Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1982-07-14
1984-07-17
Ng, Jin F.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
455188, 455190, 334 15, H04B 126
Patent
active
044610386
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a tuner used in a television receiver or the like, and more particularly to a tuner which is performable of band switching by use of a one-circuit triple-contact switch.
BACKGROUND OF THE INVENTION
Generally, a tuner for a television receiver or the like, employing a variable capacity diode as a tuning element, has a band switching circuitry which selectively switches a VHF low band, a VHF high band and a UHF band. Therefore, from the viewpoint of simplifying an electric power source for switching the band, it has been proposed that the one circuit triple-contact switch, which applies DC supply voltage selectively to each contact for the VHF low-band, VHF high band or UHF band to perform the band switching, is used for switching voltage.
The band switching mechanism in the conventional turner using the one-circuit triple-contact switch, however, has a switching diode at a VHF tuner unit kept in not-biased condition when UHF is received, so that a VHF signal at high level given to a VHF input terminal, especially the non-linearity of the switching diode in a VHF input tuning circuit, generates the harmonic frequency identical with the UHF band, resulting in that the harmonic wave is given into the input side at the UHF tuner unit to appear as noises in the picture. Such phenomenon is remarkable in a case that the VHF tuner unit and UHF tuner unit are encased in the same casing or the UHF and VHF signals from the antenna are divided by a frequency divider to be led to the VHF input terminal and UHF input terminal respectively. The reason for the former case is that the harmonic wave is easy to jump from the periphery of the switching diode to the input side of the UHF tuner unit in the casing, and that for the latter case is that the harmonic wave generated by the switching diode enters the UHF input terminal from the VHF input terminal through an external frequency divider. Hence, a turner having both the above constructions in combination is further remarkable of the above trouble.
SUMMARY OF THE INVENTION
In the light of the above problems in the conventional tuner, this invention has been designed. An object of the invention is to provide a tuner which allows a tuning coil switching diode at a VHF input tuning circuit to be conductive when receiving the UHF band, thereby generating no harmonic wave, and which actually has such construction to switch the band in the one-circuit triple-contact system without hindrance.
In order to attain the above object, the present invention applies supply voltage to a common contact at the one-circuit triple-contact switch, the common contact connecting the compound circuit. A first contact of the switch is connected to turn on a local oscillator and a radio frequency amplifying transistor in the VHF tuner unit, a second contact of the switch is connected to actuate tuning coil switching diodes and a radio frequency amplifying transistor and a local oscillator in an interstage tuning circuit of the VHF tuner unit, and connected to apply conducting voltage for a first switching diode through a second switching diode to the node of the first switching diode turned on to void a coil for a low band at the VHF input tuning circuit and a high-frequency grounding condenser, and a third contact of the switch is connected to a UHF tuner unit and also connected through a third switching diode to the node of the first switching diode and a high-frequency grounding condenser, thereby turning on the first switching diode when receiving the UHF band.
In the tuner having the band switching mechanism employing the one-circuit triple-contact switch as foregoing, the tuning coil switching diode in the VHF input tuning circuit is turned on without being not-biased while receiving the UHF band, whereby, even when the VHF signal is given, no equivalent harmonic wave is generated by said switching diode in the UHF band, thus producing no noise in the picture.
Furthermore, the first switching diode for switching the tuning coil at the VH
REFERENCES:
patent: 3794924 (1974-02-01), Furuya et al.
patent: 4189678 (1980-02-01), Sakamoto
patent: 4291290 (1981-09-01), Ijichi
patent: 4326295 (1982-04-01), Matsumoto
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