Microwave switch wherein PIN diode is mounted orthogonal to microstrip substrate

An improved microwave multi-throw switch incorporating PIN diodes wherein the PIN diodes are mounted orthogonally to the conductive stripline.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to microwave switches and more 
particularly to a multi-throw microwave stripline switch utilizing PIN 
diodes. 
2. Description of the Prior Art 
There are various microwave multi-throw switches in the prior art. Such 
prior art switches include those utilizing striplines with beam lead PIN 
diodes extending across stripline segments. There are other structures 
utilizing PIN diodes mounted in a horizontal position with the conductive 
layer of the diode superimposed on a stripline and the junction lead 
extending across the gap between striplines to another stripline. 
In microwave multi-throw switches, performance is highly dependent upon 
isolation losses, voltage standing wave ratios and insertion losses. In 
the known prior art structures the beam lead diodes provide a structure of 
significant improved performance over that of the conventional PIN diodes 
in these three operating characteristics. However, the cost to manufacture 
beam lead PIN diodes is substantially greater than that of the 
conventional PIN diode. 
As a consequence, when operating in the two gigaHertz to eighteen gigaHertz 
frequency range either beam lead diodes are used exclusively or they are 
used in combination with a conventional PIN diode. 
SUMMARY OF THE PRESENT INVENTION 
It is an object of the present invention to provide a microwave multi-throw 
switch structure which provides electrical operating characteristics 
comparable to those of beam lead PIN diodes and which are more economical 
than beam lead PIN diodes. 
It is a further object of the present invention to provide a microwave 
multi-throw switch structure utilizing PIN diodes which provide improved 
electrical operating characteristics over switches utilizing conventional 
PIN diodes. 
In a preferred embodiment of the present invention, a microwave multi-throw 
switch incorporates a PIN diode wherein the metalized surface opposite the 
junction is placed substantially orthogonally to the stripline surface. 
The metalized surface is electrically and adhesively adhered to the 
stripline surface by a conductive conduit. Leads from the junction then 
project directly across the gap of the stripline to the adjacent stripline 
segment. 
An advantage of the present invention is that it provides improved 
isolation losses over that of microwave switches utilizing conventional 
PIN diode structures wherein the conductive surface of the diodes are 
sandwiched to the stripline surface. 
A further advantage of the present invention is that it provides improved 
isolation relative to that of microwave multi-throw switches utilizing 
conventional PIN diodes with the conductive surface adherently sandwiched 
to the stripline. 
A further advantage of the present invention is that it provides a 
microwave multi-throw switch wherein the voltage standing wave ratio is 
superior to that of microwave switches utilizing conventional PIN diode 
structures with the conductive surface sandwiched to the stripline 
surface. 
These and other objects and advantages of the present invention will no 
doubt become apparent to those of ordinary skill in the art after having 
read the following detailed description of the preferred embodiments which 
are illustrated in the various drawings and figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 is illustrated a microwave single-pole double-throw switch 
referred to as general reference character 10. The electrical circuit of 
the switch 10 is illustrated in FIG. 2. The switch 10 is structured to 
operate within the two gigaHertz to eighteen gigaHertz frequency range. 
The switch 10 incorporates an insulative base material 12. About the top 
and bottom surface of the material base 12 is a conductive metal 14. The 
conductive metal 14 may be comprised of copper and gold composition. To 
form the switch panel, portions of the conductive layer are etched away or 
removed from the insulative surface 12 so as to form (microstrip) 
striplines on the surface. For example, a first stripline referred to by 
the general reference character 16, a second stripline referred to by the 
general reference character 18, and a third stripline referred by the 
general reference character 20 are formed. As illustrated, the stripline 
16 is comprised of various segments illustrated as 22, 24 and 26. The 
stripline 18 is comprised of a plurality of segments 28, 30 and 32. Line 
20 is comprised of one segment. Thus, the stripline 20 establishes the 
common line to the switch with the striplines 16 and 18 forming the key 
different throws for the switch. Interconnecting the common line 20 to the 
segment 22 is a beam lead diode 34. Connecting the stripline segment 22 to 
the segment 24 is a conventional PIN diode structure 36. Likewise 
interconnecting the segments 24 and 26 is a conventional PIN diode 
structure 38. 
