US2287925A - Radio receiver - Google Patents

Description

s. Y. WHITE RADIO RECEIVER June 30,1942.

Filed Feb. 29, 1940 W v K MS Y ATTORNEYS Patented June 30, 1942 UNITED STATES PATENT OFFICE RADIO RECEIVER Sidney Y. White, Wilmette, n1. Application February 29, 1940, Serial No. 321,377 9 Claims. (Cl. 250-40) This invention relates to a radio receiver adapted to insure positive contact with a transmitter transmitting an unmodulated or modulated carrier wave and more particularly to the reception of speech modulated carrier waves having a frequency above 20 megacycles.

The diiiiculties of establishing positive contact between a receiver and a transmitter working at such high frequencies are well recognized and contact is not easy to establish even when the carrier frequency is left on, as in the present broadcasting practice. When the carrier is on only while talking, as in some systems, the period of communication may be very short, as for example when the entire message may comprise the single word yes, the diihculty of tuning in the receiver becomes even greater. It is a broad object of the present invention to establish positive contact with a high frequency transmitter by automatically scanning the portion of the frequency spectrum which includes the frequency of the desired transmitter.

The amount of seaming coverage, namely, that percentage of the frequencyspectrum which is covered by each passage of the automatic scanning device must include the zone of error" which includes the necess zry tolerances both of the transmitter and the receiver involved in communication of this type. These tolerances are made up of 1) thermal drift of tubes and other heated components of both transmitter and receiver, (3) voltage variations which affect the frequency, (3) errors in calibration of scales used on dials, etc., (4) personal errors in making adjustments of tuning instrumentalities, etc. The total zone of errorarising from such causes may be substantially 0.4% or the transmission frequency at the present stage of the art. While under normal circumstances .a scanning sweep, either manual or automatic, of this amount would establish positive contact between a transmitter and any receiver, by increasing the scanning range we may add to the zone of positive communication. By employing a total scanning range of 1% of the carrier frequency, when receiving carriers in the region of 150 megacycles, we may so arrange it that 0.2% is allocated at each end of the range to overcome the several errors making up the "zone of error." This leaves 0.6% free in the middle of the scanning zone in which other transmitters may be located, all of which may be simultaneously operated on different non-interfering frequencies. In the example illustrated, 6 such transmitters could be employed whose carrier frequencies diifer by kilocycles, so that no interference would result between them if each employed the usual 10 kilocycle modulation band. It is an object of the present invention to provide such a receiver which will automatically scan a portion of the spectrum in which several transmitters may be operating and to quickly provide positive contact with one of them and automatically maintain such contact while the transmission continues.

Where the receiver is of the superheterodyne type, the scanning may be effected by any means which varies the oscillator frequency and may include such mechanical elements as motordriven variable condensers, movable ferro-magnetic cores or short-circuited rings. The motor could be a rotating device of any known type, a Rochelle salts crystal, a condenser of a telephone diaphragm type, etc. Where such a motor device is used, however, it is open to the serious objection that it has substantial inertia and it is practically impossible to stop it at the proper position to insure correct tuning and it also tends to hunt. According to the present invention, this objection is overcome by providing a scanning device which has no moving mechanical parts and can be made practically inertialess.

It is a further object of the invention to provide a means for use in association with such a scanning device which operates on passing over a carrier and is responsive to the frequency and not to the amplitude of the received wave. If it were responsive to the amplitude, it might pass over several carriers and only stop on the strongest carrier. This would result in delay in establishing communication between a receiver and one of several transmitters operating simultaneously on different frequencies and all desiring to contact the receiver. According to the present invention, indicating means are provided to indicate the first carrier encountered and thus provide for a more rapid setting up of the communication channel.

According to a further object of the invention a director device is provided which becomes immediately effective to control the tuning of the receiver once a carrier has been located by the scanning device. This director is arranged to overpower the scanning device so that it is not necessary to stop the latter, which is left in operation. This feature becomes important in designing receivers for communication and mobile work where rapid response of a receiver to a desired carrier is necessary from the following sively on a plurality of carrier frequencies ineither regular or random sequence. In each of these cases, when the carrier disappears, the receiver should instantly respond and commence scanning again to pick up the carrier as soon as it lays down a usable field strength.

