TECHNICAL FIELD

5 The present invention is related to heat distribution in electric circuits. More specifically it is related to temperature stabilization of components in an electric circuit.

BACKGROUND

o The main key to get all telecommunication equipment to work is that all parts in a network follow the same clock frequency over time from a single source, the master clock. Usually, the master clock is based on a frequency stable atomic clock signal or synchronized to a highly accurate free running clock oscillator, such as the clock oscillator in a GPS (Global Positioning System) signal. This clock is called the primary reference 5 clock for a network system and is positioned at the top of a master-slave clock synchronization hierarchy, where each level of the clock hierarchy is synchronized with reference clock at the closest higher level. This is described more in detail in the final draft of ETSI EN 300 462-1 V 1 .2.1 , January 2002. 0 In certain cases a lower level slave device may for some reason loose its connection to the primary reference clock or its higher level reference clock and enter a so called holdover mode. In the holdover mode the device shall use a high quality internal oscillator to maintain the stability and accuracy of its output signals during the time of holdover without impacting network operation experienced by the end-user due to the loss of5 synchronization.

Today this high quality internal oscillator may be a so called Oven Controlled Crystal Oscillator (OCXO) or a similar oscillator. A schematic illustration of an OCXO is given in Fig. 1 . 0

Often an OCXO is an expensive component to use in commercial telecom products due to its high degree of complexity and integration into one component. Additionally, a high quality OCXO is often a rather big component both regarding its footprint area and height, and requires relatively lot of space on a PCB (Printed Circuit Board) board, where space is precious. Moreover, mounting an OCXO onto the PCB during production requires special handling and/or some remedial measure outside the normal production process, for instance the soldering process.

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When trying to construct a high quality free running oscillator the single most important parameter is temperature stability around the crystal oscillator, since its oscillation frequency is temperature dependent.

o One way to achieve a constant temperature for the crystal oscillator often used by vendors of OCXO ^' s is an advanced form of temperature regulated oven built over a "sandwich" structure between two PCBs (Printed Circuit Boards) beneath and above the crystal oscillator (PCB1 and PCB2 in Fig. 1 ). These PCBs comprise the power supply and control circuits (CU) for the crystal oscillator, while the oscillator itself (OCXO in Fig. 1 ) is 5 placed inside a metal capsule (MC) with a type of heating device around it. This construction is built on a pillar stand connected to a third PCB (PCB3), which is to be soldered onto a user application PCB board. The temperature inside an OCXO's oven is usually kept in the interval 70-80 ⁰ C. One reason why this temperature range is selected is because the crystal is the least0 sensitive to temperature variations in this interval.

SUMMARY

A way to reduce the size and cost of the internal oscillators used in most synchronization5 network devices of today is much wanted and this invention is a part of a possible way to replace it with cheaper type of oscillators, e.g. a temperature compensated crystal oscillator (TCXO), and create a home designed oven around the TCXO encapsulated with a shield-pot. 0 This requires a temperature regulating mechanism and one or more suitable heater element(s) for heating the TCXO and the volume inside the shield-pot to a high stable operational temperature, of which this invention regards the heater issue and is an integral part of. One element in such an oven arrangement described above would be to use the fact that leading a current through a thin inner-layer copper trace (e.g. a strip line) or similar generates heat at relative modest currents due to its encapsulation, e.g. in a PCB's pre- preg material. The idea is to make use of this extra heater inside the PCB or similar under 5 the home designed oven to reach a temperature of the same order as used in an OCXO, which is about +7O ⁰ C.

In general this has been accomplished according to a first aspect of the invention providing a crystal oscillator package comprising a current transporting structure having o an inner conductor arrangement enclosed by an insulating material. In addition the package comprises a crystal oscillator configured to generate a signal at a predefined frequency mounted on the current transporting structure and a temperature preserving unit enclosing the crystal oscillator mounted on the current transporting structure. The inner conductor arrangement is arranged under the temperature preserving unit, where 5 the inner conductor arrangement is configured to carry a current for heating the insulating material and thereby the crystal oscillator to a temperature within a target temperature interval.

