Wave transmission lines and networks – Resonators – Dielectric type
Reexamination Certificate
2000-08-22
2002-07-02
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Resonators
Dielectric type
C333S202000
Reexamination Certificate
active
06414571
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a composite resonator and particularly, but not exclusively, to a composite resonator for use in devices operating at microwave frequencies in the field of cellular telecommunications.
Microwave resonators have a wide range of applications. In particular, in cellular telecommunications, microwave resonators are utilised in filters, multiplexers and power combining networks.
Filters are required with exacting specifications; e.g. narrow band pass filters with low pass band loss. For cellular base station applications combline filters are often used but have maximum resonator Q factors of a few thousand.
On the other hand dielectric resonators have Q values of up to 50000. However, they suffer from poor spurious response; i.e. the first spurious mode of the resonance is close in frequency to the fundamental mode. Consequently the low pass filtering required to clean up the stop band is very difficult to achieve. Further, conventional TE01&Dgr; resonators are not suited to bandwidths above 5 MHz at 900 MHz because the field is mainly confined to the dielectric, consequently it is difficult to achieve strong input coupling.
The problem of spurious resonances may be addressed by using a low pass filter in conjunction with a band pass filter so that the low pass filter cuts off spurious resonance signals. However, such an approach requires a very sharp low pass filter characteristic as the separation in frequency between the desired resonance and spurious resonances is very small. This requires low pass filters which will transmit from DC to the highest frequency of the pass band, e.g. of order 1 GHz, but then cut off within approximately 100 MHz. The corner of the low pass filter must be sufficiently sharp that the low pass filter does not add to the loss in the pass band. A total loss of 1 dB at the central frequency of the pass band is typically required. Such requirements place severe demands on the design of the low pass filter if conventional dielectric resonators are to be employed.
Hence, there is a need for a resonator with a high Q, so that sufficiently sharp band pass characteristics can be achieved, and which does not have the associated problem of closely spaced spurious resonances which require the use of further filters with very severe filter characteristics in order to provide the desired overall filter response.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a microwave frequency composite resonator comprising:
a metal housing having an internal surface and defining a resonator cavity;
a dielectric member; and
a conducting plate, in which the dielectric member is located within the resonator cavity on the internal surface of the metal housing and the conducting plate is located on top of the dielectric member.
By providing a resonator having a dielectric member inside a resonator cavity and between a conducting plate and an internal surface of the metal housing forming the resonator cavity, the frequency response of the resonator provides a desired fundamental natural mode of the resonator having a resonant frequency which is well separated in frequency from the next natural mode of the resonator and the desired natural mode comprises a pair of orthogonal modes. The Q of the resonator at its resonant frequency is at least equal to that of a coaxial resonator in a similar physical volume.
Such a resonator has a resonant frequency which is well separated in frequency from the frequency of the next nearest natural mode. Hence a low pass filter with a less sharp response can be used in conjunction with a filter comprising resonators according to the current invention so as to provide a desired overall filter response.
The mode of resonance is a dual mode with a mode Q similar in value to that of a combline resonator of similar physical size. Consequently this provides for a 2:1 improvement in Q per unit volume. Thus a filter can be constructed with approximately half the physical size of a combline filter with the same electrical performance, or with similar size and much improved performance, i.e. lower loss.
The dielectric member may directly abut the internal surface of the housing. The conducting plate may directly abut a top surface of the dielectric member.
The dielectric member may be a substantially right angular cylinder. The conducting plate may be circular. Preferably, the values of the dielectric constant of the dielectric material is between 30 and 44, more preferably between 36 and 44.
The resonator may be configured such that at resonance the resonator sustains a dual TM mode resonance. The geometry of the resonator may be arranged such that at a desired resonant frequency of the resonator, the resonator sustains a dual TM mode standing wave microwave resonance at the desired resonant frequency.
The resonator may sustain a dual TM mode resonance in which the TM mode resonance comprises a pair of orthogonal modes. The geometry of the resonator may be arranged so that the TM resonance sustained has two modes which are sufficiently close in frequency that at the resonant frequency of the cavity both modes are excited. This provides an enhanced Q of the resonator, approximately double, compared to a similarly sized co-axial resonator which sustains a resonance having a single excitable mode.
The resonator may be configured such that a TE mode resonance of the resonator has a resonant frequency higher than the resonant frequency of the TM mode. By arranging the geometry of the resonator in a suitable way, the frequency separation of a TM mode of the resonator and a next nearest TE mode of the resonator may be inverted. In a freely suspended dielectric resonator, the TE mode is lower in frequency than the next nearest TM mode. However, the arrangement and geometry of the resonator may be suitably chosen, such that the TM mode becomes lower in frequency than the TE mode, i.e. the two modes cross over in frequency, and the separation in frequency of the TE mode and TM mode can be increased compared to the situation when the TE mode is lower in frequency than the TM mode.
The resonator may have an input coupling which couples input electrical signals to the conducting plate. This provides a means of coupling an electrical signal into the resonator and coupling electrical energy into the resonator so as to excite the resonator.
The input coupling and conducting plate may be arranged such that at resonance the radial component of the electric field of the resonant mode is directed diametrically across the conducting plate from the input coupling. Owing to the arrangement and geometry of the conducting plate, dielectric member and resonator cavity, the input coupling attached to the conducting plate establishes an electric field, the radial component of which extends diametrically across the conducting plate from the point where the input coupling attaches to the conducting plate.
There may be a notch in the circumference of the conducting plate. Providing a notch in the circumference of the conducting causes a second radial component of the electric field to be generated across the plate; i.e. a component of the second of the two orthogonal modes of the dual mode TM resonance. The angular position of the notch around the circumference determines the orientation of the second radial component of the electric field with respect to the first radial component of the electric field. Hence the single physical resonator can act as a pair of coupled resonators.
The notch may be located at an angle of 45° from the direction of the radial component of the electric field. Such an angular position of the notch generates a second radial component of the electric field in a direction orthogonal to the first radial component of the electric field; i.e. the second orthogonal component of the dual TM mode. The strength of the second transverse resonance is then maximised and approximately the same as that of the first resonance.
The resonator may have an output coupling which coup
Hunter Ian
Rhodes John David
Filtronic PLC
Fulwider Patton Lee & Utecht
Jones Stephen E.
Pascal Robert
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