The desirability of not stoppin the scanning device arises from the necessity of extremely rapid action in locating a carrier and from the property of scanning means requiring a rather long time to become operative and to stop. The first point becomes apparent when it is considered that in the case of the generation of a half cycle wave by a multi-vibrator, it requires about seconds to build up to full amplitude. If it is shut off, as by disconnecting it from its source of plate voltage, large charges are left in its condensers which require a substantial period of time to be dissipated through the very large resistances employed. The present invention provides means whereby the scanning device is continually operative so that immediately upon the disappearance of a carrier which has been tuned in, it is effective to tune the receiver over its tuning range in its search for a new carrier. It is a further object of the invention that the time interval required to pick up the carrier be kept to a small value, preferably a fraction of a second.

The voltage generated by the scanning device may be of any desired form such as saw-tooth, square, sine, or triangular. The triangular wave shape is ideal in that the spectrum may be scanned steadily in one direction up to the end and then instantly reversed and scanned in the opposite direction at the same uniform speed. There is some objection, however, to generators which generate this wave shape in that they. are usually bulky and heavy, as now known in the art.

The saw-tooth wave shape provides a steady sweep in one direction and a snap back return. This has the advantage that the carrier is always approached from one direction allowing the more effective use of a beat frequency oscillator at the final detector. be an advantage in picking up a preferred station toward the beginning of the sweep, so that if two stations appear simultaneously, the receiver will probably pick up the preferred one.

The sine wave sweep obviously slows up at either end and is most rapid in the middle. This is advantageous with the observed characteristics of apparatus of this nature, the operation tends to be more diificult out toward the ends of the sweep range, since in certain cases the receiver may be operating on either side of the peak of the radio frequency amplifying stages. It is accordingly a further object of the invention to provide a scanning device for use in a receiver of this type in which the scanning voltage is generated by a multi-vibrator which generates in the plate circuits a very high amplitude wave of substantially square shape and filtering it with an enormous filter which pro- There may possibly This type of .sine wave generator is very light and portable and has the further distinct advantage of requiring no moving parts such as the rotors employed in several types of electric generators. tion will become apparent to those skilled in the art as the description thereof proceeds. For a better understanding of the invention, however. reference is made to the accompanying drawing in which:

The single figure is aschematic circuit diagram of a radio receiver embodying the invention.

Referring to the drawing, the invention is shown as applied to a radio receiver of. the superheterodyne type comprising a tunable radio frequency amplifier l whose input terminals are connected to a receiving antenna circuit 1. a first detector'or mixing device 3, an intermediate frequency amplifier 4, a second detector 5, audio amplifier 6, loud speaker I and automatic volume control device 8 which controls the output level of the receiver in a known manner. The aforementioned receiver parts may beof any known type and a detailed description thereof is not necessary to an understanding of the present invention. The beat frequency is supplied to the mixing device 3 by an oscillator 9 whose frequency is controlled in a manner to be explained later by a reactance control device Ill which is connected to a line H, which is also connected to ground through a large condenser Cl and to a scanning oscillator l2 and also to a director device l3. Suitable operating voltage is supplied to the director device II from the intermediate frequency amplifier 4 through a lead I.

Still referring to the drawing for a more detailed description of certain features of the invention and wherein the radio frequency amplifler I is shown as comprising a circuit including a condenser l5 and coil l6 which may be permeability tuned to pick up-desired carrier frequencies as indicated at 11, as by means of a powdered ferro-magnetic core. The director unit l3 consists of a driver tube V." whose inner control grid is connected through lead M to the output of the intermediate frequency amplifier l,

duces a very close approximation to a sine wave.

and whose plate circuit comprises the low impedance primary windings l8 and I9 which are coupled respectively to the secondary transformer windings of the circuits 'ICl, T02, one of which is tuned to a frequency of from 5 to lo kilocycles above the intermediate frequency, and the other of which is tuned by the same amount below the intermediate frequency. These circuits feed their diodes VT! to develop the differential voltage across their resistors R2 and RI. As shown, one end of the resistor RI is grounded and one end of resistor R2 is connected by lead H to apply the director voltage to the inner grid of the oscillator control tube VT. 1

