US3882434A - Phase equalized filter - Google Patents

The band-pass filter is constructed from a group of directcoupled resonant elements forming resonant cavities arranged in consecutive order, and at least one alternative path formed by a cross coupling. The filter comprises a first array or series of a plurality of intercoupled resonant cavities having an input end and an output end, and a second array or series of fewer resonant cavities intercoupled with the cavities of the first array. This arrangement provides a band-pass filter having a flat group delay and linear phase over the central portion of the pass band.

United States Patent [1 1 [111 3,882,434

Levy May 6, 1975 [5 PHASE EQUALIZED FILTER 3,737.816 6/1973 Honicke 333/73 w [75] Inventor: Ralph Levy, Newton, Mass.

[73] Assignee: Microwave Development Laboratories, Needham Heights, Mass.

[22] Filed: Aug. 1, 1973 [21] Appl. N0.: 384,719

[52] US. Cl 333/73 W; 333/31 A; 333/83 R {51] Int. Cl. HOlp 5/04; l-lOlp 7/06; H03h 7/08 [58] Field of Search... 333/31 R, 73 R, 73 S, 73 W, 333/83 R, 70 S, 82 R, 82 B, 31 A [56] References Cited UNITED STATES PATENTS 2,749,523 6/1956 Dishal 333/83 R 3,597,709 8/l97l Rhodes 333/73 R Primary Examiner-James W. Lawrence Assistant Examiner-Marvin Nussbaum Attorney, Agent, or FirmWolf, Greenfield & Sacks [57] ABSTRACT The band-pass filter is constructed from a group of direct-coupled resonant elements forming resonant cavities arranged in consecutive order, and at least one alternative path formed by a cross coupling. The filter comprises a first array or series of a plurality of intercoupled resonant cavities having an input end and an output end, and a second array or series of fewer resonant cavities intercoupled with the cavities of the first array. This arrangement provides a band-pass filter having a flat group delay and linear phase over the central portion of the pass band.

11 Claims, 6 Drawing Figures PATENIED AY 61975 3.882.434

sum IN 2 PHASE EQUALIZED FILTER FIELD OF THE INVENTION This invention relates in general to frequency selective apparatus for filtering electro-magnetic energy. More particularly, this invention pertains to a bandpass filter characterized by linear phase over a central portion of the pass-band. In comparison to a simple direct-coupled resonator filter. the filter of the present invention is characterized by a flattened passband attenuation and a doubling of group delay band-width.

BACKGROUND OF THE INVENTION The simple type of direct-coupled resonator filter typically may comprise a series of interconnected resonant elements. The eIectro-magnetic wave energy passes by way of one path from the input to the output of the filter; that is apart from multiple reflections that may occur. This type of a filter provides minimum phase linearity and is characterized by phase and delay characteristics which are completely prescribed by the amplitude characteristic.

A filter having both direct and cross-coupled resonators is disclosed in U.S. Pat. No. 3,597,709. In comparison with the simple direct coupled filter, the filter shown in this patent is provided with a plurality of extra cross-coupling paths. Hence, the electro-magnetic wave can reach the output from the input via multiple alternate routes. A filter of this type, as discussed in U.S. Pat. No. 3,597,709, may be designed to provide a wide variety of characteristics, e.g.. maximally-flat or equiripple group delay.

One of the primary disadvantages associated with filters of the type shown in U.S. Pat. No. 3,597,709 is that the direct and cross-coupling coefficients between resonators must generally be very close to the theoretical values if a satisfactory performance is to be achieved. This means that a filter with two or more extra crosscouplings may be difficult to tune and consequently quite expensive to produce. In particular, it may be quite difficult to tune a narrow-band filter of this type over a broad frequency band, as required for economic design of filters for communications systems, for example. Another disadvantage associated with these prior art direct and cross-coupled filters is that the out-ofband attenuation is considerably reduced by the presence of the extra cross-couplings. Hence, a greater number of cavities are required than for a conventional coupled filter. This in turn increases the insertion loss in the pass-band of the filter. For cases where between 60% and 100% of the pass-band is required to have a linear phase then a filter of the type described in U.S. Pat. No. 3,597,709 is most desireable. However, there are many applications wherein only the central 50% of the pass-band is required to have linear phase. For example, this is the case in most communication links where flat delay over the central 50 percent of the passband may result in major improvements in systems per formance since most of the energy is transmitted in the central region of the pass-band.

