US2029035A - Multirange superheterodyne receiver - Google Patents

Description

Jam. 2", 11936. w. VAN B. ROBERTS MULTIRANGE SUPERHETERODYNE RECEIVER Filed Oct. 21, 1933 VVVIIVI INVENTOR WALTER VAN B. ROBERTS BY 7% Z QEE BWQ -53 ATTORNEY Patented Jan. 28, 1936 UNITED STATES MULTIRANGE SUPERHETERODYNE RECEIVER Walter van B. Roberts,

Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 21, 1933, Serial No. 694,545

8 Claims.

My present invention relates to radio receivers, and especially to a simplified superheterodyne method for wide ranges of wave lengths.

It is well known that in any superheterodyne type of receiver there are always two frequencies which may be received for a given oscillator frequency. The one of these which is not the desired signal frequency is called the image frequency, and lies on the other side of the oscillator frequency from the desired signal frequency and at an equal distance. Interference caused by the simultaneous reception of an undesired station lying in this position is called image interference. The only way to avoid such interfer- 5 ence is, at the present time, to use a fairly high intermediate frequency so as to separate the image frequency from the desired frequency by a considerable amount, and then to suppress currents of the image frequency by some sort of electrical filtering or tuning. In general, such tuning or filtration involves variable elements. In the case of a receiver designed to cover a wide range of wave lengths, the number of tuned circuits and switching arrangements for the different wave length ranges becomes excessive for ordinary purposes.

Let it be assumed that image interference troubles were non-existent. In such a case the most desirable intermediate frequency would be a very low one which would permit a very high degree of selectivity, and allow large and easy amplification. If the intermediate frequency is very low, the tuned circuit of the oscillator will be very close to resonance with the desired signals, so that almost maximiun resonant rise for signal voltages may be obtained by coupling the signal source to this tuned circuit, thus dispensing with the necessity for more than one high frequency tuned circuit and, hence, avoiding all problems of uni-control tuning. Such a condition of freedom from image interference might be illustrated by supposing that transmitting stations were spaced 50 kilocycles apart, and the intermediate frequency of the receiver were chosen at kilocycles.

According to the present invention, there is provided a superheterodyne receiver wherein surprisingly little image interference has been found when using an intermediate frequency of 30 kilocycles. Thus, for some classes of reception, and in case the difficulty of building an elaborate receiver is to be avoided, it would appear that the advantages of this type of receiver more than outweigh the disadvantages involved by its vulnerability to occasional image interference. Furthermore, even when such interference does occur, it may usually be dodged by choosing the better of the two possible oscillator frequencies, and/or slightly varying the tuning of the intermediate frequency selecting system by a simple uni-control means, and it is another object of the invention to provide the latter feature in a superheterodyne receiver.

In addition, there is one more important element in the design of a receiver, embodying the present invention, which is adapted to operate over an extremely wide frequency range with minimum switching arrangements. This is the provision of what may be called a two-point connection oscillator circuit. That is, the oscillator circuits are so arranged that the coil and condenser of the tunable circuit are connected only at two points, rather than at three, as in the case of the Hartley or Colpitts circuits, or the like. An example of a two-point oscillator circuit is the dynatron oscillator. Any device which presents a negative resistance characteristic between two terminals may be used in connection with a tuned circuit as a two-point oscillator system, provided that the negative resistance is not accompanied by too much reactance effects, and provided that the negative resistance is of a correct magnitude over the desired range of frequencies. The only objection to the dynatron circuit is that its negative conductance is not suificient to produce oscillations in very high frequency circuits of ordinary construction. An improvement upon the dynatron is the reversed current feedback circuit described in Q. S. T. for February 1932.

Hence, it can be stated that another object of the invention is to provide a two-point connection oscillator in a superheterodyne receiver, whereby a single moving contact connecting successively with the high potential ends of a number of tuned circuits is all that is necessary to switch from one wave length band to another.

And other objects of the invention are to im prove generally the simplicity and efficiency of superheterodyne radio receivers, and to particularly provide a superheterodyne receiver which is not only reliable in operation over a wide range of wave ranges, but economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, will best be understood by reference to the following description, taken in connection with the drawing, in which l have indicated diagrammatically a circuit organization whereby my invention may be carried into eifect.

Referring now to the accompanying drawing, wherein there is shown in diagrammatic manner a radio receiver system embodying the present invention, it is pointed out that the circuit shown in the drawing illustrates the essential features of a superheterodyne receiver embodying the invention. Thus, the signal collector is shown as the usual grounded antenna circuit, and this cir cuit embodies a pair of switches l, 2, and an adjustable condenser 3 which functions as an antenna capacity reducer. The antenna circuit, also, includes the resistor 4 connected between one side of the condenser 3 and the negative terminal of the grid bias source 5, the positive terminal of the latter being grounded. The input electrodes of the first detector tube 6 areconnected across the resistor 4 and source 5.

