US2267173A

US2267173A - Signal-collecting system for radio receivers and the like

Info

Publication number: US2267173A
Application number: US319673A
Authority: US
Inventors: William A Schaper
Original assignee: Johnson Laboratories Inc
Current assignee: Johnson Laboratories Inc
Priority date: 1940-02-19
Filing date: 1940-02-19
Publication date: 1941-12-23
Anticipated expiration: 1958-12-23
Also published as: GB547367A

Description

Dec. 23, 1941. w. A. SCHAPER 2,267,173

SIGNAL-COLLECTING SYSTEM FOR RADIO RECEIVERS AND THE LIKE Filed Feb. 19, 1940 Ww w ATTORNEY *frequency of the signal.

Patented Dec. 23, 1941 PATENT oFncE SIGNAL-COLLECTING SYSTEM FOR RADIO RECEIVERS AND THE LIKE William A. Schaper, Cicero, 111., assignor to Johnson Laboratories, Inc., Chicago, 111., a corporation of Illinois Application February 19, 1940, Serial No. 319,673

4 Claims.

This invention relates to high-frequency circuits, such as those employed in radio receiving systems. More particularly, the invention relates to the portion of such systems which constitutes means for collecting the high-frequency signals radiated from relatively distant transmitting, stations. This invention incorporates an improved.signal-collecting means.

Signal-collecting systems generally include a resonant circuit tunable over a desired range of frequencies and classifiable as series or parallel resonant circuits depending upon how the signal voltage is generated in or applied to the circuit Collector systems of the series type usually em- .ploy a so-called loop antenna or its equivalent to intercept the signals. It is to this type that the present invention is addressed.

In their practicable forms, series collector systems have heretofore been tuned by variation of the circuit capacitance. In the present application, and in my co-pending applications, Serial Numbers 319,671 and 319,672, both filed February 19, 1940, to which I shall refer in greater detail later in this specification, I disclose highly advantageous and commercially practicable series collector systems tuned by variation of the circuit inductance, preferably by the employment of a ferromagnetic core of suitable characteristics movable relatively to an inductance coil in series with the loop or other exposed inductive element of the system. This method of tuning has the additional advantage of providing means for controlling the high-frequency resistance of the system in substantially any desired manner as the system is tuned over the frequencyrange.

Resonant circuits tuned by inductance variation by means of ferromagnetic cores movable relativelyto the inductive element in the circuit,

and possessing the advantage of simultaneous controlof the circuit-resistance are disclosed by Polydorofi in United States Re. Patent No.

21,282, in which a resonant circuit having an inductance coil and capacitor is adjusted over a range of frequencies by movement of a compressed comminuted core relatively to the inductance coil. This method of tuning is called permeability tuning. An improved form of such a system is disclosed in my United States Patent No. 2,051,012. Both Polydorofis original system and my improved system readily cover an adequate range-of frequencies and may easily be ganged to provide multiple unit systems.

' -In general, the signal voltage generated in a collector system is directly proportional to the Thus asignal at the upper end of the broadcast range will provide approximately three times as much signal voltage in the collector system as a signal at the lower'end of this range. Various expedients have been employed in broadcast receivers in an effort to compensate for this inherent deficiency of the collector system. In my oopending applications above referred to,'and' in the present application, I disclose ,collectorisystems in which this deficiency is avoided by employing simultaneous variation of circuit inductance and resistance, preferably as provided by permeability tuning, in particular circuit arrangements. In accordance with application Serial Number 319,672 filed February 19, 1940, I employ an additional exposed inductive element in an untuned portion of the system. In the present application, as I shall later more fully describe, I employ an additional unexposed inductive element arranged to regeneratively increase the delivered voltage in a desired manner through a coupling variation also simultaneously produced by the relatively movable ferromagnetic core in a permeability-tuned circuit.

An object of my invention is to provide an improved signal-collecting means for radio receivers,

An additional object is to provide a signal-collecting means which may be successfully employed in the most compact forms of radio receivers, and which may be placed in close proximity to the receiver chassis without serious detriment.

