GB2292839A - Microwave slow wave structure - Google Patents

Abstract

A microwave slow wave structure comprises a microstrip line formed with a meandering gap so as to form a conductive loop (8) defining two parallel conductors each having a plurality of fingers (10) extending towards the other conductor and interdigitated with the fingers of the other conductor. <IMAGE>

Description

SLOW WAVE STRUCTURE AND MICROWAVE DEVICE INCLUDING SUCH A STRUCTURE.

The present invention relates to a slow wave structure and to a microwave device including such a structure.

Many microwave devices utilise propagation delays within the device in order to achieve their function. Other devices rely on the formation of standing waves therein in order to achieve their desired operation. In each case the minimum size of the device is constrained by the requirement to include a waveguide of a predetermined length which corresponds to a predetermined transit time or a predetermined proportion of a design wavelength.

Many microwave circuits are implemented using microstrip. The microstrip half-wavelength resonator is a frequently used component. A half-wavelength resonator is shown in Figure 1(A) of the accompanying drawings. The ground plane of the resonator is omitted for clarity. The resonator comprises a length of microstrip 2 having first and second ends 4 and 6, respectively, separated by a predetermined distance d. The resonator 2 has a resonance when d is equal to half a wavelength of a signal within the microstrip. Signals are supplied to and removed from the resonator by way of feeding pads 3 which are separated from the resonator by 1 mm. gaps. The size of such a resonator may make it unsuitable for application in microwave integrated circuits where the utilisation of area in a highly efficient manner is required. In order to overcome this problem, a hairpin resonator may be used.The hairpin resonator can be viewed as a folded version of the microstrip halfwavelength resonator. The hairpin resonator does not reduce the area within the microwave integrated circuit required to form a halfwavelength resonator, although it can enable circuits to be more conveniently shaped.

According to a first aspect of the present invention there is provided a microwave slow wave structure, comprising first and second conductors spaced apart from one another, each of the conductors carrying a plurality of fingers extending towards the other conductor and interdigitated with the fingers of the other conductor.

The fingers alter the electrical properties of the transmission line formed by the conductors and reduce the velocity of propagation therein.

Preferably the first and second conductors are formed of microstrip line.

Advantageously a microstrip line or a section of microstrip line has a meandering gap formed therein which acts to divide the microstrip line or section of microstrip line into the first and second conductors and also defines the interdigitated fingers.

Each of the fingers acts as a stub and increases the capacitance per unit length of the microstrip line. Since the velocity of propagation V of signals within the microstrip line is given by V=(LCYIA, where L and C are inductance and capacitance per unit length, respectively, increasing the capacitance decreases the velocity of propagation.

The structure can be defined within the normal dimensions of a microstrip line, that is the width of the microstrip line need not be increased. The first and second conductors may be connected together at each ends of the slow wave structure.

According to a second aspect of the present invention there is provided a resonator comprising a slow wave structure according to the first aspect of the present invention.

The present invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a known half-wavelength resonator; Figure 2 illustrates a loop resonator; Figure 3 illustrates a capacitively loaded resonator constituting an embodiment of the present invention; Figure 4 is a graph of finger capacitance as a function of finger width; Figure 5 is a graph comparing the frequency response of resonators constituting an embodiment of the invention with known resonators; Figure 6 is a graph showing the phase response of resonators constituting an embodiment of the present invention and a known half-wavelength resonator; and Figure 7 is a graph comparing the insertion loss versus frequency response of a resonator constituting an embodiment of the present invention with the response of a known half-wavelength resonator.

The conventional half-wavelength resonator shown in Figure 1 can be modified by removing the internal region of the microstrip so as to leave a ring 8 of conductor. Such an arrangement is shown in Figure 2 and is described by J. Wolff and N. Knoppik, "Micro Strip Ring Resonator and Dispersion Measurement on Microstrip Lines", Electronics Letters, 1971, 7, pp.779-781. Such a structure undergoes resonance when a wavelength of a signal within the microstrip is approximately equal to the mean circumference of the loop, provided that the strip width of the loop is much smaller than the width of the associated half-wavelength resonator. The resonance frequency of the loop resonator does not change much from that of the equivalent half-wavelength resonator.

The device shown in Figure 3 has a plurality of fingers 10 extending inwardly of the ring 8.

