GB2302446A

GB2302446A - Ovenized crystal assembly and method of manufacture

Info

Publication number: GB2302446A
Application number: GB9512387A
Authority: GB
Inventors: Moshe Avidan; Yehuda Eder; Yair Shapira
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1995-06-17
Filing date: 1995-06-17
Publication date: 1997-01-15
Anticipated expiration: 2015-06-17
Also published as: DE19623799A1; IL118522A0; CN1150718A; GB9512387D0; GB2302446B

Description

1 1 1 OVENEM CRYSTAL ASSEMBLY AND METHOD OF?4ANUFACTURE

Field of the Invention

2302446 This invention relates to an ovenized crystal assembly for an oscillator. It is particularly applicable to, but not limited to use in an oscillator for a single side band (SSB) radio. The invention also relates to a method of manufacture of a crystal assembly.

Backeround of the Invention An ovenized compensated crystal oscillator (WX0) comprises a crystal encased in a closed housing with an internal heater which ' stabilizes the crystal and other oscillator components at a uniform and constant high temperature. By maintaining the crystal at an artificially elevated temperature, the frequency of oscillation of the crystal is stabilized and is not subject to external temperature fluctuations. Such oscillators are useful in SSB receivers, in which the high stability of reference oscillator oscillation is required.

An existing WX0 for an SSB radio is known to comprise a crystal in a metallic can housing clipped onto a metal bracket, on which a power transistor is bolted. The crystal and power transistor are mounted on a printed circuit board (PCB), with their leads mounted through holes in the PCB. The metal bracket extends upwards and outwards from the PCB. The metal bracket with its crystal and power transistor are encased in a closed housing. The entire construction is bulky and does not lend itself to modern surface-mount manufacturing techniques.

There is a need for a smaller, cheaper oscillator assembly.

Summaj:y of the Invention According to the present invention, an ovenized crystal assembly for an oscillator is provided comprising: a crystal having a metallic casing, a heat conducting substrate, having a first surface on which the casing of the crystal is mounted in thermal contact with the substrate and a resistive 2 heating element disposed on a second surface of the substrate, opposite to the first surface.

By providing the heating element and the crystal on opposite surfaces of the substrate, the whole assembly is very compact. The crystal can be mounted on the substrate by a surface mount technique.

By a preferred feature of disposing the heating element across a substantial area of the second surface of the substrate, efficient, even and uniform heat transfer to the crystal is achieved.

Brief Description of the Drawings

Fig. 1 is a stereoscopic view of an oscillator assembly in accordance with the invention, without its surrounding housing.

Fig. 2 is a stereoscopic view of the assembly of Fig. 1 from below.

Detailed DescriDtion of the DrawiWs Referring to Fig. 1, a PCB 10 is shown on which a crystal-housed in. a 2D metallic casing (hereafter "crystal 11") is mounted, with leads 12 and 13 soldered to tracks on the upper surface of the PCB 10. Other oscillator components 14 are mounted on the PCB 10 by surface mount device (SMD) manufacturing techniques. The crystal 11 is mounted by similar techniques, that is to say by first gluing the crystal 11 to the PCB 10, with the leads 12 and 13 positioned in contact with their respective tracks and then reflow soldering the entire assembly, so that the leads 12 and 13 and the components 13 are soldered in place. The PCB 10 is made of alumina and acts as a heat conducting substrate.

The assembly comprises connection leads 15, connecting to edges of the PCB 10, for mounting the entire assembly on to another PCB.

A temperature sensitive element 16 (for example a thermistor) is mounted on the PCB 10 between the leads 12 and 13.

Referring to Fig. 2, it is shown that a resistive heating element 20 is formed across the undersurface of the PCB 10 by a carbon deposition technique ie. a printing technique known in the art of PCB and multi-chip module manufacture as a printed resistor. The resistive heating element occupies a substantial area on the underside of the PCB 10, ie. an oven at 3 least or approximately as large as the area occupied by the crystal 11 on the upper surface. The resistive heating element 20 is disclosed between connector rails 21 and 22, which have leads connecting them to the upper surface of the PCB 10.

In operation, a current is applied to the leads 21 and 22, causing the resistive element 20 to heat up. The heat is conducted easily through the thickness of the PCB 10 and heats up the crystal 11, as well as the other associated elements 14 and 16. The entire assembly is encased in a housing (not shown) with suitable heat insulating properties, so that the temperature is stable and unaffected by outside influences.

The entire assembly is inexpensive to manufacture and is compact.

The heat sensitive element 16 controls, by other components 14, the current supplied to the rails 21 and 22 and maintains the temperature constant.

The frequency of the crystal 11 is most sensitive to temperature changes in the vicinity of the leads 12 and 13. By mounting the heat sensitive element 16 between these leads, the heat sensitive element 16 is in a position at which it has greatest control over frequency sensitive temperature changes in the oscillator assembly.

2D The PCB 10 is preferably made of alumina of 96% purity with a thickness of 0.6 mm or less and a heat conductance of at least 0.07 cal/sec.cm.'c, ie. 29.3 j/s.m.'c.

Claims

1.

4 An ovenized crystal assembly for an oscillator comprising:

a crystal having a metallic casing, a heat conducting substrate, having a first surface on which the casing of the crystal is mounted in thermal contact with the substrate and a resistive heating element disposed on a second surface of the substrate, opposite to the first surface.

2. A crystal assembly according to claim 1, wherein the resistive heating element is formed on the second surface by a printing technique.

3. A crystal assembly according to claim 1, wherein the crystal has leads emerging from the casing which are connected to the substrate and wherein a temperature sensing element is disposed on the substrate between the leads.

4. A crystal assembly according to claim 1, wherein the resistive element is disposed on the second surface by a printing technique.

5. A crystal assembly according to claim 1, wherein the substrate is 25 aluminium oxide.

6 A method of manufacture of a crystal assembly comprising the steps of printing a resistive heating element onto a first surface of a substrate and surface mounting a metallic casing containing a crystal onto a second 30 surface of the substrate, opposite to the first surface.