Compact voltage controlled oscillator for microwave applications

Abstract: In a voltage controlled oscillator, a dielectric resonator (13) is mounted on a base of a conductive housing, and a dielectric substrate (15) is secured to sidewalls (11) of the housing in a position spaced from the dielectric resonator. A first stripline (16) is secured to one surface of the substrate and electromagnetically coupled to the dielectric resonator, and an oscillating element (17) is electrically connected to the first stripline. A second stripline (18) is secured to the opposite surface of the substrate and capacitively coupled to the first stripline through the substrate. The second stripline is adapted to receive a frequency control voltage from an external circuit. A voltage-controlled variable capacitance element (19) is secured to the substrate and electrically connected to the second stripline.

Description: The present invention relates to a voltage controlled oscillator for use in the microwave region. A conventional voltage controlled oscillator for use in the microwave region includes first and second striplines secured to the base of a conductive housing, and a dielectric resonator located between the striplines to establish electromagnetic coupling between each stripline and the resonator. A varactor diode is electrically connected to one of the striplines and an oscillating element is connected to one end of the other stripline. By varying a voltage applied to the varactor diode, the oscillation frequency of the VCO is controlled. Because of the side-by-side arrangement of striplines with respect to the resonator, the overall size of the VCO is large, and implementation of a compact voltage controlled oscillator has been desired. US-A-4 871 983 discloses an electronically tuned dielectric resonator stabilized oscillator in which first and second striplines are mounted on the same surface of an alumina substrate. On the first and second striplines a fused silica spacer is arranged on which a third stripline and a dielectric resonator are mounted so that the third stripline extends orthogonally across the ends of the first and second striplines. The first and second striplines are connected to a field effect transistor, and a varacator tuner is connected to the third stripline. The third stripline provides a feedback path between the first and second striplines. The first and second striplines are spaced apart from each other by almost the length of the third stripline to generate a circular electric field in the dielectric resonator. Thus, the arrangement of US-A-4 871 983 is similar to the side-by-side arrangement of striplines discussed above, and has the same drawbacks. It is therefore an object of the present invention to provide a compact voltage controlled oscillator. This object is achieved by the voltage controlled oscillator according to the claims. According to the present invention, there is provided a voltage controlled oscillator comprising a dielectric resonator and a dielectric substrate spaced from the dielectric resonator. The substrate has a first surface facing the dielectric resonator and a second surface opposite to the first surface. A first stripline is secured to the first surface of the substrate and electromagnetically coupled to the dielectric resonator, and an oscillating element is electrically connected to the first stripline. A second stripline is secured to the second surface of the substrate and capacitively coupled to the first stripline through the substrate. The second stripline is adapted to receive a frequency control voltage from an external circuit. A voltage-controlled variable capacitance element is secured to the substrate and electrically connected to the second stripline. The stacked arrangement of the first and second striplines in relation to the dielectric resonator makes possible the reduction of the voltage controlled oscillator to one half of the prior art VCO. The present invention will be described in further detail with reference to the accompanying drawings, in which: Fig. 1 is a schematic plan view of a voltage controlled oscillator of the present invention; Fig. 2 is a cross-sectional view taken along the line 2-2 of Fig. 1; Fig. 3 is a cross-sectional view taken along the line 3-3 of Fig. 1; and Fig. 4 is a circuit diagram showing an equivalent circuit of the voltage controlled oscillator of this invention. Referring to Figs. 1 to 3, there is shown a voltage controlled oscillator according to the present invention. The oscillator comprises a conductive housing formed by a base member 10, a side-wall portion 11 and a top plate 12. A dielectric resonator 13 of cylindrical structure is supported from the base member 10 by means of a cylindrical conductive support member 14. A rectangular substrate 15 of dielectric material is secured to an inwardly extending upper portion 11a of side-wall 11 in a position spaced from the upper surface of the dielectric resonator 13. To the lower surface of substrate 15 is secured a first stripline 16 which extends along the length of the substrate 15 and is electromagnetically coupled with the dielectric resonator 13 to form part of an oscillating circuit. An oscillating element 17 is secured to the substrate 15. The oscillating element 17 has two dc supply terminals, one of which is connected to the left end of stripline 16 which is spaced a distance λ/4 from a point A aligned with the center axis of dielectric resonator 13, where λ is the wavelength of oscillations generated by the oscillating element when no frequency control voltage is supplied to the voltage-controlled oscillator. The right end of stripline 16 terminates at an arbitrary point. On the upper surface of substrate 15 is a second stripline 18 which is capacitively coupled to the first stripline 16 by substrate 15. The second stripline 18 extends along the length of substrate 15, with the right end thereof being terminated at a point spaced a distance of λ/4 from the point A and the left end being terminated at an arbitrary point The right end of stripline 18 is soldered as at 21 to an insulated conductor which extends through a hole 22 to a terminal 23. A pair of dc supply conductors also extends through hole 22 from terminal 23, one of the dc supply conductors being soldered to the right end of stripline 16 as shown at 24 and the other dc supply conductor extending alongside of stripline 16 to the other dc supply terminal of oscillating element 17. A voltage-controlled variable capacitance element, such as varactor diode 19 is secured on the substrate 15 and is electrically connected at one terminal thereof to the second stripline 18, the other terminal of the diode being coupled to a conductive strip 20 which is coupled to the housing. Microwave energy is extracted by means of a third stripline 25 which is secured to underside of dielectric substrate 15 and runs parallel to the stripline 16 so that stripline 25 is electromagnetically coupled with dielectric resonator 13 and stripline 16. Stripline 25 is soldered at 26 to a conductor 27 which extends through a hole 28 to a terminal 29 from which microwave energy is withdrawn to an external utilization circuit, not shown. In an equivalent circuit of the voltage controlled oscillator of this invention, shown in Fig. 4, oscillating element 17 has two equivalent output terminals, one of which is connected through a first half section of stripline 16 and through the dielectric resonator 13 to a first output port 30. The other output terminal of oscillating element 17 is connected through a second half section of stripline 16 to a second output port 31. An external load is coupled between output ports 30 and 31. As illustrated, the upper stripline 18 and varactor diode 19 constitute an LCR circuit which is capacitively coupled by dielectric substrate 15 to dielectric resonator 13, so that a variation of the capacitance value of varactor 19 causes the total capacitance of the voltage controlled oscillator to vary accordingly to alter the output frequency. By the stacked arrangement of the present Invention, the total volume of the voltage controlled oscillator is substantially reduced to one half of the prior art VCO. Since the upper stripline 18 is capacitively coupled with the lower stripline 16 which is strongly coupled electromagnetically with dielectric resonator 13, the output frequency of the device can be controlled with a lower control voltage than in the case of the prior art Experiments showed that the output frequency of the voltage-controlled oscillator linearly changed with a rate of 7 MHz per volt and the variation of the output power was within 3 dB when the control voltage is at 15 volts.

