US2255680A

US2255680A - Variable permeability tuning system

Info

Publication number: US2255680A
Application number: US335708A
Authority: US
Inventors: William F Sands; Paul F G Holst
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1940-05-17
Filing date: 1940-05-17
Publication date: 1941-09-09
Anticipated expiration: 1958-09-09
Also published as: GB549459A; NL61935C; DE971975C

Description

Sept. 9, 1941. w. F. SANDS ETAL 2,255,580

I VARIABLE PERMEABILITY TUNING SYSTEM Filed May 17, 1940 PER CENT Fwzpuz/vcy zwwmva/v l N N o u FREQUENCY A C Patented Sept. 9, 1941 2,255,880 VARIABLE PERMEABILITY TUNING SYSTEM William F. Sands, West Coillngswood, and Paul F. G. Holst, Oaklyn, N. 1., alsignors to Radio Corporation of America, a corporation of Delaware Application May 17, 1940, Serial No. 335,708

Claims.

This invention relates to variable permeability tuning systems for radio signal circuits, and has for its primary object to provide an improved tuning system of the character referred to,

wherein two or more signal circuits are tunable through predetermined frequency ranges in such a manner as to provide substantially a constant frequency difierence between the resonant frequencies of two of said circuits.

In a superheterodyne radio signal receiving system, it is a further object of the invention to provide an improved variable permeability tum ing system for causing the oscillator and signal input circuits to track accurately one with the other throughout a predetermined frequency range and to compensate the tendency for such circuits to depart from the desired constant frequency difference.

There are several known methods and means for effecting tracking in the tuning of two or more circuits by variable permeability tuning means, some of which are impracticable because of cost or manufacturing difiiculties, and others of which are adaptable for commercial application.

As an improvement over certain of the latter, it has been found that two or more variable permeability tuned circuits may be caused to tune through the same or differing frequency ranges with accurate tracking by the combination of certain features as will hereinafter be described,

by way of example, in an application to the tuning of a superheterodyne receiving system.

It is also a further object of the present invention to provide an improved tuning system for a superheterodyne receiver which provides for the alignment of and a desired tracking relation between two tunable circuits at a plurality of points in the tuning range.

It is a further object or the invention to provide an improved oscillator tuning-inductance which is adapted for causing alignment or tracking of the oscillator circuit with a second tunable signal circuit at a plurality of points in a predetermined frequency range.

The invention will be further understood, however, from the following description, when com sldered in connection with the accompanying drawing, and its scope is pointed out in the appended claims. 1

In the drawing,

Figure 1 is a schematic circuit diagram of a tuning system for a superheterodyne I receiver emb dy the invention,

stantially full size, of the tuning elements of Fig. 1 arranged in operative relation to each other,

Figure 3 is a modification of a portion of the tuning arrangement of Fig. 2 on the same scale and in cross section, and

Figure, 4 is a graph showing a curve illustrative of the tracking relation between two of the circuit systems shown in Fig. 1.

Referring to Fig. l, the radio signal tuning system shown comprises a tunable signal input circuit 5 and a tunable oscillator circuit 6 pr0- vided with variable permeability tuning inductance and shunt capacity means.

In the signal input circuit, the variable tuning inductance l is provided with a movable tuning core indicated at 8, while in the oscillator or second circuit 5, the tuning inductance is indicated by a two part, continuous winding 9 and i0, having a common movable tuning core H connected with the first tuning core 8, as indicated by the dotted line i2, for unitary tuning control movement.

The circuit 5 includes a shunt tuning capacitor l3 for the inductance l and a shunt trimmer capacitor 14, the latter being adjusted to establish the low frequency end of the tuning range when the core 8 is moved fully within the windmg The circuit 5 is connected to ground as indicated at I5 and is connected at its high potential side to any suitable signal source, such as an antenna is, through suitable coupling means, such asa coupling capacitor I l. l

The second tuned circuit 6 likewise includes shunt capacity means for the inductance 9l0 comprising two series connected capacitors l8 and IS, the latter being adjustable in a similar manner to the capacitor H, for adjusting the tuning at the low frequency endof the tuning range of the circuit 6.

The oscillator circuit 6 in the present example is of the Colpitts type, having a cathode connection 20 between the capacitors l8 and I9, and being grounded as indicated at 2|, at the low potential side thereof.

The input circuit 5 is connected at its high potential side to a signal input grid 22 of a combined detector-oscillator tube 23. The cathode 24 is connected to ground 25 through a suitable choke coil 26," the cathode terminal 21 of which is connected with the cathode tap 20 of the oscillator circuit.

