US 2443195 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
June 15, 1948. D, PENSYL 2,443,195
ELECTRONIC CIRCUIT Filed Oct. 15, 1943 ll8 119 1 I09 3 3 In INVENTOR D. S. PENSYL BY AM.
ATTORNEY Patented June 15 1948 ELECTRONIC CIRCUIT Daniel S. Pensyl, Garden City, N. Y., assignor to lhe Sperry Corporation, a corporation of Delaware Application October 15, 1943, Serial No. 506,361
'My invention relates generally to electronic circuits, and, more particularly, to electronic switching circuits.
' As an object of my invention I wish to provide a gating-circuit which will pass a desired signal only, with no part of the gating voltage appear ing as a pedestal in the circuits output.
It is another object of my invention to provide aigating. circuit in which the desired signal is reproduced in the output thereof with high gain and without compression of the signal in its low amplitude range.
Another object of my invention is to provide a gating circuit in which the gating voltage is applied to a balancing network interposed between the signal receiving elements thereof and the output terminals in such-manner that no component of the gating voltage appears in the output.
As another object of my invention, I wish to provide a method and apparatus for gating a signal which limits the action of the gating voltage employed substantiall to that of initiating electron flow.
Still another object of my invention is to provide an electronic circuit comprising a signal channel and a gating voltage channel, both of which are interconnected in such a manner that a double loop circuitis formed, one loop of which provides a bucking voltage equal in magnitude to the fraction of the gating voltage applied to the circuits switching means, so that the net gating voltage appearing in the output of the circuit is equal to zero.
It is a further object of my invention to provide agating circuit in which the signal input and output is unbalanced to ground, and in which the signal is gated with a gating voltage through a balanced switching network in such a manner that the value of the gating voltage in the output of the circuit is equal to zero.
Another purpose of my invention is to provide an electronic switching circuit which may be used as a rejection gating circuit.
It is also a-purpose of my invention to provide a switching circuit responsive to a gating voltage which. in addition to providing a voltage for operating the switching elements of the circuit, will also supply a biasing voltage thereto.
As another object of my invention, I wish to provide a gating circuit having ungated signal receiving terminals, gated signal output terminals .and a switching circuit interposed between the input terminals and output terminals which isresponsive to a balanced gating voltage applied 2. in such manner that it does not appear in the output of the circuit.
Other objects of my invention will become apparent as the description proceeds.
In carrying out my invention in a preferred embodiment thereof, I provide a circuit upon Whose input terminals are impressed ungated signals, among which is the desired signal to be passed. From this point they are delivered to a balanced bridge circuit whose four arms are made up of unilateral current-conducting elements, such as diodes, so as to form a switching circuit. Across one pair of terminals of the bridging circuit is impressed the gating voltage which is received from a pulse transformer in synchronization with the signal which is to be passed by the gating circuit. 0n the other terminals of the switching circuit are respectivel connected the aforementioned signal receiving means and the gated signal output means. The gating voltage appearing across the bridge circuit renders the normally non-conducting unilateral current conducting elements conducting, thereby permitting the passage of the desired signal across the bridge circuit. The switching means being arranged as a balanced bridge balances the gating voltage to ground, and no part of the same appears in the signal output of the gating circuit.
In a modification of my invention I provide a similar circuit which employs a plurality of impedance matching devices between which is interposed a single unilateral current conducting device connected to operate as the switching portion of the circuit, A gating voltage is impressed across this device to make it conducting during the period of reception of the desired signal. The gating voltage circuit is so arranged that two loops are formed, one of which supplies a bucking voltage equal to the gating voltage. This limits the'action of the gating voltage to that of firing the switching tube and thus prevents it from appearing as any part of thesignal which is passed by the gating circuit.
In a third embodiment of my invention I arrange a circuit which, unlike the previous two modifications mentioned, balances the signal to ground. This first function is accomplished by passing the ungated signals through an impedance matching device to the primary winding of a pulse transformer whose secondary is connected to balance the signals to ground. From the secondary winding 'of the transformer the signals are fed to a plurality of unilateral current conducting elements which are connected for balanced operation and which are responsive to the gating voltage that renders them conducting for the period of reception of the desired signal. As in the previous two mentioned modifications, the switching elements are so connected that no part of the gating voltage appears in the ouput of the circuit.