Another aspect of the invention provides a method for temperature control of a crystal0 oscillator. According to the method a current is led through an inner conductor arrangement of a current transporting structure comprising an inner conductor arrangement surrounded by an insulating material. A current temperature is measured inside a temperature preserving unit mounted on the current transporting structure where the temperature preserving unit is enclosing a crystal oscillator. The measured 5 temperature is compared with a target temperature range for the crystal oscillator and the current temperature of the crystal oscillator is adjusted to a target temperature range for the crystal oscillator by adjusting a current through the at least one resistive elements enclosed by the temperature preserving unit or the current through the inner conductor arrangement. 0

These and other advantages will be more readily understood by studying the following detailed description and the enclosed figures. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an illustration of an Oven Voltage Controlled Crystal Oscillator (OVCXO) according to known technology.

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Fig. 2 shows a diagram over temperature stability as a function of temperature for a Voltage Controlled Crystal Oscillator (VCXO).

Fig. 3 illustrates a perspective view of the crystal oscillator package according to a first o embodiment of the present invention.

Fig. 4 illustrates a stripline according to known technology

Fig. 5 illustrates a part of the crystal oscillator package according to a second 5 embodiment of the present invention.

Fig. 6 illustrates an example of a flow chart describing a controlled heating principle of the invention. 0 DETAILED DESCRIPTION OF EMBODIMENTS

The following example embodiments are for illustration only. It should be mentioned that many other embodiments of the present invention are possible without departing from the scope and spirit of the invention as defined by the independent patent claims. 5

An example temperature diagram for a crystal oscillator type denoted Voltage Controlled Crystal Oscillator (VCXO) is shown in Fig. 2. for 10 temperature samples. Even though this is one of the simplest crystal oscillators available and does not have any internal temperature controlling function its crystal characteristics as such are in principle the0 same as those for an OCXO or similar.

One can see that in the vicinity of the temperature marked by T the VCXO will have its highest temperature stability. Hence ambient temperature variations around an oven enclosing the VCXO will have a low impact on the temperature of the crystal inside of the component.

Thus, the aim of a crystal oscillator package according to the present invention is to reach 5 the suitable temperature for the chosen crystal oscillator some degrees above the specified operational temperature for the device, and keep it substantially stable at that temperature.

Fig. 3 illustrates a perspective view of the crystal oscillator package COP comprising an o oven OV, where the temperature compensated crystal oscillator TCXO, a current limitation resistor R1 and a heating resistor R2 are enclosed by the oven OV. Furthermore, the crystal oscillator package comprises an external temperature regulation unit TRU and a current transporting structure CTS comprising an inner conductor arrangement INC embedded into a block-shaped insulator INS. The inner conductor 5 arrangement INC is preferably mounted under the oven OV, preferably so as to comprise an area which is at least as large as the area of the crystal oscillator TCXO. Also, the crystal oscillator TCXO, the current limitation resistor R1 and the heating resistor R2 are preferably mounted on an upper plane UP (e.g. the upper surface plane) of the insulator block INS and covered by the oven OV. 0

The task of the oven OV is to substantially isolate the TCXO from the outer environment and other components on the insulating block INS, e.g. formed as a printed circuit board (PCB) or similar, on which the crystal oscillator package COP is mounted in order to minimize their influence on the temperature of the TCXO. 5

Since ovens of this type are known as such to the skilled person, the oven OV as such will not be elaborated further.

Using a temperature compensated crystal oscillator (TCXO) instead of an OCXO reduces0 the cost of a crystal oscillator package dramatically. This invention provides the option to replace the OCXO by other crystal oscillators which are less costly.