Still another object of the invention is to provide asignal-collecting circuit whose performance characteristics may be readily controlled with respect to variation with frequency.

It is also an object of the invention to provide a signal-collecting circuit which may be tuned by inductance variation, for example by means of a movable ferromagnetic core, and in which the advantages of this method of tuning may be realized in accordance with the invention in a manner which will be; readily understood from the following description taken in connection with the drawingwhich is. a schematic diagram of the basic circuitarrangement.

Referring to the drawi'mti, ;the improved signal-collecting system according to the present invention comprises an inductive element I, a capacitance 2 and a variable inductance 3 arranged to form a series resonant circuit. This circuit is arranged to supply signal voltage to the grid 4 of vacuum tube 5, which may be the first vacuum tube in a radio receiver, and which may be arranged to act as a high-frequency amplifier, as a detector or as a modulator in a superheterodyne system. Inductive element I is exposed to the passing radio signals, and may take the form of a so-called loop antenna. Variable inductor 3 is of the type in which inductance variation is secured by movement of a ferromagnetic element 6, which may be an internal core, relatively to the winding thereof.

The portion of my improved signal-collecting system just described is similar to the system described in my copending application, Serial Number 319,671 filed February 19, 1940, of which the present application is a continuation inpart. In thatapplication I- describe in some detail the performance of the conventional loop circuit tuned by capacitance variation, and I point out the performance deficiencies therein. I also point out how these deficiencies may at least in great measure be avoided by tuning a series resonant signal-collecting circuit by inductance variation ratherthan capacitance variation."

It will be apparent that since the variably- -tuned circuit includes inductances I and 3, the variable inductor 36 must provide greater inductance variation than would be required if the inductance I were absent, in order to tune the system over the required frequency range. It may be shown that, if L is the effective external series inductance, p is the ratio of the highest to the lowest frequency in the required tuning range, and La is the minimum inductance of the variable inductor 3-6, then the required effective permeability a of ferromagnetic element 6 is Again referring to the drawing, the system according to my present application also includes a winding 1 inductively coupled to variable inductance 3 and connected between the cathode 8 of vacuum tube and ground, so thatit is efiectively in the output or plate circuit ofthe tube. Winding I acts as a regenerative or feed-back coupling, in a particular manner later to be described, to materially increase the signal voltage developed across capacitor 2 particularly at those frequencies at which, in the absence of winding 1, the developed voltages would tend tolbe undesirably low. 5

The introduction of the variable inductor 3-6, with resultant decrease in the permissible inductance of the exposed portion of the collector circuit, decreases the voltage generated therein by the signal. 'I'hislmight at first be thought to constitute a serious disadvantage. It must be borne in mind, however, that, in the case of a loop, the resonant voltage across the loop at any frequency is not only directly proportional to the inductance of the loop but also inversely proportional to the resistance of the loop.

In many of the more compact designs, the loop is placed within a relatively small cabinet and in :close proximityv to the metallic chassisv of the receiver. Under these circumstances the resistance of a normalcondenser-tuned loop is greatly increased, with .resultant decrease in resonant gain and selectivity. The total. inductance in my improved circuit may be as great, even at the highest frequency, as in a conventional condenser-tuned loop, and will then be much greater at the lower frequencies. Additionally, I may design the variable inductance portion of my circuit to have relatively very low resistance, and I preferably shield it from the loss-increasing effect of the metallic chassis. Thus the total resistance of my circuit will be materially lower,

and the inductance as high or higher, than in a conventional condenser-tuned loop in the same receiver, with'the result that I thus secure much greater resonant gain and much better selectivity. The increased resonant gain more than compensates for the reduced voltage generated by the signal in the smaller inductance of the exposed portion of the circuit.

Again referring to the drawing, movement of ferromagnetic core 6 relatively to windings 3 and I automatically varies the inductive coupling between them. In order to effectively compensate for the lower voltage generated in the inductive element I by the lowerfrequ'ency signals, windings 3 and I are so arranged that the inductive coupling between them is materially increased as the ferromagnetic element 6 is advanced to tune the circuit to the lower frequencies.