The circuit 8 is connected with a second signal Figure 2 is a view, in cross section and subgrid 28 in the tube 23 for electronically mixing quency of the R.-F.

the signals from the circuit with oscillationsto produce a predetermined intermediate irequency in the output anode circuit 29 or the detector-oscillator tube 23. Feedback for the production of oscillations is provided in connection with the screen grid 36. which functions as an anode coupled to ground through a bypass capacitor ii in connection with a filter resister 38 in the screen grid supply circuit indicated at 33. The oscillator grid is provided with the usual grid coupling capacitor t4 and grid leak 85 to ground.

In connection with the circuit 5 and the signal input grid 22, a coupling capacitor 88 and grid resistor 31 are provided, the latter being connected to a bias supply lead 88, with a suitable by-pass to ground through a lay-pass capacitor as.

Signals are derived from the detector-oscillator and the output circuit 39 through a suitable tuned intermediate frequency coupling transformer 80.

Referring now to Fig. 2 along with Fig. 1, the same reference numerals are used to designate like parts as in the preceding figure, the signal input circuit inductance being indicated at l and the oscillator tuning inductance being indicated at 9-40 and their respective tuning cores being shown at 8 and H. The inductance windings are supported on suitable insulating

cylindrical coil forms

45 and 46, respectively, which are secured in spaced parallel relation to each other to a fixed mounting frame 41.

The cores are connected with operating rods 40 extending into the open ends of the coil forms and are adiustably connected with a movable frame member 49 having a body or main portion 50 which passes through guide openings 5! in the frame 41 to permit unitary movement of the two cores into and out of the windings. This tuning movement is provided as the frame 50 is moved to the right and left, as viewed in the drawing, under control of a tuning control knob 52 which serves to drive a wrapped cord 58 connected at its ends with the frame 50 representing any suitable arrangement for this purpose.

The present preierred arrangement for actuating the movable core elements is of the type wherein a plurality of. core elements are con-- nected with a common actuating means providing for the same length of travel for all of the core elements and, therefore, substantially the same coil or winding length in association with each coil, since it is necessary for maximum tuning range, for the core to move from a position at the entering end of the winding to a position at which the core is fully entered in the winding. Therefore, since all of the cores must move the same distance, the coils are all substantially of the same length.

This arrangement is satisfactory for a plurality of circuits which are tuned through the same frequency range. However, in case that kc. or in a 3:1 frequency ratio while the Oscillator must be tuned through a frequency range of 1000 to 1950 kc. or in a 2:1 frequency ratio.

Since the frequency of a tuned circuit changes inversely as the square root of the inductance, it is apparent that a smaller inductance variation will be required for the oscillator circuit than for the antenna or input circuit. The inductance becomes a maximum when the iron core fills the space within the solenoid, as nearly as practicable. An increase in the diameter of the oscillator coil. therefore, will decrease the inductance variation range. In the present example, the diameter of the oscillator coil is increased to a value providing substantially exact tracking at the ends of the tuning range.

For this reason, the diameter of the oscillator winding 8l0 is made greater than the R.-F. winding as shown in Fig. 2. Assuming that the windings l and 9 are of substantially the same length, as hereinbefore described, for unitary control of the tuning cores, the diameter of the oscillator inductance is made slightly larger than that of the R.-F. inductance; so that the R.-F. oscillator circuits will be in alignment or provide the desired frequency difference at 550 and 1700 kcs. in the present example. For such a broadcast receiver, the oscillator coil form diameter may be approximately .400 inch, while the coil form diameter of the R.-F. winding may be .275 inch approximately, both windings being approximately 1% inches long.

While this relation between the diameters of the windings provides tracking at the extreme ends of the tuning range, it tends to cause a departure from a desired tracking relation in the midrange and this is compensated or corrected by winding one of the inductances, preferably the oscillator tuning inductance, with a variable pitch winding, the number of turns per inch increasing either uniformly or in steps toward the high frequency end of the winding which is the end at which the core enters. If in steps, a, plurality of difiering winding pitches may be provided on one winding.

However, a stepped or variable pitch winding is diflicult to produce and it is, therefore, more one circuit is tunable through a different fre 1 quency range, as in the present example, where the circuit 6 is that of the oscillator and the circuit 5 is that for the R.-F. input signal, means rangesand with a predetermined tracking or frequency difference relation.

