Local oscillation circuit for reducing oscillation voltage variations between high and low frequency bands

Claim 2

A local oscillation circuit used in a tuner of a receiver whose reception frequency is variable, comprising a resonance circuit including a variable capacitance diode to change the resonance frequency of the circuit and tuning coils for low and high frequency bands; an oscillating transistor connected with said resonance circuit; a first switching diode; first biasing means for selectively supplying said first switching diode with a bias voltage to render said switching diode on and off alternatively according to said high and low bands of reception frequencies; means for changing over said tuning coils of said resonance circuit according to the conduction and cut-off of said first switching diode; second biasing means for supplying said transistor with a bias voltage so as to start the oscillation operation of said transistor; a second switching diode provided in said second biasing means and adapted to be rendered on and off alternatively according to the value of said bias voltage supplied from said first biasing means; and means for changing the collector current of said transistor according to the conduction and cut-off of said second switching diode so as to increase the collector current during reception of a signal of said high frequency band.

Claim 3

A local oscillation circuit used in a tuner of a receiver whose reception frequency is variable, comprising a resonance circuit including a variable capacitance diode to change the resonance frequency of the circuit and series-connected tuning coils for low and high frequency bands; an oscillating transistor connected with said resonance circuit; a first switching diode for short-circuiting in view of an alternating current one of said tuning coils when said first switching diode is rendered conductive; first biasing means for selectively supplying said first switching diode with a bias voltage so as to render said first switching diode on and off alternatively according to said high and low frequency bands; second biasing means for supplying said transistor with a bias voltage so as to start the oscillation operation of said transistor; a second switching diode provided in said second biasing means and adapted to be rendered on and off according to the value of said bias voltage supplied from said first biasing means; and means for changing the collector current of said transistor according to the conduction and the cut-off of said second switching diode so as to increase the collector current during reception of a signal of said high frequency band.

Claim 4

A local oscillation circuit used in a tuner of a receiver whose reception frequency is variable, comprising a resonance circuit including a variable capacitance diode to change the resonance frequency of the circuit and series-connected tuning coils for low and high frequency bands; an oscillating transistor connected with said resonance circuit; a first switching diode for short-circuiting in view of an alternating current one of said tuning coils when said first switching diode is rendered conductive; first biasing means for selectively supplying said first switching diode with a bias voltage so as to render said first switching diode on and off alternatively according to said high and low frequency bands; second biasing means for supplying said transistor with a bias voltage so as to start the oscillation operation of said transistor; a second switching diode provided in said second biasing means and adapted to be rendered on and off according to the value of said bias voltage supplied from said first biasing means; and means for changing the collector current of said transistor according to the conduction and the cut-off of said second switching diode so as to increase the collector current during reception of a signal of said high frequency band.

Claim 5

A local oscillation circuit used in a tuner of a receiver whose reception frequency is variable, comprising: a parallel resonance circuit including a variable capacitance diode, a fixed capacitor connected in series with said variable capacitance diode, series-connected tuning coils for low and high frequency bands, said series-connected coils being connected in parallel relation with the series circuit of said variable capacitance diode and said fixed capacitor, one end of said parallel resonance circuit being grounded; means for supplying a variable reverse bias to said variable capacitance diode for changing the resonance frequency of said parallel resonance circuit; a series connection of a first switching diode and a capacitor, said series connection being connected in parallel with one of said series-connected coils; a switching power source for selectively rendering said first switching diode conductive and non-conductive in accordance with the low and high frequency bands of the reception frequency; means for applying a voltage from said switching power source to said first switching diode; an oscillation transistor connected to the other end of said parallel resonance circuit through a capacitor; a power supply for supplying a bias voltage to said transistor; series-connected resistors connected Between said power supply and the ground for dividing the voltage of said power supply; means for applying a divided voltage obtained at the intermediate junction point of said series-connected resistors to the base electrode of said transistor; means for applying a voltage of said power supply between the collector and emitter electrodes of said transistor; and a second switching diode connected between the base electrode of said transistor and said switching power source through a resistor, said second switching diode being selectively rendered conductive and non-conductive at the same time as said first switching diode being rendered conductive and non-conductive, respectively.

Claim 6

A local oscillation circuit used in a tuner of a receiver whose reception frequency is variable, comprising: a parallel resonance circuit including a variable capacitance diode, a fixed capacitor connected in series with said variable capacitance diode, series-connected tuning coils for low and high frequency bands, said series-connected coils being connected in parallel relation with the series circuit of said variable capacitance diode and said fixed capacitor, one end of said parallel resonance circuit being grounded; means for supplying a variable reverse bias to said variable capacitance diode for changing the resonance frequency of said parallel resonance circuit; a series connection of a first switching diode and a capacitor, said series connection being connected in parallel with one of said series-connected coils; a switching power source for selectively rendering said first switching diode conductive and non-conductive in accordance with the low and high frequency bands of the reception frequency; means for applying a voltage from said switching power source to said first switching diode; an oscillation transistor connected to the other end of said parallel resonance circuit through a capacitor; a power supply for supplying a bias voltage to said transistor; series-connected resistors connected Between said power supply and the ground for dividing the voltage of said power supply; means for applying a divided voltage obtained at the intermediate junction point of said series-connected resistors to the base electrode of said transistor; means for applying a voltage of said power supply between the collector and emitter electrodes of said transistor; and a second switching diode connected between the base electrode of said transistor and said switching power source through a resistor, said second switching diode being selectively rendered conductive and non-conductive at the same time as said first switching diode being rendered conductive and non-conductive, respectively.