TRIMMABLE MULTI-TERMINAL CAPACITOR FOR A VOLTAGE CONTROLLED OSCILLATOR

Abstract: The invention provides a trimmable multi-terminal capacitor (202). The multi-terminal capacitor (202) comprises a plurality of capacitors (C1, C2) with each of the capacitors having a common terminal. The capacitors are formed on a layer of dielectric material (210) having a common plate of conductive material (212) disposed on its top surface, and a plurality of separate plates of conductive material (214, 216) disposed on its lower surface. The capacitance value of each of the capacitors is separately tunable to tight tolerances. The capacitor is particularly suited to be used in a voltage controlled oscillator (VCO) (200) to couple a plurality of devices to a resonator (204).

Description: TRIMMABLE MULTI-TERMINAL CAPACITOR FOR A VOLTAGE CONTROLLED OSCILLATOR BACKGROUND OF THE INVENTION Field of the Invention This invention relates to capacitors and, more particularly, to a trimmable multi-terminal capacitor for use with a voltage controlled oscillator (VCO). History of the Prior Art Progress in electronics technology has resulted in increasingly smaller electronic devices capable of performing increasingly sophisticated functions. One area in which this progress has been evident is the telecommunication area. Miniaturization in telecommunications electronics has allowed systems designers to design sophisticated systems using smaller and lighter equipment. The results of this are seen in the worldwide proliferation of cellular and related telecommunications systems. Electronic circuitry used in modern day equipment often requires accurate, small capacitance values. An example of this type of circuitry is a voltage controlled oscillator (VCO). VCOs are used to generate radio frequency (RF) signals for transmitters and receivers in telecommunications equipment. A VCO typically comprises a VCO module, that includes a resonator and coupling capacitors as part of the module. In a VCO two or more discrete coupling capacitors are needed to couple an output terminal ofthe resonator to other components in the circuit. Typically the resonator would be coupled to a varactor diode and transistor, with the varactor diode and transistor being connected to other components of the circuit. During the manufacture of a VCO module it is necessary to tune the oscillating circuit to the right operating conditions. In order to tune the module, the capacitance values ofthe coupling capacitors must be adjusted. With discrete capacitors this is a difficult process. Before the circuit is assembled the capacitors must be carefully measured and selected. At the small values used in typical VCO applications, which may be less than IpF, the capacitors may have a tolerance value of up to +/-0.1 pF. If a capacitor of 0.2pF is needed, this level of tolerance allows the value of the capacitor to be from 0.1 to 0.3pF. A tolerance of this level is a +/-50% deviation that could critically affect circuit performance. Additionally, the two discrete capacitors are not only purely capacitive, but have a more complex effect in a circuit. FIG. 1 illustrates the conventional technique for connecting a resonator 104 in a voltage controlled oscillator (VCO) circuit 100. Output terminal 102 of resonator 104 is connected to capacitors 106 and 108 at points 1 12, 1 14 and 1 16. A varactor diode and transistor (not shown) for the oscillator circuit 100 are connected to capacitors 106 and 108 at points 120 and 1 1 8, respectively. The two discrete capacitors 106 and 108 require two soldering points each for connection to the circuit 100. These soldering points introduce stray inductances and resistive losses into the circuit. Also, the capacitances have a complex component model including stray inductances and some resistive losses in the device packaging that adds further losses in the circuit. This results in a deteriorated Q value for the coupling of the resonator to the circuit. Q is defined as l/(2πf c CR s ) where f is the operating frequency of the circuit, C is the capacitance ofthe capacitor, and R s is the equivalent series resistance of the capacitor. A low Q value may cause undesirable losses within the circuit and effect the signal to noise performance of the VCO at high frequencies. The effect is more pronounced at frequencies above 1GHZ. Also, when discrete capacitors are used, there is transmission phase deviation caused by the longer signal paths resulting from using two separate capacitors. Because ofthe above, once the capacitors 106 and 108 are measured, selected and soldered into circuit 100, the actual in-circuit capacitance value may not be acceptable for the particular application. In order to tune the VCO 100 of FIG. 1 for optimum performance within a specified frequency band, the oscillator parameters must be tuned after assembly. The VCO may be tuned by hand scratching material away from resonator 104 or by changing the values of capacitors 106 and 108. Scratching material away from resonator 104 shifts the operating frequency range of the VCO up or down depending on where the material is removed from. Changing the value of capacitor 106 has the effect of both shifting the