Assuming, for example, that the tuning system shown is adapted for the present broadcast band of 550 to 1500 kc. and that an intermediate ire- 450 kc. is desired, it will be seen that circuit must be tuned from 550 to 1500 costly than the usual solenoid winding having a uniform winding pitch. It has been found, however, that means may be provided for effecting a variable pitch or obtaining the efiects of a variable pitch in a tuning inductance of the variable 1 permeability type having a movable core element.

This includes providing a plurality of back wound turns at the high frequency or entering end of the inductance winding, such as indicated at H! in Figs. 1 and 2, the portion I0 being a continuation of the portion 9 in the same winding direction. This produces the effect of a. difierent winding pitch in the winding section 9 covered by the section 10.

By way of example, an oscillator coil form 46 of .403 inch diameter may be wound as at 9 for 1% inches approximately, with No. 33enameled wire,. double spaced and equal to 64 turns per inch, and then may be back wound as at I for' spect to the 3.4 tuning core, for example,

inch, so that the R.-1". tuning core enters the-extreme high frequency end of the R.-l". inductance slightly. in advance of the entry of the oscillator core into the oscillator tuning inductance, this relation being shown in the present example and provided by threading the ends of the rods 48, as indicated at 55, and fitting the threaded ends with suitable lock nuts 58 for securing the rods after the desired relative spacing of the cores and coils is provided.

The arrangement is such that one of the cores enters the winding with which it is associated in advance of the other core entering its associated winding. In the case of the oscillator and R.-F. circuits assumed herein, by way of example, the R.-F. core enters in advance of the oscillator core.

With the arrangement shown and as described, the tracking deviation between the oscillator and R.-F. tuning circuits is substantially as indicated by the curve 58 shown in Fig. 3, the crossover points 59 and 60 being due to the back-wound turns, and the crossover point 6! being due to this arrangement and the fact that the cores enter the windings in slightly spaced consecutive order as described. As previously referred to, the tracking points 62 and 63 along the zero axis at the low and high frequency ends are provided by the predetermined relation between the diameters of the two windings.

It will be seen that this tracking relation is highly desirable since the frequency deviation curve crosses the zero axis at a plurality of points between the high and low frequency ends of the tuning range and that this tracking relation is obtained by a comparatively simple circuit and tuning element arrangement involving comparatively low cost in production.

The relation between the incremental change in inductance for an incremental change in the position of the tuning core, or the derivative of the tuning response 'curve for a permeability tunable system has been found to be greatest toward the center of the tuning band, and in the band chosen herein, for example, occurs in the region of 1200 kcs.

It is, therefore, desirable to adjust the position of the back-wound turns with respect to the tuning core at this frequency. As indicated in Fig. 3, the oscillator winding 9 is provided with the hack-wound turns III on a movable short coil form or sleeve 65, whereby the turns Ill may be moved along the winding 9 and positioned as desired, for maximum control of the tracking response, when the coil form 65 may be cemented or otherwise secured in place by any suitable means.

It will be noted that, in the arrangement of Fig. 3, the winding 9 is of larger diameter than the antenna or R.-F. inductance as in Fig. 2, as is required for producing the required shortening of the oscillator tuning range, and that the backwound turns ID are, inany case, located at or adthecombination of two iacent to the lower potential or ground end of the winding, which is also the high frequency end, since the core enters at that end. The windings 9 and III are serially connected in aiding relation so that they form one. continuous winding, the winding I0 having a fewer number of turns than the winding 9 and being shorter in axial length.

One method of effecting circuit alignment with this arrangement comprises adjusting the 'trimmer capacitors I4 and It, as shown in Fig. l, at the low frequency end of the tuning band with the cores fully penetrating the inductance windings of the coil, thereby providing circuit alignment at the low frequency end of the tuning range or ranges, as the case may be.

The tuning cores are adjusted for maximum response at a frequency at which the derivative of the tuning curve has the highest value, such as approximately 1200 kcs. in the present frequency band under consideration. The relative position of the windings Ill and S then may be adjusted for maximum sensitivity, and the winding I 0 is then secured in place on the winding It will be noted that this arrangement has the effect of changing the pitch of the turns of the winding 8 which lie directly under the turns of the winding Ill without necessitating winding the oscillator coil with a variable pitch or a multiple pitch winding.

We claim as our invention:

1. In a variable permeability tuning system, the combination of two tuning inductance windings each having a movable tuning core and one of said windings having a plurality of back wound turns adjacent one end for effecting a variable pitch in said windings thereby to provide a predetermined tracking relation in the tuning response of said windings with unicontrol tuning movement of said cores.

