US2416911A

US2416911A - Carrier-exalted receiver

Info

Publication number: US2416911A
Application number: US536093A
Authority: US
Inventors: Murray G Crosby
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1944-05-18
Filing date: 1944-05-18
Publication date: 1947-03-04
Anticipated expiration: 1964-03-04

Description

March 4, 1947.

M. G. cRosBY CARRIER-EXALTED RECEIVER Filed May 18, 1944 Patented Mar. 4, 1947 OFFICE 2,416,911 CARRIER-EXALTED RECEIVER Murray G. Crosby, Riverhead, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application May 18, 1944, Serial N o. 536,093

(Cl. Z50- 27) 9 Claims.

My present invention generally relates to car--A rier-exalted receivers, and more particularly to a system for receiving phase modulated (PM), or amplitude modulated (AM) carrier waves with carrier exaltation.

Carrier-exalted reception should come into more and more general use in all wave bands in which multiepath fading distortion is encountered. This includes the present AM broadcast band of 550 to 1600 kilocycles (kc.) in which fading distortion is experienced in a region about 50 to 100 miles from the transmitter due to interference between the sky wave and the ground wave. Such fading causes a cancellation of the carrier frequency, leaving the sidebands to beat together and cause unintelligible distortion. This distortion is severe, and is responsible for the fading wall which limits the night-time coverage of a broadcast station in the 550 to 1600 kilocycle band. Carrier-exalted reception is very f effective in removing this distortion.

Another wave band affected b-y multi-path distortion is the band from 1.5 to about 25 megacycles (me). This band is used for point-to-point communication, or for short-wave broadcasting. The multiplicity 4of paths is caused by unequal numbers of ricochets of thesignal between the ionosphere and the earth. The distortion that results is the same type that is experienced in AM broadcast reception, except that it is present at all times instead of just at night. Hence carrier exaltation is even more important inthis band.

When carrier exaltation is applied to certain known forms of AM receiver circuits, it is a simple matter to convert them to phase modulation reception by inserting a 90 phase shifter either in an unfiltered signal energy circuit or in a.

filtered carrier circuit. Phase modulation then becomes the most economical transmission to use because of its advantages at the transmitter. Carrier-exalted reception is especially advantageous in plane-to-ground communication where the band from 1.5 to 20 megacycles is used, and where the decreased weight of a phase modulation transmitter relative to other` types of transmitters is of importance. In addition, the elimination ofthe carrier-fading distortion would be realized. It has been found that in the presence of strong noise, use of carrier exaltation gives V improved reception. This improvement is apparently due to the elimination, brought about by the carrier exaltation, of the beats which otherwise occur between the individual noise com-v ponents. In particular, carrier-exalted reception gives an improved signal-to-noise ratio in the presence of strong man-made noise of the impulse type. In the case of this type of noise carrier-exalted reception appears to provide an elimination of low-frequency noise components, which improves the overall character of reception.

In-my Patent No. 2,397,840 dated April 2, 1946, I have disclosed a novel detection network for PM, or AM, wave energy. The detection network utilizes a piezo-electric crystal filter element to provide substantially unmodulated carrier energy. Automatic frequency control (AFC) Voltage is, also, provided by the network in response to a, shift in mean frequency of applied signal waves from a predetermined desired reference or center frequency. In my application Serial No. 489,924, filed June 7, 1943, I have shown a carrier-exalted receiver system utilizing some of the principles of the type of detection described in said Patent No. 2,397,840.

In said application Serial No. 489,924 there is disclosed and claimed -a compact carrier-exalted receiver circuit which is capable of receiving either PM or AM signal waves with a minimum of tubes; the circuit being readily adapted to varicus types of receiving systems. In the latter application there is provided a receiving circuit employing a converter to reduce the mean or carrier frequency of applied PM or AM carrier waves. There'is further provided a multi-grid'detector tube of the type shownin-my U. S. Patent No. 2,063,588, granted December 8, 1936, which has the signal waves applied to one grid thereof, and

, a piezo-electric crystal filter network providing filtered carrier energyfor a second grid of the detector tube. AFC voltage is derived from a balanced rectifier circuit connected to the crystal filter networkforfcontrolling the effect of a reactance-simulation tube on the oscillator section` of the converter. i

It is one of the main objects of my present invention to improve and simplify the carrierexalted receiver circuit of my ,aforesaid application Serial No. 489,924'with a view towards providing a carrier-exalted'adapter for any receiver system, the adapter being capable of confinement in a single transformer can.

