GB2295277A - RF circuits with microstrip coupler - Google Patents

Abstract

In RF circuits having a microstrip coupler with interdigitated fingers (1 to 4), resonances may occur on the coupler characteristic within the band pass of the coupler. These undesired resonances can be reduced and even entirely removed by adopting an interdigitated structure in which no intrusion by any finger of one of the transmission lines (12) occurs between the pair of inner fingers (1, 2) from the other transmission line (11). The reduction and/or removal of these resonances appears to result from an electrical shielding effect of each of these inner fingers (1, 2). Thus, each one (1, 2) of the inner fingers effectively shields the neighbouring outer finger (3, 4) of the one transmission line (12) from the other inner finger (1, 2) of its transmission line (11), and so no inter-coupling of the RF field occurs between said outer finger (3, 4) of one transmission line (12) and said other inner finger of the other transmission line (11). Thus, this interdigitated structure provides coupling between the first and second transmission lines (11 and 12) along only one (and not both) of the edges of these interdigitated fingers (1 to 4). <IMAGE>

Description

DESCRIPTION RF CIRCUITS This invention relates to RF circuits comprising a dielectric substrate carrying first and second RF transmission lines and a broadband RF microstrip coupler between the lines. The circuit may be part of, for example, a front-end or down-converter of a receiver in a microwave television transmission system or in a personal control or communications system. Such a coupler may, for example, serve as a simple DC break, instead of using an expensive RF quality capacitor.

RF circuits are known comprising a dielectric substrate carrying first and second RF transmission lines having respective first and second facing ends, and a microstrip coupler formed on the substrate between the facing ends to provide broadband RF coupling between the transmission lines. The coupler may comprise interdigitated fingers formed by interdigitated end portions of the first and second transmission lines on one face of the substrate, a ground plane being present on the opposite face of the substrate. Such an interdigitated coupler can provide an attractive means of realising a DC break in the circuit (e.g for separating biasing currents of different RF components in the circuit) by virtue of its low insertion loss and simple structure.

The coupler may also be designed to form a band-pass filter in a microwave integrated circuit (MIC).

Such a microstrip coupler is known from, for example, published UK Patent Application 2 153 155, the whole contents of which are hereby incorporated herein as reference material. Figure 3 of GB-A-2 1 53 1 55 illustrates an interdigitated structure in which two fingers from one end portion are disposed between three fingers from an adjacent end portion. In this case the RF coupling between the end portions occurs along both edges of three of the five fingers.

It is conventional practice to adopt an interdigital structure in order to obtain RF coupling along both edges of the inner interdigitated finger or fingers. The use of both edges for the coupling provides a more efficient coupling between the two end portions. However, the Applicants find that when such an interdigitated structure is adopted, two resonances may occur, superimposed on the coupler characteristic within the band pass of the coupler. These resonances appear to result from interactions at the free ends of the fingers.

It is an aim of the present invention to remove or at least to significantly reduce such resonances from the coupler characteristic.

According to the present invention there is provided an RF circuit comprising a dielectric substrate carrying first and second RF transmission lines having respective first and second facing ends, and a microstrip coupler formed on the substrate between the facing ends to provide broadband RF coupling between the transmission lines, the coupler comprising interdigitated fingers formed by interdigitated end portions of the first and second transmission lines on one face of the substrate and having a ground plane on the opposite face of the substrate, characterised in that the fingers are arranged with respect to each other to form a pair of inner fingers from the first transmission line extending between a pair of outer fingers from the second transmission line, without any finger of the second transmission line intruding between the inner fingers of the first transmission line.

The present inventor has found that the undesired resonances can surprisingly be significantly reduced and even entirely removed by adopting such an interdigitated structure in accordance with the present invention in which no intrusion by any finger of the second transmission line occurs between the pair of inner fingers from the first transmission line. The reduction and/or removal of these resonances appears to result from an electrical shielding effect of each of the inner fingers.

Thus, each one of the inner fingers effectively shields the neighbouring outer finger of the second transmission line from the other inner finger of the first transmission line, and so no inter-coupling of the RF field occurs between said outer finger of the second transmission line and said other inner finger of the first transmission line. Thus, this interdigitated structure in accordance with the present invention provides coupling between the first and second transmission lines along only one (and not both) of the edges of these interdigitated fingers.

A versatile design which can be readily simulated for good characteristics is achieved when the coupler comprises at least four interdigitated end portions of the first and second transmission lines which are so arranged symmetrically with respect to a central axis along the microstrip coupler that the pair of inner fingers from the first transmission line extend between the pair of outer fingers from the second transmission line and that the inner fingers are separated by a space which is free of intrusion by any finger of the second transmission line.