The scanning device comprises the oscillator I! which preferably generates a current of a low audio frequency, the frequency of one-half cycle per second having been found to be very suitable for the desired purpose. This oscillator comprises the tubes V' 15 and VTG, the plate of one tube being connected to the grid of the opposite tube through the condensers Cl and C2 having 9. capacity of one-half mfd., and the grids of the tubes being connected to their cathodes through the 2.0 megohm resistors R5 and R6, to thereby produce a half cycle sweep voltage. This voltage is supplied to the lead ll through condenser C3 which blocks off the direct current, and the re- Still further objects of the invensistor R1, so that the sweep oscillator voltage and the director voltage are supplied in parallel through the resistor R9 to the inner grid of the control tube VTI. It will be understood, however, that it is not necessary to modify the transconductance of the control tube by applying the director voltage to the inner grid as it may equally well be applied to another grid of the tube as, for example, the screen grid or the suppressor grid. Or alternatively, the director voltage may be applied to the inner control grid while the sweep oscillator voltage is applied to an outer grid, such as the suppressor grid.

It has been found that in receivers of this type, and especially when designed for the reception of carrier frequencies of the order of 100 megacycles, especial attention must be given to the design of the oscillator circuit. Experience has shown that the cathode of the oscillator tube, herein shown as VT3, must be grounded in order to eliminate heater-to-cathode cyclic effects, when the source of heater voltage is an alternating current or a direct current from the battery which also supplies a motor generator or vibrator type of plate supply voltage. At these high frequencies the grid of the tube has extremely poor admittance effects, and if attempts are made to tune the grid circuit, conditions almost invariably occur where the system will commence oscillating at a parasitic frequency determined by the constants of the plate circuit. It is, therefore, preferred to use an oscillator whcse plate circuit is tuned, and a tickler circuit provides a grid voltage of the proper phase to generate the oscillations. The resonant plate circuit may have lumped constants comprising the coil Li and condenser 22, as shown, or may comprise a quarter-wave line or somemodification thereof, such as two nested cups. Where the constants of the circuit are lumped, a variable condenser is unsuitable for tuning due to the varying L-C ratio, which gives much more stability at one end of the tuning range than at the other, thereby providing greatly exaggerated variations over the range of sweep of the control tube. By making the condenser 22 of a fixed value and varying the inductance of coil Ll so as to change its permeability, as by means of a powdered ferromagnetic core, as indicated at 23, substantially equal percentage tuning effects are secured throughout the band of frequencies. An additional advantage of effecting the tuning of the oscillator by change of permeability lies in a more simplified switching procedure where different coils are used for the reception of different frequency ranges. Each such coil may have attached to it permanently its own tuning condenser, thus forming a circulating path of low resistance; and the only switching required being the switching connections at the grids and plates of the tubes.

' It is found that the ordinary hydrogen reduced iron cores are not at all suitable for permeability tuning at frequencies of the order of 100 megacycles. However, it has been found that ferromagnetic cores known under the trade name Aladdinite" are very satisfactory. These cores are made from a synthetically produced ferromagnetic mass powder consisting substantially wholly of magnetic oxide of iron in the form of minute particles, substantially all of which, as they appear under a microscope, are of generally rounded form. These particles are preferably to that. of the plate.

molded into cores by mixing with several per cent of Bakelite as a binder.

The grid of the oscillator VIIi is shown as being energized through condenser C6 and coils L3 and L2 connected in series. Since the accuracy of calibration of a superheterodyne receiver depends largely on the oscillator, stable oscillators are, therefore, highly desirable. It is also highly desirable that there be a 180 phase shift between the plate and grid voltages of the tube, and, where a tickler coil is used to provide the feed-back, this is always given a polarity such as to reverse the phase of the grid relative The mutual inductance oi. the tickler coil with coil LI of the tank circuit is designated as L2 and its leakage inductance by the reference character L3. Depending upon the physical placement of the component parts, the length of leads employed and numerous other factors, the ratio between these two inductances Lil-L2, as well as their actual values, may vary between wide limits.