However, since the TCXO is more temperature sensitive than the OCXO it is important to keep its temperature and the temperature around it as stable as possible. Moreover, it is5 preferred to keep the TCXO within a temperature interval where temperature variation of the surrounding environment exhibits the least influence on the frequency stability of the TCXO. This interval may differ from one type of crystal oscillator to another, but is similar for crystal oscillators of the same type.

5 As an example, if the temperature of the surrounding environment fluctuates around, say 4O ⁰ C, then, it may be preferable to keep the crystal oscillator TCXO around a temperature of about 60-80 ⁰ C if the crystal oscillator TCXO is most frequency stable in this temperature interval.

o By means of the current transporting structure CTS comprising an inner conductor arrangement INC, the crystal oscillator can be heated to the desired temperature in an indirect way. This can be done by sending a current through the inner conductor arrangement INC of the current transporting structure CTS which gives rise to heat and warms up the surrounding insulator INS which may be a PCB pre-preg material. 5

Also, the planes on top and at the bottom of the current transporting structure CTS shown in Fig. 3 are heated up. Since the crystal oscillator TCXO and the oven OV are mounted onto the upper plane, they are heated indirectly by the current through the inner conductor INC. 0

Moreover, the temperature regulation unit TRU is connected to the oven OV and is adapted to sense the current temperature of the oven by means of a temperature sensor (not shown). Additionally, the current through the inner conductor INC is limited by the current limitation resistor R1 which is connected to the temperature regulation unit TRU.5

In order to reach the required operating temperature for the crystal oscillator TCXO - e.g. within the interval of approximately 60-70 ⁰ C above zero - a corresponding current through the current limitation resistor R1 and thereby the inner conductor INC is chosen. The relation between current in an inner conductor arrangement INC of a current transporting0 structure CTS and the resultant temperature of the surrounding dielectric or similar, which in this case is the pre-preg material, is known as such to the skilled person and will not be discussed here. It suffices to say that the use of the current transporting structure CTS has the advantage that quite substantial temperature increases in the PCB pre-preg material can be achieved at moderate amounts of current through the inner conductor INC. As an example, a current through the inner conductor INC in the range of 150 mA may give rise to a temperature increase of about 3O ⁰ C in the PCB pre-preg material.

It should be mentioned however, that a critical issue in heating the crystal oscillator TCXO 5 is the PCB pre-preg material resistance against heat. One commonly used PCB material is called FR4 comprising a composite of a resin epoxy reinforced with a woven fiberglass mat. It is critical to set the current through the inner conductor INC, such that the resulting temperature of the FR4 is kept essentially below the interval 1 10 - 200 ⁰ C, since FR4 in this temperature range transforms into graphite and becomes electrically conducting, o which in turn could create short circuits and result in fire.

Thus in general, the current through the inner conductor INC should not be so high as to cause overheating of the surrounding dielectric.

5 Now, once the correct value of the current through the inner conductor arrangement INC required for heating the crystal oscillator TCXO to the desired temperature is chosen, the current through the inner conductor arrangement INC may be kept constant. To be able to reach and keep the desired temperature without considerable power dissipation due to a large volume to be warmed up), one approach is to make the volume enclosed by the0 oven OV and the area on the PCB where the crystal oscillator TCXO is mounted small.

Additionally, it is desirable to thermally isolate the crystal oscillator package from the rest of the PCB board by means of the oven OV for the purpose of minimizing heat loss to the environment. Then, depending on the changes in environmental temperature which may5 affect the oven OV and therefore indirectly the TCXO, the temperature regulation unit TRU, placed outside the OV, is adapted to keep the temperature of the crystal oscillator in the target temperature range by controlling the current flow through the heater resistor R2 and/or the current through the current limitation resistor R1 . Variants of this could be to keep the current through either the inner conductor arrangement INC or the heater0 resistor R2 constant and have the other heating element regulated. Preferably, to enable this flexibility of controlling the heating, the heating circuitry of the OV may be realized by feeding the current limitation resistor R1 and the heater resistor R2 by different voltage sources. The control of the current through one or both of the heating elements (the inner conductor arrangement INC or the R2) can then be realized by means of a transistor or a5 potentiometer or some other means performing an equivalent function separately. It should be mentioned that the current limitation resistor R1 for the inner conductor arrangement INC may be located in the oven OV area for utilize the power dissipation from that resistor as well to heat the oven.

