US2690538A - Frequency-stabilizing and frequency-modulating system for oscillators - Google Patents

Description

Sept. 28, 1954 L. E. NORTON 2,690,538

FREQUENCY-STABILIZING AND FREQUENCY-MODULATING SYSTEM FOR OSCILLATORS Filed Dec. v29, 1949 5 Sheets-Sheet 2 INVENTOR jowe]! Mrlbn ATTORNEY Sept. 28, 1954 2,690,538

L. E. NORTON FREQUENCY-STABILIZING AND FREQUENCY-MODULATING SYSTEM FOR OSCILLATORS 5 Sheets-Sheet 3 Filed Dec. 29, 1949 fi INVENTOR 25 ZOWeII EZVOIZ'OH ATTORNEY Sept. 28, 1954 L; NQRTQN 2,690,538

FREQUENCY-STABILIZING AND FREQUENCY-MODULATING SYSTEM FOR OSCILLATORS Filed Dec. 29, 1949 5 Sheets-Sheet 4 INVENTOR lawgjl EJVoo i ATTORNEY Sept. 28, 1954 2,690,538

L E. NORTON FREQUENCY-STABILIZING AND FREQUENCY-MODULATING SYSTEM FOR OSCILLATORS Filed Dec. 29, 1949 5 Sheets-Sheet 5 iNVENTOR Z0 WaZIENMZ' ATTO R N EY Patented Sept. 28, 1954 OFFICE FREQUENCY- STABILIZIN G AND FRE- QUENCY MODULATING SYSTEM FOR OSCILLATORS Lowell E. Norton, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1949, Serial No. 135,780

8 Claims. 1

This invention relates to methods of a system for frequency-modulation of oscillators, particularly microwave oscillators, whose mean carrier frequency is stabilized.

In copending applications including Serial Nos. 4,497, filed January 2'7, 1948, 35,185 filed June 25, 1948, Patent No. 2,584,608, dated February 5, 1952, 68,648, filed December 31, 1948, Patent No. 2,663,798, dated December 22, 1953, 73,626, filed January 29, 1949, 119,119, filed October 1, 1949, Patent No. 2,555,150 dated May 29, 1951, there are disclosed various forms of two-channel servo systems in which there are produced two series of pulses jointly containing frequency-error information whose time or phase relation is compared and the deviations from a predetermined phase relation utilized for stabilization of the frequency of an oscillator.

In accordance with the present invention, the carrier frequency of the stabilized oscillator is modulated, as at audio or video frequencies, by injection of the modulating signal into one or both of the aforesaid stabilizing channels in advance of a network which integrates the output of the pulse-comparing network. The resulting variation in amplitude of the pulses, or in conductance of paths for the pulses, effectively superimposes the desired modulation upon the frequency-stabilizing voltage derived from the pulses with the result that the mean carrier frequency of the oscillator is stabilized at the desired output frequency and that the instantaneous frequency of the oscillator varies with the desired modulation. More specifically, and in some forms of the invention, the percentage modulation so obtained progressively decreases for the higher and higher modulating frequencies: when wideband modulation is desired, the higher modulating frequencies are injected into the servosystem beyond the aforesaid integrating network in compensation for the deemphasis of the higher modulating frequencies characteristic of the combined frequency-stabilizing and frequency-modulating sys-' tem.

The invention further resides in systems having the novel features of combination and arrangement hereinafter described and claimed.

For a more detailed understanding of the invention, reference is made to the accompanying drawings in which:

Fig. 1 is a block diagram of a frequency-modulated, frequency-stabilized oscillator system;

Fig. 2 is a block diagram referred to in explanation of operation of the system of Fig. 1;

Fig. 3 schematically illustrates one arrangement for injecting modulation into aphase-comparator network;

Figs. 4a and 4b are explanatory figures referred to in discussion of the operation of Figs. 3 and 7;

Fig. 5, a modification of part of Fig. 3, illustrates another modulation-injecting arrangement;

Figs. 6a and 6b are explanatory figures referred to in discussion of Fig. 5;

Figs. 7 and 8, modifications of part of Fig. 3, illustrate still further arrangements for injection of the modulation;

Figs. 9a and 9b are explanatory figures referred to in discussion of Fig. 8; and

Figs. 10, 11 and 12 illustrate other arrangements for injection of modulation into another type of phase comparator.

