US 3678414 A
A shield enclosed microstrip circuit microwave multi-diode switch with undesired waveguide signal transmission minimized. Diodes are spaced and connective microstrip sections dimensioned for optimum signal isolation in the OFF state. Further, diode dc bias isolation is provided for multi-diode sections of such switches by signal path coupling capacitors, along with dc bias supply RF rejection filtering minimizing loss of signal power and that minimizes disturbance of signal line impedance over a relatively broad band range of operation.
Claims available in
Description (OCR text may contain errors)
United States Patent Hallford [451 July 18, 1972  MICROSTRIP DIODE HIGH ISOLATION 3,474,358 l0/l969 Geddry et al. ..333/7 SWITCH 3,538,465 11/1970 Manning et al ..333/84 M  Inventor: Ben R. Hallford, Dallas, Te'x.  Assignee: Collins Radio Company, Dallas, Tex.  Filed: Oct. 19, 1970 [2 1] Appl. No.: 81,943
 U.S. Cl. ....333/7, 333/84 M, 333/8  Int. Cl. ..H0lp 5/12  mid of Search ..333 7, 8, 84 M, 84
56 2 References Cited UNITED STATES PATENTS 3,223,947 12/1965 Clar ....333/7 3,475,700 10/1969 Ertel 4333/? 3,553,409 l/l97i Lehrfeld .Q ..333/84 M TRANSMITTER Primary Examiner-Hennan Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Attomey-Warren H. Kintzinger and Robert 1. Crawford 57 ABSTRACT operation.
17 Claims, 14 Drawing Figures TRANSMITTER SWITCH -V SUPPLY V SUPPLY PATENIED JUL 1 8 m2 SHEET 1 BF 6 com 6 IN VENTOR.
R. HALL/"0RD ATTO NEY PATENTED JUL 1 8 I972 sum 2 OF 6 INVEKNTOR.
5 Dow vn 38 v O O A '0 O O mwv G awn V 15 V w, W! Fr 5 mm m NW w m can \A 4/// Sm J ohm 0mm xrx rum hm mm BEN R. HALLFORD BY ATTORNEY MICROSTRIP DIODE HIGH ISOLATION SWITCH This invention relates in general to RF switches, and in particular, to microstrip multi-diode high isolation microwave signal path switches.
Heretofore, high isolation switch systems in microwave signal paths have been waveguide switching devices as the only such microwave frequency signal switch giving as much or more than 90 db isolation. Furthermore, such microwave waveguide switches have been very narrow-band in the attainment of desired isolation levels and are rather expensive. Additional factors such as VSWR and insertion loss with such approaches are higher than desired, although lived with for some time since better solutions did not appear to be available. A stripline approach has been tried but, with this approach, a prototype 2 GHz 90 db isolation switch, the VSWR andinsertion loss were prohibitively high; Furthermore, with preexisting systems, a junction mismatch has been a significant problem where two diode switches are joined to a common line where a mismatch would exist at the common tie point if no corrections are made such as would be the case where two transmitters are operating, one switched to deliver to acommon single antenna and the other switched off in the hot standby state. The mismatch under these operational conditions exists since the presence of the OFF switch transmission line alters the impedance of the ON" switch transmission line section. An approach to this problem in minimizing such mismatch problems and disturbances arising therefrom is to locate a short (or low impedance) of the OFF switch transmission line substantially one quarter wavelength from where it joins the transmission line of the ON side. This quarter wavelength long line makes the OFF switch short appear as, effectively, an open circuit where it joins the ON side tr .nsmission line.
There is a problem with matching of a switched ON transmission line connected to a common junction with respect to another transmission line connected to the same junction in the switched OFF state with respect to quarter wavelengths being a detenninate factor since such a length exists only at one frequency with special matching techniques called for when a rather broad band range of frequencies is to be utilized. Such an idealistic matching technique has been achieved through an optimization routine in a computer program actually trying different line lengths and irnpedances at such a common junction and calculating the total switch VSWR. The computer program is such that difierences in the VSWR between changes are compared with the next calculation being made in a direction tending to improve VSWR. Such a programming approach has been employed utilizing good equivalent circuits for PIN diodes, or the firnctional equivalent thereof, with the only component elements ignored being connectors and capacitors since their impedance versus frequency relationship is not known. With this approach, the matching stub on a final optimized model is adjusted to compensate for the contribution of these two elements to the overall switch VSWR inherently with the optimization programming approach.
