GB2394365A

GB2394365A - Circuit board constructions

Info

Publication number: GB2394365A
Application number: GB0224354A
Authority: GB
Inventors: Niels-Henrik Lai Hansen; Hans Christian Blomberg; Anders Stensgaard Larsen
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2002-10-19
Filing date: 2002-10-19
Publication date: 2004-04-21
Anticipated expiration: 2022-10-19
Also published as: GB2394365B; GB0224354D0

Abstract

A circuit board construction comprises a circuit board 1 having an oscillator circuit 3 mounted on an oscillator mounting portion 5 of the circuit board 1. A slot or groove 7a,b,c is formed around the oscillator mounting portion 5 to provide a measure of mechanical isolation of the portion 5. The oscillator circuit 3 may be a voltage controlled oscillator circuit. The oscillator mounting portion 5 of the board 1 may be attached to other regions of the board 1 from which it is partially mechanically isolated by breaks or bridges 9a,b,c formed across the slot or groove 7a,b,c. There may be no more than three breaks or bridges 9a,b,c. The average depth of the slot or groove 7a,b,c may be at least 50% of the thickness of the circuit board 1. Alternatively the slot or groove 7a,b,c may extend though the entire thickness of the circuit board 1. The slot or groove 7a,b,c may extend around the oscillator mounting portion 5 to form a straight sided closed figure which may be a square or rectangle.

Description

1 2394365

TITLE: CIRCUIT BOARD CONSTRUCTIONS

FIELD OF THE INVENTION

5 The present invention relates to circuit board constructions, in particular constructions in which an oscillator device such as a voltage controlled oscillator is mounted on a printed circuit board.

10 BACKGROUND OF THE INVENTION

Electronic circuit arrangements are commonly manufactured by mounting the circuit components on a so called printed circuit board (PCB). Such a board is made of 15 an insulating material such as a glass - epoxy laminate having holes through it to facilitate mounting of circuit components on both faces so that they can be electrically connected to other components on the same face by soldering or to components on the opposite face by conducting pins 20 through the holes. In some cases, the circuit arrangements include components which form an oscillator circuit, especially a circuit including a solid state oscillator such as a quartz oscillator. In particular, the oscillator circuit may form a VCO (voltage controlled oscillator).

25 VCOs are widely used for example in radio communications terminals such as mobile telephones, radios and the like.

Where a VCO is mounted on a large PCB and the PCB is subjected in use to mechanical vibrations, the vibrations can cause minute changes to the VCO centre frequency to 30 occur. This results in noise known in the art as microphonics (translation of mechanical vibrations into

electrical noise). In general, the problem of microphonics associated with a VCO becomes greater with increasing operating frequency.

Some of the traditional ways of solving the problem of 5 microphonics arising when a VCO is mounted on a PCB include: l) applying vibration damping material at the VCO mounting positions; 2) fixing the PCB to a very stiff material using a 10 large number of fixing devices such as screws; 3) using a very stiff material (e.g. having ceramic-

like properties) for the PCB; 4) providing the VCO as a separate small module; 5) fixing the critical VCO components to the PCB using 15 a hardenable adhesive such as an epoxy or hard silicone composition. All of these known procedures are cumbersome, expensive and not easy or convenient to apply in a large scale manufacturing facility.

SU=ARY OF THE PRESENT INVENTION

According to the present invention in a first aspect there is provided a circuit board construction including a 25 circuit board having mounted thereon an oscillator circuit characterized in that the oscillator circuit is on an oscillator mounting portion of the circuit board around which is formed a slot or groove in the circuit board adapted to provide mechanical isolation of the oscillator 30 mounting portion.

The oscillator circuit may comprise a voltage controlled oscillator (VCO) circuit. An example of such a circuit is described in GB-A-2361122 by Motorola.

Individual components of the oscillator circuit may be 5 fixed to the circuit board. This may be by soldering which provides also circuit connections between the components.

The oscillator mounting portion of the circuit board may be attached to other regions of the circuit board from which it is mechanically isolated by breaks or bridges 10 formed across the slot or groove. Desirably, there are at least two, preferably three, such breaks. Desirably, there are not more than three breaks so that torsion of the circuit board in other regions is not transferred to the oscillator mounting portion.

15 Preferably, the average depth of the slot or groove is at least 50 per cent of the thickness of the circuit board.

Desirably, the slot or groove extends through the entire thickness of the circuit board (except of course where the breaks are present).