Referring to Fig. 1, the oscillator I0 to be frequency-stabilized and frequency-modulated may supply high-frequency energy to a suitable load ll through a transmission line I2. The output frequency of a search oscillator I3 is intermittently or periodically varied in any suitable manner repeatedy to sweep over a range of frequencies including the resonant frequency of a suitable standard: specifically, part of the output of the search oscillator I3 may be impressed upon a chamber or cell I 5 containing gas, such as ammonia, exhibiting sharp molecular resonance at a frequency within the range repeatedly swept by oscillator I3. The high-frequency energy transmitted by the gas is impressed upon a suitable demodulator or rectifier I6 whose output is therefore a series of pulses, each occurring as the frequency of the search oscillator I3 passes through the resonant frequency of the standard Part of the outputs of the oscillators I0 and I3 is impressed upon a mixer I9 to produce a I beat-frequency which varies at the repetition or sweep-frequency of the search oscillator. For purposes of explanation, it is assumed that the oscillators I0 and I3 are microwave generators in which case the transmission lines I2 and I4 may be waveguides and the lines 20, 20 for coupling them to the mixer 19 may be directional couplers of waveguide type.

The beat-frequency output of the mixer I9 is impressed upon a frequency-selective network 2| whose output is demodulated by a rectifier 22 of suitable type. Thus, the output, of the demodulator 22 is a series of pulses, each occurring as the beat-frequency of oscillators l0 and 13 passes through a preselected frequency. Various different arrangements in which network 2| of Fig. 1 may be an intermediate-frequency amplifier, a low-pass filter, or an intermediate-frequency discriminator are shown in the aforesaid copending applications to which reference may be made for a more detailed explanation. Further by way of example, the desired operating frequency F of oscillator [0 may be 23.900 kilomegacycles; the center frequency of the range swept by oscillator I3 may be 23.870 kilomegacycles, corresponding with the 3, 3 line of ammonia, and network 2| may favor transmission of the difference beat-frequency 3O megacycles.

In the basic arrangement as thus far herein briefly described, there are produced two trains of pulses, the pulses of one train, transmitted by channel A, each occurring as the frequency of the search oscillator passes through the resonant frequency of gas in chamber [5 and the pulses of the other train, transmitted by channel B, each occurring as the beat-frequency of the oscillators ill and I3 passes through a predetermined value. Assuming the pulses of the two trains are coincident when the frequency of oscillator I0 is of the desired value, any deviation from that frequency will cause the pulses of one series to occur before or after the pulses of the other series, depending upon the sense of the frequency-deviation. For utilization of this effect in stabilization of the frequency of oscillator It, the two pulse trains may be respectively impressed upon shaping networks ll and 23 whose output pulses PA, PB are impressed upon the two input circuits of a phase-comparator H! of any suitable type and specific forms of which are later herein described, and general discussion of which is provided in the Proceedings of the Institute of Radio Engineers, vol. 31, January 1943, at pages 7-15. In the phase-comparator, the two channels A and B have common branches so that pulses of both series, as understood in the art, jointly determine the current passed by non-linear resistances, or rectifiers, comprised in the comparator.

The output of the phase-comparator or coincidence detector I8 is an undirectional voltage which varies in accordance with the time relation of the pulses of the two series and therefor with the sense and extent of the frequency deviations of oscillator ID. The output voltage of the comparator is utilized automatically to stabilize the frequency of oscillator l6: specifically, the output voltage may be applied as by control line or cable C to a frequency-controlling electrode of the microwave generator 10.

As thus far described, the system is a twochannel servo-system, generically of the type shown in the aforesaid applications, and serves rigidly to stabilize the carrier-frequency of oscillater 10.

To effect modulation of the instantaneous fre quency of oscillator [0 for transmission of intelligence, at audio or video frequencies, for example, the modulating signal S, as produced by a suitable modulator 25, is injected into the comparator-network I8 effectively to introduce the modulating signal into either one or both of the channels of the servo-system and so vary the instantaneous value of the output voltage of the comparator in accordance with the modulation. The average value of the output voltage is not affected and consequently the modulation so introduced does not impair the stabilization of the mean carrier-frequency.

The repetition rate, or sweep-frequency, of the search oscillator l3 should be high compared to the highest modulation-frequency: for example, if the highest modulation frequency is 40 kilocycles/second, the sweep-frequency may be about kilocycles/second.