As is Well known, the effective high-frequency resistance of a permeability-tuned"circuit increases as the core is advanced into the winding to tune the circuit to lower frequencies. Delpending upon the materials employed and the process of producing the core, this increase in resistance may be controlled, and, if: desired, may bemade quite small. It is impractical, however, to attempt to entirely compensate for the lower voltages generated by the signal at the lower frequencies, by employing a verylow-losscore material and construction. As stated byJacob in United States Patent No. 2,153,622, a series resonant circuit may be arranged to produce constant gain by employing a ferromagnetic core which will maintain the ratio of inductive re.- actance to resistance in the circuit, that; is the circuit Q, substantially constant. In carrying out my invention I prefer to employ such a core, and to so arrange the coils 3 and Ithat the'variation in coupling produced by the movable. core 6 will substantially compensate for the lower voltages generated by the lower frequency signals. Such an arrangement is provided byrthe circuit constants for .a practical embodiment which I shall later give. It will be understood, however, that since the system provides two performance controls, namely, the resistance variation'and the coupling variation, both provided automatically by movement of the core, other elections may be made within the scope of the invention. For example, if a core is to be em? 'ployed which, with the chosen. coil, produces some performance variation other than that'cor responding to constant-circuit Q, then coil'l may be so chosen with respect to its winding andposition relatively to coil 3 and movingcore 6 as to producea compensatory variation of the coupling b'etween coils 3 andl. Additionally, and also by virtue of the two controls provided by the invention, any reasonable, desired relation of generated and delivered signal voltages may be secured,

As is well known, by suitable design of the winding 3'and of the'core 6, I may secure any d8:- sirable control of the quality factor Q of theloop circuit,'that is to say, I am able to regulatedts performance over the frequency range in any desired manner. 'However, 'the relations differ from those in a simple inductance-tuned circuit, and control of the operating characteristics therefore is to be achieved in a somewhat different manner, as I shall now set forth. Let

r l ml 2 v y=ratio of high-frequency to low-frequency resistance of the loop and also of the winding of the variable inductor. z=ratio of resistance of variable inductor winding to resistance of loop at any frequency. =ratio of resistance of variable inductor winding, with the core inserted so as to tune to any frequency lower than the highest frequency of the tuning range, to the resistance of the variable inductor winding without the core, both values being measured at said lower frequency, Neglecting the effect of distributed capacitance, it may be shown that,

(1) For constant resonant voltage across the circuit with constant signal strength, the circuit must have constant resistance, and

(2) For constant resonant gain in the circuit, Q=wL/R must be constant, and

(3) For constant selectivity, L/R must be constant, and

2 yp iz- 1 (5) The above relations do not take into account the fact that the distributed capacitance of the variable inductor varies as the core is inserted. This effect may be minimized in the design of the variable inductor, and may be compensated for in the development of the core for a particular design, the above equations serving as a basic guide.

In a successful embodiment of the invention the loop I comprised 17 turns of No. 28 plain enameled wire in a rectangular solenoid 5%" by 11%", the winding being long, and having an inductance of 155 ,uh. The variable inductor coil 3 comprised a progressive universal winding of single silk enameled litz wire, 1.24" long on a tube of 0.2" inside diameter, 0.007" wall, the inductance being 246 h.

Ferromagnetic element 6 may comprise a compressed comminuted core of hydrogen-reduced powdered iron that has been sifted through a screen having 400 meshes to the inch. For use with the above described inductor, it may be 0.200" in diameter and 1 long, being preferably hot-molded at 180 F. with 0.5% particle insulation and 3% of powdered Bakelite binder, and cured at 290 F. for 3 hours. Such a core will have an effective permeability of about 11.5.

Winding I comprised 2 /2 turns of No. 36 plain enameled wire wound to a length of on a tube of 0.325 outside diameter and mounted concentrically over the right-hand or grid end of coil 3, As indicated in the drawing, ferromagnetic core 6 was arranged to enter the left-hand or loop end of coil 3. It will be apparent that by other dispositions of winding 1, or by arranging to have core 6 enter the opposite end of coil 3, or both, the variation of the inductive feedback coupling may be altered.