The widths of the fingers and of the spaces between the fingers (as well as the length of the fingers) can be optimised to achieve minimal insertion loss and minimal return loss for the bandwidth of the microstrip coupler. Thus, the inventor finds that a low return loss (input reflection co-efficient) can be achieved by making the space between the pair of inner fingers of the first transmission line with a width which is substantially the same as that of spaces between each inner finger and the neighbouring outer finger of the second transmission line.

Each of the inner and outer fingers may have a width which is substantially the same as (but not necessarily equal to) that of spaces between each inner finger of the first transmission line and the neighbouring outer finger of the second transmission line. This width may correspond to the minimum dimension achievable with the microstrip technology used to form the coupler. Good coupling and low insertion loss can be obtained in this manner.

In order to achieve efficient coupling and low insertion loss, the length of the fingers may be substantially equal to a quarter wavelength at a frequency (such as the centre frequency) within the pass band of the broad band coupler.

These and other features of the present invention, and their advantages, are illustrated specifically in embodiments of the present invention now to be described, by way of example, with reference to the accompanying diagrammatic drawings. In these drawings: Figure 1 is a plan view of part of an RF circuit comprising a microstrip coupler of known interdigitated form between two RF transmission lines; Figure 2 is a plan view of part of an RF circuit having a similar microstrip coupler also of known interdigitated form; Figure 3 is a plot of the s-parameters s21 and s1 1 in dB with frequency in GHz, for the coupler of Figure 2; Figure 4 is a cross-sectional view on the line IV-IV in Figures 1, 2, 5, 7 and 9, showing the RF circuit part of these respective Figures; ; Figure 5 is a plan view of part of an RF circuit comprising one example of an interdigitated microstrip coupler in accordance with the present invention; Figure 6 is a plot of the s-parameters s21 and s1 1 in dB with frequency in GHz, for the coupler of Figure 5; Figure 7 is a plan view of part of an RF circuit comprising another example of an interdigitated microstrip coupler in accordance with the present invention; Figure 8 is a plot of the s-parameters s21 and s1 1 in dB with frequency in GHz, for the coupler of Figure 7; Figure 9 is a plan view of part of an RF circuit comprising a further example of an interdigitated microstrip coupler in accordance with the present invention; and Figure 10 is a plan view of part of an RF circuit comprising yet another example of an interdigitated microstrip coupler in accordance with the present invention.

It should be noted that, except for Figures 3, 6 and 8, all the drawings are diagrammatic and not drawn to scale. Relative dimensions and proportions of the various parts of these Figures 1, 2, 4, 5, 7, 9 and 10 have been exaggerated or reduced in size for the sake of clarity and convenience in the drawing. The same reference signs are generally used to refer to corresponding or similar features in the different embodiments of the prior art of the invention. It should be noted that Figures 1 to 3 relate to microstrip couplers in the prior art and so these Figures do not relate to embodiments of the present invention The RF circuit part of Figure 1 comprises a dielectric substrate 10 carrying first and second RF transmission lines 11 and 1 2 respectively.

A multistrip coupler 13 is formed on the substrate 10 between the facing ends of the lines 11 and 12 to provide broadband RF coupling between the lines 11 and 1 2. This coupler 1 3 comprises five interdigitated fingers 1 to 5, arranged in a known configuration, for example as illustrated in Figure 3 of GB-A-2 1 53 1 55. These fingers 1 to 5 are formed by interdigitated end portions of the first and second transmission lines 11 and 1 2 on one face of the substrate 10, a ground plane 1 5 being present on the opposite face of the substrate 10 as illustrated in Figure 4.The ground plane 15 may extend over the opposite face of the substrate 10 below the transmission lines 11 and 12, and so these lines 11 and 1 2 may be designed in microstrip technology.

Figure 2 shows a modification of the known interdigitated configuration, in which the coupler 13 has only four interdigitated fingers 1 to 4. This Figure 2 embodiment of the prior art is included for comparison with the four-finger embodiments of the present invention illustrated in Figures 5 and 7. In a specific example of Figures 1, 2, 5 and 7, the broadband RF coupler was designed to have a centre frequency of 5.8GHz and a bandwidth of 3GHz at 10dB return loss (sol). The dielectric substrate 10 was a loaded polymer having a dielectric constant of 3.0 in this specific example. The ground plane 1 5 is a conductive film, which was of copper on the dielectric material in the specific example. The transmission lines 11 and 12 and the fingers of the coupler 13 are conductor patterns which were of copper in the specific example.In the specific example, this dielectric substrate 10 was 760,um (micrometres) thick, and the thickness of the copper films 11, 1 2 and 1 5 was 20yam. It will be evident that a wide variety of dielectric materials and conductive materials may be used for the substrate 10 and conductive films 11,12 and 15, depending on the RF band frequencies. In the specific example used for the measurements, the microstrip lines 11 and 1 2 were accessed from co-axial line transitions and the source and load impedances were 50 ohms.