A more comprehensive understanding of my invention will be afforded from the following de-. tailed description when taken together with the accompanying drawing in which like reference numerals have been used throughout to designate like parts, and in which Figs. 1A and 1B are representative characteristic curves of an electronic valve showing its use as an electronic gate.
Fig. 2 is a schematic circuit diagram of a preferred embodiment of my invention.
Fig. 3 is a schematic circuit diagram showingmeans for self-biasing the switching elements of the circuit shown in Fig. 2.
Figs. 4 and 5 are circuit diagrams of modified forms of my invention, and
Fig. 6 is a graphicshowing of acomposilte wave form which may be applied conveniently to any of the circuits shown in Figs, '2, 4 or 5 to provide negative bias for the switchin elements thereof.
In various applications of ultra high frequency reception, such as'that of radio detection and ranging, it is desirable to condition a receiver for passing selected signals only from a group of reflected signals. The circuit which performs this function is referred 'to in the art as a 'fgating circuit, or gate, and is responsive to a voltageof a particular wave form which maybe a termed as a ".ga'ting voltage, also referred to as a gate. For :the purpose of this description, however, the term gatewill'be appliedto the circuit which permits the passage of the desired signal, and the term gating voltage will its receiver circuit. Thus, a switching circuit P must be provided which will turn on to pass signal 12' and turn off to prevent the passage of all other signals such as H and .13. This circuit, as stated before, is called a gate since it opens for signal I2 to pass through and closes to prevent the passage of undesired signals-Il and I3.
In the prior art, it has been common practice to use a single electronic valve, with a plate .load resistor, as the gate and to impress upon the control gridthereof a gating voltage which willerase a fixed grid bias, make the valve conducting, and permit a pulse similar to the selected signal to go through. For example, such .an electronic .device may have a characteristic curve similar to that illustrated in .lA. Here the signals II, and i3 are received 'on the :control igrid thereof together with the gating voltage [5 which is superimposed. The control grid of the electronic valve is biased beyond cut-off by the fixed bias I6 applied thereto and, as illustrated, the gating voltage pulse [.5 is synchronized to arrive with selected signal [2. This, in efiect, "elevates signal 2 to a higher level, and, as :signal I4 it swings .the grid of the tube pflsitively to produce a pulse from positive source which is passed through the gate to appear in the output as M, the gating voltage pulse l5 being sufficient to erase a part of the bias voltage l6 and render the tube conducting.
It will be noted from the figure that such operation forces the gate to pass the signal l4 while operating on the curved portion of its characteristic ClllVefWhiCh reproduces the-desired signal in the output circuit with very little, if any, gain, and also forces the signal to be compressed in the range of its low amplitude. In other words linear operation is not obtained.
By changing the value of the fixed bias [6 it 'is possible toovercome this difiiculty and move the operating point of the valve to a point on its characteristic curve wherein the signal M will be reproduced as signal M with high gain, Fig. IE. I
It will be noted, however, that any bias supplied to the control grid of the electronic waive which will force the same to operate-on the linear portion of its characteristic curve will, by necessity, cause a reproduction ofa :portion of the gating voltage 1-5 which Will appear inthe output voltage as apedestal l6.
Where it is desired to pass the selected signal having a linear relationship between input and output, with relatively high gain, and wtihout the appearance of pedestals in the circuit :output due to the gating voltage, other means must be employed for gating purposes.
These requirements are metby the circuit amustrated in Fig. 2 which is a preferredembodiment of my invention and in which a' balance'd switching means 31 is employed to pass signals selected from the group of ungated signals received on input terminal 11. Under the influence of a gating voltage which is received on the primary winding of pulse transformer 39, the switching circuit. is rendered conducting topass the desired signals to the impedance matching device 41.
