1.  Field of the Invention
The present invention relates a microwave oscillator which performs oscillation at several GHz and which has an oscillation frequency that can be changed. 
2.  Description of the Related Art
FIG.  2 illustrates a microwave oscillator circuit of the related art.  A transistor 21, which is an active device, for oscillation has a collector connected to one end of a first strip conductor 22 and has a base connected to one end of a second strip conductor 23.  The first strip conductor 22 and the second strip conductor 23 are provided on an insulating substrate (not shown) so as to oppose each other.  A cylindrical dielectric resonator 24 is provided between the first strip conductor 22 and the second strip conductor 23.  Consequently, the first strip conductor 22, the dielectric resonator 24, and the second strip conductor 23 form a feedback loop with the base as an input terminal and the collector as an output terminal. 
In addition, a third strip conductor 25 is provided on the insulating substrate to couple with the dielectric resonator 24.  The third strip conductor 25 has one end connected to ground through a varactor diode 26, and a control voltage is applied to the other end. 
When the control voltage is changed to alter the capacitance of the varactor diode 26, the magnetic field distribution in the third strip conductor 25 changes relative to the dielectric resonator 24.  This causes a change in coupling between the dielectric resonator 24 and the third strip conductor 25.  Thus, the changes in the capacitance of the varactor diode 26 and in the coupling cause a change in the total resonant capacitance including the dielectric resonance 24, thereby changing the oscillation frequency. 
However, in the above microwave oscillator of the related art, the dielectric resonator is coupled with the first through third strip conductors, thus reducing the apparent Q factor of the dielectric resonator.  Thus, the microwave oscillator has a problem in that the phase noise of an oscillation signal is reduced to about 60 dB/kHz. 
In addition, the dielectric resonator is coupled with the first and second strip conductors to form the feedback loop and is also coupled with the third strip conductor for changing the oscillation frequency.  It is therefore difficult to set the positions thereof so that the couplings with the respective strip conductors are balanced. 
Accordingly, it is an object of the present invention to provide a microwave oscillator having an improved phase noise of an oscillation signal and allowing easy setting of the coupling between a dielectric resonator and strip conductors for feedback. 
To this end, according to an aspect of the present invention, there is provided a microwave oscillator.  The microwave oscillator includes a transistor for oscillation, an insulating substrate on which the transistor is mounted, a first strip conductor which is provided on the insulating substrate and which has a first end connected to an output terminal of the transistor, and a second strip conductor which is provided on the insulating substrate and which has a first end connected to an input terminal of the transistor.  The microwave oscillator further includes a varactor diode and a TE01-mode dielectric resonator which is provided on the insulating substrate and which couples with the first strip conductor and the second strip conductor.  A first end of the varactor diode is connected to a second end of the first strip conductor or a second end of the second strip conductor.  A second end of the varactor diode is RF-grounded, thereby changing the capacitance of the varactor diode.  The dielectric resonator may be coupled only with the first and second strip conductors, the microwave oscillator allows the apparent Q factor to be maintained at a high level and allows an improvement in the phase noise of the oscillation signal. 
In addition, an optimal coupling can be achieved by shifting the dielectric resonator along the first and second strip conductors. 
Preferably, the microwave oscillator further includes a third strip conductor having a free end.  A second end of the varactor diode is connected to the strip conductor.  The length of the third strip conductor is one quarter of the wavelength of an oscillation frequency.  With this arrangement, the second end of the varactor diode can be securely RF-grounded. 
Preferably, the transistor is a bipolar transistor.  The bipolar transistor has a base as the input terminal, a collector as the output terminal, and an emitter that is RF-grounded.  With this arrangement, a higher gain is achieved even at a transition frequency of about 20 GHz, thereby allowing oscillation at a frequency of 10 GHz or higher.