operating frequencies up and down and, widening or narrowing the band of operable frequencies, depending on whether the change in capacitance is an increase or decrease. Changing the value of capacitor 108 has the effect of lowering the Q value or, changing the oscillation properties ofthe VCO, depending on whether the change in capacitance is an increase or decrease. The value of capacitor 106 determines the capacitive coupling of the varactor to the resonator 104. Increasing the value of capacitance for capacitor 106 increases the total capacitive coupling of the varactor to the resonator 104. A higher total capacitive coupling for the varactor decreases the value ofthe operating frequencies and increases the width ofthe frequency band within which the VCO may be tuned. Decreasing the value of capacitance for capacitor 106 decreases the total capacitive coupling ofthe varactor to the resonator 104. A lower total capacitive coupling for the varactor increases the value ofthe operating frequencies and decreases the width ofthe frequency band within which the VCO may be tuned. The capacitor 108 determines the capacitive coupling ofthe resonance circuit to the resonator transistor and, therefore, the Q value of the resonance circuit. Increasing the value of capacitance for capacitor 108 results in a lower Q value for the circuit and, therefore, a lower signal to noise ratio (C N). Decreasing the value of capacitance for capacitor 108 decreases the capacitive coupling ofthe resonance circuit to the transistor. If the capacitive coupling is too low, the VCO may not oscillate because of low feedback to the resonator transistor. Because ofthe above, when tuning the VCO it may be necessary to change the values of capacitors 106 and 108 to adjust the width of the frequency band within which the VCO operates or tune the Q value of the circuit. The replacement of capacitors such as capacitors 106 and 108 is a time consuming and difficult. Also, it would be difficult to determine the effect of a replacement capacitor of a different value. Since the replacement capacitor would require new soldering, which replaces the stray inductances and resistive losses of the existing capacitor with the stray inductances and resistive losses ofthe replacement capacitor, the in circuit capacitance value of the replacement capacitor could vary considerably from its predicted value. It would provide an advantage then to have capacitors with small capacitance values of fine design tolerances, with the capacitance values being tunable at high RF frequencies. It would provide a further advantage if these capacitors were easily and accurately tunable while in the circuit and could be used with a minimum number of soldering points and minimum lead length, thereby providing a higher Q value and lower phase deviation. Additionally, it would be advantageous if the capacitors were relatively easy to manufacture and package. SUMMARY OF THE INVENTION The present invention provides a trimmable multi- terminal capacitor for use in radio frequency (RF) circuits. The multi-terminal capacitor provides a plurality of capacitors of small capacitance values while avoiding the problems associated with using discrete capacitors. The capacitors are easily tunable while in the circuit, after circuit assembly, and may be tuned to the fine tolerances required for RF applications. Since the capacitors may be manufactured to be tunable within a specific range for a certain application, the range may be set wide enough so that it is not necessary to replace the capacitors to tune a circuit. Use of the multi-terminal capacitor requires a minimum number of soldering points and reduces lead lengths, resulting in a higher Q value and less losses within the circuit. The multi-terminal capacitor comprises a plurality of capacitors, with one terminal of each capacitor being in common. The capacitors are formed on a layer of dielectric material having a common plate of conductive material disposed on its top surface, and a plurality of separate plates of conductive material disposed on its lower surface. The separate plates of conducting material on the lower surface of the dielectric material do not contact each other. The common plate of conducting material forms a common terminal of the capacitors. The separate plates each form the remaining terminal of one of the capacitors. The capacitance value of each ofthe capacitances is separately tunable to tight tolerances. The tuning is done by removing material from the portion ofthe common conducting plate that directly opposes the separate plates, across the dielectric layer, of the capacitors it is desired to tune. The common conducting plate includes calibrated trimming tabs that may be removed for the purpose of tuning. The capacitor may be used in a voltage controlled oscillator(VCO) to couple a plurality of devices to a resonator. The coupling is accomplished by connecting the common terminal of the multi-terminal capacitor to the resonator terminal and, the remaining terminal of each capacitor to one of the devices. The terminal of one capacitor is connected to the VCO transistor and the terminal of the other capacitor is connected to the VCO varactor. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a conventional technique for connecting a resonator to a voltage controlled oscillator circuit; FIG. 2 illustrates the use of a trimmable multi-terminal capacitor according to the invention to couple a resonator to a voltage controlled oscillator circuit; FIG. 3 illustrates a top view of a trimmable multi- terminal capacitor in accordance with the present invention; and FIG. 4 illustrates a bottom view of a trimmable multi-terminal capacitor in accordance with the present invention. DETAILED DESCRIPTION FIG. 2 illustrates a trimmable multi-terminal capacitor according to the invention implemented within a voltage controlled oscillator (VCO) circuit. VCO circuit 200 comprises multi-terminal capacitor 202 and ceramic resonator 204. Ceramic resonator 204 comprises a central terminal 206 and a casing 208. Capacitor 202 comprises dielectric layer 210, a common conducting plate 212 disposed on the top surface 218 ofthe dielectric layer 210, and two separate conducting plates 214 and 216 (shown by dotted line) disposed on the bottom surface (not shown) of dielectric layer 210. Multi-terminal capacitor 202 is used to couple resonator 204 to a varactor diode (not shown) and a transistor (not shown) in the VCO circuit 200. To implement the coupling, capacitor 202 is connected to the center terminal 206 of ceramic resonator 204 by connecting terminal 206 to common plate 212, and connecting one of each of the two conducting plates 214 and 216 on the lower surface of dielectric layer 210 to the varactor diode and transistor, respectively. Referring now to FIGS. 3 and 4, therein are illustrated a top view and a bottom view, respectively, of an embodiment of a trimmable multi-terminal capacitor 202 in accordance with the invention. Capacitor 202 comprises a dielectric layer 210, a common conducting plate 212, and two separate conducting plates 214 and 216. Common conducting plate 212 is disposed on the top surface 218 of dielectric layer 210 (FIG. 3). Conducting plates 214 and 216 are disposed on the bottom surface 220 of dielectric layer 210 so as not to contact each other (FIG. 4). Conducting plates 214 and 216 form two capacitors Cl and C2, respectively, with the portions of conducting plate 212 aligning the area of each of plate 214 and 216. Conducting plate 212 comprises a terminal common to both capacitors. Dielectric layer 210 may comprise any suitable type of dielectric material such as, for example, A1 2 0 3 . Conducting plates 212, 214 and 216 may be any suitable conducting material such as, for example, Ag. Because dielectric layer 210 may be constructed with an accurate and uniform thickness and the surface area of the conducting plates 212, 214 and 216 may be accurately defined, capacitances Cl and C2 can be constructed with a low design tolerance. Conducting plate 212 comprises a plurality of tabs, 222, 224 226, 228, 230, 232 and 234, which may be selectively removed in order to tune each ofthe individual capacitors Cl and C2 to a desired capacitance value. Tabs 222, 224, 226, and 228 are removed to tune the value of Cl and, tabs 230, 232 and 234 are removed to tune the value of C2. Tabs 222, 224, 226 and 228 are each of an equal surface area, that is calculated to allow tuning of Cl by decreasing the value of Cl in equal increments. Tabs 230, 232 and 234 are also each of an equal surface area, that is calculated to allow tuning of C2 by decreasing the value of C2 in equal increments. The tabs may be removed by handtrimming with a cutting tool, for example, or by laser trimming. For the application shown in FIG. 2, a trimmable multi-terminal capacitor for use at PCS frequencies in the 1800-2000 MHZ range has been manufactured having values for initial capacitances C 1 and C2 of .6pF and .3pF, respectively. These values for Cl and C2 may then be trimmed down as far as .3pF and .15pF, respectively, in order to tune the VCO circuit to the correct coupling factors for both the varactor and oscillator transistor. In the VCO of FIG. 2 it is not necessary to replace Cl and C2 to tune the circuit. It is also not always necessary to tune the resonator by hand scratching. The initial values of Cl and C2 can be set to values larger than necessary, and the circuit resonant frequency can be measured and C 1 and C2 trimmed down in value to tune the circuit. The process of measuring and trimming can be repeated until the desired results are reached. It is not necessary to wait for solder to cool down before measuring after changing the capacitance values. The process of tuning the VCO is quick, saves work and allows accuracy in tuning. The multi-terminal capacitor 202 also provides the advantage of being easily packagable in tape and reel. With tape and reel packaging the multi-terminal capacitor could be automatically mounted. It is believed that the operation and construction of the present invention will be apparent from the foregoing description and, while the invention shown and described herein has been characterized as a particular embodiment, changes and modifications may be made therein without departing from the spirit and scope ofthe invention as defined in the following claims.