Element 22, illustrated as a triode, is employed for the receiving impedance matching device and receives ungated signals from the input terminal IT on its grid 2| through the coupling elements comprising the capacitor t8 and the resistor Il9. Positive voltage is received on plate 23 of this element, and the cathode '24 thereof is connected to ground through the cathode resistor .25, I Element '22 operates as a cathode 'followerand de-v livers the signals received on its grid without inversion to input terminal 29 of the balanced bridge circuit 531, through blocking capacitor 2 and resistor 28, thelatter providing a direct cur-- rent return path for the bridge. Balanced switching means 3|, which comprises unilateral current conducting elements 32, 33, 34 and 35pis connected to receive a gating voltage on its terminals 36 and 31, from pulse transformer 39. Connected in parallel with thesecondary'winding 42 of transformer 39 is abalancing poten-- tiometer 43 having a grounded tap 44. This po tentiometer improves the operation of the circuit by balancing out capacitances to ground which may be present when transformer 39 is operated over a wide range of "frequencies. To
further assist in .balancingbridge '3 l, a balancing condenser '30 is coupled between terminal 31 and ground. I
In order to prevent the gating voltage transformer 39 from shunting some of the signals received on the input'terminals |1 of the "circuit and amplified in element 22, series resistors 45 and 46 are connected in the gating voltage leads to gating circuit 3|.
The output of balanced switching circuit 3| is taken from terminal 38 and fed to an impedance matching circuit including electron discharge device 41 through the coupling elements 48 and 52 to-grid 5| thereof. Positive energy is supplied plate 53, and cathode '54 is connected to ground through cathode load resistor 55. Resistor 43, similar'to resistor 28, also performs the function of a low level impedance device at the output of bridge '3 I. The output of the circuit is taken from the cathode 54 at junction point 56 and delivered to the output terminal 51.
Discrimination of the bridge circuit 3| against undesired pulses such as H and IS in Fig. 1 depends onplate-to-cathode capacities of the unilateral current conducting elements 32, 33, 34 and 35 and also on the bias potential of these elementswhen no gating voltage is supplied through transformer 39. Accordingly, for the purposes of discrimination control, means for introducing self-bias to these elements is provided by shunt resistance-capacitance portions 58, 59 and 6|, 62 in the circuit illustrated in Fig. 3, which may be substituted for the circuit portion of Fig. 2 shown above the line a--a. In this alternative circuit the gating voltage is supplied to the bridge 3| in identically the same manner as explained in connection with the circuit illustrated in Fig. 2 with the exception that the R.-C. combinations comprising the capacitor 58 and the resistor 59, and the resistor 6| and the capacitor 62, are interposed respectively in the leads serving terminals 3! and 36. respectively, of the bridge circuit 3|.
In operation, the unilateral current conducting elements 32, 33, 34, and 35, comprising the four arms of the gate 3|, are normally cut off. Under the influence of a gating voltage which is received on the terminals of primary winding 4| of transformer 39 and is of such polarity that terminal 36 of the bridge 3| is made positive with respect to terminal 31 thereof, all four of these elements are rendered conducting and thus .they become effective as resistances for the duration of the gating pulse. During the time that the elements 32, 33, 34 and 35 are conductive. the point 38 of the balanced bridge assumes the same potential as that of point 29, and voltage variations with respect to ground at point 29' are reproduced at point 38. Further, since the gating voltage is balanced to ground, no portion thereof will be passed from the switching circuit 3|. to appear as a part of the gated signal on the output terminal 51.
In the brief interval during which junction 36 is maintained positive with respect to junction point 31, by the gating voltage supplied through transformer 39, unidirectional current flow is maintained through the two paths including series diodes 32 nd 34 and series diodes 33 and 35. During this interval, a small potential change, positive or negative. at point 29 results in potential'changes in the same direction at points 35 and 31, and thus in a potential change in the same direction at the bridge output point 38. Thusas suggested in the above paragraph, the elements 32, 33, 34 and '35 are not employed in the. circuit as unidirectionally conductive devices. but instead are so biased as to be non-conductive as point 36 is made slightly negative with respect to point 31, or effectively bi-directionally conductive as point 36 is made greatly p sitive with respect to point 31, in accordance with the gating voltage variations supplied through trans former 39.
In this arrangement, the impedance matching electronic elements 22 and 41 may be useful for operation as a video gating circuit. The former element performs the function of reducing the effects of stray capacitances and distributed constants of the source and minimizes their effectiveness by operating at a low impedance level. The cathode follower impedance transformer 22 may therefore be omitted from the circuit when the impedance requirements looking into the input terminals of the circuit are of such nature that its use is unwarranted. Similarly, the oathode follower 41 may be omitted from the circuit, if the application is such that a high-impedance load is presented to the output of the bridge 3|.
Just as the circuit may be operated to gate a selected signal, it may also be operated as a rejection gate to suppress a selected signal or group of signals. To operate the circuit as a rejection gate simply requires a reversal of the polarity of the gating voltage applied by transformer 39 to the terminals 36 and 31.
