US3428918A - Multiplexer channel units - Google Patents

Description

Feb. 18, 1969 G. L.VMATTHAEI 3,

MULTIPLEXER CHANNEL UNITS Filed May 26, 1966 INVENTOR, GEORGE L. MATT HA E I BYm 4. M Mam M M W ATTORNEYS- United States Patent 3,428,918 MULTIPLEXER CHANNEL UNITS George L. Matthaei, Santa Barbara, Calif., assrgnor, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed May26, 1966, Ser. No. 553,604

US. Cl. 333-6 Int. Cl. H01p /12 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates to multiplexer channel units, and more particularly to a filter unit for separating out bands of microwave frequencies from a transmission line carrying a large range of microwave frequencies.

Microwave multiplexers are required to take many signals of different frequencies which are propagating on a single transmission line and separate them out onto a number of transmission lines according to their frequencies. In principal this can be done by using a bandpass filter to separate out each of the desired frequencies. If the band-pass filters for the various frequencies are connected in parallel, then the many frequencies on the main transmission line are separated out into the individual filters. In practice, this is very difiicult to do, particularly at microwave frequencies because conventional band-pass filters will interact with each other. Interaction of the filters has an adverse effect on the performance of each filter. The undesirable interaction effects can be reduced somewhat by connecting all the filters to a common point. Unfortunately, when waveguides or other forms of microwave transmission lines are used, only a very limited number of filters can be joined together at one point.

Directional filters have been used with some success to multiplexer units. These filters use a main transmission line and the signals to be separated out are taken away by a directional filter structure which condutcs them out to a side transmission line. The main transmission line has the property of always presenting a matched impedance. For this reason, in theory any number of directional filter channel units can be cascaded, without undesirable interaction effects.

Although in concept directional filters appear to be ideal as multiplexer filters, they have some practical disadvantages. In order to obtain directional 'filter action, each resonator of the filter must support two orthogonal resonant modes at the same time. Because of this, directional filters are quite difficult to tune if there are more than one or two resonators in the filter, and usually more than two are necessary. Also, directional filters require a relatively tight coupling to the main transmission line. This tight coupling causes a very sizable voltage standing Wave ratio (VSWR) at frequencies off of the pass-band of the directional filter, particularly in the case of Waveguide directional filters. Thus, while in theory directional filter channel units should present a perfect impedance match at all frequencies, in practice the mismatch introduced by practical filter units may be quite large.

I have invented a multiplexer channel unit that has all the advantages of directional filter units, with considerable fewer of their disadvantages. Basically, my multiplexer channel unit comprises a pass-band filter and a stop-band filter. The unit is designed to select a single frequency or a band of frequencies from a transmission line carrying a large number of frequencies. Any number of my channel units can be cascaded together so that all frequencies carried by a given transmission line can be selectively taken from the line. There is essentially no interaction between the units and not tuning problem exists when my channel units are cascaded.

Therefore, an object of my invention is to provide a multiplexer channel unit.

Another object of my invention is to provide a system for selecting by frequency one or more signals from a transmission line carrying a large number of signals of different frequencies.

A further object of my invention is to provide a multiplexer channel unit so designed that any number of these units can be cascaded together without any adverse effects.

The above mentioned and other objects of the invention will become apparent from the following detailed description and accompanying drawing in which:

FIG. 1 shows a strip line version of my invention;

FIG. 2 shows a waveguide version of my invention; and

FIGS. 3(a)3 (0) show in detail the tuning screw and the resonant iris construction of the stop-band resonators of FIG. 2.

Referring to FIG. 1, input signals are applied to atransmission line 4 at the input terminal 1. Assume that a large number of signals having frequencies of f to f are so applied totransmission line 4 and that the signals travel in the direction of the arrow at input 1. It will also be assumed in the following discussion that all the signals except those having a frequency of f are to be present at theoutput terminal 2. Theelements 5, 6, 7 and 8 are resonators which are a quarter wavelength long at frequency h, in this case, and which are short-circuited at one end and open-circuited at the other end.Elements 5, 6, 7 and 8 form an interdigital band-pass filter so designed that only those signals ontransmission line 4 having a frequency i, will appear at theoutput terminal 3.

Theelements 9, 10 and 11 are also resonators that are open-circuited at one end and short-circuited at the other end. These elements form a band-stop filter. The band-stop filter rejects all those signals having a frequency of f While permitting all the other signals to pass to output The combined action of the band-pass filter and bandstop filter is such thatoutput 2 is effectively isolated from all those signals having a frequency of f andoutput terminal 3 is effectively isolated from all those signals having a frequency different than f Any number of units constructed similar to the unit shown in FIG. 1 may be connected together. The only difference between the units will be the frequency characteristics of the filters. Each unit is designed to select a different frequency. Of course the filters of each of the units can be so designed that each unit will select those signals falling within a particular frequency band rather than just those signals of a given single frequency.

'In practice, the strip line version of my multiplexer channel unit is constructed in accordance with conventional design techniques. By proper design the input impedance looking in from terminal 1 can be made to be nearly a perfect constant resistance. For this reason,

any number of units can be cascaded without harmful interaction effects.