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Whichever control means is chosen, the temperature regulation unit TRU may be adapted to sense the current temperature inside OV by means of a temperature sensor located inside the oven OV and to compare it to a target temperature interval in which the crystal oscillator is most frequency stable. Then the temperature regulation unit may by means of o the two principles described in the previous two paragraphs keep the temperature of the crystal oscillator TCXO in the target temperature range.

While the temperature regulation unit TRU described in Fig. 3 may be a separate component on the PCB, the temperature regulation function may also be implemented in 5 an FPGA (Field Programmable Gate Array) or a CPU (Central Processing Unit) usually already existing on the PCB outside the crystal oscillator package and normally used for other tasks. The latter will involve SW that might be more flexible and simplify revision if to change conditions or different applications compared to a FPGA. This may simplify the construction of the crystal oscillator package and also reduce its price. 0

An advantage of the crystal oscillator package of the present invention is the cost aspect, since a much cheaper crystal oscillator is used than the usual OCXO. Also, the usage of the crystal oscillator TCXO frees area on the PCB, due to the size of the component. This freed PCB area may be used for other components. 5

Moreover, the inner conductor INC develops heat efficiently due to its encapsulation inside the PCB and does not add to the cost of existing crystal oscillators.

As a further advantage, the use of the current transporting structure CTS does not take up0 mounting space on the PCB which makes it possible to keep the area of the synchronization circuitry small.

Also, keeping the volume of the oven small makes the power needed to heat it moderate, since a smaller volume reduces the time needed for the temperature rise in the dielectric5 surrounding the inner conductor to take effect on the temperature on the oven volume. The smaller oven volume to heat the shorter the time for regulating the temperature will take effect and by having a fast regulator the temperature can be kept constant.

In addition, if the current limitation resistor R1 and the heater resistor R2 are chosen as 5 surface mounted resistors, the volume of a home created oven can be kept small, and thus ensure replacement of an OCXO with an TCXO without jeopardizing the frequency stability needed in a telecommunication network device.

Fig. 4 illustrates the current transporting structure CTS from Fig. 3 in more detail. o Selecting a current transporting structure CTS having a certain inner conductor thickness H1 , width W, a suitable dielectric D or isolator and its thickness H2 (and thus its volume) the amount of current needed to heat the crystal oscillator to the desired temperature may be varied. Especially the width W and the thickness H1 of the inner conductor arrangement INC may be chosen such that a current through the inner conductor 5 arrangement INC may lead to a rapid temperature rise in the crystal oscillator mounted on one of the planes next to the inner conductor arrangement INC. One way of achieving this is to match the dimensions of the inner conductor arrangement INC to the dimensions of the oven OV enclosing the crystal oscillator and the temperature regulation unit TRU from Fig. 3. 0

The mathematical relation between the dimensions of the current transporting structure CTS, the current through the inner conductor arrangement INC and the resulting temperature rise in the dielectric D surrounding the inner conductor arrangement INC is known as such to the skilled person and will not be explained in detail. It may be added5 here, that faster temperature regulation (if the current through the inner conductor INC is used for the temperature regulation) may be achieved by placing the inner conductor INC closer to the upper plane UP of the current transporting structure CTS than to its lower plane LP. 0 Turning now to Fig. 5, a second embodiment of the crystal oscillator package according to the present invention is shown. For simplicity, the components of the crystal oscillator package that are identical to the ones in the second embodiment of the crystal oscillator package have been omitted from Fig. 5. Here, the current transporting structure CTS shown in Figs. 3 and 4 comprises layers 1 , 2, 3 with the inner conductor arrangement INC located on layer 2 sandwiched between layers 1 and 3. Even though not shown in Fig. 5, the oven OV, the current limiting resistor R1 and the heater resistor R2 as well as the crystal oscillator TCXO are mounted on the 5 plane of layer 1 .