Accordingly, it is an object of the present invention to provide a band-pass filter of the directcoupled type preferably having one extra cross-coupling added to the direct coupled resonator filter. This arrangement provides an extremely flat group delay; e.g., about 0.3 nano-seconds variation in the central 50 percent of the pass-band (defined to the equi-ripple points) of a filter having an absolute group delay on the order of 50 nano-seconds.

SUMMARY OF THE INVENTION To accomplish the foregoing and other objects of this invention there is provided a band-pass filter which generally comprises a first series of resonant elements, a means for coupling an input signal to a resonant element of the first series, means coupling to a resonant element in the first series for obtaining an output signal and a second series of resonant elements smaller in number than the first series and arranged with each resonant element opposite and in substantial alignment with a different resonant element in the first series. The second series of resonant elements preferably numbers two and provides a single cross-coupling alternate path. The resonant elements in both the first and second series are directly coupled in consecutive order. The means for coupling an input signal is disposed prefera bly at one end of the first series and the output signal is obtained at the other end of the first series.

In one embodiment of the invention the first series of resonant elements is defined by a waveguide structure partitioned off by iris-defining plates. The second series of resonant elements includes two resonant cavities separated by an iris-defining plate and intercoupled to corresponding resonant cavities of the first series by means of an iris. In addition to the use of a rectangular waveguide the concepts of the present invention may also be applied with circular cavities, coaxial cavities, or with interdigital, comb-line, or helical-line resonators.

BRIEF DESCRIPTION OF THE DRAWINGS Numerous other objects, features and advantages of the invention will now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of one form of a filter constructed in accordance with the principles of the present invention;

FIG. 2 is a cross-sectional view of the filter shown in FIG. 1 and including the tuning screws;

FIG. 3 is a low-pass prototype network of the filter shown in FIGS. 1 and 2;

FIG. 4 is a circuit representation of the low-pass prototype for an even ordered Chebyshev filter;

FIG. 5 is another circuit representation similar to that shown in FIG. 4 for the admittance-inverter form of the Chebyshev low-pass prorotype; and

FIG. 6 shows the even mode form of the low-pas prototype network shown in FIG. 5.

FIG. 2 is a cross-sectional view of the filter of FIG. 1 and also including the tuning screws that may be associated with each cavity of the filter. Regarding the filter shown in FIGS. 1 and 2 the extra cross-coupling may be considered as existing between resonant cavities Q2 and Q5. The filter of the present invention is actually only slightly more complicated than the conventional direct-coupled filter and is easily tuned to any frequency range. Another advantageous feature of the presentinvention is that the extra cross-coupling between cavities Q2 and Q5 may be made variable by a tuning screw or other means. In FIG. 2 each of the cavities Ql-Q6 have associated therewith respective tuning screws TIT6. When the cross-coupling between cavities Q2 and Q5 is changed, the filter may be rematched by tuning the main coupling between cavities Q3 and Q4, followed by minor adjustments to the tuning of the adjacent resonant cavities which should have been disturbed only slightly. If the cross-coupling is tightened. then the main coupling by way of cavities Q3 and Q4 is to be loosened. The theory of the present invention which is discussed in more detail hereinafter, shows that the sum of these two couplings remains almost constant. Hence. it is relatively simple to tune the filter to give group delay characteristics which are either undercoupled, critically-coupled, or over-coupled. This is a useful feature for many systems applications of the present invention.

Regarding the embodiment shown in FIGS. 1 and 2, since only one extra cross-coupling is used therein, the out-of-band attenuation is not reduced as much as in the case of the filter of the type shown in U.S. Pat. No. 3,597,709. It can be seen from FIG. 1 that the out-ofband attenuation of an n-cavity filter cannot be worse than that of an (11-2 )-cavity conventional filter as disclosed in the prior art patent referred to herein. In fact, since the coupling between cavities Q2 and O5 is very loose, then the attenuation will be higher than that of an (11-2 )cavity conventional filter.

The insertion loss at mid-band with the filter of the present invention is increased in comparison with that of the conventional direct and cross-coupled filter. At the same time, however, the insertion loss is relatively flattened over the central 50% of the pass-band corresponding to the flat group delay band-width. Typically, for an insertion loss of one dB this insertion loss is constant to Withiin 0.05 dB over the central 50 percent of the pass-band. Although the mid-band insertion loss of THEORY AND DESIGN PROCEDURE ln U.S. Pat. No. 3,597,709 a design procedure was disclosed relating to the filter structure disclosed therein for a band-pass filter. The theory of the present invention is examined using a low-pass prototype lumped constant embodiment as depicted in FIG. 3. The extra cross-coupling is represented by the admittance inverter Jml.