Connecting the common line 20 to the junction 28 of the segment 18 is a 
beam lead diode 40. Interconnecting the segments 28 and 30 is a 
conventional PIN diode 42. Interconecting the segments 30 and 32 is a 
conventional PIN diode 44. 
Stripline segments 16, 18 and 20 are generally in the form of fifty-ohm 
transmission line structures. These structures are all biased. For 
example, the line 16 is biased by means of a bias applied to a bias 
terminal 46 which is common to an inductive element 48 and to one plate of 
a capacitor 50. The element 48 is tied to the stripline segment 26 and the 
other side of the capacitor 50 is adhered to the metal 14 which is the 
common ground reference. Likewise, the stripline 18 is biased by means of 
a bias applied to a bias terminal 52 which is tied to an inductor 54 
extending to the segment 32. The inductor 54 is common to a capacitor 56 
of which one side is adhered to the metal 14 which is a common ground. 
The common segment 20 is biased by a source applied to the bias terminal 58 
which is common to the inductor 60 extending to the line 20. Also, the 
capacitor 62 has one side common to the metalized surface 14 which is 
common ground reference. 
The switch 10 is tied to an incoming line 64 through a capacitive element 
66. The capacitive element 66 adheres to the input line from the exterior 
to the stripline 20. 
The output from the line 16 is taken to an output line 68 through a 
capacitive element 70 which is adhered to the exterior line 68 and the 
stripline 16. 
The stripline element 18 is tied to the exterior line 72 through a 
capacitive element 74 which is adhered to the line 18 and the exterior 
line 72. 
In the illustrated embodiment the conventional type PIN diodes are shown as 
36, 38, 42 and 44. FIG. 3, is an enlarged cross-sectional view of the 
diode 38 taken along the line 3--3. The diode 38 includes a silicon chip 
76. Generally, this chip is in the form of a rectangular member with a 
junction 78 about the top horizontal surface and a metalized coating 80 on 
the bottom surface opposite to the junction. The junction 78 is connected 
by a lead wire 82 across the gap of the segment 24 to the segment 26. 
Thus, when the segment 16 is biased, current can flow through the diode 38 
intermediate the plate 80 and junction 78 and through the lead 82. At the 
high microwave frequencies, for example in the two gigaHertz to eighteen 
gigaHertz range, there are losses due to the stray capacitance between the 
lead 82 and the silicon member 76. For example, this is illustrated in 
FIG. 3 as "C.sub.1 ". Also, the line 82 has inductance L.sub.s1. At the 
microwave frequencies, these inductances and capacitances create 
capacitive and inductive losses. These features cause for isolation 
losses, insertion losses and voltage standing wave ratio losses. Thus, 
performance wise, it becomes desirous to minimize such losses and at the 
least economical cost. One way in which to decrease these undesirable 
characteristics is to utilize beam lead PIN diodes such as illustrated by 
the PIN diodes 34 and 40. However, the economic cost of such diodes 
impairs the desirability of such diodes. Frequently, it becomes a matter 
of trade-off between economics and operating characteristics whether to 
use beam lead diodes. 
FIGS. 4 and 5 illustrate an improved structure as represented by the 
present invention. In FIG. 4 a microwave single-pole double-throw switch 
similar to that of FIG. 1 is illustrated but wherein improved PIN diodes 
are utilized. For clarity and convenience, the elements of FIGS. 4 and 5 
common to those of FIGS. 1 and 3 utilize the same reference numeral 
distinguished by a prime designation. In FIG. 4, all of the segments of 
the striplines 16', 18' and 20' are interconnected by a PIN diode 84. A 
cross-sectional view of one of the diodes 84 and interconnecting the 
segments 24' and 26' is illustrated in FIG. 5, which is taken along lines 
5--5 in FIG. 4. The diodes 84 are similar to the diode 38 in that they 
incorporate a silicon substrate 76', a junction 78' and a conductive layer 
80'. However, as illustrated in FIG. 5, the conductive layer 80' is 
substantially orthogonal to the stripline 24'. The conductive layer 80' is 
conductively adhered to the stripline 24' by a conductive conduit 86. In 
FIG. 5, this conductive conduit is in the form of a silver epoxy or a gold 
epoxy. The junction 78' is connected to the stripline 26' by a conductive 
line 88. The line 88 extends directly from the junction 78' to the layer 
26'. As such, it is substantially shorter in length than the line 82 of 
FIG. 3. Furthermore, the line 88 has very little, if any, overlap with the 
silicon substrate 76' such that the only significant stray capacitance is 
that between the line 88 and the stripline 24'. 