It is, therefore, a principle object of this invention to provide a microstrip microwave multi-diode switch capable of relatively fast switching speeds, relatively high RF power handling capabilities up to, for example, watts CW, with a low VSWR factor, low insertion loss, and high isolation.
A further object is to provide such a microwave multi-diode switch in the form of a shield enclosed microstrip circuit single pole, double throw diode switch.
Another object is to provide such a microstrip multi-diode switch that lends itself to economical fabrication techniques resulting in high reliability and consistently predictable performance characteristics along with performance reliability with minimal service maintenance requirements quite adequately satisfying commercial market requirements.
Still another object with such a microstrip multi-diode switch is to provide selective low loss connection to one transmitter while providing a high isolation to the signal of another transmitter connected to the switch circuit.
A further'object with such a microstrip multi-diode switch is the minimizing of microwave RF current leakage between input and output connectors and to prevent any significant signal coupling between an RF rejection filter connected to the transmitter ON section of the switch and a transmitter connector of an OFF portion of the switch.
Still another object is the obtainment with such a microstrip multidiode switch of a relatively wide frequency operational range.
Features of the invention useful in accomplishing the above objects include in a shield enclosed microstrip circuit single pole double throw diode switch, a plurality of diodes in each of two transmitters legs of a switch with each leg of the switch as a switch activated or deactivated transmission line interconnected by a microstrip transformer section to a single antenna connective leg thereof. The diodes employed are PIN semiconductor diodes having a P layer, an intrinsic or I layer, and an N layer. These PIN diodes are mounted in shunt with the line and with no bias current appearing in the diodes, they are in a high resistance state. However, as bias current appears and is increased, the shunt resistance rapidly decreases to a very low value. With no bias applied to the PIN diodes of a switch leg, each of the diodes appears as a 50 ohm impedance with a low loss and represent thereby a good impedance match to a 50 ohm circuit line with application of such voltage bias to the diodes of a switch leg resulting in, for example, a milliamperes bias current flow through each diode of that leg. The impedance therethrough becomes very low, in the neighborhood of one ohm thereby resulting in high insertion loss to RF of the transmitter connected to that leg of the diode switch to thereby accomplish the switchedpfi state for the RF transmitter of that switch leg. DC blocking capacitors are positioned in series with the transmission line circuit path at each end of the diode section of each switch leg to prevent other than desired bias current from flowing into the respective diode switch legs of the switch. Voltage bias connection is provided to each of the respective switch legs from different level voltage supplies via a bias voltage control switch and a connection to and through an individual respective RF rejection filter to each of the diode switch sections of the respective legs between the dc blocking capacitors of the respective switch legs. End-line fed type of coax connectors are provided through walls of the shielded box containing the microstrip diode switch for connection to the three pairs of terminals of the switch with connectors being matched to 50 ohm impedance in accord with design practices well known to those skilled in the art. The switch is formed with a common T- shaped transformer section. The attainment of desired operational results through optimized impedance matching entails spacing between diodes at an optimum as well as spacing through transformer legs in accord with the desired frequency range objectives for the switch and with the spacing distance being approximately a quarter wavelength between diodes at the center operating frequency. This is with the exact spacing distance being determined empirically through measuring the frequency where the isolation is the highest when the diodes are mounted a quarter wavelength apart, at the center operating frequency, and then correcting the distance thereof by sealing the measured peak isolation frequency to the desired center operating frequency. The two diode switch legs are joining to the common T-transformer output leg while maintaining a low insertion loss and a good match to the ON side with irnpedances and lengths of the T-shaped transformer being adjusted for least VSWR over the operating frequency range; for example, in a specific switch 1,700 to 2,300 MH Bafile plates are provided within the shield box of the switch that prevent undesired leakage of RF currents between the input and output connectors and to suppress undesired coupling of RF rejection filters from the ON side to the transmitter connector on the OFF side thereby insuring maximum isolation possible.