Claims: WHAT IS CLAIMED IS: 1. A multi-terminal capacitor, comprising: a dielectric layer having a first and a second surface, said surfaces being substantially parallel in relation to one another; a common conducting plate disposed on said first surface, said common conducting plate comprising a terminal of said multi-terminal capacitor; and a plurality of separate conducting plates disposed on said second surface, wherein said separate conducting plates and said common conducting plate are aligned to form a corresponding plurality of capacitors, and each of said separate conducting plates comprises another terminal of said multi- terminal capacitor. 2. The multi-terminal capacitor of claim 1 further comprising a plurality of tabs formed within each portion of said common conducting plate aligned with, and forming a capacitor with, one of said separate conducting plates, said tabs being selectively removable to tune the capacitance values of each of said plurality of capacitors. 3. The multi-terminal capacitor of claim 2 in said common conducting plate comprises a first terminal, and in which said plurality of separate conducting plates comprises a second and third plate, said second and third plates comprising a second and third terminal, respectively. 4. The multi-terminal capacitor of claim 3 in which said plurality of tabs formed within the portion of said first conducting plate aligned with , and forming a capacitor with, said second plate comprises a first plurality of tabs, each of said first plurality of tabs being of a first surface area, and said plurality of tabs formed within the portion of said first conducting plate aligned with, and forming a capacitor with, said third plate comprises a second plurality of tabs, each of said second plurality of tabs being of a second surface area. 5. The multi-terminal capacitor of claim 1 in which said common conducting plate comprises a first terminal, and in which said plurality of separate conducting plates comprises a second and third plate, said second and third plates comprising a second and third terminal, respectively. 6. A voltage controlled oscillator circuit comprising: a resonator having an output terminal; and a multi-terminal capacitor comprising. a dielectric layer having a first and a second surface, said surfaces being substantially parallel in relation to one another; a common conducting plate disposed on said first surface, said common conducting plate comprising a terminal of said multi-terminal capacitor, said terminal being connected to said output terminal of said resonator; and a plurality of separate conducting plates disposed on said second surface, wherein said separate conducting plates and said common conducting plate are aligned to form a corresponding plurality of capacitors, and each of said separate conducting plates comprises another terminal of said multi-terminal capacitor. 7. The voltage controlled oscillator of claim 6 further comprising a plurality of tabs formed within each portion of said first plate aligned with, and forming a capacitor with, one of said separate conducting plates, said tabs being selectively removable to tune the capacitance value of each of said plurality of capacitors. 8. The voltage controlled oscillator of claim 7 in which said common conducting plate comprises a first terminal, and in which said plurality of separate conducting plates comprise a second and a third conducting plate and said second and third separate conducting plates comprise a second and third terminal, respectively. 9. The voltage controlled oscillator of claim 8 in which said plurality of tabs formed within the portion of said first conducting plate aligned with, and forming a capacitor with, said second plate, comprises a first plurality of tabs, each of said first plurality of tabs being of a first surface area, and said plurality of tabs formed within the portion of said first conducting plate, aligned with, and forming a capacitor with, said third plate comprises a second plurality of tabs, each of said second plurality of tabs being of a second surface area. 10. The voltage controlled oscillator of claim 6 in which said common conducting plate comprises a first terminal, and in which said plurality of separate conducting plates comprises a second and a third conducting plate and said second and third separate conducting plates comprise a second and third terminal, respectively. 1 1. A multi-terminal capacitor, comprising: a dielectric layer having a first surface and a second surface, said first and second surfaces being substantially parallel to one another; a first conducting plate disposed on said first surface, said first conducting plate having a first surface area; a second conducting plate disposed on said first surface, said second conducting plate having a second surface area, wherein said second conducting plate is separate from, and noncontacting with, said first conducting plate; and a third conducting plate disposed on said second surface, said third conducting plate having a third surface area that is greater than said first and second surface areas combined, wherein said first conducting plate aligns with a first portion of said third conducting plate to form a first capacitor and, said second conducting plate aligns with a second portion of said third conducting plate to form a second capacitor. 12. The multi-terminal capacitor of claim 1 1 further comprising: a first plurality of tabs formed within said first portion of said first conducting plate; and a second plurality of tabs formed within said second portion of said second conducting plate; wherein each tab of said first and second plurality of tabs is selectively removable to tune the values of said first and second capacitors. 13. The multi-terminal capacitor of claim 12 in which said first plurality of tabs comprises a plurality of tabs each being of an equal first surface area, and said second plurality of tabs comprises a plurality of tabs each being of an equal second surface area.

Inventors: AARLIN MICHAEL, AARLIN, MICHAEL
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1996
Main classification: H01G4
IPC codes: H01G4, H01P7, H03B5, H03H9