20 Each slot is preferably as wide as possible to give maximum isolation. However, the maximum length of the breaks across the slot is limited by the need to maintain strength of the circuit board at the breaks or bridges.

Preferably, the average width of the slot is between 0.5 mm 25 and 5 mm, especially between 1 mm and 3 mm.

The shape of the slot extending around the oscillator mounting portion of the circuit board preferably approximates to a straight-sided closed figure, especially a rectangle or square. Preferably, the breaks are formed 30 across the groove on two or three different sides of such a figure. Preferably, the breaks have a width, measured along the straight side of the closed figure formed by the slot,

which is not greater than one third of the length of the slot along the closed figure side. Ideally, the breaks, where three in number, would be formed at the apexes of an equilateral triangle and, where two in number, would be 5 formed at opposite ends of a diameter passing through the centre of the oscillator mounting portion. However, in practice the breaks need to be formed at positions as near as possible to the ideal positions but which suit the layout of other circuits on the board to facilitate 10 connections to the oscillator circuit from such other circuits. The slots may be formed in the circuit board in a known manner, e.g. by a known machining process such as milling. Thus, the board in the circuit board construction 15 according to the invention may be produced from a conventional circuit board machined to form the slots, the breaks being provided by unmachined parts of the board formed between the individual slots.

Desirably, the oscillator mounting portion of the 20 board is as small as possible to minimise problems caused by self resonance of that portion.

Beneficially, the invention allows the construction of an oscillator on a circuit board to be carried out in a manner which effectively minimises the problem of 25 microphonics described earlier and which can be applied inexpensively and efficiently.

According to the present invention in a second aspect there is provided a radio communications device including a circuit board construction according to the first aspect.

30 The communications device may be a radiotelephone device.

The device may for example be operable according to TETRA standards (as defined later).

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: 5 BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure l is a schematic plan view of a PCB on which is mounted a VCO circuit in accordance with an embodiment of the invention.

10 Figure 2 is a graph of vector error (EVE or error vector magnitude) probability distribution p(x) measured for a VCO circuit mounted on a PCB in a known manner.

Figure 3 is a graph of vector error (EVE) probability distribution p(x) measured for a VCO circuit mounted on a 15 PCB in accordance with an embodiment of the present invention. DESCRIPTION OF EMBODIMENTS OF THE INVENTION

20 AS shown in Figure l, a PCB 1 is constructed for use in a mobile radiotelephone. The PCB 1 has a VCO circuit 3 in a VCO board portion 5 mounted thereon. Individual components of the VCO circuit 3 are attached to the PCB l and to each other by soldering in a conventional manner. A 25 metallic shield 4 is also formed around the VCO circuit 3 and is soldered at its edges to the PCB 1. The board portion 5 is an island formed in the PCB 1 and separated from other regions of the PCB by slots 7a, 7b and 7c formed in the PCB 1. The slots 7a, 7b and 7c have been formed by 30 machining a conventional PCB. The slots 7a, 7b and 7c each occupy two sides of a right angled bend. The slots 7a, 7b and 7c a form a closed rectangular figure around the VCO

circuit 3 and VCO board portion 5 with the right angled bends forming the three of the corners of the closed figure. Breaks 9a, 9b and 9c are formed in the closed figure formed by the slots 7a, 7b and 7c and thereby attach 5 the VCO board portion 5 to other neighbouring regions of the PCB l beyond the slots 7a, 7b, 7c. In other words, the original board forming the PCB l is not machined in the regions where the breaks 9a, 9b and 9c are provided. The breaks 9a, 9b and 9c are provided on three different sides 10 of the rectangular figure formed by the slots 7a, 7b and 7c. The dimensions of the slots 7a, 7b and 7c and the breaks 9a, 9b and 9c are selected according to the criteria described earlier.

15 In use, when the PCB l having the VCO circuit 3 mounted thereon in the manner shown in FIG. l is used in a mobile radiotelephone, the problem of microphonics mentioned earlier is substantially eliminated or reduced.

We demonstrated this as follows.

