Input circuit of television tuner

#1. Field of the Invention

#This invention relates to an input circuit of a television tuner for receiving television signals of a VHF band or television signals of a UHF band.

#2. Description of the Related Art

#FIG. 1 is a circuit diagram showing a construction of an input circuit of a television tuner according to the invention;

#FIG. 2 is a relational diagram between a reception band and an ON/OFF state of a switch diode in the input circuit of the television tuner according to the invention;

#FIG. 3 is an equivalent circuit diagram when a television signal of a low range of a VHF band is received in the input circuit of the television tuner according to the invention;

#An embodiment of an input circuit of a television tuner (hereinafter called merely “input circuit”) according to the invention will be explained with reference to FIG. 1. A VHF tuning circuit 2 is connected to an input terminal 1 to which television signals of VHF and UHF bands are inputted The VHF tuning circuit 2 uses a band-switching type tuning circuit designed for tuning with each channel of a low or high range of the VHF band and shares similar circuitry characteristics as that of the VHF tuning circuit 42 of the introduced prior art. In other words, this tuning circuit 2 includes four inductance devices 2a to 2d connected in series, a tuning varactor diode 2e connected in parallel with all the inductance devices and a first switch diode 2f connected in parallel with the two in series inductance devices 2b and 2c at an intermediate part of the inductance devices 2a to 2d.

#The junction between the two inductance devices 2b and 2c at another intermediate part of the four inductance devices 2a to 2d is connected to the input terminal 1. A voltage VB is applied to an anode of the first switch diode 2f through the inductance devices 2a to 2c and its cathode is connected to a collector of a switch transistor 3. An emitter is grounded. A high- or low-level switch voltage Vs is applied to a base of the switch transistor 3. An anode of the tuning varactor diode 2e is grounded, and a tuning voltage Vt is applied to its cathode. A cathode of a coupling varactor diode 4 is connected to a cathode of the tuning varactor diode 2e. An anode of this varactor diode 4 is grounded by a bias resistor 5 and is connected to a VHF high frequency amplifier 6.

#A UHF tuning circuit 9 is connected to the input terminal 1 through a series circuit of a second switch diode 7 and a coupling inductance device 8. The voltage VB is applied to an anode of the second switch diode 7 and a cathode is connected to the collector of the switch transistor 3. The UHF tuning circuit 9 tunes with channels of a UHF band and shares similar circuitry characteristics as that of the UHF tuning circuit 47 of the introduced prior art. The UHF tuning circuit 9 includes an inductance device 9a and two tuning varactor diodes 9b and 9c the cathodes of which are connected to each other and which are connected in parallel with the inductance device 9a. The tuning voltage vt is applied to the cathodes. The cathodes are connected also to a UHF high frequency amplifier 10.