A desirable oscillator for use with a control tube in a system of this type would be one without phase angle, that is, one in which the plate energy exactly reinforces the grid energy with no reactive component. This ideal condition would require a vacuum tube which is a simple generator of negative resistance, an arrangement which may be closely approximated in practice. If the oscillator is well designed with respect to this feature, its plate supply voltage may vary within rather wide limits without substantially affecting the frequency of the oscillations generated, although their amplitude will be affected to some extent. With respect to the requirements of the oscillator control tube, these are exactly the opposite. It is desired that this generate pure reactance only and neither negative nor positive resistance. It is necessary, therefore, that the control tube be coupled into the tube circuit to be controlled in such manner that great phase shifts are secured either in the energy feed to the grid or the energy of the plate circuit, or both.

A standard means of shifting phase is to place a reactance in series with a resistance and take the voltage drop across the reactance. In practice, only a limited phase shift can occur, since a shift would require an infinite resistance a and a perfect reactance. In practice, approximations of 90 can be reached as a limit, but since in .radio frequencies only relatively small resistances can be employed, it is only practical to secure a 60 or 70 phase shift instead of the desirable 90". A further phase shift, however, may be secured by connecting another phase shifting network in series with the first, the second network producing an additional phase shift approximating 90. Since the two phase shifting means in cascade have the possibility of approximating an phase shift, it is possible to utilize a portion of each in any desired combination to give a resultant phase shift of 90, which, of course, represents a pure reactance.

Now considering the circuit which is grounded at the lower end of coil Ll, the grid receives charging current through L2 representing the mutual inductance of the tickler coil with Li. The grid current at the end of L2 is thus exactly 180 out of phase with the circulating current in Ll. Before reaching the grid of tube VT3,

the current must pass'through some series leakage inductance, represented by L3, which produces a reactance of rather small amount, but

phased in relation to that of the plate tank circult as to cause the tube to oscillate. The amount of phasing may be varied as desired by making L3 of the proper value, even, if necessary, by winding an additional uncoupled coil in' series with the grid, or, by more loosely coupling L2, and by making this coil larger we can in- .crease the leakage inductance to any desired amount of without having any physical coil, as represented by L3. With this energy, preliminarily phased, as described, the grid energy of tube VT3 may now be further phased in the same direction by means of the circuit comprising resistor R8 and the grid-cathode capacity of the control tube VT. It will be understood that condensers C5 and C8 are merely for the purpose of blocking the direct current and are 'so large that the phasing effect is negligible.

By a suitable choice of the values of L3. and R8 it is possible to produce the desired degree of reactance which we wish with a maximum stability of the oscillator and a minimum loading effect due to the connection of the control tube VTl with the oscillator. Considerable difficulty is met with in complicated networks of this type where the oscillator is required to supply voltage to the mixer device, such as 8, as well as having a control tube, such as VT, connected across it. The system is prone to be so heavily overloaded as to cease oscillation, entirely, or if excessive amounts of negative resistance are generated in the control tube, parasitic oscillations are likely to occur at a frequency determined by some loop, which may be almost anywhere in the system, or even coupled rather loosely to the circuit system. With the circuit arrangement described, phase relations may be established so that the control tube VTl acts substantially as a pure reactance device in controlling the frequency of the oscillations generated by tube VT3.

Instead of securing the double phasing effect by the arrangement previously described, it may be secured in a somewhat satisfactory manner by the inclusion of a resistor RH in series with the plate of tube V'Il and the tank circuit comprising coil Ll, in which the preliminary phasing is secured by resistor R8 alone at the input of the tube and additional phasing secured in an additive sense by RH. In cases where it may be desired to eliminate the effect of resistor RH,

a suitable short circuiting switch may be provided therefor as indicated at 24.

