Low phase noise voltage controlled oscillator circuit

#1. Field of the Invention

#The invention relates to oscillator circuits, and more particularly to a new circuit structure for reducing the phase noise of an oscillator circuit using an LC tank.

#2. Description of the Prior Art

#FIG. 1 is a schematic diagram of a prior art oscillator circuit.

#FIG. 2 is a schematic diagram of a voltage controlled oscillator circuit according to the present invention.

#FIG. 3 is a detailed schematic diagram of an oscillator core within the VCO circuit.

#Please refer to FIG. 2 and FIG. 3. FIG. 2 is a schematic diagram of a voltage controlled oscillator (VCO) circuit 20 according to the present invention. FIG. 3 is a detailed schematic diagram of an oscillator core 22 within the VCO circuit 20. The oscillator core 22 comprises an LC tank 28 for generating a sinusoidal wave and a noise reducing circuit 30 for reducing phase noise generated by the LC tank 28. In addition, the oscillator core 22 includes a positive feedback circuit 32 electrically connected to both the LC tank 28 and the noise reducing circuit 30 for amplifying the sinusoidal wave.

#The positive feedback circuit 32 comprises a differential pair of bipolar junction transistors (BJT) Q11 and Q12, each transistor connected to the LC tank 28 for amplifying the sinusoidal wave generated by the LC tank 28.

#The noise reducing circuit 30 further comprises a T-shaped resistor configuration 34 that includes two first resistors R2 and a second resistor R3. Each first resistor R2 is connected to a base of one of the BJTs Q11 and Q12, which are labeled as nodes M and N. The second resistor R3 is connected to the first resistors R2 and to a DC bias voltage source VBIAS, which is supplied to the positive feedback circuit 32 through the T-shaped resistor configuration 34.