2. In a variable permeability tuning system,

tuning inductance windings each having a movable tuning core adapted to enter one end thereof in tuning, means for moving said cores in unison to vary said tuning, and one of said windings having a plurality of back wound turns adjacent said end for effecting a variable pitch in said winding, thereby to provide a predetermined tracking relation in a tuning response of said winding with unicontrol tuning movement of said cores.

3. In a variable permeability tuning system, the combination of a plurality of tuning inductance windings each having a movable tuning core, said cores being interconnected for unitary control of the tuning of said windings, and means for effecting a variable pitch in one of said windings thereby to effect tracking in the tuning response of said winding with respect to that of the other of said windings, said means comprising a plurality of back-wound turns forming a continuation of and :being located adjacent to one end of said last-named winding,

4. In a variable permeability tuning system,

the combination of a plurality of tuning inductance windings each having a movable tuning core, said cores being interconnected for unitary control of the tuning of said windings, means for effecting a variable pitch in one of said windings thereby to eifect tracking in the tuning response of said winding with respect to that of the other of said windings, said means comprising a plurality of back-wound turns forming a continuation of and being located adjacent to one end of said last-named winding, and shunt capacitor tuning means for each of said windings connected in circuit therewith, said capacitor means being adjustable for effecting a predetermined tracking relation between the tuning of said windings at one end of the tuning ranges thereof.

5. In a variable permeability tuning system, the combination of aplurality of tuning inductance windings of substantially the same winding length, each having a movable tuning core of substantially the same diameter, said cores being interconnected for unitary control of the tuning of said windings through predetermined frequency ranges with the same length of travel of said cores, and means for effecting a variable pitch in one of said windings thereby to eflect tracking in the tuning response of said winding with respect to that or the other of said windings, said means comprising a plurality of back-wound turns forming a continuation of and being located adjacent to one end of said last-named windings, and fixed shunt capacitor tuning means for each of said windings connected in circuit therewith.

6. In a variable permeability tuning system,

' the combination of a plurality of tuning inductthereby to effect tracking in the tuning response of said winding with respect to that or the other of said windings, said means comprising a plurality of back-wound turns forming a continuation of and being located adjacent to one end of said last-named windings, and fixed shunt capacitor tuning means for each of said windings connected in circuit therewith, said capacitor means being adjustable for effecting a predetermined tracking relation between the tuning of said windings at one end of the tuning ranges thereof. v y

7. In a variable permeability tuning system, the combination of a pair of tuning inductance windings of relatively differing diameters, the larger diameter winding having a plurality of back-wound turns adjacent one end thereof for effecting a variable winding pitch therein, a pair of movable tuning core members of comminuted ferro-magnetic material one for each of said windings, means for moving core elements in unison with respect to said windings, one of said cores being displaced in the direction or" movement to enter its associated winding in advance assaeso winding.

8. In a variable permeability tuning system,

vthe combination or a pair 0! tuning inductance windings, one of said windings having a plurality of back-wound turns adjacent one end thereof for efiecting a variable winding pitch therein, a pair of movable tuning core members of comminuted term-magnetic material one for each of said windings, means for moving said core elements in unison with respect to said windings, one of said cores being displaced in the direction of movement to enter its associated winding in advance of the entry of the other core into its associated winding.

9. In a variable permeability tuning system, the combination of a plurality of tunable signalconveying circuits and means in one of said circuits for efiecting a predetermined tracking relation in the tuning of said circuit with at least one other of said circuits, said means comprising an inductance winding of the solenoid type having a plurality of back-wound turns forming a continuation thereof adjacent and surrounding one end and a movable tuning core of comminuted term-magnetic material movable into the said end of said winding to vary the tuning thereof.

10. In a variable permeability tuning system,

' the combination of a plurality of tunable signal conveying circuits and means in one or said circuits for eflectinga predetermined,'tracking relation in the tuning of said circuit with at least one other of said circuits, said means comprising in inductance winding of the solenoid type having a plurality of back-wound turns forming a continuation thereof adjacent and surrounding one end and a movable tuning core of comminuted term-magnetic material movable into the said end of said winding to vary the tuning thereof, the diameter of said core element being so related to the diameter and length or the winding that the tuning range of said wniding is predetermined and fixed with relation to the tuning of others of said circuits.

. F. SS.

PAUL F. G. HOLST.