Voltage-controlled oscillator mounted on laminated printed circuit board

Abstract: A voltage-controlled oscillator (VCO) mounted on a laminated printed circuit board (10) which has a surface layer (12) a plurality of intermediate layers (14,16), and a back layer (18). Various component parts of the VCO are arranged on the surface layer (12). One of the intermediate layers (14,16) which underlies the surface layer (12) constitutes a ground potential layer and is removed in a part thereof corresponding to the area (4) where the parts of the VCO are positioned.

Description: The present invention relates to the mounting of oscillators on printed circuit boards, and it will be illustrated by reference to a voltage-controlled oscillator (VCO) mounted on a laminated circuit board. In parallel with the miniaturization of electronic equipment, the trend toward a laminated printed circuit board, or multilayer wiring board, is accelerated for accommodating electronic parts and circuits in a dense arrangement. With a printed circuit board for high frequency applications, for example, it is a common practice to connect one of intermediate or internal layers to ground, so that high-frequency circuitry on the surface of the board may operate stably. A substrate layer of glass epoxy or similar material intervenes between nearby ones of the successive layers. When a VCO, for example, is mounted on such a laminated printed circuit board, one of the intermediate layers underlying the layer on which an oscillating smiconductor, or transistor, of the VCO as well as various electronic parts determining the oscillation frequency thereof are arranged is connected to ground to serve as a ground potential layer. In this configuration, a stray capacitance is developed between each of the the leads of the semiconductor, metallic parts and printed circuit pattern and the ground potential layer. Assuming that the printed circuit board has a given thickness, such stray capacitances are greater than in an ordinary printed circuit board whose back serves as a ground potential layer. The oscillation frequency, therefore, changes due to the scattering among the mounting conditions of the individual parts and the change in the configuration of the printed circuit pattern. Generally, the parts that determine the oscillation frequency of a VCO are implemented as parts having great Q values in order to increase the C/N (Carrier-to-Noise) ratio of the generated signal. In practice, however, the oscillation frequency of a VCO mounted on a laminated printed circuit board is determined by composite impedance of the impedances of the parts with great Q values and the stray capacitances ascribable to the glass epoxy layers. In the case that such stray capacitances have great influence on the oscillation frequency, the overall Q value and, therefore, the C/N ratio is criticially lowered since glass epoxy has a small Q value as a substrate. In the specification of European Patent Application No. 86402026.8, which was published under No. 216699 on April 1 1987, and in the specification of corresponding United States Patent No. 4,792,773, which was published on December 20 1988, there was proposed an ultra-high frequency circuit having circuit components on a substrate, which was formed either by a ceramic wafer, or by a semi-conductor material wafer which had been made semi-insulating and which had an air layer in a recess in the substrate, or in both a sole plate and a metallised layer, between the dielectric substrate and a ground plane so as to introduce an air capacity in series with a parasitic capacitance. The extent of protection sought is conferred by the terms of the appended claims which define a new invention involving an inventive step. In a particular embodiment of the present invention which will be described by way of example, a VCO is mounted on a laminated printed circuit board which generates a signal with an improved C/N ratio and stable frequency. In the particular example to be described a VCO is mounted on a laminated printed circuit board having a surface layer, a plurality of intermediate layers, and a back layer, at least one of the plurality of intermediate layers functioning as a ground potential layer, and the VCO having a semiconductor for oscillation and electronic component parts determining the oscillation frequency thereof and being on the surface layer of the printed circuit board. A part of the ground potential layer only is removed over an area corresponding to a part of the area where the semiconductor and electronic component parts are disposed. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a sectional side elevation showing a specific structure of a laminated printed circuit board to which the present invention is applicable; Fig. 2 is a circuit diagram representative of a VCO embodying the present invention; Fig. 3 is a plan view showing various component parts of the illustrative embodiment which are mounted on a first layer of the circuit board shown in Fig. 1; and Fig. 4 is a block diagram schematically showing a specific arrangement for measuring the C/N ratio of a signal generated by the illustrative embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1 of the drawings, a laminated printed circuit board to which the present invention is practicable is shown and generally designated by the reference numeral 10. As shown, the circuit board 10 has a first layer or surface layer 12, second and third layers or intermediate layers 14 and 16, and a fourth or back layer 18. Various component parts of a VCO including an oscillating transistor are mounted on the first layer 12. The second layer 14 is connected to ground while the third layer 16 is adapted for a power source. An electric circuit pattern is formed on the fourth layer 18. Substrate layers made of glass epoxy 20, 22 and 24 intervene between the first and second layers 12 and 14, between the second and third layers 14 and 16, and between the third and fourth layers 16 and 18, respectively. A VCO embodying the present invention and having a Colpitts configuration is shown in Fig. 2. The VCO, generally 30, has an oscillating transistor 31, capacitors 32, 34, 36 and 38, an APC terminal 35, a dielectric resonator 40, a choke coil 42, and a varactor diode 44 which are the fundamental component parts for oscillation. The VCO 30 also has a resistor 46 and a choke coil 48 cooperating to apply a bias to the transistor 30, and a capacitor 50 for connecting the VCO 30 to the next stage. When an APC voltage is applied to the varactor diode 44 via the APC terminal 35 and the choke coil 42, the capacitance of the varactor diode 44 changes in matching relation to the APC voltage. The change in the capacitance of the varactor diode 44 is subjected to impedance conversion by the dielectric resonator 40 and capacitor 36 and then imparted to the collector of the transistor 30 as a change in inductance. As a result, the oscillating frequency of the transistor 30 is changed. Fig. 3 shows the various component parts of the VCO 30 which are mounted on the first layer 12 of the printed circuit board having the specific structure depicted in Fig. 1. The second or ground potential layer 14 undelies the first layer 12 with the intermediary of the glass epoxy substrate layer 20, as shown in Fig. 1. A characteristic feature of the present invention is that a part of the second or ground potential layer 14 corresponding to an area A enclosed by a dashed line and indicated by hatching in Fig. 3 is removed. Generally, the stray capacitance between the ground potential layer 14 and the components provided on the surface layer 12 is inversely proportional to the distance therebetween. When the ground potential layer 14 is absent in the area A where the various components are positioned as stated above, the stray capacitance is reduced. Therefore, the ground potential layer 14 does not lower the Q values of the component parts such as the transistor 30 and resonator 40 mounted on the surface layer 12. This is successful in preventing the C/N ratio from being lowered and, therefore, in insuring stable oscillation frequency at all times. Fig. 4 schematically shows a specific arrangement for measuring the C/N ratio of the output signal of the VCO 30. As shown, the arrangement, generally 60, has a power source 62, a low-noise linear amplifier 64, and a spectrum analyzer 66 as well as the VCO 30, Fig. 2. The power source 62 supplies a DC voltage to the VCO 30 and an APC voltage is applied to the VCO 30 via the APC terminal 35. Assume that the printed circuit board 10 on which the VCO 30 is mounted has a four-layer structure which is 0.8 millimeter thick and has the ground potential layer 14 thereof removed over the area A shown in Fig. 3. Then, the area in which the VCO 30 is mounted is about 2 centimeters square, so that the area A is about 1.5 centimeters square. When the VCO 30 is caused to oscillate at a frequency of 1 gigahertz, the oscillation is deviated by 25 kilohertz. The C/N ratio of the oscillation signal was measured to be about 85 decibels by the arrangement 60 and in the resolution bandwidth of 1 kilohertz. By contrast, when use was made of the ground potential layer 14 not removed in the area A as in the prior art, the C/N ratio was measured to be about 82 decibels. The illustrative embodiment, therefore, achieves an improvement in the C/N ratio by about 3 decibels over the prior art. In summary, it will be seen that the present invention provides a VCO capable of generating a signal with an improved C/N ratio and stable frequency. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope of the invention as defined in the appended claims.

