Oscillator having unidirectional current flow between resonant circuit and active devices

This invention relates to reduction of Q loss in oscillators, and particularly to reduction of Q loss in wideband voltage controlled oscillators (VCOs).

Voltage controlled oscillators (VCOs) employ a resonant circuit excited by active devices. The resonant circuit often employs an inductor and variable capacitor, coupled in a resonant LC relation. Adjustment of the variable capacitor alters the LC resonant frequency, and hence the frequency of the oscillator. Wideband VCOs (those tuning ¼ octave or more) exhibit a significant change in the resonant impedance of the LC circuit due to the changing quality factor, or Q, of the inductor with frequency and/or the changing Q of the capacitor with frequency and applied voltage.

In the design of VCOs employing bipolar, metal oxide (MOS) and gallium arsenide (GaAs) active devices, the loop gain is designed to ensure oscillation of acceptable magnitude under worst-case conditions. Under more favorable conditions, the oscillation may be so robust that the active device effectively saturates or “bottoms out” over an appreciable portion of the frequency cycle. When the active device bottoms out, it effectively shorts the resonant circuit to ground, reducing the Q of the resonant circuit (the ratio of reactance to loss resistance) and degrading the oscillator phase noise and jitter.

To overcome this problem, oscillators have been operated from current sources, rather than voltage sources. As a result, the average current through the oscillator is limited, and the oscillator voltage drops under more robust conditions. This technique maintains a higher Q, but it also decreases oscillation amplitude, resulting in degradation of phase noise and jitter by decreasing the ratio of the oscillation amplitude to the circuit noise sources.

The suitability of an oscillator for a given use is normally governed by the worst performance the oscillator may provide at any given frequency at which it is expected to operate. Even if an oscillator operates favorably under some conditions and/or frequency settings, the suitability of the oscillator is still measured by its worst-case performance. Consequently, there is a need to improve the worst-case performance of an oscillator to make the worst-case performance less different from better-case performance.

A circuit according to the present invention is arranged to provide output signals over a range of oscillating frequencies. The circuit includes a resonant circuit, at least one active circuit device operatively coupled to the resonant circuit to supply energy to the resonant circuit, and at least one unidirectional device coupled to the active circuit device. The unidirectional device permits current to flow between the active circuit device and the resonant circuit when the active circuit device adds energy to the resonant circuit and serves to impede draining energy from the resonant circuit due to increased output signal amplitude.

The sole FIGURE is a circuit diagram of a voltage controlled oscillator according to one embodiment of the present invention.

The FIGURE illustrates a single-ended wideband voltage controlled oscillator (VCO) 10 having a power supply 12 supplying power to the center tap of coil 14 . In preferred embodiments, oscillator 10 is fabricated in an integrated circuit by well-known IC fabrication processes. Active N-channel MOSFET devices M 1 and M 2 are coupled through diode-connected N-channel MOSFET devices M 3 and M 4 , respectively, to opposite ends 16 and 18 of coil 14 . End 16 of coil 14 is coupled to one side of variable capacitor C 1 , and to the control electrode (gate) of MOSFET M 2 . Similarly, end 18 of coil 14 is coupled to one side of variable capacitor C 2 , and to the control electrode (gate) of MOSFET M 1 . The opposite ends of capacitors C 1 and C 2 are coupled together, and through resistor R 1 to a tuning voltage source 22 , which in turn is coupled to a common potential, such as electrical ground. In preferred embodiments, capacitors C 1 and C 2 are varicaps, which may be junction diodes whose insulating barriers widen with increasing reverse voltage to change the capacitance between the anode and cathode terminals of the diode, or MOS devices that exhibit a voltage-dependent capacitance between the gate and channel.

MOSFETs M 3 and M 4 are diode-connected such that their drain electrodes are coupled to their gate electrodes and to the respective ends 16 and 18 of coil 14 . Active MOSFET M 1 has its drain electrode coupled to the source electrode of MOSFET M 3 , its gate electrode coupled to the drain electrode of MOSFET M 4 and to end 18 of coil 14 , and its source electrode coupled to ground. Similarly, active MOSFET M 2 has its drain electrode coupled to the source electrode of MOSFET M 4 , its gate electrode coupled to the drain electrode of MOSFET M 3 and to end 16 of coil 14 , and its source electrode coupled to ground. The body or substrate of each MOSFET M 1 , M 2 , M 3 and M 4 is coupled to electrical ground, and the body or core of coil 14 is coupled to electrical ground.

In operation, the voltage from supply 12 operates the active MOSFETs M 1 and M 2 and the resonant circuit 20 formed by coil 14 and varicaps C 1 and C 2 . The capacitance of varicaps C 1 and C 2 , and hence the resonant frequency of circuit 20 , is established by the tuning the bias voltage source 22 to the varicaps through resistor R 1 .

The diode-connected MOSFETs M 3 and M 4 prevent the active MOSFETs from shorting the resonant circuit. More particularly, during normal operation, the resonance of series capacitors C 1 and C 2 with the inductor of coil 14 causes a circulating current to flow at the resonant frequency. The transfer of charge due to the circulating current reverses the conductive states of MOSFETs M 1 and M 2 . The circuit oscillates at the frequency established by parameters of resonant circuit 20 . If the oscillation amplitude becomes too great, a risk exists that one or both of MOSFETs M 1 and M 2 become saturated (bottoms out) during a portion of the frequency cycle. If this occurs, the saturated MOSFET will try to hold the coil potential at that end constant, shunting to ground the circulating current that would otherwise flow between the capacitor and inductor. Thus, without diode-connected MOSFETs M 3 and M 4 , MOSFETs M 1 and M 2 might remove energy from the resonant circuit thereby reducing its Q. Should this occur, the slope of the resonant circuit phase verses frequency curve is reduced, resulting in a reduced ability to maintain a steady frequency in the presence of noise sources. The inclusion of diode-connected MOSFETs M 3 and M 4 , or such other diode devices that the fabrication process may allow to be constructed, prevents current flow that might reduce energy in the resonant circuit, thereby maintaining the resonant circuit phase slope at its small-signal value.

The phase noise spectrum decreases with frequency separation from the carrier frequency at about −6 to −9 dB/octave with a noise floor asymptote that differs from the carrier frequency by an amount approximately equal to the frequency of oscillation divided by twice the resonant.circuit Q. Hence, the distance of the noise floor asymptote from the carrier frequency varies inversely with the Q. With unidirectional devices, such as MOSFETs M 3 and M 4 , the Q is increased, thereby reducing the distance of the noise floor asymptote from the carrier and proportionately lowering the noise level at lesser offsets. Hence, the oscillator with the unidirectional devices exhibits a lower phase noise level at any given offset within the resonant circuit half-bandwidth. Consequently, the “worst case” performance of the oscillator (which would occur when the oscillation is most robust) is improved.

The present invention employs uni-directional devices in series with the normal current flow of the active device to allow normal flow of current when the active device adds energy to the resonant circuit and to inhibit a drain of energy from the resonant circuit as oscillation amplitude grows. While the embodiment described herein employs diode-connected MOSFETs, any type of uni-directional, or current directional, device may be employed, including diodes, Schottky diodes, and low-threshold devices. Moreover, while the embodiment described herein is a balanced circuit, the invention is equally applicable to single-ended circuits as well. Nor is the invention limited to any particular type of semiconductor technology, as the invention might be implemented in P-channel, complementary or bi-polar devices. Moreover, the specific oscillator design is not limiting on the invention, so center-tapped coil 14 , as a source of voltage, might be eliminated with some other supply arrangement, as in the case of complementary MOS.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.