In the modification of my invention illustrated in Fig. 4, I employ a cathode follower illustrated as 22, similar to the input cathode follower in the circuit shown in Fig. 2. The output of this element is taken from the point 26 and delivered to the cathode $4 of a unilateral current conducting element 63 which is illustrated as a diode. From the plate 55 of this element a signal received from point 26 is passed to the cathode coupled stage 14, illustrated as a triode, which has its grid I6 grounded as shown. Positive voltage is supplied to plate T5, and the output of the circuit is taken from cathode 13 thereof at point 12. A gating voltage which is supplied to the primary winding ll of transformer 69 induces in secondary winding 68 a voltage having the polarity shown. This voltage is impressed across the switching element 63 through the resistors 36 and 61. A balancing resistor 10 is connected to ground from point 69 as shown. In this manner the gating voltage induced in secondary winding 58 of transformer 63 is conducted through two loops, one of which is traceable through secondary winding 68, thence through resistor 61 and the non-linear switching element 63, thence through resistor 66 and back to the secondary winding. The second loop is traceable through the secondary winding 68 of transformer 63, across the resistor 10 to ground, thence from ground through a parallel combination of the cathode resistor 25 and the plate-to-cathode resistance of tube 22, then through the resistor 66 and back to the secondary winding of the pulse transformer. The values of the circuit parameters within these loops are not critical, but for correct operation the resistors 66 and Ill should be so chosen that the gating voltage in the two loops will be balanced to prevent any part thereof from appearing in the output signal at point 12.
The operation of this circuit. as may be in ferred from its description. is as follows.
element 63 renders the same conducting for the period of the gating voltage and thereby permits the passage of the signal received on input terminal I! to the output terminal 12 of the circuit. The portion of the gating voltage which is traceable through the secondary winding 68 of pulse transformer 69, resistor 10, ground, resistor 25 and element 22 and back through the Thegating voltage impressed across the switchingamass:
resistor I56;' 153 through; the: balancing; arranges 1 ment of resistors 66 and 19, equal to thesgating f voltage through thaloop; comprisingrthe secondary. winding: of: the pulse transformerr' II; the-- resistor: 67!; the: switching: element 63; andrthe' resistor: 56%. According-13 the potential of the gating: voltage is: balanced torground at the point 'IZ-..:and does notappear in-theroutputof the Cir? cuitas any portion-of the signali I In: this embodiment of: my invention, the -elementa GSidu-ring: relatively-long. periods;- is so biasedti by; the output: voltage; of: transformer 69' that? the anode: do ismaintained negative with respect to cathode-:64. During. these periods; a sncal-l'ipotential:variationrat junction point-miresuits-sin no appreciable potential .;variationatathe anode: 65' and point-l2; During. therelatively brief: gate conduction intervals; however; the
anode 65 is maintained positive with respect to cathode; in accordance: with thepolarity :symbolsshown in Fig." 4-.-- Dilll'IlgJiJhGSE intervals; anyslig ht: potential variation,-v positive or negative;
produced; at junctiompoint 26ahrough the "action of element 22, is transmittedthrough-the element fifiito'theoutputjunctionpointl2;
A; careful study of -the circuit of :Fig.v 4 reveals that thenon-linearresistance element 53 and the resistor; til form, two series connected legs of a bridge circuit between terminals 2 G and 15-9: Awhile resistors 25; and: Hi a-rev two further series-con" nectedilegsof thebr-idge between these-terminals. T3181 gating 5 transformer 69 connected, inseries withpresistor fifi isprovided for applying controlling ;voltage pulses between terminals 59 and 26, in accordance with which the resistance" of thernon-ilinear resistance: element 63;- is varied I abruptly from alnearly infinite Value to a relatively smallresistance.