The control tube V'Il is provided with the usual supply voltage, its inner grid being given a normal bias of approximately -6 volts by connecting its cathode to the bleeder resistor 25 at a point approximately this number of volts above ground. This permits the operation of the tube at the mid-point of its control range, as determined by plotting a curve of grid bias against oscillator frequency, which results in an S-shaped curve, so that we secure operation about the mid-point thereof. This mid-point usually occurs at about --6 volts and if this value is changed to -3 volts, the tube becomes much more effective and if changed to -9 volts, the control tube becomes practically ineffective. The control tube is then in condition to respond to sweep. In director voltages which are in the form of positive and negative voltages of various amounts, the polarity of the applied voltage determines the sense of direction of the control tube and the amplitude thereof determines the amount of the control. The voltage generated by the sweep oscillator is arranged to be a total peak to peak of 6 volts so that the effective voltage at the grid of control tube VT from the sweep oscillator is :3 volts. This voltage swing sufllces to sweep the receiver through a range of frequencies approximating 1% of the carrier frequency. The director, including its resonant circuits TC I and T02 and driver tube VTl, is so designed that on the reception, of any usable signal strength it generates :20 volts even on the weakest signal for which the system is designed, and if the signal strength is increased by as much as 100 decibels, the director voltage will only increase by 5 volts. This limited characteristic, here and at other portions of the receiver, serves to prevent the building up of high amplitude-surge voltages which might give rise to motor-boating phenomena.

The director device is designed to generate a voltage approximately 7 times greater than the control tube can usefully employ, namely, 3 volts, since the higher the voltage generated by the director, the closer to actual resonance the set will be automatically tuned. Furthermore, if the sweep oscillator encounters a carrier at the extreme end of its range, as for instance at 3 volts, the sweep oscillator can generate 6 volts from that point, which voltage must be overcome by the director and still leave a sufflcient margin of voltage to tune the receiver to the incoming carrier within approximately 1 kilocycle, at the carrier frequency of 150 megacycles.

Since receivers of this type must operate at points low down in the noise level in making an utmost effort to pick up a carrier and secure contact with a transmitter, and since at any given instant there is equal distribution of noise throughout the spectrum, it is preferable to arrange the intermediate frequency amplifier and director system so as to have a symmetrical operation with respect to the center frequency. Otherwise more noise may be generated on one side than on the other and a small and fluctuating noise voltage will appear at the output of the director tube. Due to the fact that it only requires a few millivolts applied to the grid of the control tube VT4 to cover an entire channel, while the director supplies voltages of many times this amount, symmetrical operation about a center frequency serves to reduce the noise component in the output of the receiver.

It has been noted after considerable experience with receivers of this type that as the frequency is raised we can attain such a frequency that the ordinary noises which prove so troublesome in ordinary reception, such as static, diathermy machines, sparking commutator motors and ignition noises, gradually die out, and at some vague point, probably below megacycles, become extremely rare in a sharp receiver. There is left, of course, the thermal agitation noise in the circuits and the noise and shot eifects in the tubes. This is a more or less regular hiss type of noise of quite a definite and dependable character and is easily recognized by anyone skilled in the art.

Where space and weight are no consideration in designing a receiver of this type, we should probably always prefer a definite sweep voltage such as that produced by the 0.5 cycle oscillator shown, but experience has shown that if the filter condenser C4 be lessened in value to emphasize the noise component in the output of the director connected from the control tube and connected due to the amplified noise arising from the thermal agitation in the circuit lS-Hi, that the 0.5 cycle oscillator can be omitted with rather good results. In this case the fluctuating noise voltages produced in the output of the director by the amplification of the thermal agitation and shot effect current of the first stage in the differential circuits TCI and T02 are sufficient to sweep the control tube through its entire range quite rapidly and at random, but in a given period we find we cover the entire useful range of sweep in less than a second, although it is recognized that several pulses of the same polarity may follow each other rather than the smooth pulses of always alternate polarity which we obtain from the more disciplined action of the 0.5 cycle oscillator. The above condition obtains only when the receiver is of great sensitivity, 1. e., suflicient to develop on peaks a voltage sufllcient to fully control the control tube.