The first detector tube 6 is shown as being a tube of the screen grid type, and the plate circuit thereof includes a tunable resonant circuit 7. A variable tuning condenser 8 is in this tunable circuit l, and the latter is magnetically coupled to a second tunable circuit 8' which includes the variable tuning condenser 5. The input electrodes of an intermediate frequency amplifier it, of the screen grid type, are connected across the resonant circuit 8, while the anode circuit of the tube It includes a tunable circuit II.

The second detector of the receiver, which is conventionally represented, since those skilled in the art are well aware of the construction of such 'rotorsof the condensers 8, 9, l2 and [3 are arranged for mechanical uni-control by any well known type of device It, and the latter is conventionally represented since this construction is well known to those skilled in the art. The output of the second detector is to be understood as being impressed upon a succeeding audio frequency amplifier, and the audio output of the latter is then impressed upon any well known type of reproducer such as a loud speaker of the electrodynamic or electromagnetic type, or even a pair of head telephones. r

The local oscillator of the receiver embodies a two-point connection oscillator circuit. That is the oscillator circuits are so arranged that the coil and condenser of the tunable circuit are connected only at two points. Any device which presents a negative resistance characteristic between two terminals may be used in connection with a tuned circuit as a two-point oscillator system, provided that the negative resistance is not accompanied by too much reactance effects, and provided further that the negative resistance is of a correct magnitude over the desired range of frequencies.

The preferred form of two-point oscillator consists of two tubes {5 and 56, both of the screen grid type. These two tubes are eifectively coupled by pure resistance. The pure resistive coupling is obtained by using the input impedanceof a multisection low-pass filter as the coupling resistance,

' numeral ll, while the inherent tube capacities are represented by a single capacity in dotted lines and designated by the reference numeral IS. The electrical nature of a network such as designated by numeral ll has been described by me in U. S. Patent No. 1,925,340, issued September 5th, 1933.

The grid of tube I5 is connected to the plate of tube [6 by a condenser is, while the plate of tube I5 is connected to the grid of tube l6 by a condenser 20. The oscillator tuned circuits are represented for the purpose of illustration by three tunable networks 2 I, 22 and 23, the low potential sides of these circuits 2!, 22 and 23 being connected in common to the positive terminal of a potential source B, the negative terminal of which is grounded, it being noted that the cathodes of tubes l5 and 16 are grounded. The high potential side of each of the oscillator tunable circuits is connected to the plate circuit of tube It through a switch. Thus, three switches 24, 25 and 26 are shown. It will be understood that the closing of a given one of these three switches connects its associated tunable circuit with the plate circuit of tube It. It will, therefore, be seen that the tuned circuits are switched into the plate circuit of tube It, and that the potential of the plate circuit of this tube is fed back to the grid of tube 15, the feed back function being performed through the condenser I9. A condenser 2'! functions as the high frequency coupling means between the plate of tube l6 and the grid of the first detector tube 6.

The arrangement whichincludes tubes [5 and I6 and their associated circuits comprises a negative conductance device, whose conductance is fairly free of susceptance up to the cut-off frequency, and is fairly constant in magnitude up to about two-thirds of the cut-off frequency, after which it continues torise. This is a desirable feature, as the oscillator tunable circuits for ultra-high frequencies require larger values of negative conductance to make them oscillate. When any one of switches 24, 25 or 26 is closed to connect a desired one of the tunable circuits 2|, 22 or 23 to the local oscillator of the set, the variable tuning condenser in the chosen oscillator circuit may then be varied to select a desired signal frequency. Thus, the reference numerals 2|, 22' and 23 represent the variable tuning condensers disposed in the circuits 2|, 22 and 23 respectively. Of course, the three switches 24, 25 and 26 can be replaced by a single moving contact which connects successively with the high potential ends of the three tunable circuits 2!, 22 and 23, and it will thus be seen that actuation'of such a movable contact is all that would be necessary to switch from one wave length band to another.

In case a receiver, such as shown in the drawing, is made to include operation in some frequency bands, such as the broadcast band, where image interference cannot satisfactorily be avoided by the methods described, a tunable selecting system for that particlar band of frequencies may be switched in between the antenna and the receiver, for the purpose of eliminating the image frequency. Such a tunable selecting system is conventionally represented in the drawing by the numeral 30. The switches l and 2 are used in this caseto connect the selecting system 30 between the antenna and the adjustable condenser 3 when it is desired to include the selector 30 in the system.

r If it is desired to cover a frequency range of from 500 kilocycles to 30 megacycles at least four oscillator tunable circuits may be required. In that case there would be arranged four tuning knobs on the operating panel of the receiver, and.

adjacent each tuning knob would be disposed the dial associated with its tuning knob, each dial being calibrated for the particular frequency range through which its tuning knob is to be adjusted. The tuned circuits 1, 8, l I and l I are each tuned to the operating intermediate frequency, and this frequency is chosen by adjustment of the uni-control device l4. Preferably, the uni-control tuning should permit shifting the intermediate frequency between limits of approximately to 70 kilocycles to dodge image interference when necessary.