20 Experimental data was taken as follows. The output signal from a VCO under test was amplified and used as an injection to a mixer. The mixer was provided with an input signal from a signal generator creating a clean O/4 DQPSK modulated signal of the kind produced according to TETRA 25 (TErrestial Trunked Radio) standards as defined by the ETSI (European Telecommunications Institute). The mixed output was then fed to a Vector Signal Analyzer which measures the vector error at the sampling points. Close in phase noise from the VCO is immediately seen as vector error at the 30 Vector Signal Analyzer. We found that picking up a distribution function of vector error by doing many tests

of the vector error was the best way of analyzing the impact of VCO being microphonic in the TETRA application.

The graphs of FIGs. 2 and 3 show vector error (EVM) probability distribution p(x) measured for a VCO circuit 5 mounted on a PCB with and without the slots 5a, 5b, 5c shown in FIG. 1. The curve labelled A in each of FIG. 2 and FIG. 3 shows RMS error and the curve labelled B in each of FIG. 2 and FIG. 3 shows peak error. Plotted against the X-

axis (horizontal axis) in each of FIG. 2 and FIG. 3 is the 10 vector error (EVM) measured as a percentage (%). EVM is defined in ETSI standard EN 300-392-2 clause 6.6.1.2.

During measurement of the EVM, the results are normalized so that the symbols lie on a circle with radius of 1. So the EVM as a percentage is measured relative to the symbol 15 distance from the centre of this circle.

The Y-axis (vertical axis) in FIG.s, 2 and 3 plots the probability distribution p(x). It is actually a measured number. EVM is measured many times on the same test object.

Each time a slightly different result is obtained owing to 20 noise from the VCO. These numbers are collected in bins of 0.25% size and the number in each bin is counted effectively creating a histogram. The histogram is normalized so that the area under the curve equals 1.

This can be used to calculate the probability that a 25 measurement will be at or below a defined limit simply by integrating the area under the curve from O to that limit.

Those skilled in the art will appreciate that an improvement is clearly shown by using the invention by reference to FIG. 3 compared with FIG. 2. For, instance, in 30 FIG. 3, the curve A is taller and much more narrow and centred at a lower vector error value than the

corresponding curve A in FIG. 2. The curve B is also centred at a lower vector error value.

Claims

1. A circuit board construction including a circuit board having mounted thereon an oscillator circuit characterized 5 in that the oscillator circuit is on an oscillator mounting portion of the circuit board around which is formed a slot or groove in the circuit board adapted to provide mechanical isolation of the portion.

2. A circuit board construction according to claim l and 10 wherein the oscillator circuit comprises a voltage controlled oscillator circuit.

3. A circuit board construction according to claim l or claim 2 and wherein the circuit board comprises a printed circuit board and individual components of the oscillator 15 circuit are fixed to the circuit board.

4. A circuit board construction according to any one of the preceding claims wherein the oscillator mounting portion of the circuit board is attached to other regions of the circuit board from which it is mechanically isolated 20 by breaks or bridges formed across the slot or groove.

5. A circuit board construction according to claim 4 wherein there are at least two such breaks.

6. A circuit board construction according to claim 5 and wherein there are not more than three breaks.

25

7. A circuit board construction according to any one of the preceding claims wherein the average depth of the slot or groove is at least 50 per cent of the thickness of the circuit board.

8. A circuit board construction according to claim 7 30 wherein the slot or groove extends through the entire thickness of the circuit board.

9. A circuit board construction according to any one of the preceding claims wherein the average width of the slot or groove is between 0.5mm and 5mm.

10. A circuit board construction according to claim 9 5 wherein the average width of the slot or groove is between 1 mm and 3 mm.

11. A circuit board construction according to any one of the preceding claims wherein the shape of the slot or groove extending around the oscillator mounting portion of 10 the circuit board approximates to a straight-sided closed figure.

12. A circuit board construction according to claim 11 and wherein the shape of the slot or groove extending around the oscillator mounting portion of the circuit board 15 approximates to a rectangle or square.

13. A circuit board construction according to claim 12 wherein breaks are provided across the slot or groove on two or three different sides of the closed figure.

14. A circuit board construction according to claim 13 20 wherein breaks have a width measured along the straight side of the closed figure formed by the slot which is not greater than one third of the length of the slot along the closed figure side.

15. A circuit board construction according to any one of 25 the preceding claims wherein the slots have been formed in the circuit board by a machining process.

16. A circuit board construction according to claim 1 and substantially as herein described with reference to the accompanying drawings.

30

17. A radio communications device including a circuit board construction according to any one of the preceding claims.