Waveguide filter

Details Waveguide filter Waveguide filter

2001-10-18

2003-12-02

Ham, Seungsoek (Department: 2817)

Wave transmission lines and networks

Coupling networks

Wave filters including long line elements

C333S212000

Reexamination Certificate

active

06657520

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to waveguide filters and in particular, but not limited to waveguide filters for RF waves.
BACKGROUND OF THE INVENTION
Radio transmitters and receivers require filters to remove or suppress unwanted frequencies from being transmitting or received. The transmitter portion of the radio may generate frequencies which will interfere with the radio system, or which may be prohibited by the radio frequency spectrum governing body. The receiver may need to suppress unwanted signals at different frequencies generated by the transmitter, or received from an external source, which would adversely affect the performance of the receiver.
At millimeter-wave frequencies sources of unwanted frequencies include the local oscillator frequency, image frequencies from the mixer, and the transmitter frequencies (in the case of the receiver). The frequencies generated by the mixer and the local oscillator are functions of the selected radio architecture. The closer the oscillator frequency (or its harmonics) is to the transmitter frequencies, the more difficult it is to remove the undesired frequency. However, wider spaced frequencies may result in more complex circuitry resulting in a more expensive radio implementation. A small separation between the transmit and receive frequencies can result in unwanted high power transmit frequencies leaking into the receiver. The separation between the transmit and receive frequencies is usually specified by the licensing bodies and the system operators. The radio designer may not have control over this specification.
To suppress the unwanted frequencies below an acceptable power level, a filter element is required in the signal path. The filter element discriminates between the desired and undesired frequencies based on the wavelengths of the signals. At millimeter-wave frequencies the difference between the wavelengths is very small, resulting a in very high manufacturing tolerances.
A common millimeter-wave filter is based on the metal rectangular waveguide, an example of which is shown in
FIGS. 1
a
and
1
b.
The waveguide
1
comprises a series of resonant cavities
3
separated by partitions S. Each partition has an aperture or iris
7
to permit coupling of electromagnetic energy between the resonator cavities
3
. Adjustable posts or tuning screws
9
extend into each cavity to provide a means of adjusting the resonant frequency of each cavity which is dependent on the cavity volume. A rectangular waveguide is used for its low loss characteristics. The resonant elements which when combined generate the filter response are formed in the waveguide mainly through the use of irises and posts. The resonant sections
3
are formed from lengths of waveguide multiples of one half wavelength long, with the size and placement of the irises or posts determining the coupling between the resonators and hence the frequency behaviour of the filter.
For the filter to discriminate between closely spaced frequencies, the physical dimensions must be extremely accurate. In practice it is difficult and costly to achieve the required dimensional accuracy. Historically many millimeter-wave applications did not require high volume production, and thus the investment to achieve the necessary accuracy was not warranted. Adjustable tuning screws were included in the design and after manufacture and assembly each filter was individually tuned, manually or automatically, to achieve the desired frequency response.
The use of tuning screws results in increased costs when compared to machined or case filters due to the more complicated assembly and tuning steps in the manufacturing process. Examples of these filters are disclosed in the publications of commercial millimeter-wave waveguide filter or diplexer component manufacturers, such as Microwave Development Company Inc., Lark Engineering, or X&L Microwave Inc.
A typical metal insert or E-plane filter is shown in
FIGS. 2
a
and
2
b.
The waveguide housing
10
is split into two sections
12
,
14
, along the middle of the long dimension. The metal insert piece or septum
16
behaves as a series of posts when the filter is assembled. The accuracy of fabrication of the metal insert piece, which is normally etched and has a dimensional accuracy of ±0.1 mil (i.e. 0.0001 inches) is sufficient to ensure that there is no significant affect on the filter response at millimeter-wave frequencies. The frequency of operation of the filter is therefore set by the accuracy of the depth “d” of the waveguide housing, as shown in
FIG. 2
a.
A benefit of the metal insert filter is that the same housing can be used for different filters at different frequencies. Only the metal insert piece needs to be changed, and there is not a significant setup charge for changes in the metal insert piece.
To achieve the tight filtering requirements for closely spaced local oscillator and transmit frequencies, or between transmit and receive frequencies, for LMDS (Local Multipoint Distribution Service) radio applications the accuracy of the depth of the waveguide housing needs to be better than +/−1 mil. This level of accuracy can be achieved with quality machining, but is expensive to achieve for volume production.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided an E-plane waveguide comprising a housing having opposed walls and defining a waveguide channel therebetween, at least two elements spaced apart in a direction along the waveguide channel and disposed between and spaced from said opposed walls and defining a resonant cavity therebetween with said waveguide channel, wherein at least one of said walls has first and second regions separated in a direction along said waveguide channel, at least one of said first and second regions being disposed opposite the aperture formed between said elements, and a third region between said first and second regions, and wherein said first and second regions protrude into said waveguide channel relative to said third region, and the spacing between said first and second regions in a direction along said waveguide channel is less than half a wavelength of the resonant frequency of said resonant cavity.
Advantageously, the inventors have found that providing the cavity wall with a structured surface having protrusions which extend into the cavity can significantly reduce the frequency shift of a waveguide filter due to errors in the width of the waveguide channel caused by the manufacturing process.
In one embodiment, at least one of the first and second regions comprises a discrete protrusion extending into the cavity relative to the third region.
In one embodiment, at least one dimension of the first region or protrusion is different from at least one corresponding dimension of the second region or protrusion. Advantageously, this feature may promote statistical variations in the manufacturing error by increasing the chance that protrusions having different dimensions will be subjected to slightly different manufacturing process conditions.
In one embodiment, the maximum dimension of the or each discrete protrusion transverse to a line perpendicular to the plane of the wall from which the or each protrusion extends is less than or equal to the spacing between itself and the other region or protrusion. Advantageously, this feature allows the protrusions to be rotated, for example for the purpose of adjusting their height and is particularly advantageous in the design and testing of a template of a waveguide housing section, which may be used to produce a cast or mold.
According to another aspect of the present invention, there is provided a housing section for an E-plane waveguide comprising a first wall and a second wall for forming part of said waveguide, said first wall adjoining said second wall, and in use, said first wall spacing said second wall from a septum of said E-plane waveguide, and wherein the side of said second wall which, in use, faces the waveguide channel, has said first and second

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