The tuning ranges of receivers of the type described are of two distinct classifications: first,

for comparatively large tuning ranges that are to be covered, where operation may be expected on any one frequency throughout these large ranges; and, second, where specific point-to-point service might be set up on an assigned frequency. In this latter case, no tuning range at all in the ordinarily accepted sense is required, but merely excursion around the assigned frequency to establish contact between transmitter and receiver at will. In this latter case probably much closer tolerances can also be expected with respect to the accuracy of the transmitter frequency, as it would probably be crystal controlled, and only the receiver errors would remain. We thus have the two conditions, one a large tuning range and a small sweep range; the other, no tuning range at all, but a sweep range probably less than 1%. In a receiver designed for operation over a tuning range of from 100 to 200 megacycles the following circuit constants are found suitable.

Vacuum tubes of the following types are found suitable for use'with the circuits described:

VTI=type 6J7 pentode VT2=type 6H6 double diode VT3=type 955 acorn triode VT4=type 954 acorn pentode VT5, VT6=type 6N7 double triode In using the receiver, the operator tunes the input circuit of the radio frequency amplifier l so that it will pass the carrier frequency or frequencies of the transmitter or transmitters with which it is desired to establish contact. In the illustrated embodiment this tuning is effected by manual adjustment of the permeability tuning means I1. Since the frequency of oscillator VT3 is being cyclically varied through the action of the control tube VT4 whose reactance is in turn being cyclically varied by the sweep oscillator VT5, VTG, as soon as a carrier appears which forms a beat frequency with the oscillator frequency which lies within the frequency band passed by the intermediate frequency amplifier 4, an energizing voltage is applied through lead H to the control grid of the director driver tube VTI. If the output of the director VT! is disto a voltmeter and a signal is tuned through, a voltage of or -20 volts is generated in the director when the receiver is 5.0 kc. ofi tune, in other words, when the intermediate frequency impressed on the director is 5 kc. above or below the mid-intermediate frequency. This voltage is developed for all signal strengths down to the noise level of the receiver, which is equivalent to a voltage of approximately 1 microvolt on the grid of the first radio frequency amplifier l and is prevented from rising substantially above this value for stronger signals by the limiting action of driver tube VTI.

Tube VT4 is an acorn type 954 tube and has a characteristic curve such that at a plate voltage of 150 volts, its maximum Gm. is at about -3 volts on its control grid while its Gm. becomes negligible at about --9 volts on its control grid. The maximum output voltage of the director is, therefore, many times greater than that required to fully control the control tube VTI and gives a steep slope of output voltage vs. frequency change so that a departure of only 1 kc. from the mid-intermediate frequency will develop 3 volts of appropriate polarity to tune the oscillator VT3 toward the correct frequency.

Should a carrier appear when the voltage of the grid of control tube VT4 is increasing and is, for example, at 8 volts and which would be tuned in when the grid voltage of VT is at 4 volts, as the'grid voltage of VT passes successively through 7, 6 and -5 volts under the control of the oscillator I2 as the tuning position is being approached, no intermediate frequency appears in the output of the amplifier 4 and, therefore, no energizing voltage is developed by the director VTZ. As the oscillator l2, however,

