Radio transmitters and receivers require filters to remove or suppress unwanted frequencies from being transmitted 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 millimetre-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, to operate at wider spaced frequencies may require 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. A common millimetre-wave filter is based on the metal waveguide.
Waveguide filters are used at microwave frequencies due to their low loss characteristics. Low loss in the resonant sections corresponds to a higher-Q, faster rolloff outside the passband and lower transmission loss in the passband. A typical waveguide filter consists of multiple coupled resonators, where the volume of a resonator is proportional to the frequency of operation.
An example of a conventional waveguide filter comprises a housing containing a series of resonator cavities arranged in a straight line, where adjacent resonator cavities are separated by an apertured partition which forms a coupler. The resonator cavities are typically rectangular or cylindrical and have a length corresponding to one half wavelength or multiples of one half wavelength of the centre frequency.
Another implementation of a waveguide filter is the E-plane filter, an example of which is shown in FIGS. 1A and 1B. Referring to FIGS. 1A and 1B, the waveguide filter 1 includes a filter housing 2 which forms an elongate channel 4. The housing is split into two parts 6, 8 along the length of the channel to receive an apertured thin metal sheet 10 therebetween. The apertured metal sheet 10 is called a septum.
The rectangular apertures 12 formed in the thin metal sheet 10 each define a resonator and the metal strips 14 remaining between the resonators function as couplers and are known as coupling sections. Each coupling section of the septum effectively divides the waveguide into two halt waveguides having a reduced width of less than half the center frequency wavelength so that the reduced size waveguide does not permit propagation of the electromagnetic wave.
In microwave communications at moderately high frequencies, for example carrier frequencies in the range of 24 to 31 GHz, the frequency band for each of the receive and transmit channels may have a width of only one percent of the center frequency and the center frequencies may be separated by a frequency band of similar width. Thus, a waveguide filter suitable for such an application must provide a relatively narrow pass band with a sharp roll-off, and therefore such a filter requires a relatively large number of resonator cavities and coupling sections. One problem in conventional filter design is that as the number of resonators and coupling sections increases, the waveguide becomes longer and therefore requires a larger housing which adds to the cost and makes it difficult to integrate with other system components.
Various designs for a resonator cavity-type waveguide filter have been proposed to accommodate the resonators and couplers into a smaller space. For example, Japanese Patent Application No. 57041702, Publication No. JP-A-58161403 and Japanese Patent Application No. 57070942, Publication No. JP-A-58187001 each discloses a band pass filter having a series of coupled cylindrical resonator cavities, each centered at the corner of a square. This design takes advantage of the cylindrical symmetry of the resonators to permit the output coupler of each resonator to be oriented at 90° with respect to its input coupler.
U.S. Pat. No. 6,181,224 (Glinder) describes a resonator cavity-type waveguide filter having a series of resonator cavities interconnected by coupler channels in which opposite sides of the coupler channels are the same length, but opposite sides of the resonator cavities have different lengths, so that the input of each resonator cavity is angled relative to its output. In one example, a number of similar resonator cavities having dissimilar length sides are arranged to form an S-shaped waveguide which is accommodated in a space whose length is shorter than that needed for a linear waveguide having similar characteristics. The mechanical length of a resonator cavity having dissimilar length sides which determines the pass center frequency is based on the length of the arcuate center line through the resonator cavity between the input and output couplers. Due to the shape of the resonator cavity, the length of the curved center line is different from that of a linear resonator cavity and is calculated by first calculating the required mechanical length of a linear resonator cavity and then applying a correction factor to the mechanical length. The correction factor is calculated based on the guide wavelength for a linear resonator, the desired pass center wavelength for the non-linear cavity, the width of the waveguide and the radius of curvature of the center line. Although the design disclosed in U.S. Pat. No. 6,181,224 allows the length of a waveguide filter to be reduced, it may be difficult to implement a high-Q, narrow pass band filter using this design since the required dimensions of the filter become more difficult to calculate as the number of cavities increases.