A compact, wide range inductor capable of being trimmed to a desired frequency value, comprising at least two individually tunable inductive elements of different resolution, disposed upon an insulative support. The inductor is usually placed within a hybrid circuit and trimmed after component population.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of inductors and more 
particularly to hybrid circuit tunable inductors. 
2. Description of the Related Art 
In the manufacture of electronic equipment inductors are often used. 
Particularly, the microelectronics manufacturing industry frequently uses 
them, therefore the miniaturization of such components is of critical 
importance. In many cases during manufacturing, a circuit must be 
assembled first and thereafter tested. If upon testing the circuit is not 
within operational or desired limits, component replacement is required. 
Such replacement is both time consuming and expensive. 
In recent years variable and tunable inductors have been used in the 
manufacturing process. Such an inductor may be formed within a hybrid 
circuit (one wherein some of the components are formed by conductors on an 
insulator or substrate) or a separate adjustable inductor element can be 
used. The need for miniaturization and performance has made the separate 
adjustable inductor ineffective. Due to its miniaturization, the inductor 
formed within a hybrid circuit is usually capable of being tuned using a 
laser or electron beam to remove or alter the inductor. 
These variable inductors are manufactured into a circuit and then tuned to 
within operational limits. There have been a number of basic ways of 
achieving this goal. Often a spiral shaped inductor or a ladder or "U" 
shaped inductor with parallel shorts is used. Both of these have been 
useful in the current art, and both are commercially and industrially 
feasible. 
The spiral inductor is space efficient; for a given tunable range the 
inductor takes very little space. The drawback to the spiral inductor is 
that the breaching of shorts across the spiral segment produces results 
that are not precisely predictable and of coarse granularity; thus it is 
not finely tunable. Such inductors, if designed for precision, with many 
tunable shorts, are very difficult to manufacture and add significantly to 
the cost. When the inductor is designed and manufactured with a smaller 
number of shorts, the inductor is useful and cost effective for 
applications where a broad range of values must be accommodated and 
component space is critical. It is not useful for applications where 
precision is critical. 
The ladder or "U" shaped inductor is useful where fine tuning is required 
but space is not a premium consideration. Its inductance can be varied by 
breaching a short across its vertical legs. Here, however, the variance is 
substantially predictable and correlates highly to the number of, and 
spacing of the rungs. While such an inductor is useful for applications 
where precision is mandated, the space required per unit change of 
inductance is much greater than that of the spiral inductor. 
Thus, using either of the aforementioned techniques has serious 
limitations; the former in terms of tuning precision, and the latter in 
terms of size and space requirements. 
SUMMARY OF THE INVENTION 
In practicing the invention, an electronic circuit is formed by disposing a 
conductive strip upon an insulative substrate. Selected adjacent sections 
of the conductive strip are shorted together by disposing additional 
conductive strips upon the substrate, forming a tunable inductor. The 
tunable inductor being formed such that there are at least two separately 
tunable inductive elements. Other components may also be added to form the 
complete circuit. 
The tuning of the aforementioned tunable inductor may be performed either 
before or after the population of the circuit components. In the preferred 
embodiment it is tuned after the circuit is otherwise complete and 
components are added. Once the circuit comprising the tunable inductor is 
formed, it is tested to determine its current value of inductance. The 
current value of inductance is subtracted from a target value of 
inductance to determine the desired increase in inductance. The element 
with the coarsest resolution which is not greater than the desired 
increase is selected. If an element is selected, the outermost short on 
that inductive element is breached. The circuit is again tested, an 
element selected, and a short breached until no element can be selected 
without causing the value of inductance to rise above the target value; 
thus producing a circuit tuned to within the available resolution of the 
target. 
Another method of tuning the circuit is equally as precise and may also be 
utilized. This method relies upon knowing the minimum tunable range of 
each inductive element. Once the circuit comprising the tunable inductor 
is formed, it is tested to determine its current value of inductance. The 
current value of inductance is subtracted from a target value of 
inductance to determine the desired increase in inductance. An inductive 
element is selected which is the finest resolution element in which its 
minimum tunable range, plus the sum of the minimum tunable range of all 
finer resolution elements, if any, is greater than the desired increase; 
if the resolution of the element is less than the desired increase, the 
outermost short of said selected element is breached. The circuit is again 
tested, an element selected, and a short breached until no short can be 
breached without causing the value of inductance to rise above the target 
value; thus producing a circuit tuned to within the available resolution 
of the target. 
The foregoing and other features and advantages of the invention will be 
more readily understood upon consideration of the following detailed 
description of the invention, taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The word "spiral" as used herein is intended to include a broad class of 
shapes which exhibit a winding path beginning at a substantially 
centralized location, wherein each successive winding circumscribes the 
previous winding. This definition is intended to include shapes that are 
irregular but generally spiral. 
The word "ladder" or "U", as used herein to denote the shape of an 
inductor, is intended to include a broad class of shapes in which there 
are two lines carrying electrical current in opposite directions between 
which there is a negative mutual inductance. Usually such lines are 
substantially parallel and connected at one end. Further, the variable 
inductor in this shape would have shorts interconnecting or bridging these 
two lines. 
For the purpose of this description, a hierarchy of separately tunable 
inductive elements exists in the instant invention such that all such 
elements may be ordered beginning with the element having the greatest 
total range of tuning. If two or more elements have the same total range, 
they are ordered according to the size of the discrete change that may be 
made, larger first. If two or more elements are otherwise equivalent, 
ordering is arbitrary. 
Within any tunable inductive element, only the outermost remaining 
short--the short closest to the entry of electrons into the element--is 
breached. The change that can be made by breaching any short is measured 
as the change made by breaching that short when it is the outermost 
remaining short. 
FIG. 1 shows a substrate 100, which may be made of ceramic or of other 
suitable insulating material on which a conductor 99 is provided by any 
known method forming a spiral element 106 and ladder shaped elements 107, 
108 and 109. The conductor 99 is attached at its ends 151 and 191, 
in-circuit to other components, usually disposed on or attached to the 
same substrate. The conductor 99 may be manufactured upon its own 
substrate and its ends attached to conductors upon another insulative 
support. 
The spiral element 106 operates on the theory of a positive mutual 
inductance. The adjacent spiral winds of element 106 carry current flowing 
in the same direction which causes a positive mutual inductance. For a 
given length of wire, the spiral element will provide more inductance than 
a "U" shaped element or straight wire due to this positive or sympathetic 
mutual inductance. 
The ladder elements 107, 108 and 109 also make effective use of mutual 
inductance, specifically negative mutual inductance. The ladder shaped 
elements 107, 108 and 109 have adjacent sides which carry current in 
opposing directions causing a negative mutual inductance. For a given 
length of wire, a ladder shaped element will provide less inductance than 
a spiral shaped element or a straight wire due to this negative mutual 
inductance. 
Such mutual inductance is affected by the distance separating the current 
carrying elements, the closer the current carrying elements, the larger 
the positive or negative mutual inductance; conversely, the farther apart 
the current carrying elements, the lower the positive or negative mutual 
inductance. 
Spiral 106 has an inner plate 101 connected through the substrate 100 by a 
conductive connector 190 to attach to a conductor 192 disposed on the back 
surface of the insulative support or on another adjoining insulative 
support. 
Ladder element 107 has shorts 103 which form shortened conducting paths 
across the element. The outermost short 103a allows the majority of the 
current to flow directly across; as a result, the remaining shorts 103 
will have relatively little effect since the majority of the current will 
flow through short 103a. Short 104a and short 105a have a similar effect 
upon ladder element 108 and ladder element 109 respectively. Once an 
outermost short 103a, 104a or 105a is severed, the inductance of the 
element will rise and the current will flow primarily through the next 
outermost short 103b, 104b, 105b. 
Spiral element 106 has shorts 102 which form a conducting path to its 
center plate 101. The outermost short 102a allows the majority of current 
to flow through a sequence of shorts 102 directly to the center plate 101. 
The remaining shorts 102 will have relatively little effect since the 
majority of the current will flow through the short 102a. Once the 
outermost short 102a is severed the inductance of the element will rise 
and the current will flow primarily through the next outermost short 102b. 
When the circuit, including the conductor 99 and the shorts 102, 103, 104 
and 105 is assembled with other components to form a complete operable 
circuit it may thereafter be tested to determine its characteristics. The 
present invention contemplates the trimming of the shorts 102, 103, 104 
and 105 sequentially to tune the complete operable circuit to the desired 
frequency. It is readily understood that the largest increase in 
inductance in each section may be achieved by breaching the outermost 
short 102a, 103a, 104a and 105a of a tunable inductive element. It is also 
readily understood that, due to mutual inductance, for any outermost short 
breached, the spiral element 106 will produce larger changes in inductance 
that the ladder elements 107, 108 and 109. 
There should be no value of inductance within the range of the inductor 
which the instant inductor cannot be tuned to within its resolution. To 
accomplish this, the present invention is formed such that the largest 
tunable change that can be made in any element should be less than the 
total increase that can be made by all finer tunable elements. When 
selecting an inductor in accordance with the present invention, it is 
important to consider the resolution of such an inductor. The shorts 105 
in the last tunable inductive element must be close enough together to 
obtain the target resolution. 
FIG. 2 shows another variable inductor, as a fragment of a circuit, in 
accordance with the present invention. Here, however, the inductor has two 
spiral elements 106 and 206 disposed upon a substrate 100. Unlike FIG. 1, 
terminals 151 and 191 both appear on the same surface of the substrate 
100. The plate 101 is connected to conductor 192 by feed through connector 
190. The conductor 192 is disposed on the back surface and is connected to 
the center of the spiral 206 by another feedthrough connector 190. 
Producing a circuit tuned to a target (or desired) frequency can be 
performed using the tunable inductor illustrated in FIG. 1. The inductance 
of an inductor in a circuit is inversely related to the frequency of the 
circuit. By increasing the value of inductance a circuit will have a lower 
frequency; and by decreasing the value of an inductor a circuit will have 
a higher frequency. 
Methods of producing a circuit tuned to a targeted frequency may utilize a 
circuit comprising a tunable inductor such as the tunable inductor 
represented in FIG. 1. Specifically, it is required that the tunable 
inductor must have at least two separately tunable inductive elements. A 
target for the value of inductance of the tunable inductor (and therefore 
the frequency of the circuit) must be known or selected. The maximum 
change in inductance for each inductive element due to the breaching of a 
short should additionally be known. It is also useful to know the minimum 
tunable range of each inductive element. 
As a circuit is tuned, testing is required to determine the current value 
of inductance. Testing is performed prior to removal of any shorts and 
again each time one or more shorts are removed. After testing, the current 
value of inductance is subtracted from the target value to determine the 
desired increase in inductance. 
In a first method, a short is selected for breaching by determining which 
unbreached short will produce the largest change in inductance that is not 
greater than the desired increase. The selected short is then breached. It 
is also possible, and often desirable, to breach a number of such shorts 
as long as the total increase in inductance will not be greater than the 
desired increase. The selected short or shorts are then breached. 
Another method involves quite a different way of selecting a short for 
breaching. In this latter method, if the minimum range of the element with 
the finest resolution is greater than the desired increase, that element 
is selected. Otherwise, that minimum range is subtracted from the desired 
increase producing a remaining desired change, and the minimum range of 
the element with the next finest resolution is compared to the remaining 
desired change. If the minimum range of that element is greater than the 
remaining desired change, that element is selected. Otherwise, that 
minimum range is subtracted from the remaining desired increase; and so on 
until an element is selected. The outermost short of the selected element 
is breached if the maximum change is less than the desired increase (not 
the remaining desired increase). It is possible, and often desirable, to 
breach a number of such shorts in the selected element as long as the 
total increase in inductance will not be greater than the desired 
increase. 
These methods are more easily understood by example. For simplicity, the 
numbers supplied are not actual data. A particular inductor, such as the 
inductor in FIG. 1, has four separately tunable inductive elements. Table 
1 lists empirical data expressed in frequency (inversely related to the 
inductance of the tunable inductor) which is known for a particular 
tunable inductor. 
TABLE 1 
______________________________________ 
Minimum Maximum Number of 
Range Khz Change Khz Shorts 
______________________________________ 
Spiral 15000 2300 16 
Coarse 920 210 6 
Medium 920 110 9 
Fine 920 70 13 
______________________________________ 
The target value for the circuit is 19950 Khz. 
By testing the circuit, the current value for the circuit is determined to 
be 29826 Mhz. By subtracting this from the target value we obtain a 
desired decrease in frequency (increase in inductance) of 9876 Khz. 
In first tuning method, a short would be selected which will produce the 
largest change not greater than the desired decrease in frequency. The 
outermost short of the spiral meets this requirement because it produces a 
maximum of a 2300 Khz change (no other short has a larger change) and this 
change is no greater than the desired decrease in frequency of 9876 Khz. 
In the preferred embodiment, the outermost four shorts in the spiral 
element would be cut before subsequent testing. The maximum change that 
could be produced by this action is four times 2300 Khz, or 9200 Khz; and 
this is less than 9876 Khz, the desired decrease. 
After the shorts are breached, the circuit is again tested and the current 
value is found to be 21732 Khz, and by subtracting the target value, the 
desired increase is found to be 1782 Kkz. Next, select a short which will 
produce the largest change not greater than the desired decrease in 
frequency. The short selected would be one of the coarse shorts as their 
maximum change is only 200 Khz, much less than the 1782 Khz remaining. 
Note that the spiral short must not be selected because its maximum change 
of 2300 Khz is greater than the desired decrease. While mathematically, 
eight of the shorts would be breached from the coarse element, there are 
only six, thus all six are to be breached. The maximum change that could 
be produced by this action is six times 200 Khz or 1200 Khz, well below 
the 1782 Khz remaining. 
Testing now reveals a current value of 20645 Khz leaving a desired decrease 
of 695 Khz. Since there are no more coarse shorts, medium shorts have the 
largest change less than the desired inductance. Breaching the outermost 
five will yield a change of less than 695 Khz remaining, thus these are 
breached. This time, testing the circuit reveals 20156 Khz leaving a 
desired decrease of 206 Khz. One medium short may be breached. Testing 
after breaching that short yields 20067 Khz, or a desired decrease of 117 
Khz. Only one fine short may be breached, leaving a current value of 20031 
Khz and a desired decrease of 81 Khz afterwards. Again one fine short 
being breached the current value becomes 19988. Now the desired decrease 
is only 38 Khz. 
As a final and optional step, once no more shorts have a largest change 
less than the desired decrease, the short with the smallest maximum change 
may now be breached if its maximum change is less than twice the desired 
decrease. In the example, a fine short remains, and its maximum change is 
70 Khz, this is less than 76 Khz (two times 38 Khz, the desired decrease.) 
Thus this short is breached, and after testing the current (and final) 
value is 19940 Khz. This final value is within 10 Khz of the target. 
In another method of tuning, an element would first be selected. The 
selected element must be the finest resolution element in which the 
minimum range, plus the minimum range of all elements of finer resolution, 
summed together are greater than the desired decrease. Starting with the 
same example, the circuit is tested and the current value is 29826 Khz, 
thus the desired decrease will be 9876 Khz. The spiral element will be 
chosen because the sum of the minimum range of all the elements of finer 
resolution is 2400 Khz (800 Khz+800 Khz+800 Khz) plus the minimum range of 
the spiral (15000 Khz) yields a total of 17400 Khz which is greater than 
the desired decrease. The four outermost shorts will be breached in the 
spiral (the selected element) because the maximum change this will yield 
is 9200 Khz, and this is below the desired decrease. 
Testing shows an actual change of 8094 Khz to a current value of 21732 Khz, 
leaving a desired decrease of 1782 Khz. The next element selected is the 
coarse element because the sum of the minimum range of the coarse element, 
plus the sum of the minimum range of all elements of finer resolution is 
2400 Khz, and this is sufficient to decrease the frequency by the desired 
decrease of 1782 Khz. As in the previous method, even though 
mathematically eight shorts should be selected, the physical limitation of 
six exists. Therefore all six of the shorts in the selected (coarse) 
element are breached. After testing the current value is 20645 Khz leaving 
a desired change of 695 Khz. 
Since the fine element has a minimum tunable range of 800 Khz, more than 
sufficient to tune to the desired decrease of 695 Khz, it is the selected 
element. The outermost nine shorts of the finest element are breached (9 
times 70 Khz being less than 695 Khz). And subsequent testing gives a 
current value of 20099 Khz, or a desired decrease of 149 Khz. The selected 
element is again the fine element, and two shorts are breached leaving a 
tested current value of 19968 Khz or a desired decrease of 18 Khz. 
As a final and optional step, once no more shorts have a largest change 
less than the desired decrease, the short with the smallest maximum change 
may now be breached if its maximum change is less than twice the desired 
decrease. In the example, a fine short remains, and its maximum change is 
70 Khz, this is not less than 36 Khz (two times 18 Khz, the desired 
decrease.) Thus this short is not breached, and the current (and final) 
value remains 19968 Khz. This final value is within 18 Khz of the target. 
Even though the above examples demonstrate that the first method tuned the 
circuit closer to the target value, this is not the case. Both methods are 
equally capable of tuning the circuit to within one half of the maximum 
change of a short in the finest element. 
Thus it can be seen that a new, improved, variable inductor that is compact 
and has a wide dynamic range for an inductor of its precision, and new, 
improved methods of tuning a circuit comprising a variable inductor have 
been provided by the present invention. The disclosed inductor is easily 
and precisely tunable to increase the inductance value thereof in a 
pre-assembly or post-assembly operation. 
It will be understood that the invention may be embodied in other specific 
forms without departing from the spirit or central characteristics 
thereof. The instant examples and embodiments, therefore, are to be 
considered in all respects as illustrative and not restrictive, and the 
invention is not to be limited to the details given herein.