An important object of this invention is to provide in conjunction with a piezo-electric crystal iilter type of .discriminator-rectier circuit,'a simple diode rectifier functioning to produce carrierexalteddetection of PM or AM carrier waves.

A further object of my invention is to provide acompact and simple form of adapter, employing a minimum number of a carrier-exalted PM or AM wave receiver.

Still other objects of my invention are to provide circuits which may be employed either for carrier-exalted AM reception, carrier-exalted PM reception, or for balanced PM reception, thereby to improve the efficiency and reliability of PM-AM. receivers; and more especially to provide an economical system for PM reception or for carrierl application. The input transformer T may be supplied withsignal energy from the output circuit of a pentagrid converter tube which may be of the GSA'T type. Its circuits are very well known. The converter functions to reducey the carrier', mean or center frequency of applied' PM, or AM, carrier wave energy in conventional manner. circuits prio-rV to the converter is immaterial to this invention. The collected PM, or AM?, carrier waves may be in the megacycle (mc.) or in thel kilocycie (kc.) ranges.l Prior to the converter the collected waves may be heterodyned, either one or more times, until the mean or center frequency of signal energy' at the signal input grid of the nal converter is equal, forv example, to 450 kc. This is a suitable intermediate frequency (I. F'.) value of the usual superheterodyne receiver'.

Whether the wave energy at the converter input grid is PM- or AM, the treatment thereof according to my invention is insome respects the same. It may be desired first to reduce the mean or center frequency of the received signal `energy tubes not exceeding threev in number, to convert a standard AM receiver tov The specific nature ofthe signal receiver` 550 kc., so that the applied 450 kc. signal energy may be reduced to the proper 100 kc. value.

The signal wave energy appearing across secondary winding 8' is passed through a crystal filter to provide substantially unmodulated carrierV energy. The filter consists of piezo-electric crystal P tuned to 100 kc. The inter-electrode capacitance ofr the crystal filter is neutralized by shunt condenser Cn. The input and output electrodes of the crystal P may be separate metal electrodes, or they may be metallic coatings provided on the opposite faces of the crystal. My

aforesaid Patent No. 2,397,840 has described the functioning of the general type of crystal filter network and its. associated opposed rectiers shownherein. The ltered carrier energy output of crystal P is transmitted through condenser I2 to the anode I4 of a detector tube I5 of the diode type. Condenser I'2 is large, and has substantially no effect on the prase of the filtered carrier Y energy' transmitted therethrough.

The cathode I6 of diode I5 is connected to ground through a current indicator I1 whichY functions visually to indicate the magnitude of space current of diode I5. The indicator I'I may, therefore, be employed as a mean for visually indicating the carrier intensity of received signals, i. e. as a tuning indicator. The loadv resistor I3 is connected from the anodeV |14' to ground, and is traversed by the diode space current. In addiy tion to ltered carrier energy, there is applied un'- to a lower value. ThereaftenA it isdesired to primary winding 8 of transformer T. If desired,

thetransiormer may be ofthe iron core type. Shuntl condenser 9 tunesLwinding 8- to the frequency of the signal energy impressed on the circuit 9-8. This may be a beat frequency which maybe lower or higher than that of the signal .energy applied to the input ofv a final convertertube,Y andi maybe assumed tobe a frequency of 100km. There will therefore bedeveloped across tuned primary circuit 9--3 signal energy whose mean frequency is that. of the resonant frequency ofl the circuit. The passband of transformerT should be sufficiently wide to. pass allV the sideband. components. in the case of AM or PM carrier energy; Of course, in the case mentioned above by way of illustration, the converter tube will be provided with a local oscillator section producing local oscillations'ofa suitable frequency, er g.

ltered signal' energy to the anode I4. This is accomplished through switch S which may be connected either to the contact designated AM or that marked PM, depending upon whether AM or PM signals are being received'. `'Ihe YPM contact is connected to the lowerY end of winding 8 through condenser I3. The AM contact is connected by series-arranged resistor I9 and condenser 23- to the lower end of winding 8. Hence,

either AM or PM wave energy in unfiltered formmay be applied to diode rectifier I5, and operation of switch S determines the general phase relation of the unfiltered signal energy relative to the filtered carrier energy at diode I5. f

