GB2254195A

GB2254195A - Balanced printed ciruit board transformer.

Info

Publication number: GB2254195A
Application number: GB9203902A
Authority: GB
Inventors: Esko Satomaki
Original assignee: Nokia Mobile Phones Ltd
Current assignee: Nokia Oyj
Priority date: 1991-03-19
Filing date: 1992-02-24
Publication date: 1992-09-30
Anticipated expiration: 2012-02-24
Also published as: GB2254195B; FI911336A0; FI911336A; FI91930C; FI91930B; GB9203902D0

Abstract

A transformer provided on a printed circuit board (PCB) comprises a pair of parallel closely spaced conductive strips (11,15) provided on the PCB. The two ends of the conductive strips are coupled to a respective pair of interface ports (13, 14; 17, 18) via a conductive track on the PCB. The conductive strip (11, 15) and the associated conductive track are configured as a substantially closed rectangular loop (20, 21 respectively). A respective capacitor (12,16) is provided between the interface ports (13 and 14, 17 and 18) of each conductive strip (11, 15). The conductive strip (11, 15) and associated capacitor (12, 16) form a respective LC resonant circuit with a centre frequency preferably in the range between 50 MHz and 2 GHz. <IMAGE>

Description

PRINTED CIRCUIT BOARD TRANSFORMER The invention relates to a printed circuit board transformer particularly for high frequency applications.

In high frequency applications, as for instance in mixer and modulator circuits used in radio telephones, transformers, e.g. symmetrization transformers mounted on printed circuit boards are used to make an unbalanced signal into a balanced signal and on the other hand to galvanically separate the primary and the secondary. Traditionally the transformers are made as separate components having primary and secondary coils wound on a magnetic core. The manufacture of such a traditional transformer involves many stages, some of which are usually carried out by hand, and so consequently is expensive.

Coupled strips (coupled lines) on the printed circuit board itself are also known. The coupled lines presented in the literature generally have a length of a quarter wave. For instance at a frequency of 100MHz in most cases it is impossible to use quarter wave coupled lines, because their length on a fibreglass board (FR4) would be about 40 cm, depending to some extent, however, on the printed circuit board material.

i For example, the European patent application EP-A-0,324,240 discloses a balanced planar transformer with symmetric configuration. In order to obtain sufficient coupling the strip conductors are made as long as possible. The symmetry requires the provision of cross-overs of the conductors.

Cross-overs are avoided in US Patent No. 4,376,274 where the primary and secondary tracks are located adjacent each other over a long distance. However, here the ends of the secondary track are located remote from each other and on different sides of the primary track. For instance, when used in a mixer, it is difficult to connect a surface-mounted diode to the secondary track. Also, it is difficult to fully symmetrize the transformer due to the positioning of the secondary terminals. For this purpose, there has been provided trimming means which, however, is not conducive to a mass production environment.

According to the present invention there is provided a transformer comprising primary and secondary conductive strips provided in mutually confronting spaced-apart relationships in a substrate, each of the primary and secondary strips being coupled to a respective pair of interface ports, and respective capacitive means coupled between the interface ports of the primary conductive strip and between the interface ports of the secondary conductive strip1 wherein each conductive strip and the capacitive means coupled to the interface ports thereof constitute a respective LC resonant circuit.

Thus it is a requirement of the present invention that both the primary and the secondary strips constitute resonance circuits at the operation frequency. Hence, the coupling between the strips is improved and the strips can be made shorter. Therefore, the transformer can be used for lower frequencies.

The resonant circuits are produced with the help of added capacitors, the strips constituting the inductive elements of the resonant circuits. The resonance causes an increased current strength in the strips and, hence, a shorter strip length is sufficient for the coupling. The strips need not constitute long windings, but can be made straight, whereby symmetrization of the configuration is readily achieved.

Preferably the straight transformer strips form a part of a rectangular continuous strip loop. The strip loops in accordance with the invention may be formed on the surface of the printed circuit board or in some intermediate layer of the printed circuit board, e.g.

by the same methods and at the same time as other conductor circuits printed on the circuit board.

When the strip loops are in the same place they may be situated one within the other, or on each side of the center line between them. Similarly, if the strip loops are in different layers of the printed circuit board, then their projections can be within each other or on both sides of the center line between them. A printed circuit board transformer in accordance with the invention is suitable to couple from the primary to the secondary a signal at a frequency between about 50 MHz and 2 GHz, preferably between about 100 MHz and 400 MHz. The transformer strips can be substantially shorter than the wavelength of the signal coupled through the transformer because the transformer strips can be considered as coils rather than transmission lines.The transformer strip and a capacitance coupled between the interface points arranged in the middle of the long side of the strip loop together form an LC resonance circuit.