High power handling capabilities of the switch are in large measure the result of large breakdown voltage characteristics of the PIN diodes used along with good heat sink action attained between the gold plated copper bars of the diode semiconductor assemblies that are mounted in intimate contact with the relatively thick aluminum plate that fom1s the backing plate of the polyolefin microstrip board employed for the switch. This is with a slot milled through the polyolefin substrate and directly into the aluminum of the ground plane plate for each individual diode in providing the setforth good metal to metal contact for each of the diodes. The dc blocking capacitors employed in the switch are chip style resting above gaps in the microstrip conductors of each switch leg with each capacitor being, for example, a 100 picofarad capacitor as required for a 2 GH, center frequency switch for the series impedance to be negligibly small. The dc blocking capacitors have many plates, separated by thin dielectric with a high dielectric constant, and the capacitors are mounted with the plates perpendicular to the microstrips conductor. The chip capacitors electrically lengthen the transmission line paths at 2 GH approximately 0.040 inches per capacitor with this length being a significant factor subtracted from the length of T-shaped transformer legs in satisfying functional operational design requirements. The operational control switching waveform to the diodes is a negative voltage that supplies the necessary bias when turning a switch leg off. A positive back voltage is applied to quickly remove the stored charges in the individual PIN diode junctions of a switch leg when switching to the ON state. Further, this positive voltage also reduces the insertion loss factor in the diodes by depleting the diode junctrons.
A specific embodiment representing what is presently regarded as the best mode of carrying out the invention is illustrated in the accompanying drawings.
In the drawings:
FIG. 1 represents a top plan view looking down at the top of a microstrip diode high isolation switch contained in a shield box with the lid removed showing microstrip isolation switch circuitry for a two transmission line alternate switch to common output switch circuit;
FIG. 2, a planar view of the shield box lid;
FIG. 3, a cut away and sectioned view taken along line 3-3 of FIG. 1 of the microstrip diode high isolation switch in its box shielded environment;
FIG. 4, a circuit schematic showing of the microstrip diode high isolation switch system of FIGS. 1 and 3;
FIGS. 4A and 4B, transmitter ON and OFF" diode equivalent circuit operational states;
FIG. 5, a plan view of microstrip art work circuit board results prior to further machining and the mounting of circuit components thereon for the two transmission line microstrip diode high isolation switch of FIGS. 1, 3, and 4;
FIG. 6, a partial T-junction detail view showing electrical reference measurement detail supplemental to the showing of FIG. 5;
FIGS. 7 and 8, side and top views respectively illustrating dc diode switch section bias isolating microwave signal coupling capacitors;
FIG. 9, a VSWR to frequency in GI-I perfomiance characteristic curve for the same embodiment;
FIG. 10, an isolation in db to frequency in GI-I, performance characteristic curve for the same embodiment;
FIG. 11, an insertion loss in db to frequency in OH performance characteristic curve for a microstrip diode high isolation switch system constructed in accord with the embodiment of FIGS. 1, 3, 4, and 5; and
FIG. 12, a two transmission line embodiment of a microstrip diode high isolation switch with the switching diodes of one switch section inverted from those of the other switch section, and with the diode biasing switch control system different from the diode switch bias control shown with the FIGS. 1, 3, 4, and 5 embodiment.
Referring to the drawings:
The diode high isolation switch of FIG. 1 is shown to be in microstrip circuit form on a microstrip circuit board 21 contained in a shield conductive metal box 22 with the lid 23,
shown in FIG. 2, removed that is normally fixed in place on box ledge 24 by screws extended through lid openings 25 and threaded into shield box threaded openings 26. The shield box 22 is provided with through wall inserted bias voltage coax connectors 27a and 27b, one of which appears in cross section in FIG. 3, along with the coax connector 28 for connection to an antenna 29, schematically indicated in FIG. 4. The shield box 22 is also provided with through wall coax connectors 29a and 29b for connection to RF transmitters 20a and 20b for selective switch connection via the diode switch 20 for signal connecting one or the other RF transmitter 20a or 20b to feed the antenna 29 as diode switch control selected.