Claims: A voltage controlled oscillator comprising a cylindrical dielectric resonator (13), a dielectric substrate (15), a first stripline (16), an oscillating element (17) electrically connected to said first stripline (16), a second stripline (18) capacitively coupled to said first stripline, said second stripline (18) being adapted to receive a frequency control voltage from an external circuit, and a voltage-controlled variable capacitance element (19) electrically connected to said second stripline (18), characterized in that said first and second striplines (16, 18) are stacked in relation to said dielectric resonator (13); said substrate (15) is rectangular; said first and second striplines (16, 18) extend along the length of said substrate (15); said first stripline (16) and said oscillating element (17) are secured to a first surface of said substrate (15) so that the first stripline is electromagnetically coupled to said dielectric resonator (13); said second stripline (18) and said voltage-controlled variable capacitance element (19) are secured to a second, opposite surface of said substrate (15) so that said second stripline (18) is capacitively coupled to said first stripline (16) through said dielectric substrate (15); said first stripline (16) has an end spaced from a point (A) aligned with the center axis of the cylindrical dielectric resonator (13) a distance equal to the quarter wavelength of oscillations generated by said oscillating element (17) in the absence of said frequency control voltage; and said second stripline (18) has an end spaced from said point (A) aligned with said center axis a distance equal to said quarter wavelength. A voltage controlled oscillator as claimed in claim 1, further comprising a third stripline (25) secured to the first surface of said substrate (15) and being electromagnetically coupled with said first stripline (16) and said dielectric resonator (13), and terminal means (29) connected to said third stripline (25) for withdrawing high frequency energy.

Inventors: WATANABE HIDEAKI, WATANABE, HIDEAKI, Watanabe, Hideaki, c/o NEC Fukushima, Ltd.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H03B5
IPC codes: H03B5, H01P7, H03B1