FIG. 2 shows a waveguide version of my multiplexer channel unit. Thewaveguide 4 is the main transmission line. Input signals having different frequencies are simultaneously applied towaveguide 4 at the input 1. The signals travel alongwaveguide 4 in the direction ind-icated by the arrow. The elements numbered 6 are conventional cavity resonator-s. Each resonator is provided with a tuning screw 9. Theelement 7 is a waveguide band-pass filter. The elements number '8 are specially designed resonators which will be more fully described. A residualreactance annulling screw 5 is provided at the bottom ofwaveguide 4.

It is again assumed that the multiplexer unit shown is to select only those signals having a frequency of f while permitting all other signals to pass to theoutput port 2, thencavity resonators 6 will be designed and tuned by means of tuning screws 9 to be resonant at frequency f Band-pass filter 7 is designed to pass only those signals of frequency f Therefore, only f signals will appear at theoutput port 3.

Resonators 8 are designed and tuned by means of thetuning screws 10 to reject f signals while permitting all other signals to pass on to theoutput port 2.Resonators 8 form a band-stop filter. The band-stop filter assures that no f signals will appear atport 2. Therefore, the combined action ofcavity resonators 6 andband pass filter 7 and the band-stop filter assures that only f signals will appear at aport 3 while all other signals will pass undisturbed on toport 2.

Any number of waveguide units of the type shown in FIG. 2 may be cascaded with little or no adverse effects. Each unit, of course, is designed to select a difi erent frequency or band of frequencies. The resonators are readily brought to resonance by adjusting their tuning screws. Tuning is a simple matter when compared with the difiiculties encountered in attempting to tune waveguide directional filter.

As was mentioned above,resonators 8 are specially designed devices. All the other components are conventional waveguide components. I attempted to use conventional iris-coupled band-stop resonators for the bandstop filter, but I found that because of the inductive-iris couplings of these resonators, the structure had an excessive voltage standing ratio at frequencies off of the channel separation frequency. For this reason I designed a novel type of band-stop filter resonator. This resonator is shown in detail in FIGS. 3 (a)-3 (c).

As shown in FIGS. 3(a) and 3*( b), a smallresonant iris 13 is cut out of a slab ofmetal 12. The whole slab of metal with the iris is soldered into the top wall ofwaveguide 4. Theback wall 14 behind the iris is 0.100 inch away fromiris 13 while the metal wall thatiris 13 is cut out of is 0.050 inch thick.Iris 13 is brought to resonance at the desired frequency by tuningscrew 10.Screw 10 is a chrome plate brass screw which is capped by a section of dielectric material 1'5 and anend cap 16 ofaluminum foil 16. Thus, the actual tuning is achieved by the dielectric portion of the screw with the aluminum foil tip. The main advantage of this type of resonator is that it is small and compact and operates much like a lumped-element resonator. The use of my novel resonator resulted in a much lower off-resonance voltage standing ratio than which occurred when I used conventional iris-coupled band-stop resonators.

The conventional iris coupled cavity resonators used forresonators 6 also tended to give a sizable voltage standing ratio olf of the pass-band of the band-pass filter. This problem is solved by the use of residual reactance-annullingscrew 5.

From the foregoing remarks it is apparent that I have invented a microwave frequency multiplexer channel unit that has all the advantages of the conventional prior art type of multiplexer channel units without the major disadvantages of the prior art devices. Any number of my multiplexer channel units can be cascaded without any adverse interaction or tuning problems. Furthermore, waveguide or strip line techniques can be used to con struct my multiplexer channel unit.

The invention has been described with reference to two preferred embodiments. It will be obvious to those skilled in the art that various modifications and changes can be made to the embodiments shown and described without departing from the spirit and scope of the invention as defined in the appended claim.

I claim:

1. A multiplexer channel unit comprising a waveguide transmission line; a waveguide band-pass filter coupled to said transmission line, said band-pass filter having first, second and third tunable resonators connected in series and a waveguide filter section connected to said third resonator; and a band-stop filter coupled to said transmission line, said band-stop filter having first and second iriscoupled resonators, each of said iris-coupled resonators having a slab of metal with an iris cut out of said slab, a back plate mounted in back of said iris plate in such a manner that said iris is .100 inch away from said back plate and a chrome plated tuning screw having a dielectric cap and an end cap of aluminum foil.

References Cited UNITED STATES PATENTS 1,469,832 10/19-23 Hamilton 333-6 XR 2,432,093 12/1947 "Fox 333-7'3 2,588,226 3/1952 Fox 333-73 2,816,270 12/ 1957 Lewis 333-73 XR 3,292,075 12/1966 Wenzel. 3,327,255 '6/ 1967 Bolljahn et al 33373 3,345,589 10/1967 Di Piazza 333-73 3,348,173 10/1967 Matthaei et al. 333-73 FOREIGN PATENTS 683,068 4/1964 Canada.

HERMAN KARL SA'ALBACH, Primary Examiner. M. NUSSBAUM, Assistant Examiner.

US. Cl. X.R.

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