Duringperiods of, application of the: gating voltage-iotpolarityasshowrr in Fig; 4, the element 53 is highly: conductive andsignal voltage" appliedacross resistor 25 producesv an output' voltage between-rthe junction of-l elements fisfland 61 the! grounded junction of resistors Eda-11d; Ill: The gatingwoltage applied betweenethe op posited junction points or terminals 25 and" 69 produces substantially equal voltage drops-acrossresistors- 61 i and 78, so that' substantially .-no :potentialdifference is produced betweenltermi-n'al' 12 andgrounddue to the" gating" voltage" applications-- In the-embodiment off-sm-y invention illustrated inlFig. 5, the'ungated signals-"are impressed upon thei grid 79- of electronicdischarge device: I8; whoseplat'eB I I receives positive Voltage and :whose cathode 82 -is.--connect-ed topass the signals -received" onith'ez grid'r19 of this element :through'the coupling! elements comprising the resistor 83" andfrcapacitorrfl l to the primary 'windingi85" of thetransformer 8'5: A secondary windin I06 of transformer 85"is balanced to ground through the resi-stors lil i' and Hi5; whosecenter'point IOZ' is" connected as shown. The' balanced output of the secondary windin is delivered to cathodes IDI 'and I 88;" respectively, of switching elements 81*and 88'." Bias" is appli'edto these electrodes from'souroe ml. The output of each oftheswitching elements Bland 381s taken respectivelyffom th'e plates fifi and'sl thereof and delivered across the shuntresistor $5 to the primary winding" 92'of pulse transformer 93; A pulsetransformer 98is connected to induce a gatin voltage in th'e' secondary winding'S'Fthereof; which is connected-inseries with the biasing source 101.
This-voltage is impressed upon-plates 89 and '9'I I B9I through=transtormen 93'.
The :ungated signals: receivediomgrid:'I'9 ofele; ment: 'I8-"are induced; in: the secondaryi'windingfz I 0620f pulse transformerB65whereirrtheyiarerbalt an'ced'itot ground: as shown. The" switching eles 1o mentsrtl anci 88; which are'biasedrfrom' aisourcer I BI; are r normally I non-conducting-rbut arecrenz deredriconducting under theainfluerrcesoffthe gata ingsvoltagereceived' on' the terminals of primary? winding; 99: of. plllSEi transformer: 98'; From: this:
1 point the polarity of the gating: voltageeisrassin'e dicated: which; through the plate? resistors; 96'
and- 98,: applies a positive" voltages-to? tHEE plates;
89 andifl I, respectively; of 'therswitchin'g elements: 8T'andi88i' This renders theseelements conductsing forl the durationiofthegatingpulse totthereby;
permit thetpassage 'Ofl a selected:'signalswhicl'hiss received in synchronization; therewith-ii When: tubes 81: and- 8t are-rendered conducting",v signal: which is balanced: to. ground as aforea mentioned'appears with oppositespolarity:onzthec cathodes I01" and. laser tubesy8z! and: 88: Ins; this manner the signal is additive" t0: theitgatirfg i voltage flowing in one half" of: the balancedi switching: circuit and subtractive; in' the? other:
. half. This produces apotentialrdifierence:acrossi th'e reslstor. :95 whichinzturnis induced? that; secondary winding 94" of pulse transformer 9-33 and; passed toxthetoutput' terminal-t Illfliasstlio? desired: signal; The'gatingr voltage; beingfba-lianced 'out in the two switching-1 loops doesznow appear in the output; I
As: previously mentioned in. connection": with I the modification of my: invention shoWn imFig. 2; it is possible. to operate it as a ban'di rejections lcircuiti by: simply "reversing, the polarity:- off then gating voltage impressed. on" the. switching: cit-"- cuit' 3|.- The circuits illustratedin Figs. ,4eandz5i may; also vJoe operated. as band rejectiomci-rcuitsn by choosing the circuit parameters; to apply:
proper bias' to the 'switchinggelements"thereof? Inthe embodiments of myinvent-ionillustratedgi. I have used a I pulse 7 transformer to change fromi. a balancedrto an'unbalancedfcircuit: It isftOJbB; understood; however,- that balanced :fcircuits ems- 5U ployin-gthe use of tubesmay be employed: also? The-pulse transformer 'hasthe' advantagecof res quirin-gw no: operating voltage and": is simple to? install: I
As' stated" before; the impedance matchingtransformers; illustrated in- Fig; 2: aselements 22* and 47 and in Fig. 3 as elements 22 and "T4,- mayhe -omitted," depending upoirtha requirement of thecircuit. In-=eachofthe embodiments of my invention w illustrated; it'is possible that, due to the-inherent characteristics of the switching elements used; a" smallsignal voltage may be passed througlrtlie gating' branches of the circuit when the tubes arenormally non conducting. If thisaction is 0b 65)" J'ect-ionable, the switching elements of"eaclr-cir= cuitmay be conveniently biased -by-subst'ituting a gating voltage of thewave form illustratedih Fig: 6. Here a'widenegative gating 'voltage of the formillustratedas H5 is mixed' withanar= row positive gating voltage IItfto form-acorn" posite wave I I1.