The operation of the receiver embodying the present invention should be clear from the aforegoing description taken in connection with the diagrammatic representation in the drawing. Assuming that one of the switches 24, 25 or 26 has been closed, the variable tuning condenser of the chosen oscillator tunable circuit is varied in its operating frequency range to receive the desired signal frequency. Since the operating intermediate frequency is very low, approximately 30 kilocycles, the tuned oscillator circuit is very close to resonance with the selected desired signal. There is secured, therefore, almost maximum resonant rise, because the signal collector is coupled to the operating oscillator circuit. This dispenses with the necessity for more than one high frequency tuned circuit, and hence avoids all problems of uni-control tuning. If image interference becomes troublesome at both of the possible settings of the oscillator tuning condenser, it is only necessary to adjust the control [4 until a new intermediate frequency is secured, which will dodge the image interference.

It is, of course, obvious that a blocking tube may be interposed between the antenna and the local oscillator if it is desired to prevent radiation from the oscillator. It is also to be noted that the tuning of the I. F. transformers or band filters may be varied by varying the permeability of the coil cores, or by various other means than variable condensers.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made, without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:-

1. In a superheterodyne receiver, a source of signal currents comprising the series connection of a capacity type antenna and a high resistance, a local oscillator including a tunable circuit, a first detector coupled to said source of signals, a condenser having a low impedance to the signal currents connected between the high potential end of said resistance and the high potential end of said tunable circuit, said oscillator tunable circuit including an adjustable tuning means, and an intermediate frequency network coupled to said first detector, said intermediate frequency network being tuned toa frequency of a value sc low as to permit substantially maximum signal resonance rise in said oscillator tunable circuit.

2. In a superheterodyne receiver, an untuned antenna circuit, a local oscillator including a tunable circuit, a first detector coupled to said antenna circuit, a condenser arranged to couple said first detector and said oscillator tunable circuit, said oscillator tunable circuit including an adjustable tuning means, and an intermediate frequency network coupled to said first detector, said intermediate frequency network being tuned to a frequency of a value so low as to obtain substantially maximum signal resonance rise in said oscillator tunable circuit, an additional tunable oscillator circuit, each of said oscillator tunable circuits being adapted to tune the oscillator through a different frequency range, and switching means for connecting a desired one of the oscillator tunable circuits to said local oscillator.

3. In a superheterodyne receiver, a first detector having a single grid or input circuit, a plurality of tunable circuits connected to the input circuit of said first detector, a source of signal currents consisting of an aperiodic antenna coupled to said input circuit, a local oscillator, a condenser arranged to couple said oscillator to said input circuit, each of said tunable circuits being arranged to Vary the tuning of the local oscillator through a different frequency range, and switching means for connecting any one of said plurality of circuits to said local oscillator and said first detector input circuit.

4. In a receiver as defined in claim 3, said local oscillator consisting of a pair of tubes effectively coupled by a pure resistance, and including a feedback circuit between the output circuit of one tube and the input circuit of the other tube whereby it functions as a negative conductance device.

5. In a receiver as defined in claim 3, an intermediate frequency amplifier coupled to said first detector, said amplifier being tuned to a frequency which is so low as to obtain substantially maximum signal resonance rise in the same tuned circuit as determines the local oscillator frequency.

6. In a superheterodyne receiver, the combination of a first detector, an oscillator arrangement comprising a negative resistance device having only two output terminals, a plurality of tunable circuits arranged to tune said oscillator over a plurality of difierent frequency bands of reception, 2. switching device arranged to connect either of said tunable circuits across said output terminals and means coupling said switching device to said first detector.

7. The combination defined in the preceding claim in which one end of said tunable circuits is permanently connected to one output terminal of said negative resistance device.

8. In a superheterodyne receiver, the combination of a vacuum tube oscillator having a cathode, grid and plate, a plurality of parallel resonant circuits tuned to different frequencies, a conduc'tive connection permanently connecting the low potential ends of said circuits to said catho'de, a plurality of switches arranged to connect the high potential end of either of said resonant circuits directly to the plate of said tube, a detector having an input circuit, a source of signalling current, means connecting said source to the detector input circuit and means coupling each of said switches to said detector input circuit.

WALTER VAN B. ROBERTS.

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