WO1998044583A1

WO1998044583A1 - Stable oscillator using an improved quality factor microstrip resonator

Info

Publication number: WO1998044583A1
Application number: PCT/US1998/006164
Authority: WO
Inventors: Nitin Jain; Xiangdong Zhang
Original assignee: Whitaker LLC
Current assignee: Whitaker LLC
Priority date: 1997-03-31
Filing date: 1998-03-30
Publication date: 1998-10-08
Anticipated expiration: 1999-09-30
Also published as: AU6784498A

Abstract

A resonator comprises a microwave microstrip disposed on a low dielectric substrate. The microstrip has a length substantially equal to half of a wave length of a resonating frequency. Opposite ends of the microstrip are shorted to ground potential. Another embodiment of a resonator comprises a microwave microstrip disposed on a low dielectric substrate. The microstrip has a length substantially equal to a quarter wave length of a resonating frequency. An end of the microstrip is shorted and an opposite is unterminated. A circuit having an oscillating frequency copmrises an oscillator having an input and an output. The output of the oscillator is coupled to the input of the oscillator through a resonator comprising the aforementioned half wave length or quarter wave length resonators.

Description

Stable Oscillator Using An Improved Quality Factor Micro-;tri-p Resonator

Resonators are an important component in many microwave circuits, such as tuned amplifiers, filters, and oscillators. US Patent 5,309,119 discloses use of a cylindrical dielectric resonator formed of high permittivity ceramic and disposed close to a microstrip in a feedback circuit to en ancs the frequency stability of an oscillator. Dielectric resonators have a high quality factor (Q) at high frequencies, but are difficult to physically position on a substrate contributing variability to the yield of a microwave circuit. In addition, dielectric resonators occupy a large physical area on a microwave circuit substrate which operates in contravention of an interest in miniaturization. US Pa ent 5,309,119 also discloses use of a ring shaped thin film resonator made of super conducting material arranged close to a microstrip and placed in the feedback path of an oscillator circuit. The ring resonator has a circumference having a length equal to a single wave length or an integer multiple of a single wave length of the resonating frequency. Accordingly, the diameter of t e ring is a fixed quantity. Advantageously, the ring resonator has a high loaded Q a microwave frequencies. It would be desirable, however, to achieve the same loaded Q as a ring resonator in a smaller physical size. The text book "Microwave Resonators and Filters", Section 9-2, discloses half wave length and quarter wave length coaxial resonators. This reference discloses lengths of coaxial lines that exhibit resonant behavior at frequencies where the length of the line is a multiple of a half or quarter wavelength of the resonating frequency. In the interest of miniaturization, it is desirable to place microwave circuits on small thin substrates. A length of a coaxial line is geometrically less than optimum for placement on a microwave substrate. There are also practical disadvantages to coupling a microwave circuit to a length of coaxial line.

Another conventional half wave length resonator comprises a length of microstrip unterminated at opposite ends. Coupling to the resonator is achieved by disposing conductive traces proximate to, but not in direct contact with the unterminated ends . Disadvantageously, this half wavelength resonator exhibits an undesirably low loaded Q at frequencies above 20 GHz.

Accordingly, there is a need for a smaller and higher Q resonator for use at microwave frequencies.

According to one embodiment, a resonator comprises a microwave microstrip disposed on a low dielectric substrate. The microstrip has a length equal to a half wavelength of the resonating frequency. Opposite ends of the microstrip are shorted to ground potential . According to another embodiment, a resonator comprises a microwave microstrip disposed on a low dielectric substrate. The microstrip has a length equal to a quarter wave length of a resonating frequency. An end of the microstrip is shorted and an opposite is unterminated .

Advantageously, a resonator according to the teachings of the present invention is reduced in size, exhibits an improved loaded Q, and maybe coupled to more easily than prior art resonators.

A circuit having an oscillating frequency comprises an oscillator having an input and an output. The output of the oscillator is coupled to the input of the oscillator through a resonator. The resonator comprises a microstrip having a length equal to half of a wave length of the oscillating frequency, wherein ends of the microstrip are shorted to ground potential.

A circuit having an oscillating frequency comprises an oscillator having an input and an output. The output of the oscillator is coupled to the input of the oscillator through a resonator. The resonator comprises a microstrip having a length equal to quarter of a wave length of the oscillating frequency, wherein an end of the microstrip is shorted and an opposite end is unterminated.

Advantageously, an oscillator according to the teachings of the present invention exhibits improved frequency stability and occupies a smaller physical space than prior art oscillators.

Embodiments of the invention will now be described by way of example with reference to the accompaning drawings in which,

Figure 1 is an illustration of an embodiment of a quarter wave length resonator according to the teachings of the present invention.

