GB2404091A

GB2404091A - Circuit board having first and second inductors mounted on either side of the board

Info

Publication number: GB2404091A
Application number: GB0316573A
Authority: GB
Inventors: Haim Friedlander; Eliyahu Basri; Gadi Shirazi
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 2003-07-15
Filing date: 2003-07-15
Publication date: 2005-01-19
Anticipated expiration: 2023-07-15
Also published as: GB0316573D0; GB2404091B

Abstract

A circuit board construction 15 comprises a first inductor 13 electrically connected to a second inductor 14, where the first inductor is located on a first surface of the circuit board and the second inductor is located on the other surface of the circuit board. When each inductor has the same inductance, this construction allows the total inductance of the two inductors to remain constant during any deformation of the circuit board as the increase in inductance of one of the inductors as it contracts 19, cancels out the decrease in the inductance of the other, as it expands 17. The circuit board may be used in a voltage controlled oscillator (VCO) circuit in, for example, a mobile telephone. The inductors may be air core coiled inductors and the axes of the coils of the first and second inductors may be parallel to each other.

Description

1 2404091 TITLE: CIRCUIT BOARD CONSTRUCTIONS

FIELD OF THE INVENTION

The present invention relates to circuit board constructions, in particular constructions in which components of an oscillator circuit such as a voltage controlled oscillator are mounted on a circuit board.

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 an insulating material such as a glass-epaxy 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 through the holes. In some cases, the circuit arrangements include components which form an oscillator circuit, especially a circuit including a solid state oscillator. In particular, the oscillator circuit may form a VCO (voltage controlled oscillator).

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 occur. This results in noise known in the art as microphonics (a 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 known ways of solving the problem of microphonics arising when a VCO is mounted on a PCB include: 1) applying vibration damping material at the VCO mounting positions; 2) fixing the PCB to a very stiff material using a large number of fixing devices such as screws) 3) providing the VCO as a separate small module; 4) attaching screws as close as possible to the VCO area to maximise rigidity; 5) using a very stiff material (e.g. having ceramic like properties) for the PCB; 6) fixing the critical VCO components to the PCB using a hardenable adhesive such as an epoxy or hard silicone composition.

7) increasing the loop bandwidth of a phase locked loop in which the VCO is incorporated; None of these solutions is entirely satisfactory. A number are cumbersome, expensive and not easy or convenient to apply in a large scale manufacturing facility.

SUMMARY OF THE PRESENT INVENTION

According to the present invention in a first aspect there is provided a circuit board construction including an insulating circuit board and circuit components mounted on the circuit board, wherein the circuit components include a first inductor mounted on l a first face of the board and a second inductor mounted on a second face of the board opposite to that of the first face and electrically coupled in series with the first inductor.

Preferably, the first inductor and the second inductor have are coils or windings having axes which are substantially parallel. The first and second inductors may have an elongate shape and the said axes may be the longitudinal axes of the inductors. The first inductor and the second inductor are conveniently connected directly together. Desirably the inductance of the first inductor is substantially equal to that of the second inductor.

Desirably, the first and second inductors are mounted on the circuit board in adjacent regions of the board. For example, at least part of the first inductor may be opposite at least part of the second inductor.

The first and second inductors may be included in an oscillator circuit. The oscillator circuit may be a VCO circuit. The inductors may be in a resonant or so called tank portion of the VCO circuit.

The problem of microphonics associated with a board mounted VCO circuit is caused by slight perturbations of both the component values and of parasitic values that affect the resonant frequency of the tank or resonant portion of the VCO circuit. An inductor is normally employed in a VCO circuit and usually is the most sensitive component. A perturbation of the inductance provided by the inductor when subjected to mechanical vibrations is caused by deformations of the circuit board (on which the inductor is mounted) along the longitudinal axis of the inductor. These deformations cause the spacing between the windings of the inductor to expand and contract, thus decreasing and increasing the inductance, respectively.

However, by the invention, beneficially the net inductance provided by the first and second inductors becomes more stable. This is because if the circuit board on which the inductors amounted deforms in use, one inductor expands while the other contracts, as illustrated later. The total inductance of first and second inductors when connected electrically in series is the sum of the two inductances, so the result of the deformations is that positive changes of inductance of occur in one of the inductors and negative changes of inductance occur in the other inductor and these two changes in inductance tend toward cancelling each other out. Where the first and second inductors are included in a resonant portion of a VCO circuit, their total inductance may be that required to provide suitable resonance.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Brief Description of the accompanying drawings

FIG. 1 is side elevation in somewhat schematic form of a known circuit board construction.

FIG. 2 is side elevation in somewhat schematic form of a circuit board construction embodying the present invention.