The diode detector tube I5 functions to demodul'ate either PM signal waves or AM signal waves. The elements I3 and lil- 20 may be' adjusted properly to phase Vthe modulated carrier wave energy applied to anode I4' so that thereo-` tied voltage acrossfload resistor I8 will correspond to the modulation components of the orig-1 inally-modulated signal energy for either AM or PM detection; In other words, if there is transmited through condenser I2 to the anode I4 they v filtered carrier of AM signal energy, then with switch S on the AM position there will be simultaneously applied to anode I4 through con-` denser 2@ and resistor Iii unfiltered AM signal energy which has its phase properly adjusted for AM detection.` The carrier energy transmittedY through condenser I2 will be substantially-unmodulated, andv its phase willy be substantially the'same as the phase of the carrier of the AM signal energy transmitted through condenser 2i) and resistor I9. With this. in-phase relation between the energies applied tothe tube. iii-the lat-v ter will function tol provide across output resistor I3 the desired modulation signal components. The values of condenser 20 and resistor I9 are so chosen as to attenuate the unfiltered AM signal waves, including their sideband components, but

so as not substantially to affectv the phase of the p carrier. Hence, selective fading effects are over.-

come., due to the adjustment of the relative-v am-.

plitu'de's of the AM carrier and sideband components at the anode of diode I 5. In other words, the effect of attenuatingthe AM signal energies prior to application to diode I is to exalt or augment the carrier relative to its sidebands. If desired, the filtered carrier passing through condenser I2 may be amplied for further carrier exaltation before being impressed on diode I5.

Assume,.now, that PM signal energy is transmitted to transformer T. In that case the phase modulated carrier energy applied to anode III must be adjusted to be substantially 90 out of phase, i. e. to be in phase quadrature, with the filtered carrier energy, by moving the switch S to the contact point PM. This phase quadrature relation is required for providing a resultant AM wave at diode I5 whose variations correspond to the phase variations of the PM wave. Condenser I3 is suiciently small to impart a substantially 90 phase shift to the PM Wave transmitted therethrough. The ltered carrier of the PM wave passed through condenser I2 suffers no phase shift, and due to the action of the crystal in removing phase modulation from it, is similar in phase and wave form to the carrier generated at the-transmitter. The vector sum of the filtered carrier and the unfiltered signal voltage at any instant increases with the degree of departure of the phase of the unltered Wave energy from the quadrature relation. Hence, at the anode I 4 there will be provided a resultant AM voltage whose vector length` i. e. amplitude, is proportional to the instantaneous phase devian tions of the PM signal wave with respect to the normal quadrature phase relation of the unltered PM carrier. The diode I5 is now readily capable of detecting the received energy as it has been changed into an AM signal wave.

The action of thedetector tube I5 will, therefore', be to demodulate the PM signal energy, and provide dernodulation signal components across output resistor I8. In PM signal energy detection, carrier exaltation will also result since the condenser I3 attenuates the sideband components of the PM signal wave. Such carrier-exalted detection is of especial value in connection with AM and PMr reception in the frequency range below megacycles, because in that range the effect of selective fading on the transmitted signal energy is to produce an apparent increase in percentage modulation which may result in distortion due to over-modulation. Further advantages of carrier-exalted reception have been recited heretofore in the description.

vThe modulation signal voltage, whether for AM or 'PM reception, is transmitted through resistorcapacitor lter 22--23 to any desired form of modulation signal voltage amplifier network. If the modulation frequencies are in the audio frequency range, then the filter 22-23 will remove allI. F. currents, and the audio amplier will feed the amplified audio voltage to a final reproducer. Automatic volume control (AVC) may be derived from the rectified voltage across resistor I8. The resistor 24 and condenser 2li` provide AVC filter network to remove all alternating currentcomponents. The AVC bias magnitude will substantially follow the variations in carrier amplitude, since there is applied to diode I5 through condenser I2 substantially pure carrier energy. F'

Hence, any variation in amplitude of suchA filtered carrier will produce a change in AVC bias, and the latter may be applied to one or moreof the signal. transmission tubes prior to transformer Tito controlt'he'gains of such tubes-in a sense'to overcome the carrier amplitude variation.

