US1888278A - Electrical coupling circuit - Google Patents

Description

Nov. 22, 1932. w. A. MaCDoNALD ELECTRICAL COUPLING CIRCUIT Filed July 19, 1950 l l IILIIINQIIIIIIIAII Patented Nov. 2v2, 1932 l UNITED STATES PATENT OFFICEv WILLIAM A. MACDONALD, OF LITTLE NECK, NEW YORK, ASSIGNOR TO HAZELTINE CORPORATION, OF JERSEY CITY, NEW JERSEY, A. CORPORATION OF DELAWARE i ELECTRICAL COUPLING CIRCUIT Application filed July 19, 1930. Serial No. 469,040.

This invention pertains to high-frequency ent for the amplification to increase with freelectrical coupling circuits adapted to transquency. It has been found, however, that this.

fer energy throughout a range in frequency effect goes 'too farin the opposite,directioi1, 'te I from an exciting circuit to a tunable absorbi. e., the sensitivity"'actually decreassgwith l 5 ing circuit. The coupling circuit may be increase intuning frequency. lilyV utilized for connecting an antenna system to The coupling circuit of the present inven'- the input of a radio receiver or for intertion proposes to overcome the above noted connecting successive stages of a high-freobjections by utilizing a two-winding transquency thermionic amplier or radio receiver. former for coupling a pair of, thermionic A primary object of the invention is to tubes in cascade relation, for example, where- 6 0 provide a coupling circuit of the type specified in theprimary circuit is rendered inductivewhich in association with the elements conly reactive throughout the tuning range so nected thereby, will provide a sensitivity or that normally the sensitivity will increase amplification characteristic varying autowith frequency. To partially offset this matically with the frequency of tuning in a effect, a coupling condenser is'connected be- 65 manner which is under control of the detween the primary and secondary terminals signer. More specifically thelinvention is of like open-circuit polarity, as a result of intendedto provide an approach toward unwhich there will be provided between primary form sensitivity throughout the tunable freand secondary circuits a capacitive coupling quency range. A which opposes the magneticcoupling between 7o It is well known that the connection of an windings throughout the tuning range. antenna system to a rvadO receiver OI' the It will be obvious with this arrangement interconnection of successive stages of a highthat; whereas Jehe magnetic coupling between frequency thermionicamplier by means 0f circuits will remain substantially constant the usual high frequency transformers haV- throughout the frequency range, the vcapaci- 75 ing tunable secondaries, results in an overall tive coupling will increase steadily With the sensitivity characteristic which increases with tuning frequency, and will Jhns oppose the frequency throughOnt the tllnlng Tange effect of the magnetic coupling to a greater This condition is undesirable since it renders und greater degree as the upper frequency an amplifying 0r radio IeCeVng SYStem mOst limit is approached. Thus, whereas the ca- 80 Sensitive et the uPPel fefluency hmlt WheIIe pacitive coupling has very little effect upon it is 16B-St stable If the design is such es the amplification at the low frequency limit, t0 insure adequate stability et the PPPeI' fre" it operates strongly to reduce the sensitivity quencies, the device becomes inefficient at the otherwise present et the upper frequency lower frequenCeS 0f tllnlglimit and thereby provides an approach to- 85 Attempts have been mede t0 @effect ths ward uniform sensitivity throughout the tun- COHOU by pfevdmg 11? shunt lth the able range. Itis thus possible to design the Primary transformer Yndmg e seielent ce receiver for eicient amplification at the low- Peety t0 Tender the Plmefy en eult resonant er frequencies consistent with stable operaat a frequency below the tuning range, S0 tion et the upper frequencies. 90 thatv it is capacitively reactive and hence In Order to provide a maximum opposing possesses an impedance characteristic deen'eet between the capacitive and magnetic creasing with increase in frequency throughcouplings as the upper frequenoy linut .is out the tunable range. This decrease in priapproached, it is desirable to arrange. 1n

mary circuit impedance coupled with the shunt with the primary transformer winding 95 automatic decrease in effective coupling bea sufficient oepeeity to render the primary tween primary and secondary circuits, due to circuit resonant at a frequency slightly .above the shunting' effect of the capacity, as the the tuning range. To this'end the primary tuning frequency is increased, will operate coil may be so wound that the various capacito offset the natural tendency otherwise presties naturally associated therewith, such as 100 the distributed capacity between turns, the coupling capacity betweeA primary and secondary circuits, and the capa "ity ot an antenna circuit or the anode-to-gi-ound capacity of a thermionic tube ccnnectcd thereto, will render the primary' circuit resonant at the desired frequency.

Referring now to the drm/'ing5. Figure l shows in diagrammatic forni radio receiver employing the coupiing circuits ot the present invention for connecting the antenna circuit to the receiver input and for interconnecting successive stages thereof.

Figure 2 shows a seri-es of graphs indicating the manner in which the sensitivity varies with the frequency ot tuning` for various coupling circuits in accordance with the present invention.

