US1757333A - Frequency trap for radio signaling apparatus - Google Patents

Description

y 6, 1930. R; H. RANGER 1,757,333

FREQUENCY TRAP FOR RADIO SIGNALING APPARATUS Fi led Aug. 51. 1922 2' Sheets-Sheet 1 RIC RD 0mm RANGER 351 his momma May 6, 1939. R. H, RANGER I FREQUENCY TRAP FOR RADIO SIGNALING APPARATUS 2 Sheets-Sheet 2 Filed Aug. 31. 1922 Ware Len 77% Fig. 5,

gvwento'c ICHARD HOWLAND RANGER 351 his 111M0 7 flaw Patented May 6, 1930 w gn stares RIcHARn'nowLa-Nn RANGER, or BROQKLYNQNEW YORK, ASSIGNOE To nanro' con e rie-E Formation or AMERICA, A connonnmron or nE'LAwacan FREQUENCY TRAP FOR RADIO SIGNALIITG APPARATUS it Application filed August 31, 1922. Serial No. 585,352.

Fig. 3 is a diagramof the equivalent cir- 5 cult of my coupling coil and condenser,

Fig. l is a graphicillustration of the detector current curve for the arrangement shown in Fig. 1' as compared with arrangements previously known in the art, wherein the curve for the previously known arrangements is shownas a dotted line, and

Fig. his a modification of'iny invent-ion applied to an audio frequency circuit.

Various arrangements knownas frequency trapsor rejectors have been proposed inthe past for reducing interference in'the reception of radiosignals; in particular, it has been suggested to make use of the phenomenon of parallel resonance by placing a loop resonant circuit in shunt across radio fre quency circuits thus providing a by-pass to short out all but the desired frequency. This by pass is requiredito he ofinfinite impedance to currents of the desired frequency, andof low impedance to currents of other frequencies; therefore it is usually necessary to use a low inductance Withvery low resistance and a large capacity.

y This arrangement and othersimilar ones have not proved successful in practice, because ofthe fact that itis difficult and expensive to construct capacities suitable for the purpose in a'compact, portable-andeasily adentire receiving systems is disturbed, making retuningnecessary, and even at times making it necessary to carry on all the tuning simultaneously.

and also because-of the fact the tuning of the This is due to the fact that the impedance of the tuned circuit is of substantially the same order of magnitude as that of the difcuit in which it isplaced, and achang inimpedance of the tuned circuit makes acornparable change in the impedance of the entirecir'cuit. Furthermore, since the capach ties used are necessarily very large it is difficult to tune'properly in a short time over any Wide range.

In the previously known systems oscillations received upon the antenna were im'-' pressed on the grid circuit of anaimplifier, and the output circuit of this am lifier was coup'led hy means of a coupling coil to the condenser shunted across the secendar'y of the coupling coil connecting the amplifier and detector; This arrangement has Iibt roved satisfactory for reasons that will be explained; y

[is-pointed out above, in order to obtain amaximum reduction of interference, it' is necessary to provide alarge capacityinthe" tuning circuit, but on the otherhand, in order to obtain a maximum ower diitpiihit is necessary that the impeolanceof the giidrii -cui't be matched with that-of the tuning circnit and-thisrequires that'th'e capacity" ofthe tuningcircuit should he 10w: These two re quirenients contradictory and as usual? in" suchcases,"a comprdrniseis made hetween the two: Thisis donehyiusing smaller'capaci ty int'he tuned circuit thari'in the fi'rst a e, andutilizing the'ca pacity of thetiib'e; A's is Well knowng uie tuhe capacity is not purely reactive, but has seme' leakage; we further more, the capacity varies With each tithe usedi As a result of attemptin to Inset we warm:

