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.
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.