The transfer function of the low-pass prototype takes the form:

This low-pass prototype network will now be derived by a modification of the standard lumped-element even-ordered Chebyshev low-pass prototype filter. See IEEE Transactions on Microwave Theory and Techniques, Volume MTT=l8, Number 6, June, 1970, pp.29030l by John David Rhodes. The n'" ordered prototype is shown in FIG. 4, where the element values are given by:

S is the VSWR at do, or the ratio of the output conductance to the input conductance.

Using the relationship:

where (r=0, I, 2, n)

then it is possible to transform the circuit of FIG. 4 to the physically symmetric circuit of FIG. 5 by introducing ideal admittance inverters. Electrically. this net work is actually antimetric. The central admittance inverter is given by:

VS modd l/VT Mcven This circuit is seen to be identical to that of FIG. 3 with the exception of the extra cross-coupling admittance inverter J,,. ln accordance with the theory of this invention, the invertor J,,, is now introduced in the form of a small perturbation of the Chebyshev filter (it is quite small in practice). The even-mode embodiment of the network of FIG. 3 formed by inserting an open-circuit along the physical line of symmetry is shown in FIG. 6. The only portion of the network which is changed from the Chebyshev values is to the right of plane A-A. The admittance at this plane is:

m ill-l z m The low-pass prototype is designed, therefore. by in troducing a finite value ofJ,,, into the Chebyshev prototype, adjusting .l,, from its Chebyshev value J,,,' given by equation (4), using equation (7), and analysing the filter over the low pass-band. It is then a simple matter to find a value of J giving the desired group delay response since only one parameter is varied. It is found that under-coupled, critically-coupled, or overcouplcd group delay characteristics may be obtained.

It should be evident from the following description that the invention can be embodied in many different forms. It is preferred to use only one cross-coupling path, however, especially for filters containing a number of cavities, two or more cross-coupling paths can be employed.

Because the invention can be embodied in varied structures, it is not intended that the invention be limited to the forms here illustrated or described. Rather, it is intended that the invention be limited solely by the appended claims and include those structures that do not fairly depart from the essence of the inventionv What is claimed is:

l. A microwave band-pass filter having a relatively flat group delay and linear phase over the central portion of the pass-band, said filter comprising;

means defining a first series of resonant elements,

means for coupling an input signal to a resonant element of the first series,

the resonant elements in the first series being each directly coupled in consecutive order with its adjacent element, means defining a second series of resonant elements smaller in number than the number comprising said first series and arranged with each resonant element opposite and in substantial alignment with a different resonant element in the first series,

means coupling from a last resonant element in the first series for obtaining an output signal therefrom,

the resonant elements in the second series being each directly coupled in consecutive order with its adja cent element,

said first and second series being arranged to provide a main path through all of said resonant elements with at least one cross-coupling path being provided between two adjacent elements of the first series,

means for independently tuning each resonant element, and

tuning means associated with said one extra crosscoupling path.

2. The filter ofclaim 1 wherein both the first and second series of resonant elements each include a waveguide structure partitioned by at least one iris-defining means to form a plurality of resonant cavities.

6. The filter of claim 5 including means for independently tuning each resonant element.

7. A microwave band-pass filter having a relatively fiat group delay and linear phase over the central portion of the pass band, said filter comprising;

means defining a series of resonant elements including a waveguide structure partitioned by a plurality of iris-defining means to form a plurality of resonant cavities,

said series of resonant elements being arranged so that at least two of said elements are opposite and in substantial alignment with other resonant elements of the series to define a main path through all of the resonant elements,

two other of said elements providing one crosscoupling path,

means for independently tuning each resonant element,

resonant elements.

10. The filter of claim 9 wherein a first of said resonant elements of the in line group includes means for receiving an input signal and a last of said resonant elements of the in line group includes means for coupling out an output signal.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3 2 434 Dated Ma 64 l 075 Invento Ralph Leity It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line l8, change "Jm-l" to --J,,,

Column 4, equation l should read 2(P) P 7 W Column 4, line 34, change "Cm-l" to -C Column 4, line 39, change 'Jm-l' to --J Column 4, line 53, delete the second "n" appearing in the lower part of the equation in the parentheses.

Col gmn 4 l ine 55, draw a l ine above y sin Column 4, line 60, change the first occurance of "sink" to --sinh--.

Column 4, line 60, change the second occurance of "sink" to --sinh--.

Column 5, line 9, change "M even" to -m even--.

Signed and Scaled this twentieth Day Of April 1976 [SEAL] AIIeSI.

Arresting Officer