Figure 3 shows measurements of the insertion loss and return loss for this specific example of Figure 2 in which the coupler 1 3 has the following dimensions for its fingers and spaces: w1 = w2 = w3 = 0.2mm d =8.8mm Thus, in this specific example of Figure 2 each finger 1 to 4 has the same width w1; and this finger width w1 is the same as the width w2 of the space 21 between neighbouring fingers and is the same as the width w3 of the space 22 between the end of a finger and the neighbouring transmission line.

In Figure 3, plot a is the forward transmission coefficient s21 for this specific example of Figure 2, and plot b is its input reflection coefficient s1 1. The vertical scale (ordinate) of Figure 3 covers a range of O to 20dB for both s1 1 and s21. The horizontal scale (abscissa) of Figure 3 corresponds to a frequency range from 16Hz to 1 2GHz. As can be seen from Figure 3, two resonances are superimposed on the coupler characteristic, at frequencies of about 5GHz and about 6.6GHz.

These resonances reduce the RF power coupled between the lines 11 and 12 at these two frequencies in the band-pass of the coupler 13, so distorting the transmitted RF signal. Similar resonances are found to occur with the five-finger interdigitated structure of Figure 1.

The precise mechanism resulting in these resonances is not understood. Several factors seem to be involved. One factor seems to be fringing fields due to capacitances at the free ends of the fingers, both between the narrow finger end 4,1 and the neighbouring wider part of the other line 1 1,12 (i.e across the space w3) and between the finger ends (1 and 2) and (3 and 4) of the same line 11,12. Another factor appears to be inductance effects due to current crowding at the transition between the wider part of the line 1 1 , 1 2 and its narrow fingers (1,2), (3,4). An asymmetric resonance seems to occur around the finger pairs (1 and 2), (3 and 4) due to the asymmetric pair geometry.

Substantially no such resonances are superimposed on the coupler characteristic of Figure 6, which shows measurements for the Figure 5 embodiment of the present invention. In the coupler of Figure 5 in accordance with the present invention, the four fingers 1 to 4 are so arranged symmetrically with respect to a central axis 8 along the coupler 13 that a pair of inner fingers 1 and 2 from the first transmission line 11 extend between a pair of outer fingers 3 and 4 from the second transmission line 12 and that the space 20 separating the pair of inner fingers 1 and 2 is free of intrusion by any finger of the second transmission line 1 2. This symmetrical interdigitated structure of Figure 5 may be considered to have some similarities to the interdigitated configuration of Figure 1, except that the central finger 5 of the coupler 13 of Figure 1 is not present in the space between the fingers 1 and 2. Because the coupler 13 is formed in microstrip technology, it is possible to form this symmetrical configuration for the fingers 1 to 4 at one face of the substrate 10 over the underlying ground plane area 15 on the opposite face of the substrate 10. The reduction and/or avoidance of the resonances is thought to occur due to a shielding effect of each of the inner fingers 1 and 2 with respect to the outer fingers 3 and 4 of the other transmission line. Thus, the finger 1 of transmission line 11 shields the finger 3 of transmission line 1 2 from the finger 2 of transmission line 11. Similarly the finger 2 of transmission line 11 shields the finger 4 of transmission line 1 2 from the finger 1 of transmission line 11. The measurements of Figure 6 were made for the specific example of Figure 5 in which the dimensions of the fingers and spaces in coupler 13 were as follows: w1 = w2 = 0.2mm wO = O. 8mm d =8.8mm The coupled length of the fingers 1 to 4 is 8.8mm which corresponds to a quarter wavelength at the centre frequency (5.8GHz) of the pass band of this coupler 13.Although the dielectric constant of the substrate material 10 was 3.0, an effective dielectric constant used to calculate a quarter wavelength is less than 3.0 due to (1) the coupler 13 being formed in microstrip technology with the substrate material 10 only on one side of the coupler conductors 1 to 4,11,12 and with air on the other side of these coupler conductors, and (2) the different effective dielectric constants resulting from the geometrical differences for even and odd mode coupling between the fingers.

As can be seen from Figure 6, the input reflection coefficient s1 1 is rather high in the bandpass of the coupler 13 of Figure 5, i.e there is a rather high return loss of the input signal by reflection from the coupler 13.

Figure 8 illustrates a significant reduction in this return loss, as achieved by reducing the width wO of the space 20 between the inner fingers 1 and 2 of the transmission line 11.

Figure 7 illustrates the interdigitated coupler configuration on which the measurements of Figure 8 were taken. In this case, the width wO of the space 20 between the pair of inner fingers 1 and 2 of the first transmission line 11 is substantially the same as the width w2 of the spaces 21 between each inner finger 1 and 2 and the neighbouring outer finger 3 and 4 of the second transmission line 1 2.