FIG. 3 is a circuit diagram of a VCO circuit which includes the construction of FIG. 1.

Detailed description of embodiments of the invention As shown in FIG. 1, a known construction 1 includes an air core coiled inductor 3 mounted on the upper face of a printed circuit board 5. FIG 1(1) shows the construction 1 with no mechanical perturbation applied to the board 3. The inductance of the inductor 3 is L1.

FIG. 1(2) shows a deformation applied to the circuit board 5, e.g. when a mechanical vibration is applied to the board 5, such that the deformation component which mainly affects the inductance Lo, is along a longitudinal axis of the inductor 3. The deformation of the board 5 causes the spacings between the windings of the inductor 3 to expand. The expansion, shown in exaggerated form, is indicated by arrows 7. The inductance is caused to be reduced to (1 - A) L1 where is a small fraction of unity.

FIG. 1(3) shows a deformation applied to the circuit board 5, e.g. when a mechanical vibration is applied to the board 5, is such that the spacings between the windings of the inductor 3 contract. The contraction, shown in exaggerated form, is indicated by arrows 9.

The inductance of the inductor 3 is caused to be increased to (1 + A) L1.

As shown in FIG. 2, a construction 11 embodying the invention includes a first air core coiled inductor 13 mounted on an upper face of a printed circuit board 15 and a second identical inductor 14 mounted on the opposite (lower) face of the board 15 and a conductor 16 connecting the inductors 13 and 14 through the board IS. The ends of the conductors 13 and 14 which are connected in this way are opposite one another (on opposite faces of the board 15 in same region of the board 15) . FIG 2(1) shows the construction 11 with no mechanical perturbation applied to the board 15. The inductance of the inductor 13 is 0.5L1 and the inductance of the inductor 14 is 0.5L1 making the combined inductance of the two series connected inductors L1.

FIG. 2(2) shows a deformation applied to the circuit board 15, e.g. when a mechanical vibration is applied to the board 5, such that the deformation component along a longitudinal axis of the inductors 13, 14 is similar to that shown in FIG. 1(2). For the inductor 13 on the upper face of the board 15 the deformation is such that spacings between the windings of the inductor 3 expand. The expansion, shown in exaggerated form, is indicated by arrows 17. The inductance of inductor 13 is caused to be reduced to (1 - A) 0.5L1 where is again a small fraction of unity. However, for the inductor 14 on the lower face of the board 15 the deformation is such that spacings between the windings of the inductor 14 contract. The contraction, shown in exaggerated form, is indicated by arrows 19. The inductance of inductor 14 is caused to be increased to (1 + A) 0.5L1. Thus, the overall inductance of the two inductors is still L1.

FIG. 2(3) shows a deformation applied to the circuit board 15, e.g. when a mechanical vibration is applied to the board 5, such that the deformation component along a longitudinal axis of the inductors 13, 14 is similar to that shown in FIG. 1(3). For the inductor 13 on the upper face of the board 15 the deformation is such that spacings between the windings of the inductor 3 contract. The contraction, shown in exaggerated form, is indicated by arrows 29. The inductance of inductor 13 is caused to be increased to (1 - A) 0.5L1, where is again a small fraction of unity. However, for the inductor 14 on the lower face of the board 15 the deformation is such that spacings between the windings of the inductor 14 expand. The expansion, shown in exaggerated form, is indicated by arrows 27. The inductance of inductor 13 is caused to be reduced to (1 - A) 0.5L1. Thus, the overall inductance of the two inductors is still L1 Thus, the overall inductance of the two inductors 13, 14 remains constant at L1 for both kinds of deformation shown in FIGs 2(2) and 2(3) respectively.

The construction 11 shown in FIG 2 may be used in a VCO circuit of otherwise known configuration. FIG. 3 shows an example of such a circuit 300, which is a VCO in Colpitts configuration. In FIG 3, an input control voltage is applied at an input terminal 301 and fed along a path 302 including an inductive coil 303 and capacitors 304 and 305 leading to a terminal 306 connected to the base of a junction transistor 307. A varactor diode 308 is in a connection between the path 302, between the coil 303 and the capacitor 304, and ground. An inductor 309 is in a connection between the path 302, between the capacitor 304 and the capacitor 305, and ground. A resistor 310 is connected between the path 302, between the capacitor 305 and the terminal 306, and ground. Also, series connected capacitors 311 and 312 are in a connection between the path 302, between the capacitor 305 and the terminal 306, and ground. A resistor 313 is connected between a terminal 314 connected to the emitter of the transistor 307 and ground. The junction between the capacitors 311 and 312 is connected by a connection 315 to the junction between the resistor 313 and the terminal 314.