The microstrip board 21 is constructed of a relatively thick rigid ground plane plate 30 with a relatively uniformly thick plastic dielectric laminate sheet 31 bonded to the upper surface thereof, and with RF conductive microstrip circuitry 32 formed on the upper side of the plastic dielectric laminate sheet 31, such as having been formed through patterned etching of a copper cladded side of the plastic dielectric laminate sheet 31. The microstrip art work of a microstrip board 21 is shown in FIG. 5 whereon microsnip circuit section elements are produced including a center microstrip T-shaped transfonner 33 having a transmitter output connective leg 34 and top opposite side RF transmitter switch connective arms 35a and 35b, each with a nonconductive gap spacing 36a and 36b that are bridged, respectively, by signal coupling chip capacitors 37a and 37b. The microstrip board 21 is also equipped, successively outwardly from the respective transformer arms 35a and 35b, with microstrip transmission line switch sections 38a, 39a, 40a, and 41a, and 38a, 39b, 40b, and 41b, respectively. This is with transmission line sections 38a and 38b aligned with and spaced from transformer arms 35a and 35b, respectively, for insertion therein of individual diode assemblies 42. Furthermore, additional diode assemblies 42 are inserted in the gaps between the transmission line switch sections 38a and 39a, 39a and 40a, 38b and 39b, and 39b and 40b. Diode assemblies 42 are PIN type diodes having a through conductive element bridging the respective gaps between adjacent transmission line elements and provide a connection through the diode structures in shunt with the respective transmission lines to ground at each diode location. This is with, as shown in the schematic of FIG. 4, the cathodes connected to the respective transmission lines and anodes connected to ground, that is, the ground plane plate 30 of the microstrip board 21. The PIN diodes 42 are each in the form of a semiconductor having a P layer, an intrinsic or I layer, and an N layer. As has been pointed out, these diodes are mounted in shunt with the respective transmission lines and each appear as a high resistance when there is no bias current through the respective diodes 42 and, as bias current through the diodes is developed, there is a rapid decrease in resistance therethrough to a very low value. Relatively high power handling capabilities of the switch are provided as a result of the relatively large breakdown voltage characteristics of the PIN diodes 42 coupled with good heat sink action between gold plated copper bars 43 of the diode assemblies 42 for cooling, particularly with the bars 43 being tightly mounted as by screws 44 to ground plane plate 30. This is with 43 receiving slots 45 milled through the dielectric layer 31, in switches that have been built being in the form of a laminate layer of polyolefin, and into the aluminum of the ground plane plate 30 to provide good metal to metal contact. This is with the grooves in the ground plane plate being of such depth that the through ribbon conductive leads of the diodes 42 conveniently rest on the adjacent microstrip transmission line conductor ends thereby achieving well matched impedance connections from the respective microstrip transmission line conductors to the diodes 42.
The nonconductive gap spacings 46a and 46b between microstrip transmission line switch sections 400 and 41a and 40b and 41b are bridged by signal coupling chip capacitors 47a and 47b, respectively, that are substantially the same as signal coupling chip capacitors 37a and 37b. These do blocking capacitors are placed in series with the respective circuit transmission lines to do isolate the diode switch portions of the RF transmitter switch connective transmission lines signal coupled to the microstrip transformer 33 (shown with additional detail in FIG. 6) to prevent bias current in the respective switch sections from flowing into the circuits joined to the switch 20. The particular capacitors 37a, 37b, 47a, and 47b employed, as shown in FIGS. 7 and 8, are ceramic chip style having metalized ends 48 and 49 for connection to strip conductors by soldering or brazing 50 or through the use of other highly conductive fastening media such as silver filled epoxywith the capacitors relatively small cubes of approximately 50 mils cube. The four dc blocking capacitors so employed are chip style components resting above respective gaps in the microstrip circuit conductors providing approximately 100 picofarads at a center operational frequency of 2 61-1,, with a switch that has been built, such as to cause the series impedance to be negligibly small. These capacitors have many small plates which are narrowly separated by relatively very thindielectric layers having a high dielectric constant and the capacitors are mounted with the plates perpendicular to the microstrip conductor since that it has been found that when the capacitors are mounted with the plates parallel to the RF conductors that an absorption type of resonance occurs causinga sharp rise in insertion loss..It appears that the multipath propagation through the high dielectric constant capacitor dielectric causes suflicient phase change between the energy along the lower and upper plates when the coupling capacitors are mounted with the plates parallel to the microstrip conductors to form a resonant circuit. It appears that such multipath condition exists since the alternate plates are joined together at the two ends of the capacitor of each capacitor structure with the electrical distance through each plate combination of the capacitor structure being different when so mounted. It should be noted further that while relatively small, these chip capacitors do electrically lengthen the microstrip conductor with such additional length at, for example, 2 GH being effectively measured to be approximately 0.040 inches per capacitor chip. Such length factors must be subtracted from the length of the T-shaped transformer 33 where the capacitors are mounted to satisfy the conditions called for in optimized switch design. It should be noted further, at this point, that actual location of the chip capacitors along the respective transmission lines varies the effective electrical length through the respective capacitor chips, that is, those capacitor chips within the microstrip T-transformer.