Balanced PM detection-and AFC voltage -are provided bythe opposed rectiers 26 andv 21. While these rectiers are symbolically represented as separate diodes, it is preferred that the diodes be separate diode sections of a tube of the double diode type, such as a GHG type of tube. The load resistoi's'of the rectiers 20 and 21 are denoted by numerals 26' and 21 respectively. The cathode of rectier 21 is grounded, and each of the load resistors is suitably bypassed for alternating currents. The junction of the load resistors is connected to the junction of a pair of direct current return or

isolation resistors

28 and 29.

As explained in my aforesaid Patent No. 2,397,840, the ltered carrier energy at the output electrode of crystal P is applied in like polarity, i. e. in parallel, to the anodes of the respective rectiers 26 and 21. As shown herein, this is done through respective circuits 30-40-4I, and

4I-50-5I, which will be further described hereinafter. The modulated, or unltered, signal energy existing at the opposite ends of secondary winding 8 is applied to the rectiiiers 26 and 21 in opposite polarity, or in push-pull, by virtue of the

shunt coupling condensers

32 and 33 respectively.'- Winding 8 has its midpoint grounded, and parallel resonant circuit connects the grounded midpoint of coil 8 to the output electrode of the crystal P; that is, circuit 60 is connected between the output side of crystal P and ground; Circuit S0 may be tuned to crystal frequency, or may be somewhat detuned relatively thereto, and acts as a coupling circuit of finite impedance between the output of crystal P and the anodes of rectiers 26 and 21. The circuit 60 increases the Q of the crystal beyond what it would be if the resistance of circuit 60 were innite. As shown, circuit 60 connects to lead 10 between the crystal P and rectier 21. Condenser may shunt winding 3 to tune it to crystal frequency.

As explained in my last-named application,

` each rectifier 26 and 21v has applied to it filtered and unfiltered signal energy in phase quardature relation. This phase quadrature relation of the two voltages at each rectifier results from the fact that the unfiltered signal energy is appliedI to the rectifiers by

condensers

32 and 33 which are Suniciently small to effect a phase shift, and are also of substantially equal capacities so as to produce equal phase shifts both of the un-l filtered carrier and of signal components.

Thel condensers

32 and 33 are non-selective to phase or frequency variations of the unltered carrier, and accordingly permit all signal components to pass to the rectifiers 2li and 21. The crystal'P, however, effects no substantial phase shift of the filtered carrier wave at modulation frequencies, but by its inertia effect substantially removes' the phase modulation of the signal thereby restoringV the carrier substantially to the phase and wave form which it had before modulation at the transmitter. vThe crystal P due to its sharp selectivity is, of course,` selective against frequencies" ofi resonance, but will nevertheless pass a band of' frequencies varying overa narrow-range.'

Circuit 60 which, as heretofore lndicated, is employed to control and improve-the Q,`orse' lectivity, of crystal lteriP may introduce 're-` actanc'e into the circuit of the crystal tendin'gto change'somewhat the phaseof the nltered carrier:

output from the crystal.V Theiltered output'on.

its Way 'to rectier26 4pas-sesl through a network consisting .of variable condenser 30 in Vassociation Y With condenser 40 and resistor 4l. This network may be designed to compensate for such phase change. Similarly, a network consisting'of variable condenser 3l Vin association with condenser 5l! and resistor 5l may similarly compensate for dephasing effects of circuit 80 on filteredenerey` supplied to rectifier 21. of circuit Eil, Which` may be effected to increase the Q of crystal P, cause a change from the desired normal phase quadrature relation between the filtered carrier .and unfiltered signal wave, then condensers 33 and 3i may be adjusted to compensate for the dephasing so as to restore the normal phase quadrature relation.

V The rectiiiers 2S and 2l function to rectify the respective resultant vector voltages of these aforesaid quadrature-related energies. When the ap` plied signal energy at transformer T is unmodulated, i. e. has a frequency and phase corresponding to the predetermined reference characteristics of crystal P, then the respectiveresultant vector voltages applied to rectifiers 26 and 2l are equal, each of them being the sum of the filtered carrier voltage from crystal P without phase change and of an unfiltered carrier voltage substantially 90 different in phase from the filtered carrier. Accordingly, the differential direct current voltage output of the rectiers is substantially zero, since the voltages across resistors 25' and 2l are equal and of opposite polarity.

