the collector which is Vo comprises a DC part Vodc and a ripple component Vor.

If the ripple on the power supply voltage Vbs is Vrl, and the ripple on the power supply voltage Vs is Vr2, the ripple voltage Vor on the collector is: Vor = Vr2-ZC. Ri 1-Vri<BR> R, 2 Rlo + Rll Without the diode D10, and due to the required DC operation of the oscillator, the resistor Rl 1 usually has a value (for example, 2.7 kohm) which causes the ripple originating from the base to be much larger than the ripple originating from the power supply voltage Vs, see Fig. 4A.

With the diode D10, the value of the resistor Rl 1 can be decreased while maintaining substantially the same DC operation. Now the ripple contribution to the collector ripple voltage Vor originating from the base decreases and a better compensation is achieved, see Fig. 4B.

Fig. 4 shows waveforms of signals occurring in the simplified oscillator circuits of Fig. 2 and Fig. 3. Figs. 4A and 4B show, in the left diagram, the waveforms representing the ripple voltages Vrl (in bold) and Vr2 (dashed) and, in the right diagram, the resultant ripple voltage Vor, respectively, without and with the diode D10.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. For example, it is

also possible to use a pnp transistor instead of the npn transistor shown. It is possible to have the collector coupled to earth potential and the emitter to a negative potential.

In the claims, any reference signs placed between parenthesis shall not be construed as limiting the claim. The verb"comprising"and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware.