Coarse and fine adjustment warp coil

An inductor coil consisting of two cores enclosed by a single layer of wire is disclosed. The cores may be of the same or different permeability. The coils arranged in this configuration behave like two coils in series in which the relative values of inductance can be varied independently, thus the change in inductance depends only on the number of turns and the relative permeability of the cores. Decreased sensitivity, however, is obtained because of the increased area inside of the coil provided by the two cores side by side.

BACKGROUND OF THE INVENTION 
This invention relates to warp (adjustment) coils for adjusting the 
frequency of crystal controlled oscillators in the VHF and UHF areas, for 
example, and it is an object of the invention to provide an improved warp 
coil of this nature. In such oscillators, the circuits utilizing the 
piezoelectric crystals need adjustability because the crystals themselves, 
after manufacture, do not have the precisely defined frequency required. 
Prior solutions to this problem have included the use of a toroidal 
inductor plus a warp coil in series with the piezoelectric crystal. While 
this solution solved the problem within some limits, it lacked the 
necessary sensitivity and was expensive because of the cost of the 
toroidal inductor and the warp coil. In this prior solution the toroidal 
inductor usually had a ferrite core and could thus provide sufficient 
inductance for producing the necessary coarse adjustment, but an 
additional series warp coil was necessary to produce the fine adjustment. 
In these fine adjustment coils there tended to be excessive inductance 
change per unit turn of the core in the coil to have the necessary 
decreased sensitivity for accurate adjustment of the final desired 
frequency unless specially spaced wound coils were used. 
SUMMARY OF THE INVENTION 
It is a further object of the invention to provide an improved coarse and 
fine warp coil which will eliminate the objections of the prior art 
solutions. 
In carrying out the invention according to one form an adjustable inductor 
is provided comprising a coil of a predetermined number of turns and a 
predetermined interior dimension and two cores of predetermined 
permeability adapted to be disposed side by side within the predetermined 
dimension, and at least one of the cores being movable relative to the 
other. 
According to a further embodiment of the invention an adjustable inductor 
adapted for coarse and fine adjustment is provided comprising a first tube 
of insulating material, a second tube of insulating material disposed 
along side the first insulating material, a first metallic core of a 
predetermined permeability disposed in the first tube, a second metallic 
core of a predetermined permeability disposed in the second tube, and a 
winding having a predetermined number of turns surrounding both the first 
and second insulating tubes, at least one of the first and second cores 
being movable relative to the other.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings there is shown an inductor 10 comprising two 
dielectric or insulating tubes 11 and 12 disposed side by side, a winding 
13 wrapped around both cores 11 and 12 and a pair of metallic threaded 
cores 15 and 16 disposed inside of dielectric tubes 11 and 12 
respectively. 
The cores 14 and 15 includes slots 16, 17, 18, 19, in order that the cores 
may be adjusted in position from either end by the use of an ordinary 
small screwdriver. The interior of the tubes 11 and 12 need not be 
threaded because the threads of the cores bite into the interior surface 
of the tubes. 
Referring to FIG. 2, the winding 13 surrounds both tubes 11 and 12 and, 
thus, also, the cores 14 and 15. It is therefore completely clear that the 
area within the winding 13 is over twice the area that it would be if the 
winding surrounded only a single one of the tubes 11 or 12. If the two 
coils were placed in series and the same number of turns applied around 
each coil, the area of each coil would be reduced by at least half, as 
compared with both coils, and consequently, the inductance thereof would 
be reduced. Thus, by placing the coils side by side and using a single 
coil the area within the coil is increased by a factor greater than two 
and the inductance of the combination is thereby increased by at least a 
factor of two. 
Referring to FIG. 3, there is shown a circuit having terminals 21 and 22 
between which in series are connected the coil 10 and a piezoelectric 
crystal 23. It is the frequency of the crystal 23 that needs to be warped 
to the right value by the inclusion of the coil 10. This is efficiently 
and conveniently done, according to the invention, by the coil having two 
cores surrounded by a single layer of turns as described. The toroidal 
inductor of the prior art is thus eliminated, while at the same time the 
sensitivity of the adjustability is decreased. 
Sensitivity of the coil is defined by the change in inductance per unit 
turn of the winding when either of the cores 14 or 15 is moved. Thus, 
while the inductance is increased by a factor of over two by virtue of 
having the increased area of the coil when the two cores are side by side, 
the sensitivity is only one quarter of the value of a single coil because 
of the decreased change in inductance per unit turn. Accordingly very 
accurate adjustments of the crystal frequency may be made with the single 
coil unit is illustrated. 
Referring to FIG. 1, it will be noted that the cores may occupy any 
position relative to the winding 13 from all the way into the coil to all 
the way out. The inductive effect of the core of course is the greater 
when it is completely within the winding rather than only partially in. 
The cores 14 and 15 may be of different materials, such for example, as 15 
may be of ferrite and 14 may be of aluminum. The ferrite material is 
highly magnetic and relatively small movements of the core 15 therefore 
may be used to obtain wide changes in inductance and thus relatively wide 
changes in frequency of the crystal 23 for coarse adjustment. In this 
instance the core 14 being of aluminum, a diamagnetic material, much 
smaller changes in inductance are made by one turn or less or more for 
that matter of the core 14 into the coil 13. In this way, very fine or 
small changes in inductance and frequency are obtainable. In particular 
cases, it is of course feasible, for the cores 14 and 16 to be of the same 
material, for example, aluminum. 
A very substantial advantage of the device as illustrated and described is 
that the frequency of the crystal 23 can be warped onto the desired value 
by simply adjusting the positions of the cores 14 and 15 inside the 
inductor winding 13. No selection of a toroidal core, for example, to fit 
the initial frequency of the crystal is necessary. All adjustments can be 
made by changing the cores 14 and 15 in the single coil. Moreover, this 
can be done with greatly decreased sensitivity and therefore much more 
expeditiously and accurately. 
The coils connected as described behave as two coils connected in series in 
which the relative values of inductance can be varied independently. The 
change in inductance depends only on the number of turns and the relative 
core permeabilities. 
While the coil cross-section, shown in FIG. 2 is oblong, any cross-section, 
including round, may be used. The term interior dimension is intended to 
include all of these cases. 
In one particular case, for example, the outside diameter of the tubes 11 
and 12 was 110 mils, the inside diameter of the tubes 0.083 mils, and the 
thickness of the walls of the tubes was 0.08 mils. The cores 14 and 15 
were aluminum 1/4 of an inch in length and the winding 13 and 16 turns of 
number 30 wire. Other dimensions, of course, can be used to particular 
circumstances. The particular crystal circuit was intended to operate in 
the 39 to 62 MHz range. The material of the tubes was that commercially 
available under the trade name Nylon Zytel made by Dupont.