The underlying principles of this method of obtaining both frequency-modulation and frequency-stabilization can best be understood by discussion of Fig. 2 which is a simplification of Fig. 1 with the corresponding elements identified by the same reference characters.

In Fig. 2, channel A of the servo system includes the source of the standard frequency, Fe. The other channel, B, includes the source of the output frequency, F0, and the switch 29. With switch 29 open, the output frequency F0 of the oscillator i0 is where:

eizinitial potential of the frequency-control electrode.

Fozyes transfer factor.

The output frequency F0, or its phase (since frequency is merely the time rate of change of phase) is compared with the standard frequency Fs in the comparison network l8. The output voltage 8c of the comparison circuit is, for the frequency comparison case,

8:a transfer factor,

Fs standard frequency.

The voltage 8c is effectively in series with the voltage e1 so that with the switch 29 closed to complete the servo loop system, the output frequency of the oscillator is:

It is now assumed that the initial potential e1 contains both a steady state component and a variable component and may therefore be expressed as:

where:

mzmodulation factor which has a time dependence,

E1:steady state component Under such circumstance, Equation No. 4 may be rewritten Where:

,60 is the zero frequency value of ,8, 40:21)

To introduce an incremental frequency shift AFO, it is necessary to produce a change in the second term of Equation No. 6 since E1 was defined as constant: stated differently In the frequency-stabilizing circuit of Fig. 1, when of the type shown in copendin application Serial No. 4,497 filed January 27, 1948, the factor ,u -l-ufl approaches zero at low modulating frequencies for large values of loop gain (#5) so that at low frequencies, the modulation component of voltage e1 is due almost entirely to the term m -AF +H-B since, for the same condition of large loop gain s) the factor m 1-1-14 (Equation No. 6), the modulating potential, is indicated diagrammatically in Fig. 2, must be introduced ahead of the integratin network 26 or equivalent circuit in the comparison circuit with transfer factor B.

All the circuit arrangements later herein described in detail produce such result. In general, in Figs. 3 and 10, the modulation is introduced ahead of network 26 in one of the two servo channels; in Figs. 5 and 11, the modulation is introduced unsymmetrically ahead of network 26 in one of the servo channels; in Fig. 7, modulating signals are introduced into one channel, but symmetrically, and in phase; in Fig. 8, modulating signals are introduced into one channel, symmetrically, and in phase-opposition; in Fig. 12, the modulating signals are introduced into one channel, symmetrically and are in phase.

Referring to Fig. 3, the phase-comparator circuit ISA comprises rectifiers 3|, 3|, resistors 3G, 30 and capacitors 32, 32, all connected in series as shown to form a loop. The pulses of channel B are impressed upon the loop between its input terminal 35 and ground: the pulses of channel A are impressed upon the loop between its input terminals 35, as, both ungrounded. For use in the comparator of Fig. 3, the pulses of channel A of Fig. 1 are converted, as later described, by the shaping network HA to pairs of sharp coincident pulses of opposite polarity. The pulses of one polarity are impressed through a condenser 33 upon one input terminal 34 of network [8A, and the pulses of opposite polarity are similarly impressed upon the other input terminal 34 through the associated condenser 33. The pulses of channel B of Fig. 1, may be shaped to sawtooth waveform by network 23, and impressed upon input terminal 35 of network i8A. Thus, as more fully explained in the aforesaid copending applications, the unidirectional potential at the output terminal 36 of the phase-comparator network ISA varies in dependence upon the timing or phase relations between the pulses of the two channels.

In the particular arrangement shown in Fig. 3, the pulse output of the comparator circuit I8A, as smoothed by the filtering or integrating network 26, constitutes the frequency-stabilizing control voltage of the grid potential of the D.-C. amplifier frequency control tube 46. Any change in the magnitude of this voltage varies the voltage drop across the resistor impedance 41 common to the anode circuit of the control tube 46 and to the reflector electrode circuit of the reflex klystron IIJA whose frequency is to be stabilized. The voltage of the direct current source, generally represented by battery 48, which pro vides the anode current of the tubes is regulated. The network including voltage regulator tube 49 and series resistor 58, provides an additional regulated potential for the screen electrode of tube 46. The arrangement of Fig. 3'as thus far described provides for rigid control of the oscillator i 0,

Also to obtain variation of the instantaneous frequency of the oscillator, in accordance with the desired modulation, the phase-comparison circuit IBA is provided with two resistors 38 connected in series in a branch of the network which is in shunt to the rectifiers 3|, 3 l The modulating potential is injected into the network by connection 21 from the common terminal of these resistors to the output of the modulator 25. The modulation is thus injected into the comparison circuit in advance of the integrating network 26, with the result the instantaneous value of the frequency control potential is varied at modulating frequency. Otherwise stated, the injected modulation provides the second term of Equation No.6.