However, when the received carrier is phase,- modulated, and as described more fully by aid of vector diagrams in my Patent No. 2,397,841 dated April 2, 1946, the filtered carrier remains as before, but the unfiltered signal energy is supplied to the rectiers 25 and 21 in phases differing from the 90, or quadrature, relation to an extent determined by the degree of phase modulation. If the degree of phasemodulation is small, a relatively small direct current voltage is built up at the cathode end of resistor 26 due to a component of the signal voltage adding to the filtered carrier voltage at one of the rectifiers and subtracting from it at the other. The greater the degree of phase modulation the greater the sum or vectoi1 voltage of the unfiltered signal energy andV the filtered carrier from crystal P on one of the rectiiiers and the less the sum of such voltages on the other rectifier. The polarity of the direct current voltage drop across the load resistors 26 and'21' of the opposed rectiers depends on thedirection of the phase change ofthe received signal energy. f

The direct current potentials across resistors 26' and 2l', described above as derived by rectification of a PM Wave, follow one another in frequency and polarity to accord with the modulations of the received carrier wave. Accordingly, audio or other modulation signals may be taken olf at the cathode of diode 26. Assuming that the modulations will be at audio frequency, I

have shown an audio frequency output switch 80' which may be alternatively connected with diode I5 for AM, or exalted-carrier PM, reception, or with rectiers 26 and 21 for balanced PM reception.

The circuits of rectiers 25 and 2l are also preferably employed kfor the derivation of AFC potentials. The crystal P can respond to frequency changes over a, limited range, and, in particular, to slow frequency changes such as may, for example, result from local oscillator drift in a superheterodyne receiver. As to such frequency changes within the band of frequencies passed by Should slight detuning it,.the'crysta1 P'acts as series-connected inductvance and capacity. When the received frequency is exactly that of the crystal, it produces no phase change and theV filtered carrier remains at from the unfiltered

energy passingthrough condensers

32 and 33 respectively. When, however, the incoming frequency changesfslowly toward one side of the pas-sband of the crystal, the ltered output'changes in phase in a direction toward the phase of the current in one of

condensers

32 and 33, and away from the phase of the current in the other of such condensers. In this way there is produced increased energy on one of rectiiiers-26 and 2l and decreased energy yon the other so as to create a direct current potential across rectifier load resistors 26' and 21. Upon change of the incoming frequency inthe opposite direction, the phase of the filtered output of the crystal changes in the opposite directiontherebydecreasing the energy on the rectier where it before was in creased, and increasing the energy on the other rectier so as to produce a reversed direct current' potential drop across resistors 26, and 2l.

This .frequency discrimination action is similar in certain of its aspects to that described in my Patent No. 2,363,652 dated NOV. 28, 1944. The direct current voltage output of the opposed rec.- tifiers is utilized as AFC bias, after iilteringoff of modulation frequency voltages by resistor 82 and condenser 84, and maybe applied over'lead 3d to a frequency control device ofthe reactance tube type. The reactance tube andA its associated circuits are of the well-known form, and are. shown in my aforesaid application Serial No. 489,924. The AFC discrimination action is'the same for AM and PM signal waves. In explaining the functioning of the reactance tube, it is pointed out that its plate to cathode impedance is connected in vshunt across the oscillator tank circuit of the converter tube. By suitable connections to a phase shifter connected to the oscillator circuit the plate to cathoder impedance of the reactance tube will function as ifV it was re active, and the latter effect Will appear across the tank circuit; The magnitude of this simulated reactance Will then be a function of the gain of the reactance tube. Obviously,'the direction and extent of vvariation of the reactive impedance of the reactance tubewill depend upon the variation in AFC biastransmitted over lead 34. This, of course, is the Well-known AFC action Whichis familiar to those skilled Vin the. art. Inv this case thev AFC circuit functions to maintain the mean frequencyof the signal energy applied'toV apparent to one skilled in the art that my invenv tion is by no means limited to the particular organization shown and described, butthat many modifications may be made Without'departing from the scope of my invention.

What I claim is: f

1. In combination with a diode rectifier-circuit, means providing either amplitudemodulated lcarrier waves or angle Ymodulated carrier Waves, means for deriving from either of said waves the carrier thereof substantially free of modulation, means for applying the carrier energy to said rectifier, means for selectingxeither of said modulated waves in a predetermined. .phase relation relative to the said modulationfree carrier energy, means applying the selected waves to said rectifier, and means for deriving from the rectifier circuit modulation signals corresponding to the modulation on the said selected Waves.