Figure 2 is an illustration of an embodiment of a half wave length resonator according to the teachings of the present invention.

Figure 3 is a conceptual diagram of the resonator in a feedback loop of an oscillator to improve oscillator stability. Figure 4 shows an embodiment of an oscillator stabilized with a quarter wave length resonator according to the teachings of the present invention.

Figure 5 shows an embodiment of an oscillator stabilized with a half wave length resonator according to the teachings of the present invention.

With specific reference to Figure 1 of the drawings , there is shown an embodiment of a resonator according to the teachings of the present invention in which a 2 micron thick, 400 micron wide gold microstrip 1 having a fixed length is printed on a low dielectric substrate 2. A suitable substrate comprises 200 micron thick Corning 7070 glass which has a dielectric constant of approximately 4.0. Generally, the width of the microstrip 1 is greater than 1.5 times the thickness of the substrate and is nominally 2 to 4 times the thickness of the substrate. For suitable substrates other than Corning 7070 glass as described, the appropriate width of the microstrip is limited by a dielectric constant of the substrate, an operating frequency, and the point at which there is excitation of the transverse mode. For purposes of the present description, a microstrip is a configuration comprising a conductive layer connected to ground potential disposed adjacent to a dielectric layer. A conductor carrying signal is disposed over the dielectric layer with the conductor exposed to air. The resulting layered configuration is ground-dielectric- conductor- air. A stripline is a configuration comprising two conductive layers connected to ground potential. Each ground layer is adjacent to a dielectric layer. A conductor is disposed between the two dielectric layers resulting n a layered ground- dielectric -conductor- dielectric- ground configuration. With respect to the present invention, whil< all embodiments of the invention are described for a microstrip, stripline embodiments are as appropriate and are considered to be within the scope of the description and claimed invention. For all embodiments described, there is particular advantage when used at resonating frequencies equal to or greater than 20 GHz. In the embodiment shown in Figure 1, a length of the microstrip 1 corresponds to a quarter wave length of a resonating frequency. A first end 3 of the microstrip 1 is open or unterminated and a second end 4 opposite the first end 3 is shorted to ground potential. While technical distinctions may exist, for purposes of the present description, the terms "open" and "unterminated" are used interchangeably. When the microstrip 1 is excited at the resonating frequency, the shorted and open boundary conditions at the first and second ends 3,4 set up a quarter wave length standing wave. The unterminated first end 3 of the microstrip 1 is an area of high electric field 7 of the quarter wave length microstrip 1. Coupling to the microstrip 1 is accomplished by disposing conductive traces 5, 6 proximate to and oriented perpendicularly relative to the area of high electric field 7. Input conductive trace 5 and output conductive trace 6 couple energy in and out of the area of high electric field 7 across a coupling gap 8. For a width of microstrip 1 of 400 microns at 20 GHz, the coupling gap 8 is between 2 mils and 8 mils (50 microns and 200 microns) and is nominally 3 mils (75 microns) . Advantageously, the quarter wave length microstrip resonator occupies less space than a conventional half wave length resonator having two unterminated ends while the excitation of parasitic waves such as surface and leaky waves is lower. With specific reference to Figure 2 of the drawings , there is shown an embodiment of a resonator according to the teachings of the present invention in which a half wave length microstrip 9 comprises a fixed length of two micron thick, 400 micron wide microstrip printed on a low dielectric substrate 2. A suitable substrate 2 is Corning 7070 glass. A length of the half wave length microstrip 9 corresponds to a half wave length of the desired resonating frequency. The first and second ends 10, 11 of the half wave length microstrip 9 are shorted to ground potential. Coupling to the half wave length microstrip resonator is accomplished by disposing the input and output conductive traces 5, 6 proximate to a area of high electric field 12. The area of high electric field 12 in the half wave length microstrip embodiment is positioned at a geometric center of the half wave length microstrip 9 equidistant from the first and second ends 10,11. Due to physical layout constraints, appropriate coupling may be achieved by positioning the input and output conductive traces 5,6 in adjacent parallel relation to each other and proximate to a general vicinity of the area of high electric field 12. Coupling occurs across the coupling gap 8 which is approximately 3 mils (75 microns) wide for a 400 micron wide half wave length microstrip 9. For purposes of improving circuit layout flexibility and minimizing the occupied space, the first and second ends 10,11 of the half wave length microstrip 9 may be curved off of a straight line to resemble a semi- circle. The half wave length microstrip embodiment of a resonator as described advantageously occupies less space than the conventional ring resonator with improved loaded Q and reduced parasitic waves.