In the specific arrangement shown in Fig. 3, the pulse-shaping network HA comprises a tube 4| upon whose grid circuit are impressed the amplified output pulses of the demodulator l-6 inverted to be of positive polarity. The network 42 in the anode circuit of tube 4! produces a train of pulses corresponding in number with the input pulses, but providing pulses of both -{PA and PA of opposite polarity across the resistors 53 of tube ll. Thus, in channel A of the servo system, there are produced two trains of pulses coincident in time but of opposite polarity for impression upon one input circuit of the phasecomparator, both terminals 34, 34 of which, as

above stated, are ungrounded, forming a divided The shaping networks IA, 23A are per se of known type and need not be here further described.

The steeper trailing edges of the sawtooth waves are used for coincidence or phase-comparison purposes. It may be assumed for purposes of explanation that when oscillator H] is operating at proper frequency, the positive and negative pulses of channel A as applied to rectifiers 3| occur exactly at the same time that the trailing edges of the sawtooth waves pass through their mean value as shown in Fig. 4A. Under such circumstance and assuming zero modulating signal, there is no change in the control voltage applied to the grid of tube 46. Should, however, the frequency of oscillator l shift above or below its proper value, the sharp pulses of channel A occur earlier or later with respect to the trailing edge of the sawtooth waves. Under such circumstance, the pulses of current through the rectifiers are not equal and opposite and the output voltage of network 26, at rate determined by its constants, rises or falls to a new value corresponding with the existing deviation of frequency F0 from the chosen output frequency. The time constant of network 26 is selected for effective filtering at the high sweep-frequency of oscillator l3. Equation No. 6 describes the operation of the servo loop and shows how it operates to restore the mean frequency of the oscillator IDA to its proper value.

The effect of introducing a modulating potential into the phase-comparator network of Fig. 3 is shown in Fig. 4B. During existence of modulation, the potential difference between the cathode and anode of both of the rectifiers varies in accordance with the modulation and accordingly the amplitude of the signal voltage as well as the phase relation of the pulses PA, PE determines duration and amplitude of the current pulses transmitted by the rectifiers to the integrating network. The output voltage of network 26 therefore contains the second term of Equation No. 6 which results in variation of the frequency of the stabilized oscillator at the lower modulating frequencies.

This modulating arrangement gives a progressively small frequency-deviation, smaller percentage of modulation, for higher modulating frequencies, as

departs from unity with decreasing B, which deemphasis may be desired, as in frequency-modulated transmission of broadcast programs. Where such deemphasis of the higher frequencies is not desired and wideband modulation is required, as in television transmission, the higher modulating frequencies are superimposed upon the frequency control voltage beyond the integrating network 26. Specifically as shown in Fig. 3, these higher frequencies may be transmitted from modulator 25 to the input circuit of the frequency control tube 46 in a path comprising conductor 28 and a capacitor 29 of high reactance at the lower modulating frequencies.

With the arrangement |8B shown in Fig. 5, the sawtooth pulses PB are applied in phase to the oppositely poled rectifiers 3|, 3|, and the pulses +PA, PA are applied 180 out-of-phase to those rectifiers. In this respect Figs. 3 and are similar and act similarly so far as the frequencystabilizing operation of the comparator is concerned. However, in Fig. 5, the modulating signal voltage is impressed upon a resistor 5|, or equivalent impedance, which is intermediate the resistors 38, 38 of equal high magnitude and one of whose terminals 53 is connected to the integrating network. Therefore, the modulating signal, as indicated by Fig. 6B, is unsymmetrically introduced into the comparator network affecting conduction in only one of the detectors.

symmetrically to introduce the modulation into both channels, the arrangement of Fig. 5 may be modified as shown in Fig. 7. In efiect, the resistor 5| of Fig. 5 is divided into two equal sections 5|A, 5|B, Fig. 7, with the midpoint or common terminal 53 connected to the integrating network.