The two transmitter fed diode switch 20 is joined to the common output connector 28 for feeding a single antenna 29 via leg 34 of the T-shaped transformer 33 while maintaining a low insertion loss and good match to the ON side of the diode switch. The complete switch 20 is analyzed with dimensions optimized in the T-shaped transformer 33 to attain impedances and lengths for least VSWR over the operating frequency range, in a particular instance for a particular switch 1,700 to 2,300 MH with the optimized dimensions and factors as set forth with FIG. 6. Location of electrical reference planes such as illustrated in FIG. 6 is important in determining the measurements for the legs of the T-transformer with the electrical reference planes defining the location of the respective conductor ends from the junction to determine the total physical lengths involved. It should be noted again that the chip capacitors 37a and 37b are significant in effectively electrically lengthening the microstrip conductors of the opposite side diode switch legs of the transformer 33 with, for the specific 2 6H, center frequency diode switch, the additional length to be computed for each leg being 0.040 inches per capacitor. This additional 0.040 inch length must be subtracted from the length of the diode switch arms of the T-shaped transformer in satisfying the design conditions imposed. With a T-head microstrip conductor width of .108 inches and a stem width of 0. 1045 for the antenna output connection, the electrical reference plane with respect to the ON side diode switch leg activated is displaced 0.009 inches from the center line of the common stem leg 34 in the direction of the ON side, and the elecnical reference plane for the common antenna output connective stem leg 34 is displaced 0.048 inches in the direction of the common stem leg 34. The additional resulting dimensions are 0.856 inches from the T vertical center line to a switching diode side leg end resulting in a 0.206 wavelength from the electrical reference plane to the switched ON end, and an effective impedance for the diode ON arm of 39.87 ohms. Please note that the diode switching arm ends of the transformer 33 are beveled from a width of 0.045 inches at 45 in order to provide an improved and proper impedance match with the through conductive ribbon of the respective switching diodes 42 connected thereto. The vertical dimension of the common antenna connective leg 34 is 1.058 inches from the adjacent edge of the switching diode legs of the T-transformer to the connective end, other than for a relatively narrow connective extension tab 51 of the leg 34, with this length resulting in the leg 34 being 0.2415 wavelength effectively long from the electrical reference plane therefor and having a characteristic impedance of 40.83 ohrrs. Further, capacitor 37a and 37b location gaps of 0.020 inches are located at a distance of 0.262 inches from the ad jacent edge of the common transformer leg 34. At this spacing the chip capacitors 37a and 37b look like capacitive reactances to effectively lower to some degree the operational frequency range of the diode switch 20.
In addition to critical dimensions set forth for the T-transformer 33 of the diode switch 20, it is of interest to note that in optimum spacing the equivalent of quarter wavelength distance spacings of the switch center frequency is provided for the three PIN diodes 42 in each of the diode switch legs of the diode switch 20. This is with the exact distance determined empirically through measuring the frequency where the isolation is highest when the diodes are mounted a quarter wavelength apart at the center operating frequency, and then correcting through scaling the measured peak isolation frequency to the desired center operating frequency. Bias line connections are provided from a minus voltage supply 52 and a positive voltage supply 53 through a switch 54 that, while illustrated in FIG. 4 as being a mechanical switch, has in at least one instance assumed the form of a fast acting electronic switch for alternately connecting bias line leads to the minus voltage supply 52 and the positive voltage supply 53 such as in accord with the switching waveform illustrated therewith. One switch output is connected through a lead line 55a that includes the center conductor 56a of through wall coaxial connector 27a to a RF rejection flter 57a, such as is the subject matter of copending application entitled RF Rejection Filter filed by applicant on July 24, 1970 and assigned to the common assignee hereof, and through a quarter wavelength, of the center operational frequency of the switch, line 58a from the rejection filter 57a to connection with the diode switch side section 35a of a transformer diode switch leg for bias switching ON-OFF control of the diodes 42 of that leg. In like manner, the minus voltage supply 52 and the positive voltage supply 53 are connected through switch 54 to and through line 55b including the center conductor 56b of through wall coaxial connector 27b to RF rejection filter 57b and through a quarter wavelength, of the center operational frequency of the switch, line 58b from the rejection filter 57b to connection with the diode switch side section 35b of a transformer diode switch leg for bias switching ON-OFF control of the diodes 42 of that leg. Each of the lines 55a and 55b also include jumper line sections 59a and 59b, respectively, extended from the center conductors 56a and 56b to a microstrip connective pad 60a and 60b, respectively, and also interconnective lines 61a and 61b therefrom to the respective rejection filters 57a and 57b, respectively.