With specific reference to Figure 3 of the drawings, there is shown a block diagram illustrating one of the uses of the resonator according to the teachings of the present invention. Specifically, a resonator 13 is placed in the feedback path of an active device such as a microwave oscillator 14 for improved oscillator stability. Implementations of the circuit shown in Figure 3 for embodiment of the quarter wave length microstrip and half wave length microstrip resonators are shown in Figures 4 and 5 of the drawings respectivel .

With specific reference to Figures 4 and 5 of the drawings, there is shown two embodiments of a microwave oscillator circuit comprising first and second active devices 15, 19. The active devices 15, 19 as shown in the drawings comprise high electron mobility transistors (HEMT) . Other active devices such as metal semiconductor field effect transistors (MESFET) and pseudomorphic high electron mobility transistors (PHEMT) are also appropriate. A source 16, 20 of each active device, 15, 19 is connected to each other and to ground potential 19. A drain 18 of the first active device 15 is connected to a gate 21 of the second active device 16 through a first capacitor 24 hs zing a nominal value between 0.1 and 0.2 pF . The first capacitor 24 provides DC block and impedance matching functions. A drain 22 of the second active device 19 is connected to the output conductive trace 6 as well as an output port 27.

The output conductive trace 6 is connected to the output port 27 through second capacitor 25 having a nominal value of approximately 20 pF. The second capacitor 25 provides DC blocking. In the embodiment shown, the first capacitor 24 comprises an interdigital capacitor and the second capacitor 25 comprises a metal insulator metal (MIM) capacitor.

With specific reference to Figure 4 of the drawings, there is shown an embodiment of a microwave oscillator circuit used with a quarter wave length resonator according to the teachings of the present invention in which the area of high electric field 7 of the quarter wave length microstrip 1 is positioned proximate to the input and output conductive traces 5,6. The first end 3 of the quarter wave length microstrip 1 is an open circuit and the second end 4 of the quarter wave length microstrip 1 is shorted to ground. The input and output conductive traces 5,6 are separated from the area of high electric field by the coupling gap 8 of approximately 3 mils (75 microns) . Advantageously, use of the quarter wave length resonator shown in Figure 4 provides a simpler layout and reduced size of a microwave oscillator circuit relative to conventional resonators .

With specific reference to Figure 5 of the drawings, there is shown an embodiment of microwave oscillator circuit used with a half wave length resonator according to the teachings of the present invention in which the area of high electric field 12 of the half wave length microstrip 9 is positioned proximate to the input and output conductive traces 5,6. In the embodiment shown, the input conductive trace 5 is connected to a gate 17 of the first active device 15 through third capacitor 26 comprising an interdigital capacitor having a nominal value of between 0.1 and 0.2 pF. By way of illustration, the half wave length microstrip 9 is curved at either end to loosely resemble a semicircle. The area of high electric field 12 towards a center of the half wave length microstrip 9 is essentially straight so as to be perpendicularly oriented to the input and output conductive traces 5,6 over the width of the traces 5, 6. Accordingly, the essentially straight area of high electric field 12 maintains a uniform coupling gap 8.

Changes in construction will occur to those skilled in the art and various apparently different modifications and embodiments may be made without departing from the spirit and scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only. It is therefore intended that the foregoing description be regarded to be illustrative rather than limiting, the invention being described in the appended claims .

Claims

Claims :

1. A resonator comprising a frequency dependent length of microstrip transmission line characterized in that: the resonator has first and second opposite ends and at least one of said ends is shorted to reference potential .

2. An amplifier operating at an operating frequency and having a feedback path, the operating frequency of the amplifier stabilized with a resonator in said feedback path, the resonator comprising a frequency dependent length of microstrip transmission line characterized in that: the resonator having first and second opposite ends and at least one of said ends is shorted to reference potential and said amplifier is complied to the resonator in an area of the resonator having maximum voltage potential.

3. A resonator as recited in claims 1 or 2 wherein said length of transmission line is a multiple of one half of a wavelength as defined by said operating frequency.

4. A resonator as recited in claims 1-3 wherein said first and second ends are shorted to reference potential.

5. A resonator as recited in claims 1 or 2 wherein said length of transmission line is an odd multiple of a quarter of a wavelength as defined by said operating frequency.

6. A resonator as recited in claims 1,2 or 5 wherein said first end is shorted to reference potential and said second end is unterminated.

7. A resonator as recited in claims 1-6 wherein the microstrip transmission line is disposed on a low dielectric substrate.

8. A resonator as recited in claim 8 wherein the substrate has a dielectric constant less than or equal to 5. . A resonator as recited in claim 8 wherein the substrate comprises glass having a thickness of at least 200 microns.