In this specific example the dimensions were as follows: wO=w1 = w2 =0.2mm d=8.8mm Although in the preceding and following description the transmission lines 11 and 12 are termed "first" and "second" transmission lines respectively, it should be understood that the designations "first" and "second" are merely convenient labels for distinguishing the two lines 11 and 12 in words.The designations "first" and "second" do not imply that the RF signal passes from the first transmission line 11 to the second transmission line 1 2. Thus, the direction of transmission of the RF signal between the lines 11 and 12 may be either from the first transmission line 11 to the second transmission line 1 2 or from the second transmission line 1 2 to the first transmission line 11. Thus, either of the transmission lines may have the pair of inner fingers which extend between the pair of outer fingers to form the interdigitated coupler 13.

Many other modifications and variations are possible within the scope of the present invention. Thus, for example, if the transmission lines 11 and 12 are wide or have wide end portions, the coupler 13 may comprise more than four interdigitated fingers. By way of example, Figure 9 illustrates a microstrip coupler 13 in accordance with the present invention having six interdigitated fingers.

Figure 10 illustrates another modification in accordance with the present invention, in which the area 20' between the inner fingers 1 and 2 is occupied by the same metal film as the fingers 1 and 2. In this case, the fingers 1 and 2 are the peripheral side areas of this large metal portion 1,20',2 which actively couple to the fingers 3 and 4 of the other transmission line 1 2. This large metal end portion 1,20',2 of the transmission line 11 has a width of (wO + 2.w1).

In the broadband couplers disclosed in GB-A-2 1 53 155, additional separate longitudinal portions are present between the two end portions of the transmission lines. The end portions of the transmission lines then have fingers which are interdigitated with fingers of these intermediate longitudinal portions. Such an arrangement may be adopted in accordance with the present invention. Thus, in Figure 5, reference 1 2 may designate an intermediate longitudinal portion of this type of coupler, which is interdigitated with a first transmission line 11 at one end and a second transmission line (not shown) at its opposite end (not shown).

As described above, the RF transmission lines 11 and 12 may be formed in microstrip technology. Most of the RF circuit on the substrate 10 may also be formed in microstrip technology, so that the ground plane 1 5 may be present over substantially the whole of the opposite face of the substrate 10. However, as the transmission lines 11 and 1 2 extend away from the microstrip coupler 13, they may be configured in a different microwave transmission line technology, for example in a co-planar transmission line technology on the upper face of the substrate 10.

From reading the present disclosure, other variations and modifications will be apparent to persons skilled in the art. Such variations and modifications may involve equivalents and other features which are already known in the design, manufacture and use of RF circuits, couplers and other component parts thereof, and which may be used instead of or in addition to features already described herein.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims (8)

1. An RF circuit comprising a dielectric substrate carrying first and second RF transmission lines having respective first and second facing ends, and a microstrip coupler formed on the substrate between the facing ends to provide broadband RF coupling between the transmission lines, the coupler comprising interdigitated fingers formed by interdigitated end portions of the first and second transmission lines on one face of the substrate and having a ground plane on the opposite face of the substrate, characterised in that the fingers are arranged with respect to each other to form a pair of inner fingers from the first transmission line extending between a pair of outer fingers from the second transmission line, without any finger of the second transmission line intruding between the inner fingers of the first transmission line.

2. An RF circuit as claimed in Claim 1 further characterised in that the inner fingers of the first transmission line are separated by a space which is free of intrusion by any finger of the second transmission line.

3. An RF circuit as claimed in Claim 2, further characterised in that the space between the pair of inner fingers of the first transmission line has a width which is substantially the same as that of spaces between each inner finger and the neighbouring outer finger of the second transmission line.

4. An RF circuit as claimed in any one of the preceding claims, further characterised in that the coupler comprises at least four interdigitated end portions of the first and second transmission lines which are arranged symmetrically with respect to a central axis along the microstrip coupler to form the pair of inner fingers from the first transmission line extending between the pair of outer fingers from the second transmission line.

5. An RF circuit as claimed in any one of the preceding claims, further characterised in that each of the inner and outer fingers has a width which is substantially the same as that of spaces between each inner finger of the transmission line and the neighbouring outer finger of the second transmission line.

6. An RF circuit as claimed in any one of the preceding Claims, further characterised in that the length of the fingers is substantially equal to a quarter wavelength at a frequency within the pass band of the broadband coupler.

7. An RF circuit substantially as described with reference to any one of Figures 5 to 10.

8. An RF circuit having any one of the novel features described herein and/or illustrated in the accompanying drawings.