Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1999-07-19
2001-01-30
Pascal, Robert (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S077000
Reexamination Certificate
active
06181216
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to voltage-controlled oscillators (VCOs), and more particularly a voltage-controlled oscillator for use in a communications device, such as a modulator or receiver.
In all VCO's, phase noise degrades the performance of the VCO. Newer designs of VCOs have improved the phase noise performance, consequently previously unimportant contributions to the phase noise are now more significant. It is therefore desirable to further reduce the phase noise.
One technique for reducing the phase noise is to provide an external (i.e., off-chip) filter on the supply voltage line. For example, a resistor-capacitor combination (R/C) can be coupled externally to the power supply line. However, this technique reduces the supply voltage or the supply current for a given supply voltage to the VCO, thereby degrading performance of the VCO.
Alternatively, it has been attempted to add an external inductor-capacitor (L/C) combination to avoid the voltage drop inherent in the above device. Unfortunately, this technique results in little improvement below the cutoff frequency (f
c
) of the device, resulting in unrealistic component values.
Another technique for reducing phase noise is disclosed in U.S. Pat. No. 4,621,241, in which elaborate arrangements are made to reduce the phase noise. To improve the phase noise of the circuit, a series capacitor is included in a feedback loop, which includes impedance not present in the Colpitts or Clapp oscillators. Additional impedance (in the form of a resistor) is connected between the feedback output of the amplifier and ground. In this patent, low-pass filtering of noise is accomplished using a resistor to ground and a blocking capacitor in the feedback path. However, the values of the blocking capacitor and resistor used for the filter must be selected for a variety of purposes, which requires the circuit designer to choose less than the optimum values for these elements depending on the requirements of the circuit.
The present invention is therefore directed to the problem of developing a method and apparatus for improving the phase noise performance of a VCO without reducing the supply voltage or the supply current to the VCO.
SUMMARY OF THE INVENTION
The present invention solves this problem by providing a shunt inductor-capacitor series combination to a pre-scalar port, which is directly connected to the drain of the oscillator FET. In combination with resistance between the supply voltage Vcc and the FET's drain, the resulting resistor/capacitor combination can significantly improve the phase noise performance without degrading other parameters and without additional changes to the integrated circuit.
According to one aspect of the present invention, the inductor should have high impedance over the frequency range of the oscillator.
According to another aspect of the present invention, the capacitor should be a low equivalent series resistance (ESR), high capacitance component. One example of such a capacitor is a tantalum capacitor.
According to yet another aspect of the present invention, a voltage-controlled oscillator includes a field-effect transistor, a prescalar output port coupled to the drain of the field-effect transistor, a capacitor coupled between the prescalar output port and the drain of the field-effect transistor, and a filter coupled between the drain and the pre-scalar capacitor to ground.
According to another aspect of the present invention, the filter used in the voltage-controlled oscillator preferably includes an inductor and a capacitor coupled in series.
According to another aspect of the present invention, the voltage-controlled oscillator preferably includes a resistor coupled between the drain and a voltage source.
Alternatively, the voltage-controlled oscillator includes two resistors coupled in series with one resistor coupled to the voltage source and the other resistor coupled to the drain. In this alternative embodiment, the voltage-controlled oscillator preferably includes a capacitor coupled to the series resistors and to ground.
According to another aspect of the present invention, the voltage-controlled oscillator preferably includes a resistor coupled to a voltage source and a gate of the field-effect transistor.
According to yet another aspect of the present invention, a method for improving phase noise performance of a voltage-controlled oscillator fabricated on an integrated circuit, includes the steps of: coupling an on-chip resistor between a supply line and the integrated circuit, and coupling a large, off-chip capacitor between the resistor and ground to filter noise from the supply line.
According to yet another aspect of the present invention, the above method preferably includes the step of coupling an off-chip inductor in series with the large capacitor.
According to yet another aspect of the present invention, the above method preferably includes the step of coupling the large capacitor to a pre-scalar output port through an inductor.
According to yet another aspect of the present invention, in an oscillator including a transistor in a Colpitts configuration formed on an integrated circuit, a method for reducing phase noise includes the steps of coupling a resistor-capacitor network between a supply voltage source and the transistor, and coupling a filter to the transistor and to ground.
According to another aspect of the present invention, the above method preferably includes the step of coupling the filter to a pre-scalar output port on the integrated circuit.
According to another aspect of the present invention, the above method preferably includes mounting the filter off-chip.
According to yet another aspect of the present invention, an oscillator includes a field-effect transistor in a Colpitts configuration, a resistor coupled from the source to ground, a capacitor coupled between the source and the gate, another capacitor coupled between the source and ground. In this embodiment, the two capacitors provide a phase-shifted feedback signal to the gate. The oscillator further includes a varactor tank coupled to the gate, which varactor tank has a voltage variable reactance that is used to tune oscillation of the oscillator to a desired frequency. The oscillator also includes a voltage source and a resistor-capacitor network coupled between the drain and the voltage source.
According to another aspect of the present invention, the resistor-capacitor network of the above oscillator preferably includes two resistors coupled in series and a shunt capacitor coupled to the two resistors and to ground. In this embodiment, the shunt capacitor minimizes local oscillator leakage onto the supply voltage line.
According to another aspect of the present invention, the above oscillator further includes a voltage-controlled oscillator input, a phase-locked loop generating a correction voltage output to the voltage-controlled oscillator input, and a pre-scalar output port coupled to the drain and the phase-locked loop. In this embodiment, the pre-scalar output provides a reference signal to the phase-locked loop.
According to another aspect of the present invention, the above oscillator preferably includes a filter network coupled to the pre-scalar output port reducing base-band noise from the supply voltage line. One possible implementation of the filter network includes a shunt network.
According to another aspect of the present invention, the shunt network of the above oscillator preferably includes a small inductor and a low equivalent source resistance capacitor. In this embodiment, supply noise is filtered without reducing voltage or current supplied to the oscillator.
According to yet another aspect of the present invention, inductor of the above oscillator removes the shunt capacitance at an oscillation frequency, avoiding any reduction in signal to the phase-locked loop circuit.
According to another aspect of the present invention, in the above oscillator, the low ESR capacitor works with resistance on-chip between the supply voltag
Choe Henry
General Instrument Corporation
Mayer Fortkort & Williams LLC
Pascal Robert
Wiliiams Karin L.
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