Here, a conductor arrangement in the form of a serpentine trace arrangement is preferred. Preferably, the size of the area of the serpentine trace may be chosen to have at least the size of the area occupied by the oven comprising the crystal and other components inside o the OV needed to achieve a uniform heating.

Furthermore, possible copper planes and traces inside the oven area (not shown) may be connected to the outer copper layers with as few and thin connection points as possible in order to prevent heat dissipation from the oven area. Optionally, some of the PCB 5 material next to the outside of the oven may be punched or milled for the purpose of minimizing thermal dissipation to the outside environment.

As an example, the current through the serpentine shaped trace could be constant and set to a value for a suitable temperature rise that together with the current through a0 resistor array heater provides a temperature raise enough to keep the oven at about +7O ⁰ C degrees, or at least within an interval of approximately 60-70 ⁰ C above zero. The temperature regulation unit TRU from Fig. 3 may then compensate for oven temperature variations due to ambient temperature changes by controlling the current through the heater resistor R2 or by controlling the current through the current limitation resistor R15 and thereby the inner conductor arrangement INC as mentioned earlier.

In Fig. 6, the steps of a method according to the present invention are illustrated, with the additionally option of regulating the amount of power dissipation added by the invention part. 0

At step 500, a current is sent through the inner conductor arrangement INC, e.g. having the form of a stripline or similar. This current is chosen so as to effectuate a temperature increase in the dielectric surrounding the inner conductor arrangement INC and thereby the upper and lower planes between which the inner conductor arrangement INC is5 sandwiched. In this fashion, an indirect temperature rise in a crystal oscillator TCXO mounted on one of the planes closest to the inner conductor is achieved. It should be mentioned that the current should be chosen so that the temperature of the crystal oscillator rises to a value T which is in the temperature interval where the frequency stability of the crystal oscillator is at an optimum. This optimum temperature interval may 5 differ from one type of crystal oscillator to another, but is similar for crystal oscillators of the same type..

At the next step, i.e. at 510, a temperature regulation unit, such as the temperature regulation unit TRU in Fig. 3, measures the current temperature To inside an oven OV o mounted on one of the planes closest to the inner conductor arrangement INC and enclosing the crystal oscillator. One way of measuring the current temperature in the oven may be by receiving a temperature signal measured by a temperature sensor placed inside the oven.

5 This current temperature To is then compared at step 520 by the temperature regulation unit TRU to the target temperature T mentioned earlier. One reason why temperature regulation is needed at all despite enclosing the crystal oscillator by an oven and isolating it from other components mounted on the current transporting structure CTS is that ambient temperature variations may affect the oven temperature and thus the crystal0 oscillator.

Now, if at step 520 the temperature regulation unit TRU determines that the temperature To is outside of a target temperature interval it may at step 525 either adjust the current through a current limiting resistor R1 connected to the inner conductor of the current5 transporting structure. The current regulation may be used if for example deviations of the current temperature are occurring rapidly or if a faster temperature regulation is needed. Rapidly occurring temperature variations of the oven may be for example determined by comparing a difference between two or more measured oven temperature values with a threshold temperature value. 0

On the other hand, if the temperature regulation unit at step 520 determines that the current oven temperature To is within the target temperature interval it may keep at step 530 the current through the inner conductor arrangement INC of the current transporting structure CTS and the heater transistor inside the oven OV constant. 5 It should be mentioned that the present invention is not limited to the embodiments and examples referred to in the description and the accompanying figures, but is only limited by accompanying patent claims. Thus, persons skilled in the art may after studying the above description contemplate other possible embodiments without departing from the spirit and scope of the present invention