Claims: An oscillator mounted on a surface layer (12) of a laminated printed circuit board (10), the board (10) including a plurality of intermediate layers (14) (16), at least one (14) of the plurality of intermediate layers functioning as a ground potential layer, and respective substrate layers (20) (22) (24) between the surface layer (12), an intermediate layer (14) (16) and a back layer (18), the oscillator having a semiconductor (31) for oscillation and electronic component parts (32) (34) (35) (36) (38) (40) (42) (44) determining an oscillation frequency of the semiconductor (31), a part of the ground potential layer (14) being removed over an area corresponding to at least a part of the area where the semiconductor (31) and the electronic component parts (32) (34) (35) (36) (38) (40) (42) (44) are disposed, and a space being provided between adjacent substrate layers (20) (22), which space is equal in thickness to the thickness of the removed ground potential layer (14). An oscillator as claimed in claim 1, wherein a substrate layer is made of a glass epoxy material. An oscillator as claimed in claim 2, wherein there are two intermediate layers (14) (16), one of the intermediate layers (16) being for connection to a power source. An oscillator as claimed in claim 3, wherein an electronic circuit pattern for the oscillator is provided on the back layer (18) of the printed circuit board (10). An oscillator as claimed in any one of the preceding claims characterised in that the board (10) is a multilayer wiring board and the oscillator is mounted on the surface layer (12). An oscillator as claimed in any one of the preceding claims characterised in that the oscillator is a voltage controlled oscillator (VCO). An oscillator as claimed in claim 6, characterised in that the voltage controlled oscillator is a Collpitts oscillator. An oscillator as claimed in claim 5, characterised in that the substrate layers (20) (22) (24) respectively intervene between adjoining ones of the surface, intermediate, and back layers (12) (14) (16) (18) of the multilayer wiring board. A method of mounting an oscillator circuit on a multilayer printed circuit board (10) having a ground potential layer (14) as one of intermediate layers thereof, and respective substrate layers (20) (22) (24) between surface, intermediate, and back layers (12) (14) (16) (18), including the steps of mounting the oscillator circuit on the surface layer of the board (10), and removing from the ground potential layer (14) a part above which at least part of the oscillator circuit is disposed, thereby providing a space between adjacent substrate layers (20) (22) which space is equal in thickness to the thickness of the removed ground potential layer (14).

Inventors: NORIMATSU HIDEHIKO C O NEC COR, NORIMATSU, HIDEHIKO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H05K1
IPC codes: H05K1, H05K3, H03B1, H03B5