Fig. 3 shows a coupling circuit in accordance with the present invention interposed between thermionic tubes ci the three electrode type wherein the ceuplin'g transformer is provided with a neutralizing wind.- ing and associated condenser tor neutralizing the grid-cathode tube capacity.

In the circuit of Figure i, an antenna l and ground 2 are connected to opposite terminals of a variable resistance 3. the voltage drop across which is impressed through a coupling circuit A1 upon 'the input section comprising grid and cathode of a thermionic.

tube V1 of the screen grid type operating as a stage of radio-frequency amplification. In the case of the antenna coupiine; system, the primary circuit being resonant above the tunable frequency range will be capacitively reactive throughout the range. The output sec'- tion comprising anode and cathode of tube V1 is connected through a coupling network A2 to the input section of a. second tube V2 also operating as a stage of radio-frequency amplitication. The output of tube V2 is connected through the usual detector and audiofrequency amplifier D to a loud speaker L. The proper operating potentials for tubes V1 and V2 are supplied over conductors 4 while condensers f serve to bv-pass the radiofrequency currents around the voltage supply circuit. The iilaments of the tubes may be heated by any of the well-known methods (not shown).

Each coupling` circuit A includes a transformer T having separate magneticallyT coupled primary and secondary windings L1 and L2. In shunt with the secondary winding is a variable condenser C2 which serves to tune the circuit over a range in frequency, specifically the broadcast band. The condensers Co are mechanically coupled for simultaneous adjustment by means of the unicontrol device U.

The primary and secondary windings are in each instance arranged to have their ungrounded terminals of like open-circuit polarity so that by connection of a coupling capacity C therebetween, a capacitive coupling will exist between primary and secondary circuits which opposes the effect of the magnetic coupling therebetween for the condition that the primary circuit is inductively reactive throughout the tunable 'frequency range, (except when the coupling system is a part of. the antenna circuit as previously explained) the coupling capacity C beingr sufiiciently small that the current flowing' therethrough is in phase opposition to vthe current of the primary circuit.

As the tuning condenser C2 is adj usted for higher frequencies, the magnetic coupling M remains substantially constant, whereas the opposing capacitive coupling1 due to C steadily increases. The combination of these two factors causes the resultant etiective coupling between primary and secondary circuits automatically to decrease with increase in tuning frequency and thereby to oiiset in part the normal tendency for the sensitivity to increase with frequency.,

In order to render the effect of the opposed capacitive and magnetic couplings more pronounced as the upper requencylimit is approached, the effective inductance of the primary winding L1 may be tuned by means of the capacities associated therewith to a frequency slightly above Athc upper Jtrequency limit of tuning. The effective inductance ot the primary winding is the magnitude thereof obtained with the secondary winding shortcircuited and is, in tact, equal to (l-KZ) Ll, where L1 is the inductance ot the primary with the secondary on open circuit, and K is the coefficient of magnetic coupling between circuits.

In the case of the couplingr circuit A1 the capacities operating to render the primary circuit resonant at a frequency above the tuning range are (l) the capacity to ground of the antenna system proper, (2) the distributed capacity between turns of the primary winding L1, and (3) the capacity of the coupling condenser C to ground through the tuning and by-passing condensers C2 and f, respectively, which latter are so large in comparison with C that their effects may be neglected, and (4) any stray capacitive effects existing between the terminals of the primary Windinv.

In Figure 1 all of the capacities existing across the primary winding L1 with the eX- ception of the coupling capacity C are indicated by the lumped capacity C1. The total capacity thus associated with the primary winding should be suiiicient to tune the primary circuit slightly above the tuning range, and if the capacities naturally associated with the primary winding are not suiiicient to this end, a supplemental condenser may, of course, be connected thereacross.

In the interstage coupling circuitlA2 the primary circuit is tuned to resonance above turns and the anode-to-ground capacity of e tube V1 connected thereto, theseff-latter two capacities together with any .stray capacitive'eifects being indicated by the lumped capacity C1. l

The coupling capacities CI physical condenser units or the primaryand 'secondary transformer windingslmay be so wound and so positioned thatihefnatural capacities existing between win ings--willproduce the desired results.

Figure 2 shows a set of curves'obtained eX- perimentally depicting the manner'inwhichv` ythe coupling capacity Cvcontolsthe.ampli- -cation per stage as the -frequen o adjusted throughoutthe range The f lcurve C=0 representsthe conditi l coupling capacity and shows clearly tlfxeman-` ner in which theesensitivity/nrrnally-'inwai@ and 1/ in the order named, 'showthe' esults creases with frequency. ThecurvsC obtained by utilizing coupling capa ities 'of successively increasing magnitude: From these curves it is apparent thatfth "coupling capacity has substantially noeifectfonfthe sensitivity at the low-frequenc'yfend.gf'ffthe scale, but reduces -it below its normalfvalue by a steadily increasing amount as the. fre. quency of tuning is adjusted toward the upper frequency limit. Within limits it is also apparent that the greater the coupling capacity, the more nearly a condition of uniform sensitivity is approached.