In this symmetrical system, the modulating potentials in the two channels must be in phase: if they are in phase-opposition, the resulting modulation, which is proportional to the differential potential, is zero for equal signal voltages in the two channels. Consequently, the modulation is applied as in-phase signals to the two resistors 5|A, 5|B, or equivalent. Such in-phase signals may be obtained from two separate amplifier channels 25A, 253 having a common driving stage 25 with separate output conductors 21A, 27B to the comparator network. Figs. 4A and 4B previously discussed are illustrative of the frequency-stabilizing and frequency-modulating action of the comparator-modulator arrangement of Fig. '7.

With the arrangement shown in Fig. 7, since the modulating potentials are applied in phase, if there is appreciable noise or bum in the amplifiers, undesirable extraneous modulation of the oscillator-frequency will result. To minimize such undesired modulation, there may be utilized the arrangement shown in Fig. 8 in which the signal potentials are applied in phase-opposition to the comparator, so effecting substantial cancellation of hum and noise. In this arrangement, the modulation-injecting resistor 5|C, like that of Fig. 5, is intermediate the resistors 38, 38, but in Fig. 8 both of its terminals are connected to amplifier 25 which in this case has push-pull output. Also with this modification, a resistor 54 of suitably high magnitude, or equivalent conductive impedance, is connected between the rectifiers 3 I, 3! to serve as the input of the integrating network 26.

In the preceding modifications specifically illustrated and described, the sawtooth pulses of channel B have been applied in phase to the rectifiers 3|, 3| and the sharp pulses of channel A have been applied out-of-phase to the rectifiers. In any of the modifications, and as shown in Fig. 8, the sawtooth pulse may, as an alternative, be applied out-of-phase and the sharp pulses in-phase. In such case, the sawtooth oscillator 23A of Fig. 3, or equivalent, is followed by a push-pull inverter tube 60 with connections to condensers 33, 33 of the comparator circuit from the cathode and anode resistors of the inverter tube.

With such mode of connection of the servo channels to the comparator network I8D, the time relation of the two trains of pulses, as compared in the two channels of the comparator, which exist for zero deviation of the oscillatorfrequency from the set-point frequency is shown in Fig. 9A. The effect of introducing the modulation is shown in Fig. 9B.

The foregoing methods of injecting modula- 9, tion into the frequency-stabilizing system is not limited to use of the comparator circuit shown in Figs. 3, 5, 7 and 8: they can be employed in other phase-comparator circuits by following the basic requirements above set forth. By way of further specific example, there are briefly described three arrangements in which the modulation is introduced into a four rectifier comparator network of the type shown in the aforesaid applications Serial Nos. 4497 filed January 27, 1948, Fig. 1; 119,119 filed October 1, 1949, Fig. 2; and 35,185 filed June 25, 1948, Fig. 6. The pairs of rectifiers 31A, 31B form a rectifier bridge, the common terminal of one pair of rectifiers being connected through a resistor-condenser network 26A to one input terminal 34 of the comparator network and the common terminal of the other pair of rectifiers being similarly connected through filter network 2613 to the other input terminal 34, the blocking condensers 32, 32 permitting passage of the stabilizing pulses and of the modulating frequencies but precluding passage of direct current. The direct-current path for the rectifier includes the resistance 6!! in shunt to channel A, within the comparator.

Thus, briefly described, the phase-comparators 18E, F and G of Figs. 10, 11 and 12 each provides a unidirectional output voltage which varies in accord with change in the phase relation between pulses respectively supplied to the comparator by channels A and B of the frequency-stabilizing servo-system. In each of Figs. 10 to 12, the modulation term is introduced in a different way which, however, is similar to one of the methods of Figs. 3, 5, 7 and 8 and all of which are generically comprehended by Figs. 1 and 2. and discussion thereof.

Specifically, in Fig. 10, the modulating signal is applied in a common channel of the comparator by connecting one terminal of the modulator 25, or equivalent, to the electrical midpoint, as provided by equal resistors 38, of a shunt branch of the network IBE'. Thus as in Fig. 3, the modulating signal in effect amplitude-modulates both trains PA, PB of the frequency-stabilizing pulses. In Fig. 11, the modulating signal is unsymmetrically introduced into the comparator network by resistor connected as in Fig. 5 between equal resistors 38, 38, with one terminal 53 connected to the pulse-integrating network 26'. In Fig. 12, the modulating signal is symmetrically introduced into the comparator network, by resistor 5! and identical amplifiers 25A, 25B, disposed and connected as in Fig. 8.