causes the grid voltage of tube VTl to closely approach 4 volts, the frequency of oscillator VT3 is increased, an intermediate frequency voltage of increasing value appears in the output of the amplifier 4, which voltage energizes the director VTZ and a voltage appears in the lead H which tends to increase the voltage of the grid of VT towards the desired voltage of -4.0. In other words, as the oscillator frequency approaches the proper tuning in frequency, the voltage generated by the director assists the voltage output of the oscillator I! in causing the frequency of the oscillator VT3 to approach the correct tuning-in value. As the voltage of the grid of W4 is further increased by both the oscillator l2 and director VT! it will, of course, pass through the value of 4.0 volts when the frequency of oscillator VT! is such as to properly tune in the station. At this instant since the mid-intermediate frequency is being impressed upon the director VTZ, the output voltage of the director is zero. An instant later, however, the voltage of the grid of VT is increased to a value slightly above 4.0 volts by the oscillator I2 and the frequency of the oscillator VT3 is correspondingly increased so that a frequency above the mid-intermediate frequency appears in the output of the amplifier '4 and a voltage is generated by the director VT! which opposes the increasing voltage being generated by the oscillator l2. This opposing voltage is of sufllcient amount to counterbalance the increase in voltage and to maintain the voltage of the grid of VT4 at a value only slightly above 4.0 volts, which value is such as to maintain the oscillator VT3 tuned within 1.0 kc. of the correct. frequency. These opposing voltages are balanced within extremely close limits, since to tube the receiver entirely through a station 10 kc. wide requires a change in voltage on the grid of the control tube of only 0.04 volt for a carrier frequency of 150,000 kc. As the voltage generated by the oscillator l2 gradually passes up to the limit voltage of 3 volts and returns in the opposite direction to a voltage just above 4.0 Volts, these voltage changes have small influence on the grid of tube VT! and, therefore, do not appreciably change the frequency of the oscillator VII. When the voltage generated by oscillator l2 reaches the value of 4.0 volts, the oscillator VT! is now generating the correct frequency and the output voltage of the director VT! passes through zero value. As the output voltage of oscillator I2 decreases to a value only slightly below 4.0 volts, a voltage appears in the output of the director VT2 which rises sharply and which opposes any decrease in voltage in the lead H by the oscillator l2 so that the director voltage impressed upon the lead II is sufllcient to substantially overcome the decreasing voltage applied to this lead by the oscillator i2 and to prevent the grid voltage of control tube VTI from decreasing below the desired value of 4 .0 volts by any substantial amount so that the frequency of oscillator VT! is maintained at a'proper value to maintain the station tuned in. As the output voltage of oscillator l2 passes successively through the values 5, 6, 8 and its limiting value of --9 volts, the net voltage impressed on the lead II and the control grid of tube V'Il suffers no substantial change but is maintained by the director VT2 at a value only slightly less than 4.0 volts and the receiver is thus maintained in tuned-in condition throughout the full range of sweep voltage of the oscillator l2. The receiver remains locked on the carrier even though the carrier frequency should vary considerably. Upon interruption of the carrier, the scanning action of the oscillator VT! is resumed under the control of the sweep oscillator VTS-VTG since the director device VT! is no longer generating any director voltage to operate the control tube VTl. Upon reappearance of the carrier, the receiver will again lock on it in the manner described, should its transmitter be the only one operating. However, if two or more transmitters are operating on different carrier frequencies within the acceptance band of the input circuit l-l6, the receiver will automatically lock on the first carrier passed over, since the oscillator VT3 will produce a difference frequency with such carrier frequency which lies within the acceptance band of the intermediate frequency amplifier 4.

While the receiver shown in the drawing and above described is a single superheterodyne, the apparatus in which the invention was developed was a double detection type superheterodyne with a first intermediate frequency of 17.5 megacycles and a final intermediate frequency of 0.46 megacycle. The term inertialess" as used in the following claims designates a control means having no moving mechanical parts and hence free of mechanical inertia.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. A radio receiving system for receiving carriers of different frequencies associated with dif- 7 ferent communication channels comprising in combination. a circuit tunable over a wide range of frequencies, means comprising a reactance generating device for tuning said circuit over a range of frequencies at least as wide as that consisting of a plurality of adjacent communication channels, means free of mechanical inertia connected to said reactance generating device and arranged to automatically generate oscillations of low frequency and of sufllcient amplitude to repeatedly change the reactance of said reactance generating device by an amount suiiicient to cause the tuning of said circuit through said wide range of frequencies and frequency responsive means responsive to the reception of any one of the carriers for maintaining the tuning of said circuit approximately at a desired frequency within said range of frequencies.

2. A radio receiver comprising an amplifier having an input circuit having an acceptance band at least as wide as the frequency spectrum of at least three transmitting stations operating on different carrier frequencies, a tunable circuit, means free of mechanical inertia arranged to automatically tune said tunable circuit over a range of frequencies difierent from but approximately as wide as the acceptance band of said input circuit, a mixing device connected to the output of said amplifier and to said tunable circuit, and a frequency responsive device connected to the output of said mixing device and arranged to autgmatically render ineffective the action of said automatic tuning means on the tuning of said tunable circuit in response to the impression on said input circuit of carrier current from any one of the transmitting stations.