Other factors remaining constant, the coupling capacity cannot be increased in magnitude indefinitely with attendent improveyments as regards uniform amplification. As

pointed out above, the capacity C is one of the factors determining the resonance frequency of the primarycircuit which must always be maintained of the order of the upper frequency limit, since otherwise the ampliication will rise to a maximum value within the tuning range `and then drop off-sharply `thermionic tubes V1 and V2 of the threeelement type are coupled in cascade relation by means of a coupling circuit A3 the ampli- Vfication'cr gaincharacteristics of which are similar to that of circuit A1 and A2. In order,

however, to neutralize the grid-anode capacity of tube V1 transformer T3 in addition to the primary and secondary windings L1 and L2, is provided with a neutralizingwinding LN magnetically coupled thereto. The winding LN is connected from the low potential terminal of winding L1 through a neutralizfr ing capacity `CN to the grid electrode of tube f tuning' V. "iT-liel neutralization isy accomplished in the'A well-known manner described in Patent No. 1,533,858 to L. A. Haifeltine.

What isfjclaimed is:

1. A high-frequency electrical coupling system. tunable over a range in frequency comprising separate primary and secondary circuits, said circuits being coupled magnetimeans of primary and secondary idling 'throughout "said frendsaid primaryf transformer nv the operation of said sysimary lcircuit resonant at a he hihest frequency of said ...Af l- 1 quencyelectrical coupling ,t'emtunabiep' r aran'ge in frequency comprisingjsparate"primary and secondary cireuits,.`sa circiijitsbeing coupled magnetically byvm'eiinsoftransformer windings individual theretgfand coupled capacitively by Ymeans offa' condenser connecting a terminal of eacl1,winding,'a conductive path extending between two "remaining terminals of said windings, said capacitive coupling being arranged to oppose said magnetic coupling throughout said tunable range, and said primary winding in the operation of said circuit being arranged to have capacity associated therewith rendering said primary circuit resonant at a frequency above the highest frequency of said tunable range.

3. A high-frequency electrical coupling system tunable over a range in frequency comprising separate primary and secondary circuits, said circuits being coupled magnetically by means of primary and secondary transformer windings individual thereto, respectively. and coupled capacitively by means of a condenser connected between winding terminals of like open-circuit polarity. said primaryA winding in operation of said system having capacity associated therewith renderquency slightly above the highest frequency of said tunable range, whereby the voltage amplification of said system remains substantially uniform as the tuning frequency is adjusted throughout said range.

4. A hi h-frequency electrical coupling system for interconnecting elements of a er windings individual thereto, re-` `coupled capacitively by means' ectin'gri said circuits, said' A being arranged tooppose.

pacity associa-ted therewith.

lng said primary circuit resonant at'a frese'v radio receiving system comprising a transformer having separate magnetically coupled primary and secondary windings, a variable condenser bridging said secondary winding for tuning the system over a range in frequency. a capacity7 connecting terminals of said windings of like open-circuit polarity, and a conductive path extending between two remaining terminals\thereof, said primary Winding in the operation oit said device havinjr capacity associated therewith render ing the primary circuit resonant a frequency slightly above 'the highest frequency of the tuning range whereby said capacitive coupling opposes #said magnetic coupling between 'windings and the aniplircation produced by said system approaches uniformity throughout said tunable range.

5. In combinatioin an electrical coupling circuit interconnecting an antenna circuit with the input section et a therinonic tube in a ra dio receiver, said coupling system comprising separate magnetically coupled primary and secondary transformer windings connected to said antennaand said tube respectively, a variable condenser bridging said secondary Winding for tuning said circuits over a range in frequency, a. capacitive coupling extending between said windings arranged to oppose said magnetic coupling throughout said tunable range, and capacity including the capacity of said antenna and oiz said capacitive coupling associated with said primary circuit rendering the prirnary circuit resonant at a frequency above the highest frequency of said frequency range.

6. In conibinatioin radio-frequency coupling circuit interposed. between elements of a therniionic tube radio-frequency amplifier, said circuit comprising separate 1nagnetically coupled primary and secondary transformer windings connected to said eienients respectively, a. variable condenser associated with said secondary for tuning said circuit over range in frequency7 a capacitive couu pling extending between said windings arranged to oppose .said magnetic coupling throughout said requency range7 and capacity associated with said primary Winding rendering the primary circuit resonant at a frequency of the order of the upper limit of said tunable range.

7. In combination, a pair of thermionic tubes, each including cathode and grid constituting an input section, and an anode and said cathode constituting an output section, a high-frequency electrical coupling circuit connecting the output section of one said tube to the input section of the other, said circuit comprising separate magnetically coupled primary and secondary windings connected to said sections, respectively, a Yvariable condenser bridging said secondary winding for tuning said circuit over a range in frequency, a. capacitive coupling extending between said windings arranged to 0ppose said magnetic coupling throughout said frequency range, the eective inductance of the primary Winding being such, in conjunction with the capacity associated therewith which includes the anode-to-cathode capacity of the tube connected thereto and said coupling capacity, as to render the primary circuit resonant at ire ency of the order of will@ f reef my signature. a. linoJNl-LQ 'ilO

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