2. In combination with a diode rectiiler circuit, means providing either amplitude carrier waves or phase modulated carrier waves, piezo-elehtrc crystal filter means for deriving from either of said Waves the carrier thereof substantially free of modulation, means for applying the carrier energy to said rectifier, adjustable phase shift means for selecting either of said modulated waves in a predetermined phase relation relative to the said modulation-free carrier energy, means applying the selected waves to said rectifier, and means for deriving from the rectier circuit modulation signals corresponding to the modulation on the said selected waves.

3. In combination with a rectifier consisting of a single diode, means for applying to the diodel electrodes a phase modulated carrier wave, means for applying a modulation-free barrier derived from the modulated carrier wave to said diode electrodes in predetermined phase relation, and means for deriving modulation signals from the rectier output.

4. In combination with a source of phase modulated carrier waves, a pair of opposed rectiers having a common output circuit, means including a piezo-electric crystal filter having an input electrode coupled to said source, parallel output connections from the Ioutput electrode of said crystal to the respective rectiers, separate means applying modulated carrier wave energy from said source to said opposed rectiers in pushpull relation, a rectifier circuit including a single diode, means connecting the crystal output electrode to the diode anode for applying to the latter filtered carrier energy derived from the phase modulated carrier waves, and an additional phase-determining connection between said source and said diode electrodes for applying thereto phase modulated carrier Wave energy in predetermined phase relation to said filtered carrier energy.

5. In combination with a diode rectier circuit, means providing either amplitude modulated carrier waves or phase modulated carrier Waves,

crystal nlter means for deriving from either of r said waves the carrier thereof substantially free 'l modulation, means for applying the derived carrier energy to said rectiiier, means for selecting either said phase modulated carriervwaves in a normal phase quadrature relation relative to the said modulation-free carrier energy or said amplitude modulated carrierv waves in in-phase relation to said fcarrier energy, said selecting means attenuating sideband components of the modulated waves, means applying the selected modulated waves to said rectifier, and means for deriving from the rectiiier circuit modulation signals corresponding to the modulation on the said selected waves.

t. In combination with a diode rectier circuit, means providing either amplitude modulated carrier waves or phase modulated carrier Waves, crystal filter means tuned to the carrier frequency for deriving from either of said Waves the carrier thereof substantially free of modulation, means for applying the derived carrier energy to said rectifier with no phase shift, adjustable phase shift means for selecting either said phase modulated waves in phase quadrature relation relative to the said modulation-free carrier energy or said amplitude modulated carrier waves in in-phase relation to said carrier energy, means applying the selected waves to said rectifier, means for deriving from the rectifier circuit modulation signals corresponding to the modulation on the said selected Waves, and means for deriving from the modulation-free carrier energy a variable direct current voltage for automatic volume control.

Vrectiiiers having a common output circuit,

means including a crystal filter having an input electrode coupled to said source, parallel output connections from the output electrode of said `:crystal to the respective rectiiiers, separate condensive means applying modulated carrier wave energy from said source to said opposed rectifiers in push-pull relation, a diode rectiiier circuit, non-phase shift means connecting the crystal output electrode to said rectifier circuit for applying to the latter filtered carrier energy derived from the phase modulated carrier waves, and an additional phase determining connection between said source and said diode rectifier circuit for applying thereto phase modulated carrier wave energy in phasel quadrature relation to said liltered carrier energy, said phase determining connection including an element for attenuating sideband components. y

9. In combination with a diode rectiiier, means for applying to the diode electrodes an angle modulated carrier wave, said means attenuating sideband components, means for applying a` modulation-free carrier derived from the modulated carrier Wave to said diode electrode in predetermined phase quadrature relaton, means for deriving from the rectiiier output modulation signals, and an additional means for deriving from the rectiiier output a direct current voltage Whose magnitude is a direct function of the magnitude of said modulation-free carrier.

MURRAY G. CROSBY.

REFERENCES CITED The following references are of record in the file of this'patent:

UNITED STATES PATENTS Number Name Date 2,280,525 Hunt Apr. 21, 1942 2,340,432 Schock Feb. 1, 1944-.