3. A radio receiver as claimed in claim 2 in which the means for automatically tuning the circuit comprises a low frequency oscillator arranged to generate successive oscillations of constant amplitude, said oscillator comprising a pair of similar electron discharge tubes and means for coupling the plate of each tube to the grid of the other tube.

4. A radio receiver comprising an input circuit having an acceptance band at least as wide as the frequency spectrum of a plurality of transmitting stations having different carrier frequencies, a tunable circuit, means free of-mechanical inertia for continuously and automatically tuning said tunable circuit over a range of frequencies substantially as wide as the acceptance band of said input circuit, a mixing device connected to said input and tunable circuits and a frequency responsive device connected to the output of said device and arranged to render ineffective the action of said automatic tuning means on the tuning of said tunable circuit in response to currents in the output of said mixing device of a predetermined frequency.

5. In a radio receiver of the superheterodyne type tunable to receive signaling currents of different carrier frequencies within a wide band of frequencies and each identified with a different transmitting station, said receiver comprising an oscillator and an intermediate frequency amplifier, a mixing device connected to the input of said intermediate frequency amplifier and to said oscillator, a circuit network connected to the input of said mixing device and having an admittance band sufllciently wide to transmit at any instant carrier currents from a plurality of the transmitting stationsof means free of mechanical inertia electrically connected to said oscillator and arranged to automatically vary the frequency of the currents generated by said oscillator throughout a range of frequencies substantially as wide as said band, a director device coupledto the output circuit of said intermediate frequency amplifier and responsive to the frequency of the currents passing through said amplifier and a reactance generating device connected to said director device and arranged upon the energization thereof by said director device to maintain the frequency of the currents generated by said oscillator substantially constant.

6. In combination with a radio receiver of the superheterodyne type adapted to receive ultra high frequency carrier currents of difierent frequencies and comprising a first detector and an oscillator, said oscillator comprising a vacuum tube having coupled grid and plate circuits, said plate circuit comprising a coil and a fixed condenser connected in parallel and providing lumped constants determining the resonant frequency of said plate circuit, control means for adjusting the inductance of said coil to cause said oscillator to generate currents of a predetermined frequency and means free of mechanical inertia for automatically causing the frequency of the oscillator to vary progressively from a first frequency to a second frequency and from the second frequency to said first frequency a plurality of times in succession, said first and second frequencies being adapted to combine with the highest and lowest received carrier frequencies to form a beat frequency.

7. A radio receiver adapted to receive signal currents of different carrier frequencies lying within a band of frequencies higher than 20 megacycles and the frequency difference between the highest and lowest frequency signaling currents being of the order of 200 kilocycles comprising, in combination, inertialess means arranged to automatically and cyclically tune said receiver over the frequency band, means responsive to the reception of any one of several carriers having a frequency within said band for rapidly and automatically rendering said inertialess means ineffective to continue the tuning of the receiver over said band and to automatically permit the cyclic tuning of the receiver by the said inertialess means again, in response to the disappearance of the carrier.

8. A radio receiving system comprising in combination, an input circuit having an acceptance band at least as wide as the vfrequency spectrum of a pluralii ,7 of transmitting stations of different carrier frequencies, an electron discharge tube amplifying device having its input connected to said input circuit and arranged to amplify to a very high degree the thermal agitation noise appearing in the input circuit, a tunable circuit connected to said amplifying device, a reactance generating device connected to said tunable circuit, a frequency responsive director device coupled to the output of said amplifying device and a filter circuit connecting the ouput of said director device to said reactance generating device, the time constant of said filter circuit being so small as to cause the amplified noise voltage variations appearing in the output of said director device to vary the reactance of the reactance generating device in such a manner as to cause the random tuning of said tunable circuit over a wide range of frequencies.

9. A radio receiving system comprising in combination, an input circuit having an acceptance band at least as wide as the frequency spectrum of a plurality of transmitting stations having

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