When the gating voltage-I I'lis applied' tothe s tching elements of any of' the circuits: illustrated; the' switching elements 1 will be "biasedbe-= 7 yond cut oir'for'periods' represented-by" the'por= tions of gating voltage H! which are designated by the numerals H8 and H9, and the tubes will conduct for only such period of time as the portion of the gating voltage H1 produced by the narrow gating voltage H6 is applied.
Other modifications of my invention are, of course, possible, and they may present themselves in view of the foregoing disclosure. Accordingly, the recitations and representations herein made are to be considered as illustrative and in no wise in a restrictive sense, and the spirit and scope of my invention is to be considered of such breadth as described by the ap pended claims.
What is claimed is:
1. Bridge circuit apparatus for selectively coupling a source to a load and for controlling energy transfer therebetween in accordance with a control voltage, comprising a non-linear resistance element and a first linear resistance element connected in series between opposite junction points and forming a first intermediate junction therebetween, two further linear resistance elements connected in series between said opposite junction points and forming a second intermediate junction therebetween, means for intermittently applying a control voltage between said opposite junction points for efiecting changed conductivity of said non-linear element during said voltage applications, means for coupling a signal source to the terminals of one of said elements, and means for connecting a load between said intermediate junctions.
2. A bridge circuit comprising a non-linear resistance element and a first linear resistance element connected in series between opposite junction points, second and third linear resistance elements connected in series between said junction points, said second linear resistance element being connected to said non-linear resistance element at one of said junction points and said third linear resistance element being connected to said first linear resistance element at the other of said junction points, means for applying a signal voltage across said second linear resistance element, utilization circuit means connected between the junction of said. non-linear element and said first linear element and the junction of said second and third linear elements for utilization of the signal voltage coupled thereto through said non-linear element, and means for applying a controlling voltage between said opposite junction points for abruptly varying the resistance of said non-linear element.
3. A controllable signal transmission circuit comprising a non-linear resistance element characterized by appreciable variation of conductivity in accordance with voltage impressed thereon, a first resistor connected in series with said element, control means connected to said seriesconnected element and first resistor for applying thereto a gating voltage producing a voltage drop component across said. first resistor and causing appreciable variation of conductivity of said element, neutralizing voltage drop means connected to said control means and including a second resistor connected at one end to the junction 01" said control means and said first resistor for producing a gating voltage drop component across said second resistor substantially equal to the gating voltage drop component across said first resistor, current path means being provided b-etween the end of said second resistor opposite said one end and the junction of said control means and said non-linear resistance element, means for applying an input signal between said nonlinear resistance element and said second resistor for conduction therethrough during periods of hi h conductivity of said element, and signal output utilization means connected to said first and second resistors at points thereon non-adjacent to the connection of said resistors to said control means, whereby a version of said input signal is applied to said utilization means during periods of high conductivity of said non linear resistance element and gating voltage drop components in said first and second resistors are effectively neutralized.
4. Apparatus for selectively controlled coupling between a signal source and a signal utilization device, comprising an electrical signal source having first and second terminals, a two-terminal utilization device for receiving a version of said signal voltage during selected time intervals one terminal of said utilization means being connected to said first source terminal, a non-linear impedance element connected between the other terminal of said utilization device and said second source terminal, a pair of impedance elements connected in series between the terminals of said utilization device and forming an intermediate junction therebetween, and means connected between said intermediate junction and one of said source terminals for applying a control voltage to said non-linear resistance element whereby the conductivity thereof may be varied electrically.
DANIEL S. PENSYL.
R EFEREIIQES QITED The following references are of record in the his of this patent:
UNITED STATES PATENTS Number Name Date 1,598,453 Scott-Taggart Aug. 31, 1926 1,673,002 Fearing June 12, 1928 2,025,158 Cowan Dec. 24, 1935 2,086,601 Caruthers July 13, 1937 2,233,860 Wise Mar, 4, 1%1 2,323,634 Van Slooten et a1. July 6, 1943 OTHER REFERENCES Electronics, March 1938, pages 28, 29 and 30, The Modulator Bridge, by Hellmann.