cli'c'toryrequirements; the results a r'e not cs pecially' satisfactoryfrom either pci'ritof grid circuit of a detector having a variable View, and "the arrangement presents othe'i d'e fects; oneofthe most prominent of which is an" increased tendency of the tube to break into' oscillation, and another is avariation of the tuning with a change hitches; The objects of this invention are to pro vide'arejeetoreircuit wliich will'fibt ihci' isc" the' tendency ofthe' tuhe'to "oscillate; to 'provide al rejecteno'fa law resistance giving low damping values, and to provide a circuit giving a sharp tuning characteristic and greater selectivity. Other objects are to provide interference reducing means which may be easily and cheaply constructed in a compact and readily portable form, to provide an arrangement by which the rejector circuit may be tuned without disturbing the tuning of the set, and to be very sharply tuned, without in any manner disturbing the tuning of the remaining apparatus in the set, and have further found that this circuit presents great advantages over all other known arrangements from the standpoint of efficiency of operation and ease and cheapness of construction. This arrangement of coupling coil andcondenser I term a frequency trap. I

If the proper degree of coupling between the primary and secondary of the coupling coil, and the proper ratio of turns be chosen it is possible to use considerably smaller values of capacity, making it possible to construct a very compact arrangement, which shall at the same time have a large effective value of capacity and a low resistance and inductance referred to the primary circuit,and be capable ofbeing sharply tuned overa wide range very quickly- 40 Referring to Fig. l, signals are received by means of the antenna 1, which is provided with the usual tuning means, comprising a variable condenser 2, and inductance 3. The incoming oscillations received on the antenna are directly impressed upon the grid and filament' circuit of the thermionic tube amplifier 4. In the output circuit of this tube, I place the primary 5,, of a coupling coil 5, having a variable. condenser 6 across the secondary 5 This constitutes the trap circuit. Connected across the terminals of the primary of this coupling coil, I provide a radio frequency transformer 77. The secondary circuit of this transformer is connected to a detector 8 in the usual manner. The current fromthe de tector may be indicated in the telephone receiverslO or may be led to any other suitable indicating means, or to an amplifier.

I provide the primary and secondary of the coupling coil with turns preferably in the ratio of four turns in the secondary to each turn in the primary, and I also prefer to construct this coupling coil in the manner shown in Fig. 2, that is, with the secondary 5, surrounding the primary 5,, for the reasons to be '9 for the explained later. I may also provide a switch purpose of disconnecting the. trap circuit.

For a better understanding of my invention it is permissible to consider my coupling coil and-condenser as replaced by its equivalent circuit as shown in Fig. 3.

There the primary winding is considered as divided into two parts, L S, which sets up the primary leakage flux, and L M which sets up the-mutual flux. In a similar manner, the secondary circuit may be resolved into its components L 8 and L M, the former representing the part of the secondary winding which establishes the secondary leakage flux and the latter, thatwhich is linked with L M. For the sake-of simplicity, the primary and secondary resistances are not shown.

Sincethe secondary of coil 5 completely enclosesrthe' primary, the primary leakage flux will be low, and I have therefore represented L S by a single loop, indicating that this value is as small as possible. The primary andsecondary are constructed to'have a low value of mutual inductance, but the secondary has a comparatively high leakage, which is shown diagrammatically byv the comparativelylarge number of turns in L S.

For the best reception, it is desired that four conditions shall obtain in the trap circuit; first, low effective resistance, providing low damping; second, low effective inductance giving a low impedance to signals of frequency lower than those desired; third,

large effective capacity, giving low impedance to signals of higher than desired 'frequency, and fourth, infinite impedance to signals of desired frequency. I

It will be seen that it is possibleto construct a circuit according to Fig. 1 having low effective primary resistance, since the effective resistance of the trap as a whole is equal to R where R is the primary resistance, R the secondary resistance and n the ratio of transformation given bye i 2 where I and I are respectively, the currents in the primary and secondary.

The second condition, low effective inductance, is obtained by using a low value of mutual inductance and providing against leakage in the primary.

The third-condition, large effective capacity, is made possible by the use of the condenser in the secondary circuit, since the resultant capacity of the secondary referred to the primary is given by the capacity of the secondary multiplied by the square of the transformation ratio, or stated differently, the resultantcapacitive impedance of the primary is equal to the capacitive impedance of the secondary dividedby the square of transformation ratio, assuming practically no effective capacity in the primary itself.

This is particularly important, because it makes possible the use of a standard air condenser of a capacity of .005 microfarads, giving in the primary an effective capacity of the order of .08 microfarads for a transformation ratio of four.