A switching control voltage waveform, such as that illustrated with the voltage supplies 52 and 53 in FIG. 4, with switching from a minus voltage supply of 0.95 volts to a positive voltage supply of +24 volts with the negative voltage the necessary bias to bias the diodes of one switch side to conduction and provide switch OFF of the signal of the RF transmitter of that side, and the positive back voltage of +24 volts is quite adequate to quickly remove stored charges in the PIN diode junctions of the diodes of that side to thereby accomplish turnoff of the diodes and turn ON switch connection of the RF transmitter of that side. Furthermore, the positive voltage is of such level to reduce insertion loss of the diodes through quickly depleting the diode junctions of the side being positively voltage biased. The particular diodes 42 that have been used in working diode switches to date are Hewlett Pachard Associate PIN diodes, part number 5082-3040. FIG. 4a illustrates the PIN diode equivalent circuit for a diode 42 biased OFF and transmitter biased switched ON state of operation with two series connected coils of O.25 Henry with a common junction thereof connected through a 1170 ohm resistor and a 0.01Xl0' farad capacitor in parallel to ground. Then in the diode biased to conduction state with the diodes conducting approximately a 100 milliamperes current equivalent to the transmitter switched OFF state for the transmitter of that side, the equivalent circuit of FIG. 4b shows, again, the 0.25Xl0' Henry value series connected coils with the common junction thereof connected serially through a 1 ohm resistor and a l7.5 l0 Henry coil to ground. With this being the case, when the positive voltage is applied to a diode switching side, the PIN diodes thereof appear as 50 ohm impedances with a low loss thereby presented in a good impedance match in a 50 ohm circuit line. Conversely, when a negative voltage is applied generating approximately 100 milliamperes of current through the diodes, the impedance therethrough to ground becomes very low, in the neighborhood of 1 ohm, resulting in, relatively speaking, a very high insertion loss to RF signals.
In order to attain performance objectives desired, particularly VSWR of the assembled switch, matching stubs 62a and 62b are provided approximately midway in transmission line sections 39a and 39b, respectively, approximately midway between the outermost PIN diodes 42 of the respective PIN diode switching sections of the diode switch 20. These matching stubs 62a and 62b are size and location empirically determined in looking at a swept response of the return loss of the switch at the antenna connector output to compensate for a mismatch departure from calculated performance contributed by coaxial connectors in the dc blocking capacitors in series with the RF conductors.
In addition to all the features hereinbefore presented, conductive baffle plates 63a and 63b are required within the diode switch enclosing shield box 22 in the attainment of the desired isolation of the transmitter switched OFF side of the switch. These baffle plates 63a and 63b are seated in milled grooves 64a and 64b, that are milled through the substrate dielectric 31 and into the ground plane backing plate 30 of microstrip board 21, wherein they are securely seated as by screws (detail not shown). The bafile plates also span the vertical interior dimension of the shield box 22 from the ground plane plate 30 to intimate close engagement with the inside of the shield box lid 23 that may be further biased into tight engagement contact therewith by screws extended through the lid into the baffle plates 63a and 63b (detail not shown). Further, the baffle plates 63a and 63b extend from opposite side end engagement with the outer RF transmitter connective ends of the diode switch shield box 22 to inner spaced ends 65a and 65b that are relatively closely spaced from closely adjacent bias lines 58a and 58b, respectively, and much closer toward the center of the switch than the rejection filters 57a and 57b, respectively. This baffle plate structure arrangement within the switch shield case 22 substantially prevents leakage of RF currents between the input and output connectors that would otherwise occur between the shield box floor and the metal backing plate of the polyolefin laminated board that, were such loss to be present, would be manifest as a loss in the isolation of the OFF side. Coupling of the RF rejection filter of the transmitter switched ON side of the diode switch is substantially eliminated from RF coupling to the transmitter connector of the OFF side of the switch, an RF coupling factor that would otherwise reduce the maximum isolation possible. Another problem that is substantially eliminated through the use of the baffle plates 63a and 63b is that without such plates within the switch shield box, the interior box size is sufficiently large to support the dominate TE waveguide mode, such as is well known to those skilled in the art, with RF propagation in this mode being through the box interior, not depending on microstrip conductors, with in such mode, low isolation between the antenna conductor and the OFF transmitter conductor resulting from such propagation mode.
The dominant waveguide TE mode cutofi' wavelength is Ac 2a, where a is the inside width, or opposite end to end length, of the waveguide. For the example cited, a 8.25 inches.