A supply voltage Vcc is applied from a terminal 316 through a coil 317 to a terminal 318 connected to the collector of the transistor 306. The terminal 316 is also connected via a resistor 319 to the junction between the capacitor 305 and the terminal 306 and also via a capacitor 320 of large capacitance to ground. The junction between the terminal 318 and the coil 317 is connected to an output terminal 321 from which an output voltage is extracted.

In practice, the circuit of FIG.3 operates as follows. The transistor 307 is biased active by means of resistors 319, 310 and 313. Inductor 317, which is a radio-frequency choke (RFC), together with bypass capacitor 320, insures RF isolation between the collector 318 and the DC power source 316. A portion of the signal at the emitter 314 is fed back to the base 306 through a passive feedback network which includes capacitors 311, 312, 305, 304, inductor 309 and varactor diode 308. This feedback network is frequency selective, i.e. the phase and amplitude of the transfer function is a function of frequency. The component values are selected in a manner such that at the desired operating frequency, the voltage arriving at the base of the transistor 307 is in phase with the voltage at the emitter, while the overall gain is at least unity. Under these conditions, the circuit will oscillate at the desired frequency. The frequency of the oscillations may be altered by supplying a bias voltage at input terminal 301, which goes through inductor (RFC) 303, to varactor 308. When the voltage is increased, the varactor is biased more negative, which reduces its capacitance and thereby increases the TO frequency of oscillations.

The components of the circuit 300 may be mounted and interconnected on a printed circuit board such as the board 5 (FIG 1) in a known manner. The coil 309 in the resonant part of the circuit 300 may, in accordance with an embodiment of the invention, be provided in the form of two series connected inductive coils mounted on opposite faces of the board in the manner described with reference to FIG. 2.

The benefits of using a circuit board configuration of the form shown in FIG. 2 in a circuit of the form shown in FIG. 3 were verified as follows.

A VCO circuit was constructed (a) in accordance with the prior art in the manner shown in FIG.1; and (b) in accordance with an embodiment of the invention in the manner shown in FIG. 2. Inductor 309 was used in (a) and was replaced in (b) with two inductors which were placed on opposite sides of the circuit board. The inductance of each inductor in (b) was chosen to be half the value of that of the original inductor in (a).

Apart from the change from one inductor to two, other properties of the circuit and its construction were kept the same. The board used in each case was a Motorola CM200 VHF board (a mobile radio board), using two Coilcraft type 1008HQT inductors in the case of the construction embodying the invention. The performance of the two circuit constructions when mechanical vibrations were applied thereto was measured. The vibrations were applied according to the standard vibration profile defined in MIL-STD-810; FM hum and noise was measured according to the test method described in the TIA/EIA-603 standard: this is the 'ETA hum and noise'. The results obtained are shown in the

following table:

Vibration EIA Hum and Noise (dB): Frequen cy [Hz] Single inductor Two inductors -60 -57 -56 -56 -51 -55 -45 -52 250 -38 -40 300 -35 -40 350 -37 -42 400 -35 -40 450 -39 -49 500 -35 - 2 The results demonstrate a lower hum and noise obtained by using the construction including two inductors according to an embodiment of the invention. The worst case result improved from -35 to -40 do, which

Claims

Claims 1. A circuit board construction including an insulating circuit

board and circuit components mounted the circuit board, wherein the circuit components include a first inductor mounted on a first face of the board and a second inductor mounted on a second face of the board opposite to that of the first face and electrically coupled in series with the first inductor.
2. A circuit board construction according to claim 1 wherein the first inductor and the second inductor have coils or windings having axes which are substantially parallel.
3. A circuit board construction according to claim 2 wherein the first and second inductors have an elongate shape and the said axes are the longitudinal axes of the inductors.
4. A circuit board construction according to any one preceding claim wherein the first inductor and the second inductor are electrically connected directly together.
5. A circuit board construction according to any one preceding claim wherein the inductance of the first inductor is substantially equal to that of the second inductor.
6. A circuit board construction according to any one preceding claim wherein the first and second inductors are mounted on the circuit board in adjacent regions of the board.
7. A circuit board construction according to claim 6 wherein the any one preceding claim wherein the first and second inductors are mounted on the circuit board in such a manner that the first inductor is opposite at least part of the second inductor.
8. A circuit board construction according to any one preceding claim wherein the first and second inductors are included in an oscillator circuit. The oscillator circuit may be a VCO circuit. The inductors may be in a
9. A circuit board construction according to claim 8 wherein the oscillator circuit is a voltage controlled oscillator circuit.
10. A circuit board construction according to any one of the preceding claims and substantially as herein described with reference to the accompanying drawings.