An alternative embodiment of the structure of the diode 84 is illustrated 
in FIG. 6. In FIG. 6 the surface 80' is connected to a conductive bracket 
90. One end of the bracket 90 is adhered to the conductive layer 80' by 
means of a solder joint 92 and the other end is connected to the stripline 
24' by means of a solder joint 94. Basically, the differences between the 
diode 84 of FIG. 5 and 84 of FIG. 6 is the means in which it is amounted 
to the striplines 24'. 
To comparatively illustrate the electrical characteristics of the diode 
structure 38 of FIG. 3 to the diode structure 84 of FIGS. 5 or 6, a 
mathematical comparison may be made. First, viewing the structure 38 and 
letting L.sub.1 represent the inductance per inch and h represent the 
length of the wire 82, then assuming the inductance is 5.08 nh per inch, 
and the length is 0.03 inches, the inductance 
##EQU1## 
At a frequency (f) of 18 GHz, 
EQU X.sub.L =L=0.94 ohms. 
For four diodes, X.sub.L =3.76 ohm which is approximately 1 db of insertion 
loss. 
The stray capacitance C.sub.1 may be viewed as, 
EQU C.sub.1 
=.epsilon.A/d=.epsilon..times.(0.005.times.10.sup.-3)/(0.002)=.epsilon..ti 
mes.(2.5.times.10.sup.-3),=22.12.times.10.sup.-15 farads=0.022 pf, 
where A is the cross sectional area of lead 82 (0.005.times.10.sup.-3) and 
where d is the average separation between lead 82 and silicon chip 76 
(0.002) (obtained by physical measurements and verified by microwave 
measurements). 
At 18 GHz the equivalent circuits 
EQU C.sub.T =C.sub.j +C.sub.1 ; 
where C.sub.j =0.02 pf, C.sub.j being the diode 38 junction capacitance; 
EQU C.sub.T =0.044 pf; 
and has a 10 db isolation. 
Therefore, for a four diode switch, the total isolation is approximately 40 
decibels. 
As to the structure 84 using the same type wire, but of approximately 
one-fourth the length, then the inductance of the wire 88 
##EQU2## 
At a frequency (f) of 18 GHz; 
EQU X.sub.L =L=0.25 ohms. 
For four diodes X.sub.L =1.00 ohms which is approximately 0.2 db of 
insertion loss. 
The stray capacitance C.sub.2 may be viewed as, 
EQU C.sub.2 
=.epsilon.A/d=.epsilon..times.(0.001.times.10.sup.-3)/(0.0056)=.epsilon..t 
imes.(0.17.times.10.sup.-3)=1.5.times.10.sup.-15 farads=0.0015 pf, 
where A is the cross sectional area of lead 88 (0.001.times.10.sup.-3) and 
where d is the average separation between lead 88 and silicon substrate 
76'. 
At 18 GHz, the equivalent circuit 
EQU C.sub.T =C.sub.j +C.sub.2 ; 
where C.sub.j =0.02 pf, C.sub.j being the diode 84 junction capacitance; 
EQU C.sub.T =0.0215 pf; 
and has a 14 db isolation. 
Thus, for a four diode switch, the total isolation is approximately 56 db. 
Therefore, the structure 38 of FIG. 3 has approximately 40 decibel 
isolation and 3.4 decibel insertion loss; whereas the structure 84 has 
approximately 56 decibel isolation and 2.6 decibel insertion loss. 
Although the present invention has been described in terms of the presently 
preferred embodiments, it is to be understood that such disclosure is not 
to be interpreted as limiting. Various alterations and modifications will 
no doubt become apparent to those skilled in the art after having read the 
above disclosure. Accordingly, it is intended that the appended claims be 
interpreted as covering all alterations and modifications as fall within 
the true spirit and scope of the invention.