US 2302893 A
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Description (OCR text may contain errors)
Nov. 24, 1942. w. VAN B. ROBERTS 2,302,393
VARIABLE INDUCTANCE ARRANGEMENT Filed Sept. 29, 1959 2 Sheets-Sheet 1 0 Eyd SOURCE F IRE CURRENT 0K WLY VARY ('0 0L CURRENT ENTOR WA L TER B. ROBERTS ATTORNEY Nov. 24, 1942. w V ROBERTS 2,302,893
VARIABLE INDUCTANCE ARRANGEMENT I Filed Sept. 29, 1939 2 Sheets-Sheet 2 .9 4 r0 PUSH-PULL Rf. SELECTOR .12 A F. GRIDS AND AMPL/F/ER AAA AAAA vvy vvv qllp FREQl/fA/CY CONVERT 5k AL 1. F. AMPL lF/ER v INVENTOR WALTER VAN B ROBERTS ATTORNEY Patented Nov. 24, 1942 2,302,893 VARIABLE INDUCTANCE ARRANGEMENT Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 29, 1939, Serial No. 297,032
It has been proposed to vary the inductance of a coil having a magnetic core by superposing on said core a controlling flux which is adjustable in accordance with the inductance desired. However, if the coil carrying the controlling current is coupled to the radio frequency winding losses will be produced in the radio frequency winding. The present invention provides a construction which permits both the radio frequency coil and the control current winding to produce flux in a common portion of their magnetic circuits without thereby introducing any coupling between the two coils provided that the one coil carries radio frequency current and the other coil only direct or slowly varying current.
In the drawings:
Fig. 1 shows a variable inductance embodying the invention,
Fig. 2 is a modification,
Fig. 3 shows a radio receiver circuit employing the variable inductance device at the antenna circuit,
Fig. 4 shows a superheterodyne receiver employing the variable inductance device for automatic frequency control.
Fig. 1 shows one form of the invention where l represents a high frequency core made up of a material such as powdered iron and where the dotted lines 2 bearing arrows indicate the magnetic lines of force produced by current in the radio frequency coil 3. The core I is provided with a cylindrical copper shield 4 which surrounds the magnetic core 3 and this shield prevents radio frequency variations of magnetic flux from getting outside of the shield. Member 5 which is made of ordinary iron has mounted thereon control windings 6 and I. This arrangement forms a magnetic circuit which is completed by the magnetic material inside the shield 4. Typical lines of flux are shown by lines 9 and Ill. Due to the fact that the control flux represented by lines 9 and I0 is at most only slowing varying the copper shield 4 is ineffective to prevent the control flux from passing through the shield and acts merely like a small air gap. It will be noted that in addition to its primary purpose of preventing the radio frequency field from linking the control winding or the solid iron portion of the control circuit, the shield also acts in the usual way to prevent coupling between the radio frequency coil and other coils adjacent thereto. a
It will be noted also that a small air gap H is left in the radio frequency magnetic circuit so that the control flux will pass mainly through the central portion of the radio frequency core I where its effect is the most pronounced. Windings 6 and I are connected to a suitable source of control current through terminals 8.
Fig. 2 shows another arrangement operating similarly in principle to the arrangement shown in Fig. 1 except that the radio frequency coil I2 and core l3 are elongated and the control winding I4 are wrapped upon the shield l5. An air gap may be left in the radio frequency magnetic circuit if desired but in this case an air gap will not increase the flux through the interior of the radio frequency coil.
Fig. 3 shows the application of a controlled inductance of the type described to the input circuit of a radio receiver for the purpose of preventing overloading of the first tube. The antenna circuit is is normally unbalanced in the presence of weak signals but when the signals are strong enough to produce a predetermined detector input current this predetermined current flowing through the control winding 21 of the antenna coil l8 makes its inductance decrease to a point that will bring the bridge circuit l1, l8 and IS in the antenna system to balance, thus preventing any voltage reaching the first tube 24. Obviously, the detector input can never quite reach this value and hence the input to the first tube 24 can never exceed a predetermined amount. In addition to this type of automatic control of input voltage the rectifled voltage developed across resistor 29 may be also applied to one or more of the grids of the amplifier tubes shown generally at 30 to control the gain thereof in the usual way.
Fig. 4 shows the application of a coil in accordance'with the invention to autcmatic frequency control. The coil is used in the oscillator 30 of a superheterodyne whose frequency is approximately adjusted by the variable condenser 3|. If the frequency is not quite correct a voltage will be developed by the discriminator circuit 32 and thus produce a flux in one direction or the other in the control magneto circuit 33 according to the direction of the misadjustment. A third winding 34 energized to a constant amount by a battery 35 is employed with the result that a flux in one direction produced by the discriminator increases the total flux in the control core 36 while the flux in the opposite direction decreases the total flux. Thus the effective inductance of the oscillator coil 31 is increased or decreased according as the mistuning is in one sense or the other. The action in other respects is identical with that oi known A. F. C. circuits employing a so-called reactance tube for controlling the oscillator frequency. The advantage in the present arrangement lies in the elimination of the extra tube.
A compact variable inductance unit comprising a cylindrical radio frequency term-magnetic core, a single radio frequency coil wound coaxially along solely said core, a non-magnetic 10