GB2198591A

GB2198591A - Microphonics-free voltage controlled oscillators

Info

Publication number: GB2198591A
Application number: GB08628993A
Authority: GB
Inventors: Ariel Luzzatto; Meir Haim; Gaby Fuchs
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1986-12-04
Filing date: 1986-12-04
Publication date: 1988-06-15
Anticipated expiration: 2006-12-04
Also published as: GB8628993D0; GB2198591B

Abstract

Microphonics-free frequency generation is achieved by soldering microphonics-susceptible components, and preferably the complete voltage controlled oscillator, onto conductive tracks printed directly on a rigid mass. The rigid mass is preferably a ceramic resonator. <IMAGE>

Description

MI CROPHONI CS -FREE VCO Field of Invention This invention relates to microphonics problems with Voltage Controlled Oscillators (VCOs). More particularly, this invention relates to the design of microphonics-free VCOs.

Background of the Invention Microphonics problems are ever present in the design of frequency generation circuits -- particularly VCOs.

Microphonics result in signal noise and arise from a miriad of vibration-induced circuit impedance variations.

Mechanical vibration of the VCO's internal resonator can be a source of impedance variation. Mechanical vibration of other devices and structures in the immediate proximity of the resonator can cause capacitive loading and consequent impedance.variation. Mechanical strain and bending in the circuit traces and ground paths can cause impendance variations. Mechanical vibrations in components -- such as aging multilayer chip capacitors -- can also be a source of microphonics problems.

A number of different circuit topologies and devices have been proposed and have had varying degrees of success at solving the microphonics problem.

One type of resonator, a microstrip inductor printed on an alumina hybrid substrate, has been tried as a solution. But, while the rigidity of alumina reduces mechanical vibration of the resonator, the inherent structure of the microstrip resonator creates other problems. The microstrip must be made very wide to obtain reasonable Q for the resonator (even so, the Q is still too low for good noise performance). The sizable microstrip creates long ground paths and subjects it to capacitive loading from other devices and structures in the immediate vicinity. Extensive shielding is necessary to minimize these capacitive loading effects. Thus, microstrip resonators have not satisfactorily resolved the microphonics problem.

Fortunately, helical resonator cells provide very high resonator Qs, but unfortunately present somewhat different difficulties. They require costly, heavy, cast mountings and require substantial shielding. Shield vibration and very long ground paths make for even more pronounced microphonics.

Although stripline-type resonators reduce the effect of vibrations in the immediate vicinity, it is a costly solution and is still problematical due to long, unbalanced ground paths.

Thigh permittivity ceramic resonator cells can have very high Qs (like helical cells) and, when soldered to alumina substrates, seemed to offer some promise toward solving the microphonics problem because the resonator is, itself, rigid and vibration-free. Also, it is often shielded on five sides from the effects of nearby vibrations. However, the ground paths remain long and the elasticity of the solder joint during vibration varies the strain on the ground plane of the cell, each resulting in impedance variations and contributing toward the microphonics problem. Moreover, precise cell placement is crucial to manufacturing repeatability and no convenient means of trimming cell frequency exists.

Hence, none of the foregoing alternatives has resulted in a sufficiently acceptable solution to the nagging microphonics problem.

It is, therefore, one object of the invention to provide a system for microphonics-free frequency generation.

Summary of the Invention According to the present invention, there is provided a scheme for microphonics-free frequency generation by soldering microphonics-susceptible components (preferably, the complete VCO) onto a conductive pattern printed directly on a rigid mass (preferably, directly on the ceramic resonator itself).

Brief Description of the Drawings An exemplary system in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a dual, microphonics-free VCO according to the present invention.

Figure 2 is a schematic diagram of one of the VCO's used in Figure 1.

Like reference symbols are used throughout the drawings to designate like items.

Detailed Description of the Invention An analysis of the factors contributing to the microphonics problems and a methodical elimination of each of them resulted in the present invention -- a dual, microphonics-free VCO -- by soldering multiple complete VCOs onto conductive patterns printed directly on a ceramic filter resonator.