The fourth condition is of course obtained by tuning the trap so that for the frequency desired wL Where L and C respectiveas follows: If no interfering signals are expected, switch 9 may be left open and the apparatus tuned in the usual manner to obtain the best tone in the telephones. If, however. interference appears, all that is necessary is to close switch 9, and tune the rej ector circuit by means of condenser 6 until the interfering effects are eliminated. It will be seen that changing the tuning of the trap will not affect the tuning of the set, because the plate circuit in which the trap is located is of great impedance relative to that of the trap. If desired the impedance of the plate circuit may be increased by the insertion of an impedance in the plate circuit as shown in dotted lines in Fig. 3.

As will be seen from Fig. 4-, by using my trap circuit, it is possible to obtain a much better elimination of interfering signals, especially in a range rather close to the frequency of the desired circuit. In connection with this figure, it should be noted that the detector current curve for arrangements in accordance with my invention is illustrated by a solid line and compared with the detector current curves of arrangements previously known in the art, as hereinbefore described, which are shown by the dotted line curve.

This trap may be applied equally well to audio frequency circuits, as shown in Fig. 5. Here the incoming oscillations are impressed across the filament and grid of a detector tube 8. Connected in the plate circuit of the detector-tube is a transformer 15 which may have a core of magnetic material. As in the first case, the secondary winding 15, is closed through a variable condenser 6. I use a higher ratio of turns for the trap than for the radio frequency trap, preferably about 1:8.

Across the primary of this transformer 1 connect an audio frequency transformer 17. whose secondary is connected to a vacuum tube amplifier (not shown) to which in turn the telephones may be connected.

In accordance with the foregoing it will be seen that I have provided a simple and effective means for reducing interference which may be easily and cheaply constructed in a compact form, and one which will at the same time have low effective resistance, and high of the set, and by which a large range may be quickly covered.

It is apparent that while I haveshown radio receiving-apparatus, my invention may be applied to receiving apparatus for carrier curi"entitelepliony, orso called wired wireless, and other modifications and changes which williabe apparent to those skilled in the art may be made without departing from the scope of myinvention.

Having described my invention, what I claim is:

1. In a radio signaling system, an antenna circuit, means for tuning said circuit to predetermined signaling frequencies, means for detecting and receiving said predetermined signaling frequencies, a trap circuit including a transformer having a variable capacity element connected in shunt with the secondary winding thereof for eliminating interference effects in said detecting and receiving means, and a substantially non-reactive coupling connecting said trap circuit and said antenna system.

2. A signaling system comprising in combination a signaling circuit, means for tuning the circuit to predetermined signaling frequencies, transformer means having a transformation ratio greater than unityand having a variable capacity element connected in shunt with the secondary winding thereof connected with said signaling circuit for confining predetermined frequencies thereto, and a coupling connecting said receiving circuit with said signal confining circuit for preventing reflections from said last named circuit upon said receiving circuit.

3. In a signaling system, a receiving circuit, means for tuning said circuit to predetermined desired signaling frequencies, a second circuit including a transformer having a transformation ratio greater than unity connected with said receiving circuit, a ca pacity element shunting the secondary winding of said transformer for providing together with said transformer secondary a trap circuit for interference frequencies, and a non-reactive coupling connecting said tra circuit with said signaling circuit for avoi ing reflections of undesired received frequencies as absorbed by said trap circuit from affecting said signaling circuit.

i. A signaling system including a signaling circuit, inductive and capacitive means for tuning said circuit to predetermined desired frequencies, a second circuit electrically connected with said signaling'circuit including an inductance of small value compared with said tuning inductance and normally transparent to a broad band of frequencies, a tuned circuit including a relatively small capacity and a relatively large inductance coupledwith said last named inductance for rendering said second named circuit opaque to signaling frequencies but transparent to all other frequencies for confining the predetermined desired frequencies to the signalingcircuit and Icy-passing all undesired frequencies through said second named circuit and a non-reactive coupling connecting said signaling circuit and said second named circuit for preventing reflections from said sec- 0nd named circuit from affecting the tuning of-said signaling circuit.

RICHARD HOWLAND RANGER.

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