The present invention relates in general to microwave signal processing circuitry and, more particularly, to a microwave filter illustrated in a bandpass filter implemented in microstrip circuitry for which it is initially being used.
Bandpass filters are designed to pass a desired range of frequencies and to reject others above and below the desired range of frequencies. A common characteristic of bandpass filters is that they also pass higher frequencies, usually at the third and higher odd multiples of the desired range of frequencies. When the passage of the higher frequencies is not desirable, additional filtering, such as a low-pass filter or a band-reject filter, is required to suppress the higher frequency responses of the bandpass filter. Such additional filtering requires the use of additional printed circuit board space, and the longer lengths traversed by the radio frequency signal causes additional losses.
A bandpass filter is tuned and designed to allow a passband, a range of frequencies, to pass with low loss while suppressing frequencies above and below the passed range of frequencies. Circuitry is included into the existing structure of the bandpass filter so that higher frequencies can also be suppressed to thereby reject a band of frequencies at a selected odd multiple of the passed frequency range.
In accordance with the teachings of the present application, a microwave filter comprises a plurality of vertical microstrip elements placed parallel to one another. The plurality of vertical microstrip elements have upper ends that are open circuited and lower ends that are connected to ground potential. At least one horizontal microstrip element connects each of the plurality of vertical microstrip elements to one another, and a spurline is formed in the at least one horizontal microstrip element. In this way, the filter passes a band of frequencies defined by the vertical microstrip elements, connection points of the at least one horizontal microstrip element, the location of a signal input point of the filter and the location of a signal exit point of the filter, and the filter blocks a band of frequencies defined by the spurline formed in the at least one horizontal microstrip.
The plurality of vertical microstrip elements (P) may be greater than two and the at least one horizontal microstrip element then comprises a plurality equal to the plurality of vertical microstrip elements minus one (P−1).
The frequencies of the blocked band of frequencies is substantially equal to an odd multiple of the frequencies of the passed band of frequencies. For example, the frequencies of the blocked band of frequencies may be substantially equal to three times the frequencies of the passed band of frequencies.
The following detailed description of the preferred embodiments of various embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals, and in which:
In the following detailed description of the illustrated embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of various embodiments of the present invention. The microwave filter of the present application is described with reference to microstrip technology for which it is initially being used.
Reference is made to
The filter 100 is tuned by selecting the length of each of the elements 102, the points at which each horizontal microstrip element 104 is attached to each vertical element 102 and the lengths of the horizontal microstrip elements 104, as well as the point of signal entry 106 and the point of signal exit 108 on each end of the filter 100. The frequency response of one configuration of a filter as illustrated in
As shown in
It is also known to use a “spurline” in a microstrip circuit to create a notch filter. A spurline consists of a cut in the microstrip circuit shaped like an L having one end, the short leg of the L, open to one side of the microstrip circuit and the rest of the spurline cut, the long leg of the L, entirely contained within the microstrip circuit. With reference to
In accordance with the teachings of the present application, by forming at least one spurline in at least one of the horizontal microstrip elements 104 of the bandpass filter 100 of
In the illustrated embodiment of
The bandpass filter 500 is tuned by selecting the length of each of the elements 502, the points at which each horizontal microstrip element 504 is attached to each vertical element 502 and the lengths of the horizontal microstrip elements 504, as well as the point of signal entry 508 and the point of signal exit 510 on each end of the filter 500. The band of frequencies, i.e., the suppressed higher-order odd response, that is rejected is tuned by appropriately sizing and shaping the spurlines 506. As an example, the spurlines 506 may be sized and shaped to block the third harmonic or third order response of the filter 500 so that the lengths of the spurlines 506 are set to be about ¼λ where λ is the wavelength of the third harmonic of the center frequency of the passband.
As shown in
Having thus described the invention of the present application in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.
The present application is related to a U.S. patent application entitled MIRCROWAVE FILTER (attorney docket VAL 057 PA) that is being filed on the same day as the present application, is assigned to the assignee of the present application and is incorporated by reference herein in its entirety. The present application is related to a U.S. patent application entitled METHODS AND APPARATUS FOR RECEIVING RADIO FREQUENCY SIGNALS (attorney docket VAL 060 PA) that is being filed on the same day as the present application, is assigned to the assignee of the present application and is incorporated by reference herein in its entirety.