A printed circuit board transformer in accordance with the invention can favourably be used as a symmetrization transformer, in which the center point of the transformer secondary strip is connected to the signal earth. A mixer circuit, preferably a balanced mixer circuit can also be realized with this kind of printed circuit board transformer. A printed circuit board transformer in accordance with the invention may also be used in an IQ modulator, providing a simple and favourably circuit arrangement where no DC control is required.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figures la and ib show the circuit diagram of a symmetrization transformer and the corresponding layout of the strips on the printed circuit board in accordance with the invention; Figure 2 shows the basic circuit arrangement, known per se, of a balanced mixer circuit; Figure 3 shows the basic circuit arrangement of a mixer circuit realised with a printed circuit board transformer; Figure 4 shows the basic block diagram of a known IQ modulator; and Figure 5 shows the circuit diagram of an IQ modulator realised with a printed circuit board transformer.

Figure la shows the basic circuit diagram of a symmetrization transformer 10. An unbalanced signal ASYM is connected between the signal interface 13 of the primary 11 and the signal earth 14. A capacitor 12 is connected between the interface points 13 and 14. A balanced signal SYM is obtained from the transformer secondary 15 at interface points 17 and 18. A capacitor 16 is connected between the interface points of the secondary. The center point 19 of the secondary strip 15 is connected to the signal earth.

In accordance with the invention the symmetrization transformer 10 is realised as shown in Figure 2 by forming parallel transformer strips 11 and 15 close together on the printed circuit board, at the same time as other printed conductors are made using printed circuit board manufacturing methods known per se. The transformer strips 11, 15 form respective parts of rectangular strip loops 20 and 21 (see Figure 16). In Figure lb the capacitors 12 and 16 are shown symbolically; they are assembled as separate components on the printed circuit board (for example as surface mounted components) and connected to the strips at the respective points.

The implementation of printed circuit boards is well known to a person skilled in the art, and therefore is not described in further detail here. In a specific embodiment the height d of the primary and secondary strip loops, or the length of the transformer strips 11, 15 is 16mm. The width (at the inner edge) of the primary loop 20 is lmm. The width b (at the inner edge) of the secondary loop 21 is 3mm. The width 1 of the strips is 0.5mm. The isolation gap between the transformer strips is also 0.5mm. The values of the capacitors 12 and 16 are 180 pF and 120 pF, respectively. A capacitor and the transformer strip form a corresponding LC resonance circuit. With these dimensions and values the printed circuit board transformer operates at a frequency of 100 MHz. It will be recognised by a person skilled in the art that the dimensions may be varied within boundaries so that the possible frequency range is from about 50 MHz to 2 GHz, and the best frequency range between 100 MHz and 400 MHz. It is observed that the length d of the transformer strip is substantially shorter than the wavelength of the signal to be coupled. The printed circuit board transformer shown here has a comparatively narrow bandwidth, which is an advantage in certain applications.

In practice it is possible to realise a transformer in accordance with the invention in many different ways.

Figure lb shows a simple case, where the primary loop 20 and the secondary loop 21 are in the same plane on each side of a straight line extending in the middle of the gap c. Alternatively, the loops 20, 21 may be provided one within the other in the same plane. In order to conserve the area of the printed circuit board it is also possible to locate the strip loops 20, 21 at different levels, for instance so that one loop is on the surface of the printed circuit board and the other loop on an intermediate layer. In this case the loops may be spaced apart laterally, i.e. in side by side relationship although in different planes, or spaced apart vertically, i.e. the transformer strips may be situated one on top of the other. The capacitor of the intermediate layer loop is then connected to the strip line through apertures in the surface layer.It is also possible to use several secondary strips, whereby the secondaries further could be in different layers.

It is an advantage of a transformer in accordance with the invention that its components, i.e. the strip lines and capacitors, do not require separate manufacturing steps, but the strip lines and capacitors are assembled in automatic machines in the same way as the other components mounted on the printed circuit boards.

A printed circuit board transformer in accordance with the invention is suitably used for a balanced mixer. Usually the mixer is realised with separate wound transformers, which occupy a large space and the component values are critical. Figure 2 shows the circuit of a conventional balanced mixer, where the mixing element comprises a diode pair D1, D2 (Schottky diodes) connected in series. A radio frequency signal RF is coupled to the transformer input, a local oscillator signal LO to the middle point of the secondary, and the intermediate frequency output signal IF is obtained via a low-pass filter F from a point between the diodes D1, D2.

Figure 3 shows a printed circuit board transformer 10 to replace a usual transformer in a balanced mixer circuit. The printed circuit board transformer 10 and the mixing diodes 31 and 32 in this example constitute the balanced mixer 50. The diodes 31, 32 correspond to the diodes D1, D2 of figure 2. In the circuit of figure 3 the local oscillator interface LO is connected between the diodes. The interface LO could be connected directly to the transformer strip as in figure 2, but then the interface should be arranged particularly so that it will not interfere with the symmetry of the transformer. It is possible to connect the LO, RF and IF ports in several ways, but the basic idea of providing a 180 degree phase shift for the diodes will be the same.

