Audio demodulating circuit and television broadcast receiver

[0043]The following descriptions will discuss one embodiment of the present invention in reference to FIG. 1, and FIGS. 5 and 6. FIG. 1 is a block diagram illustrating an electrical structure of an audio demodulating circuit 41 in accordance with the present embodiment. The audio demodulating circuit 41 can be used as the audio demodulating circuit 14 of the receiver 1 explained earlier. In this audio demodulating circuit 41, a trap circuit 42 is a series circuit of a coil L1 and a condenser C1. This trap circuit 42 is placed between an IF signal line 43 and ground, and is connected / disconnected by a transistor TR1 which serves as connection means.

[0044]For example, the coil L1 has an inductance of 0.68 μH, and the condenser C1 has an electrostatic capacitance of 39 pF. For the transistor TR1, for example, a switching transistor 2SC2735 available from Hitachi Ltd. may be adopted.

[0045]Then, a SIF signal is input to a mixer 44 via an IF signal line 43, where it is mixed with an LO si...

[0056]The following descriptions will discuss the second embodiment of the present invention in reference to FIG. 2. For convenience in explanations, members having the same functions as those adopted in the foregoing embodiment will be designated by the same reference numerals, and the descriptions thereof shall be omitted here.

[0057]FIG. 2 is a block diagram illustrating the electrical structure of an audio demodulating circuit 51 in accordance with the second embodiment of the present invention. This audio demodulating circuit 51 has a similar structure to the audio demodulating circuit 41 adopted in the first embodiment. However, in the audio demodulating circuit 41, the trap circuit 42 is the series circuit composed of the coil L1 and the condenser C1; whereas, in the audio demodulating circuit 51, a trap circuit 52 is constituted by a trap element utilizing the piezoelectric effect. For this trap circuit 52, for example, an MKTGA30M9AALP00B05 available from Murata Manufacturin...

[0059]The following descriptions will discuss the third embodiment of the present invention in reference to FIG. 3. For convenience in explanations, members having the same functions as those adopted in the foregoing embodiment will be designated by the same reference numerals, and the descriptions thereof shall be omitted here.

[0060]FIG. 3 is a block diagram illustrating an electrical structure of an audio demodulating circuit 61 in accordance with the third embodiment. This audio demodulating circuit 61 has a similar structure to the audio demodulating circuit 41 adopted in the first embodiment. However, in the audio demodulating circuit 41, one trap circuit 42 is adopted; whereas, in the audio demodulating circuit 61, two trap circuits 42 and 62 are adopted. This trap circuit 62 is a series circuit composed of a coil L2 and a condenser C2 as in the trap circuit 42 adopted in the first embodiment. This trap circuit 62 is placed between the IF signal line 43 and ground, and is conn...