Single conversion tuner
View Patent ↗A single conversion tuner suitable for receiving RF signals having different requirements as to selectivity, linearity and image rejection is presented. Tuners of this kind are particularly useful for digital terrestrial as well as cable reception, where different modulation schemes require very high image rejection ratios on one hand and high linearity or pass band flatness on the other hand. The inventive tuner features a double-tuned tuneable filter in combination with an image reject mixer. The double-tuned filter provides excellent pass band flatness, while the image reject mixer provides a high intrinsic image rejection. The image rejection ratio of the combination of image reject mixer and tuneable filter is very high and fulfils the requirements of commonly used modulation schemes such as 8VSB, while the pass band flatness is compliant with the requirements for modulation schemes such as QAM.
1. A single conversion tuner adapted to selectively receive modulated signals of a first and a second type at an input, the signals of the first type being modulated according to a first modulation scheme and the signals of the second type being modulated according to a second modulation scheme, wherein the first modulation scheme requires a flat amplitude response over a predetermined bandwidth, and wherein the second modulation scheme requires a predetermined ratio of image rejection, including a tuneable bandpass filter that is adapted to provide a flat amplitude response over the predetermined bandwidth for receiving signals according to the first modulation scheme and which provides a first ratio of image rejection, wherein the ratio of image rejection provided by the tuneable bandpass filter is smaller than required for the second modulation scheme, wherein an image reject mixer providing a second ratio of image rejection is connected to the tuneable bandpass filter, wherein the ratio of image rejection provided by the image reject mixer is smaller than required for the second modulation scheme, wherein the first and second ratios of image rejection are selected so that the sum of the ratios of image rejection achieves or exceeds the required predetermined ratio of image rejection for receiving signals according to the second modulation scheme, and that an intermediate frequency signal is present at an output of the image reject mixer.
2. Tuner according to claim 1 , wherein a variable gain amplifier is connected upstream of the tuneable bandpass filter or the image reject mixer.
3. Tuner according to claim 1 , wherein the input is connected to an amplifier or the tuneable bandpass filter via a band select filter.
4. Tuner according to claim 1 , wherein the output of the image reject mixer is fed to a fixed frequency bandpass filter.
5. Tuner according to claim 4 , wherein the fixed frequency bandpass filter is a surface acoustic wave filter.
6. Television apparatus including a tuner according to any one of the preceding claims.