Figure 4 is a block diagram of an IQ modulator known per se. The local oscillator signal LO is applied to the mixer M1 and after a 90 degree phase shift (PS) to the mixer M2. The signals I and Q to be modulated are coupled to the mixers M2 and M1, respectively, and the mixed signals are applied to the adder S, which produces a high frequency output signal RF.

Conventional applications have used commercial mixers, which are expensive and seldom have a local oscillator signal attenuation adequate for e.g. the GSM system.

In order to attenuate the oscillator signal it was necessary to use various balancing methods, e.q. direct current or DC control, which results in extra costs and possibly temperature problems. In an IQ modulator the use of a printed circuit board transformer in accordance with the invention will provide a preferred solution according to figure 5, regarding the manufacturing process.

In figure 5 there are used two printed circuit board transformers 50 and 50' to the unbalanced inputs of which there is applied the local oscillator signal LO and the 90 degrees phase shifted signal LO (+90 ), respectively. The way in which the phase shift is realised is not directly relevant to the present invention and therefore it is not presented in more detail here. No DC control is required in the IQ modulator described here. The signals I and Q are applied to the respective transformers via voltage dividers 61, 62 and 65, 66. The mixed signals are directed to the output RF via a capacitor 63 and resistor 64 and capacitor 67 and resistor 68, respectively. Due to the characteristics of the printed circuit board transformer the IQ modulator will have a relatively narrow bandwidth, but in most cases this is an advantage regarding the filtering of spurious signals.

In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the present invention. For example, the parallel strips are straight in the above examples, but this is not necessary. The strips can have almost arbitrary shapes, provided that the design is symmetrical about the center line between the two loops.

A printed circuit board transformer in accordance with the invention has particular application in radio telephone circuits for example, telephones compatible with the pan-European digital cellular system known as Groupe Special Mobile (GSM).

Claims

1. A transformer comprising primary and secondary conductive strips provided in mutually confronting spaced-apart relationship on a substrate, each of the primary and secondary strips being coupled to a respective pair of interface ports, and respective capacitive means coupled between the interface ports of the primary conductive strip and between the interface ports of the secondary conductive strip, wherein each conductive strip and the capacitive means coupled to the interface ports thereof constitute a respective LC resonant circuit.

2. A transformer as claimed in claim 1, wherein the primary and secondary conductive strips are disposed in substantially mutually parallel relationship.

3. A transformer as claimed in claim 1 or claim 2, wherein the primary and secondary conductive strips respectively form part of a substantially closed loop conductive strip present on the substrate.

4. A transformer as claimed in claim 3, wherein each of the primary and secondary conductive strips respectively is in the form of a substantially straight strip forming part of a rectangular closed loop conductive strip.

5. A transformer as claimed in claim 4, wherein the interface ports coupled to the primary and secondary strips are provided mid-way along the long side of the respective closed loop opposite the side constituting the primary and secondary conductive strip respectively.

6. A transformer as claimed in any of claims 3 to 5, wherein the closed loop strip including the primary conductive strip is present within the closed loop strip including the secondary strip.

7. A transformer as claimed in any of claims 3 to 5, wherein the closed loop strips are provided in a common plane on each side of a centre line between the primary and secondary conductive strips.

8. A transformer as claimed in any of the preceding claims, wherein at least one of the primary and secondary strips is provided on the outer surface of the substrate.

9. A transformer as claimed in claim 8, wherein at least one of the primary and secondary strips is on an internal layer of said substrate.

10. A transformer as claimed in any of the preceding claims, wherein the substrate is in the form of a printed circuit board including a plurality of conductive tracks not associated with said transformer and provided contemporaneously with said primary and conductive strips.

11. A transformer as claimed in any of the preceding claims, wherein the frequency of the signal to be coupled between the primary interface ports and the secondary interface ports is in the range of approximately 50 MHz to 2 GHz, and the primary and secondary conductive strips are shorter than the wavelength of the signal to be coupled.

12. A transformer as claimed in any of the preceding claims, wherein a point substantially mid-way along the secondary conductive strip is coupled to earth potential.

13. A transformer substantially as herein described with reference to Figures la and 1b of the accompanying drawings.

14. A balanced mixer including a transformer claimed in any of the preceding claims.

15. A balanced mixer as claimed in claim 14, wherein two diodes are coupled between the interface ports of the secondary strip, and a filter is coupled to a point between the two diodes.

16. A balanced mixer substantially as herein described with reference to Figure 3 of the accompanying drawings.

17. An IQ modulator including a transformer claimed in any of claims 1 to 13.

18. An IQ modulator as claimed in claim 17, including a first transformer claimed in any of claims 1 to 13 and a second transformer claimed in any of 1 to 13.

19. An IQ modulator as claimed in claim 18 wherein the secondary interface ports of the first transformer and the secondary interface ports of the second transformer are coupled to a common port.

20. An IQ modulator substantially as herein described with reference to Figure 5 of the accompanying drawings.