In the illustrative embodiment just described,

resistor 9 was of 500 ohms and capacitor It! was of 0.1 microfarad. Capacitor 2 was adjusted to tune the circuit to the highest frequency in the range with core 6 fully removed from coil 3. Vacuum tube 5 was of thekind known to the trade as the type 1232. The frequency range of the system was from 540 kc. to 1500 kc.

It will be understood that the above constants are given merely by way of illustrative example and are therefore not to be taken as in any wa limiting the scope of my invention.

My improved signal-collecting system in-. cludes a closed series resonant circuit having a fixed or adjustable capacitor, an inductive portion exposed to the signals, which may be a loop, and an unexposedinductive winding. The system is tuned to resonance with any desired signal within a wide range of frequencies solely by variation of the inductance of the winding, produced by means of a ferromagnetic core movable relatively to the winding. My closed series resonant circuit is to be distinguished from open series resonant circuits in which part of the series capacitance is the capacitance of an open-ended antenna to ground. The exposed inductive portion of my closed series resonant circuit has capacitance to ground, but this capacitance is effectively in parallel to the exposed inductive portion of the circuit. Additionally, my improved signal-collecting system provides means for producing a delivered signal voltage which is substantially independent of the frequency of the desired signal, these means including a vacuum tube and an inductive feedback coupling which is automatically varied in a desired manner by the tuning means.

In the illustrative embodiment above described, the coupling between coil 3 and winding 1 was well below the value which, with the tube employed, would correspond to incipient self-oscillation of the system, at all frequencies within the range. Couplings which will not produce selfoscillation should invariably be employed for obvious reasons.

Having thus described my invention, what I claim is:

1. A signal-collecting system for use in radio receivers and the like having a first vacuum tube, including a closed series resonant circuit having a capacitor, an exposed inductive portion and an unexposed inductive winding connected to an input terminal of said vacuum tube, a coil connected in the output circuit of said vacuum tube and positioned adjacent one end of said winding, and a ferromagnetic element movably arranged to enter the other end of said winding to tune said circuit over a, range of frequencies and to simultaneously vary the coupling between said winding and said coil, whereby the signal voltage delivered to saidvacuum tube is substantially independent of the frequency of the signal.

'2. A signal-collecting system for use in radio receivers and the like having a first vacuum tube with control electrode, cathode and plate, including a closed series resonant circuit comprising a capacitor and an exposed inductive portion having a common ground connection and an unexposed inductive winding having a terminal connected to said control electrode, a coil connected between said cathode and ground and inductively coupled to said winding and positioned adjacent said terminal, and a ferromagnetic element movably arranged to enter the end of said winding remote from said terminal to tune said circuit over a range of frequencies and to simultaneously vary the coupling between said winding and said coil, whereby the signal voltage delivered to said vacuum tube is substantially independent of the frequency of the signal.

3. A signal-collecting system for use in radio receivers and the like including a closed series resonant circuit having a capacitor, an exposed inductive portion and an unexposed inductive winding, a ferromagnetic element movably arranged to enter one end of said winding to tune said circuit over a range of frequencies and adapted to maintain the reactance-to-resistance ratio of said winding substantially constant, and regenerative means including a coil positioned adjacent the other end of said winding, whereby its eifect is automatically regulated in approximate inverse proportion to the frequency of the signal by motion of said element.

4. A signal-collecting system for use in radio receivers and the like including a first vacuum tube having a control electrode, a cathode and a plate; a closed series resonant circuit comprising a capacit'ora'nd an exposed inductive portion having a common ground connection and an unexposed inductive winding having a terminal connected to said control electrode; a ferromagnetic element movably arranged to enter one end of said winding and adapted to maintain the reactance-to-resistance ratio of said circuit substantially constant while tuning said circuit over a range of frequencies; and a regenerative coil connected between said cathode and ground and positioned adjacent the other end of said winding, whereby its effect is automatically regulated in approximate inverse proportion to the frequency of the signal by motion of said element.

WILLIAM A. SCHAPER.