Figure 1 includes a dual, microphonics-free VCO, generally designated 10, according to the present invention. It includes a conventional three-cell 900 MNz ceramic filter resonator (20) with the center cell (30) shorted. Conductive patterns (40) are printed on the remaining two cells (50 & 60). Surface mount (chip) capacitors (C1 - C10), resistors (R1 - R3), diodes (Dl),'and transistors (Ql) (Ql) are reflow-soldered to the conductive patterns (40) to construct dual 450 MHz microphonics-free VCOs (10).

This technique substantially reduces microphonics problems because the circuit traces (40) and ground paths (40) are kept to an absolute minimum (the the physical size of, and consequently the majority of the cost of, the entire VCO is reduced to that of the resonator); resonator vibrations are suppressed by the mass and rigidity of the ceramic; and microphonics due to vibrations in the vicinity are minimized by the shielding on the bottom and four sides of the ceramic resonator. Thus, each source of vibration that contributes to the microphonics problem has been systematically eliminated or substantially reduced.

Further advantages are obtained with the present invention. The center frequency of the cell (50 or 60) can now be trimmed by merely trimming the conductive pattern incorporated for that purpose (70). Alumina substrates, as used in the past, have been eliminated, reducing cost. The size of the entire VCO has been shrunk to that of the largest component -- the ceramic resonator -- saving both space and cost.

Figure 2 is a schematic diagram of one of the VCOs used in Figure 1. The following values were used in constructing a 450 MHz VCO.

ELEMENT VALUE C1 3.9Pf C2 6.8Pf C3 100Pf C4 2.7Pf C5 lPf C6 5.lPf C7 10Pf C8 100Pf C9 100Pf C10 12Pf D1 MMBV109 Q1 MMBR901 R1 500 OHM R2 10K OHM R3 1K OHM Vb 4V Vc 10V While the preferred embodiment of the invention has been described and shown, it will be understood by those skilled in the art that other variations and modifications of this invention may be implemented. For example, it is most economical, in terms of space and cost, to mount the VCO directly on the ceramic resonator. However, some, but not all, of the benefits of the present invention could be realized by increasing the ceramic surface and mounting only some of the microphonics-susceptible components there.

This modified arrangement would, however, reintroduce the disadvantage of increasing the circuit traces and ground paths. Other arrangements can be envisaged, but what has been shown and described is the preferred embodiment of the invention.

In summary, according to the present invention, there has been provided a system for microphonics-free frequency generation by soldering microphonics-susceptible components (preferably, the complete VCO) onto conductive patterns printed directly on a rigid mass (preferably, directly on the ceramic resonator itself).

Claims

Claims

What we claim and desire to secure by Letters Patent is: 1. A scheme for microphonics-free frequency generation characterized by: placing microphonics-susceptible components in electronic conduction about a relatively massive, relatively rigid foundation.
2. A scheme for microphonics-free frequency generation characterized by: means for providing a relatively massive, relatively rigid foundation, means for electronic conduction incorporated thereabout, and microphonics-susceptible means for frequency generation placed in electrical communication with the conduction means.
3. A scheme as claimed in Claims 1 or 2 wherein the foundation comprises: microphonics-free component means for frequency generation.
4. A scheme as claimed in Claim 3 wherein the microphonics-free component means comprise: ceramic resonator(s).
5. A scheme as claimed in Claims 1 or 2 wherein the microphonics-susceptible means comprise: portions of voltage-controlled oscillator(s).
6. A scheme as claimed in Claims 1 or 2 wherein the microphonics-susceptible means comprise: complete voltage-controlled oscillator(s).
7. A scheme as claimed in Claims 1 or 2 wherein the electronic conduction comprise: conductive patterns about the foundation to receive the microphonics-susceptible means.
8. A microphonics-free VCO characterized by: ceramic resonator(s) with conductive pattern(s) thereupon and microphonics-susceptible portion(s) of VCO(s) mounted thereupon and in electrical communication therewith.
9. A microphonics-free VCO substantially as shown or described herein.