What is claimed is:

1. An arrangement for frequency-modulatin and frequency-stabilizing an oscillator system including an electronic tube having an electrode Whose potential afiects the oscillator frequency which comprises means for producing two series of simultaneous constant-amplitude pulses jointly containing frequency error information, means for deriving from said pulses a control potential applied to said electrode comprising a phase-comparator including rectifier means and a smoothing filter, said phase-comparator having two input circuits to which said two series of constant amplitude pulses are separately applied, and means for continuously injecting the modulating signal for said oscillator into at least one input circuit of said phase-comparator in advance of said smoothing filter whereby the magnitude of said control potential varies both in accordance with variations of the phase-relation iii between the pulses of said two series and. with the modulation.

2. An arrangement for frequency-modulating and frequency-stabilizing an oscillator system including an electronic tube having an electrode whose potential affects the frequency of the generated oscillations which comprises means for producing a series of constant-amplitude input pulses containing standard-frequency information and a series of simultaneous constantamplitude input pulses containing frequencyerror information, all of the pulses of one of said series being of like polarity and the pulses of the other of said series comprising coincident pairs of opposite polarities, meansfor deriving from said input pulses a control potential applied to said electrode comprising a phase-comparator providing transmission channels for said pulses including rectifier means poled to produce output pulses varying with variation of the phase-relation between the pulses of the two series, and a smoothing filter for integrating said output pulses for application as said control-potential to said electrode, and means for continuously injecting the modulating signal for said oscillator into at least one of said channels in advance of said filter to efiect variation of the instantaneous magnitude of said control potential with the modulation.

3. An arrangement as in claim 2 including means coupled to the injecting means for introducing the modulating signal into a channel common to said input pulses to vary the amplitude of said pulses as applied to said rectifying means.

4. An arrangement as in claim 2 including means coupled to the injecting means for introducing the modulating signal into the channel of the paired pulses as in-phase modulating potentials whereby the amplitudes of the paired pulses are varied differentially with the modulation.

5. An arrangement as in claim 2 including means coupled to the injecting means for introducing the modulating signal into the channel of the paired pulses as phase-opposition modulating potentials.

6. In a two-channel servo-system for stabilizing the frequency of an oscillator which includes a phase-comparator having two input circuits separately coupled to the two channels of said servo-system for producing the frequency-control voltage, the combination of means for continuously injecting a modulating signal for said oscillator into at least one input circuit of said phase-comparator to vary the instantaneous magnitude of said control voltage in accordance with the modulation.

'7. In a two-channel servo-system for stabilizing the frequency of an oscillator and in which the frequency-control voltage is produced by a phase-comparator and a filter for smoothing the output thereof, said phase-comparator having two input circuits separately coupled to the two channels of said servo-system, the combination of means for continuously injecting a modulating signal for said oscillator into at least one input circuit of said phase-comparator in advance of said filter to vary the instantaneous magnitude of said control voltage in accordance with the modulation and with progressively decreasing efiect for the higher modulating frequencies.

8. A system for frequency-stabilizing and frequency-modulating the carrier of an oscillator including an electronic tube having an element whose potential afiects the oscillator-frequency F0 in accordance with the transfer factor ,u, which comprises a feedback loop from the output system of the oscillator to said element, a frequencyerror detector in said loop including a resistancereactance network having the overall transfer factor B, a source of standard frequency F5 coupled to said detector for production thereby of a feedback potential applied to said element to affeet the oscillator-frequency in accordance with the relation F I+FB a 1+m where e1 is the instantaneous magnitude of said potential, and modulating means coupled to said detector in advance of said network to vary said 12 feedback potential in accordance with the modulation to eifect incremental change of the oscillator frequency defined by both #6 ll 1 #1 1 MB where m is the modulation factor.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,279,659 Crosby Apr. 14, 1942 2,419,527 Bartelink Apr. 29, 1947 2,469,218 Thomas May 3, 1949 2,559,719 Hershberger July 10, 1951 2,562,943 Pensyl Aug. 7, 1951 2,591,257 Hershberger Apr. 1, 1952

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