Thus, Ac=2(8.25)= 17.50 inches The corresponding cutoff frequency is:
jc= V/Ac=1l.8/8.25 1.43 GH, where V is the free space velocity in inches per nanosecond. The operating frequency range is 1.7 to 2.3 GH so propagation would occur. With the bafile plates present, no waveguide path is possible between the antenna connector and the transmitter connectors.
The bias lines to the PIN diodes account for no appreciable loss of the RF signal and are of such high impedance where they are joined to the main circuit conductor that no change to RF match was observable. The effectiveness of this bias line decoupling results from the very low impedance over a broad frequency range that is possible with the special RF suppression filters acting in combination with a quarter wavelength of high impedance line which serves to invert the low filter impedance to a very high impedance where the bias line is joined to the RF circuit. Other types of decoupling schemes caused a noticeable and undesirable coupling to the main circuit conductor.
FIGS. 9, l0, and 11 illustrate actual measured performance characteristics as compared to theoretical optimum characteristics with FIG. 9 illustrating a VSWR measured waveform, FIG. 10 isolation in db, and FIG. 11 insertion loss in db and with these waveforms taken through the 1.7 to 2.3 GH, frequency range for a particular switch using the specific PIN diodes identified and other circuit component values and parameters set forth. The performance characteristics portrayed by these waveform curves are together very highly idealistically optimized operational results. With diode switches designed for operation above about 3 GH, RF absorbing material may be used under the shield box lid in the continued attainment of high isolation with the move to higher operational frequencies.
With the alternate embodiment of FIG. 12, illustrated in circuit schematic form, those portions duplicating those of the embodiment of FIGS. 1 through 8, providing the operational characteristic curves of FIGS. 9, 10, and 11, are numbered the same and those portions that include some variance therefrom carry primed numbers. With this embodiment, the diodes 42 and 42, while they may be PIN diodes such as used with the other embodiment, the switching diodes 42 of one side are reversed from those of the previously described embodiment with the cathodes thereof connected to the ground plane plate and the anodes connected to the switching transmission line section of that side that interconnects RF transmitter 20a and the rnicrostrip T-transformer 33. Consistent with these changes, a minus voltage supply 52 and a positive voltage supply 53' are connected to respective poles of single throw, double pole switch 54' that, in turn, is connected through dropping resisters 66a and 66b to respective side bias connection line systems 55a and 55b, that are otherwise the duplicates of lines 55a and 55b with the components recited therefore with the other described embodiment. With the embodiment of FIG. 12, however, the minus voltage supply 52 and the positive voltage supply 53' are of substantially equivalent equal voltage magnitudes, with only one supply at a time simultaneously applying voltage biasing to conduction voltage to one side and the reverse bias to the diodes of the other switch side, interchangeably in each instance as it is switched from one pole switched position to the other. With this particular embodiment, all other operational actions are substantially the same as with the previously described embodiment and a further description with respect thereto is not again repeated.
Whereas this invention is herein illustrated and described with respect to a specific embodiment hereof, it should be realized that various changes may be made without departing from the essential contributions to the art made by the teachings hereof.
1. In a shield enclosed microstrip diode high isolation switch, a circuit board with a rigid electrically conductive ground plane plate mounting a dielectric material layer and microstrip circuitry in laminate relation thereon; electrically conductive shield means substantially completely enclosing the microstrip circuitry above said ground plane plate; signal input through wall electric circuit connective means for interconnecting a signal source and said microstrip circuitry; signal output through wall electric circuit connective means for interconnecting signal output utilizing means and said microstrip circuitry; a microstrip section with signal coupling dc blocking means at opposite ends in the microstrip circuitry between said input and said signal output connective means; diode means in the form of a plurality of PIN diodes and assembly structures connected between said microstrip section and said ground plane plate with common diode electrodes connected to said microstrip section and opposite electrodes connected to said ground plane plate; each diode assembly structure including a highly electrically and thermally conductive mounting metal base seated at locations in the switch structure in close metal to metal engagement in said ground plane plate, said diodes spaced at substantially one-quarter wavelength of the switch operational designed center frequency apart along said microstrip section and controlled switch voltage bias circuit means connected to said microstrip section for controlled biasing said diode means to conduction for a switched OFF state of said switch between said input and said output connective means from a switched ON state diode means not biased to conduction.
2. The shield enclosed microstrip diode high isolation switch of claim 1, with a plurality of said microstrip sections in microcircuitry interconnecting a plurality of said signal input through wall connective means and said signal output through wall electric circuit connective means.
3. The shield enclosed microstrip diode high isolation switch of claim 2, wherein a plurality of said controlled switch voltage bias circuit means is provided one for each of said plurality of microstrip sections.