Each of the fingers functions as an open circuited transmission line. It is known that such an open circuited line has an input impedance Z given by: Z = -jZocot(2sL/A) when L, the length of the line, is less than A/4, where A is the wavelength of the signal within the line. Z0 is the characteristic impedance of the line and is equal to (L/Z). Thus the fingers capacitively load the ring 8 and consequently reduce the velocity of propagation within the ring. The reduction in signal velocity means that a given size ring will resonate at a lower frequency or that, for a given operating frequency, the dimensions of the ring may be reduced.

Assuming that there is little or no coupling between neighbouring fingers, the degree of resonant frequency shift depends on the total loaded capacitance Total which is proportional to the number of fingers N. When fabricating fingers within a given surface area, N is inversely proportional to the finger width when the fingers are equally spaced.

Figure 4 shows a graph illustrating the change in capacitance per metre of a finger as its width W to the thickness h of the dielectric substrate varies. Reducing the width of a finger reduces the capacitive loading C5 provided by that finger. However, even if a finger width is reduced by twenty times, C5 is only reduced by a factor of less than 2. Thus the resonant frequency can be decreased as the number of fingers is increased.

The Q factor of the resonator is also increased since the structure exhibits a lower radiation loss when compared with the conventional linear halfwavelength resonator, as shown in Figure 1.

Figure 5 shows a comparison of performance of four resonators. All the resonators were formed on an RT/Duroid with dielectric having a thickness of 1.57 mm. and a relative dielectric constant E,-2.2. All the resonators occupied the same surface area (20 mm. long and 6.6 mm.

wide) and all were fed via 1 mm. gaps in a microstrip line circuit. The curve 11 indicates the response of a loop resonator having 31 fingers.

The curve 12 indicates the response of a loop resonator having 21 fingers, the curve 1 3 indicates the response of a loop resonator having no fingers and the curve 14 indicates the response of the conventional half-wavelength resonator of the type shown in Figure 1. The loop resonator having 31 fingers was formed with a loop of 0.5 mm. thickness and a finger length of 5.2 mm. The resonator having 21 fingers was formed with a loop of 0.8 mm. thickness, a finger length of 4.6 mm. and a finger width of 0.4 mm. The loop resonator having no fingers was formed with a loop of imam. thickness. The resonant frequency of the loop resonator having 31 fingers is at 3.7 GHz, whereas the resonant frequency of a similarly sized half-wavelength resonator of the type shown in Figure 1 is 4.9 GHz.Furthermore, the resonator having 31 fingers has a better Q factor compared with the linear half-length resonator.

Figure 6 shows the phase versus frequency response of the resonators.

Line 21 indicates the response of the loop resonator having 31 fingers, line 22 indicates the response of the loop resonator having 21 fingers and line 24 indicates the response of the linear half-wavelength resonator. The rate of change of phase with respect to frequency is higher for the capacitively loaded strip line and increases with the number of fingers.

A uniform length of microstrip can be modified by selectively removing portions thereof to create a stub loaded line which does not occupy any extra surface area. Such a modified line functions as a slow wave structure. The signal velocity within the 31 finger loaded loop and the 21 finger loaded loop was reduced to 0.72 and 0.85, respectively, of the velocity in the corresponding line having no fingers.

Figure 7 compares the performance of two resonators having nominally the same centre frequency. The line 26 shows the response of a linear half-wave resonator (as shown in Figure 1), whereas the line 28 indicates the response of a ring-type resonator constituting an embodiment of the present invention having 31 fingers. Both resonators were 6.6 mm.

wide, but the linear half-wave resonator was 27.4 mm. long whereas the 31 finger loop resonator was only 20 mm. long. The Q factor for the loaded ring resonator is better than that of the linear resonator.

Furthermore, the loaded ring occupies a smaller surface area.

It is thus possible to provide a slow wave structure which enables the reduction in size of microwave resonators and other components whose physical sizes are a function of microwave propagation velocity.

Claims (6)

1. A microwave slow wave structure comprising first and second conductors spaced apart from one another, each of the conductors carrying a plurality of fingers extending towards the other conductor and interdigitated with the finger of the other conductor.

2. A structure as claimed in Claim 1, in which said first and second conductors are formed of microstrip line.

3. A structure as claimed in Claim 2, in which the microstrip line or a section thereof has a meandering gap formed therein which acts to divide said microstrip line or section thereof into the first and second conductors and also defines said fingers.

4. A structure as claimed in any preceding claim, in which the first and second conductors are connected together at each end.

5. A microwave slow wave structure substantially has hereinbefore described with reference to the accompanying drawings.

6. A microwave resonator comprising a slow wave structure as claimed in any preceding claim.