1. Field of the Invention
The present invention relates to a television tuner which is able to receive not only television signals, but also FM broadcast signals.
2. Description of the Related Art
A television signal and an FM broadcast signal have different bandwidths. The bandwidth of the television signal is 6 MHz, while the bandwidth of the FM broadcast signal is 200 KHz.
The configuration of a known television tuner is described below with reference to FIG. 4. In FIG. 4, a television signal received by a television antenna 41 and an FM broadcast signal received by an FM antenna 42 are input into an antenna duplexer 43.
The antenna duplexer 43 selects one of the input television signal or the input FM broadcast signal, and outputs the selected signal to a high-frequency tuned circuit 45. The selection of the signal by the antenna duplexer 43 is controlled by a switch circuit 44.
The switch circuit 44 includes a switch transistor 44b whose base is connected to an input terminal 44a, emitter is grounded by a resistor 44c, and collector is connected to a control terminal 43a of the antenna duplexer 43. A power supply voltage MB is applied to the collector of the switch transistor 44b via a resistor.
The input television signal or FM broadcast signal is tuned in the high-frequency tuned circuit 45 and is then input into a mixer 46. The mixer 46 is connected to the high-frequency tuned circuit 45 and is also connected to a local oscillator 47. Accordingly, the television signal is converted into an intermediate frequency having a center frequency of 44 MHz, and the FM broadcast signal is converted to 41.25 MHz, which corresponds to the audio intermediate frequency.
The frequency-converted television signal or FM broadcast signal is input into an intermediate-frequency tuned circuit system 48. The intermediate-frequency tuned circuit system 48, which is a balanced circuit, includes a first parallel tuned circuit 49, a second parallel tuned circuit 50, and a second switch diode 51 connected in series to the second parallel tuned circuit 50. The first parallel tuned circuit 49 includes a sixth capacitor 52 and series-connected third inductors 53a and 53b, and is tuned at 44 MHz, which is the center frequency of the intermediate frequency band, and the bandwidth is 6 MHz. The second parallel tuned circuit 50 includes a seventh capacitor 54 and a fourth inductor 55, and is tuned at 38 MHz, and the bandwidth is about 1.8 MHz. A series circuit of the second parallel tuned circuit 50 and the second switch diode 51 is connected in parallel to the first parallel tuned circuit 49. A voltage obtained by dividing the power supply voltage MB is applied to a node between the third inductors 53a and 53b and is also applied to the anode of the second switch diode 51 via the third inductor 53b. The cathode of the second switch diode 51 is connected to the collector of the switch transistor 44b. 
An intermediate frequency amplifier 56 is connected to the intermediate-frequency tuned circuit system 48. The intermediate frequency amplifier 56 is a balanced-input and unbalanced-output type, and the output thereof is connected to an output circuit system 57.
The output circuit system 57 includes a parallel circuit consisting of a first switch diode 58 and a first capacitor 59, a first inductor 60 connected in series to this parallel circuit, a DC-cut capacitor 63, and a fifth capacitor 61, and a resistor 62. The fifth capacitor 61 and the resistor 62 shunt nodes between the first inductor 60 and the DC-cut capacitor 63 to grounds. The parallel circuit of the first switch diode 58 and the first capacitor 59, the first inductor 60, and the fifth capacitor 61 form a peaking circuit. A predetermined bias voltage is applied to the anode of the first switch diode 58 from the intermediate frequency amplifier 56. The cathode of the first switch diode 58 is connected to the collector of the switch transistor 44b via the first inductor 60.
In the above-configured television tuner, when a television signal is received, a low level voltage is applied to the input terminal 44a of the switch circuit 44. Then, the switch transistor 44b is turned off so that the collector voltage of the switch transistor 44b is changed to a high level. Thus, the antenna duplexer 43 connects the television antenna 41 to the high-frequency tuned circuit 45.
The second switch diode 51 is turned off, and only the first parallel tuned circuit 49 becomes effective in the intermediate-frequency tuned circuit system 48. As a result, the tuned frequency is 44 MHz, and the bandwidth is 6 MHz.
The first switch diode 58 is turned off, and a video intermediate frequency undergoes a peaking operation by the first capacitor 59, the first inductor 60, and the fifth capacitor 61, and the intermediate-frequency television signal is output from the television tuner.
On the other hand, when an FM broadcast signal is received, a high level voltage is applied to the input terminal 44a of the switch circuit 44. Then, the switch transistor 44b is turned on so that the collector voltage of the switch transistor 44b is changed to a low level. Accordingly, the antenna duplexer 43 connects the FM antenna 42 to the high-frequency tuned circuit 45.
The second switch diode 51 is turned on, and thus, both the first parallel tuned circuit 49 and the second parallel tuned parallel circuit 50 become effective in the intermediate-frequency tuned circuit system 48. As a result, the total tuned frequency of the first and second parallel tuned circuits 49 and 50 is 41.25 MHz, and the bandwidth becomes 1.5 MHz.
The first switch diode 58 is turned on, and an audio intermediate frequency undergoes a peaking operation by the first inductor 60 and the fifth capacitor 61, and the audio-frequency FM broadcast signal is output from the television tuner. In this case, a bias current from the intermediate frequency amplifier 56 passes through the first switch diode 58 and flows into a ground via the resistor 62.
In the above-described known television tuner, the tuned frequency and the bandwidth are changed according to whether a television signal or an FM broadcast signal is received. However, the selection characteristics are insufficient, and adjacent-channel interference cannot be sufficiently prevented.