4. The shield enclosed microstrip diode high isolation switch of claim 3, wherein transformer means interconnects the plurality of said microstrip sections and said signal output through wall electric circuit connective means.
5. The shield enclosed microstrip diode high isolation switch of claim 3, wherein said transformer is a microstrip transformer; and RF rejection filter means is included in each of the plurality of said controlled switch voltage bias circuit means.
6. The shield enclosed microstrip diode high isolation switch of claim 5, wherein two of said signal input through wall connective means are located at opposite signal input ends of said electrically conductive shield means in the form of a rectangular elongate microwave microstrip switch box; and said signal output through wall electric circuit connective means is in the form of a signal output connection located substantially midway of said switch box opposite signal input ends.
7. The shield enclosed nficrostrip diode high isolation switch of claim 7, wherein diodes of a first microstrip section are connected cathodes to the first microstrip section and anodes to said ground plane plate; and diodes of a second microstrip section are connected anodes to the second microstrip section and cathodes to said ground plane plate.
8. The shield enclosed microstrip diode high isolation switch of claim 6, wherein said microstrip transformer is a T- shaped transformer with T-opposite side legs approaching one-fourth wavelength of the switch operational designed center frequency length, and having a common T-shank substantially one-fourth wavelength of the switch operational designed center frequency long that is output end connected to said signal output through wall electric circuit connective means.
9. The shield enclosed microstrip diode high isolation switch of claim 8, wherein said signal coupling dc blocking means are each capacitors; a gap is provided in each of the transformer T-opposite side legs; and a capacitor signal couple spans each of the transformer T-opposite side leg gaps.
10. The shield enclosed microstrip diode high isolation switch of claim 9, wherein the apparent signal electrical length of said transformer T-opposite side legs are substantially onefourth wavelength of the switch operational designed center frequence long with a gap spanning capacitor in place on each transformer side leg.
11. The shield enclosed microstrip diode high isolation switch of claim 10, wherein said dc blocking signal coupling capacitors are relatively small discreet multiplate capacitors mounted with plates substantially parallel to the longitudinal direction of the microstrip circuit and across the gaps and with the plates substantially vertically perpendicular to the plane of the microstrip circuitry in the area of gap ends.
12. The shield enclosed microstrip diode high isolation switch of claim 6, wherein electrically conductive material baffle plate means is provided within said rectangular elongate microwave microstrip switch box extended substantially through the longitudinal length of the box from end to end and generally spanning the interior of the shield box from said ground plane plate to intimate close engagement with the inside of a shield box lid.
13. The shield enclosed microstrip diode high isolation switch of claim 12, wherein the plurality of said microstrip sections are located to one side of said baffle plate means; and said RF rejection filter means are located to the other side of said baffle plate means from the side of said microstrip sections.
14. The shield enclosed microstrip diode high isolation switch of claim 13, wherein each of the plurality of said controlled switch voltage bias circuit means includes a through wall bias connector; and a substantially one-fourth wavelength of the switch operational designed center frequency long relatively high impedance line interconnecting each RF rejection filter and the respective microstrip section.
15. The shield enclosed microstrip diode high isolation switch of claim 14, wherein said baffle plate means is in the form of two bafile plates extending from opposite side switch box ends to relatively closely spaced inner ends approaching the transformer Tshank of the microstrip circuitry within the box.
16. The shield enclosed microstrip diode high isolation switch of claim 8, wherein length and impedance parameters of transformer T-opposite side legs are design selected to optimize the attainment of microstrip circuit matched characteristic impedances.
17. The shield enclosed microstrip diode high isolation switch of claim 16, wherein length and impedance parameters of the common T-shank of said microstrip transformer are designs determined to optimize the attainment of microstrip circuit matched characteristic impedances.
UNITED STATES PATENT UFFICE QEHECATE T QEUHN Patent No. 3,678,414 Dated July I8, 1972 Inven Ben R Ha] 'Ifnrd I It is certified that error appears in'the above-identified patent and that said Letters Patent are hereby corrected as shown .below:
C011umn 9, 'hne 43, after "state" insert-with sa1'd-.
Signed d sealed this 19th day of December .1972.
EDWARD M.FLETC HER, JR. ROBERT GOI'TSCHALK Attesting Officer- Commissioner of Patents FORM FO-105O (10-69) USCOMM-DC 60376-P69 1% us GOVERNMENTPRINYING OFFICE: I969 o-aee-su.