Title:
System for reducing interference
United States Patent 2294129


Abstract:
The present invention relates to reduction of interference upon systems for superselective transmission and reception employing a plurality of continuous waves or their equivalent. The system herein described is based upon the use of superaudible modulation, in which at the transmitter (1)...



Inventors:
Purington, Ellison S.
Application Number:
US35659240A
Publication Date:
08/25/1942
Filing Date:
09/13/1940
Assignee:
RCA CORP
Primary Class:
International Classes:
H04B7/02
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Description:

The present invention relates to reduction of interference upon systems for superselective transmission and reception employing a plurality of continuous waves or their equivalent.

The system herein described is based upon the use of superaudible modulation, in which at the transmitter (1) a radio carrier is modulated as to amplitude at a superaudible rate, (2) the superaudible rate is changed in frequency for signalling, (3) the radio carrier is wobbled 1 in frequency for purposes of privacy and for reducing the amount of time that any interfering transmitter can match the outgoing radiations.

A suitable receiver for this type of transmission includes a radio tuner, first detector to produce current of the modulating frequency, an intermediate or modulating frequency amplifier, a second detector of the heterodyne type to produce audible frequency currents, and usually a tuned audio frequency amplifier circuit. Such systems have been described in TU. S. Patent #1,681,293 issued to John Hays Hammond, Jr., U. S. Patent #1,690,719, issued to Chaffee et al., and U. S. Patent #1,899,527, issued to E. S.

Purington.

The freedom from interference depends in part upon the wobbler range, by which the amount of time that any undesired radiation can disturb the system is reduced.

The present invention is concerned primarily with reduction of the transmission of signals with which the interfering signals would cause disturbance, and also with reduction of the transmission of the receiver at the instant that such interferences would be produced. Either or both of these devices can be used in order to reduce the interference possible on such a system still further than that due to the superselective features previously disclosed.

It is to be understood that this invention is applicable to other systems employing a wobbler at the transmitter for similar purposes, or even to the transmission and reception of telephonic intelligence by the frequency modulation process, such as shown in U. S. Patents #1,599,586 to E. S. Purington and #1,977,439 to John Hays Hammond, Jr., in which a portion of the signal can be deleted without disturbing the transmission in which the signal is integrated from. other portions of the transmission.

The invention also consists in certain novel features of construction and combinations of parts hereinafter set forth.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the mode of its operation and the manner of its organization, may be better understood by referring to the following description taken in connection with the accompanying drawings forming a part hereof. In said drawings, Fig. 1 illustrates diagrammatically a preferred form of a transmitter circuit incorporating certain of the features of the present invention; and, Fig. 2 is a diagrammatic representation of a receiver which may be used to receive signals transmitted by a transmitter like that shown in Fig. 1 and incorporating certain feal0 tures of the invention.

In Fig. 1, a "master carrier oscillator" MCO operates through a "buffer" stage BS to excite a "modulated power amplifier" MPA, which is modulated by a "modulator" M actuated by a "modulation oscillator" MO. The "modulation oscillator" MO is provided with a "key" Ki by means of which the modulation frequency may be frequency modulated at dot-dash rate for purposes of signalling. The "master carrier oscillator" MCO is frequency modulated artificially by a "wobbler" mechanism WM which may include a motor driven "fan" type condenser F, and an operable "key" K2. The former serves to spread the entire radiation over a band of frequencies, whereas the latter serves to conceal the changes in side band location due to operation of the signalling key KI associated with the modulation oscillator MO.

The transmitted radiations of an arrangement like that disclosed comprise essentially a carrier wave and an upper and a lower side frequency. Whenever any extraneous interference matches either the carrier or a side band, the receiver response is interfered with. While the use of the motor driven "fan" type condenser portion of the wobbler prevents the interference from matching the radiations all of the time, yet the disturbance caused by the interference is not negligible.

For example, if the range of wobble due to the fan is 10,000 cycles, and the interference is effective when the interfering signal matches an outging radiation within 100 cycles it can be shown that the interference may be effective from 1.4 per cent to 6.4% of the time.

As applied to the transmitter, it is recognized that whenever there exists an interfering signal which would cause disturbance at the receiver, the transmitters also will have some of the interfering signal present. That is, any transmitter also acts as a receiver. If the differences between any continuous wave equivalent of the outgoing radiation for operating the receiver and any incoming continuous wave representing interference is for example 1000 cycles, then there will be a 1000 cycle beat note existing in the plate circuit of the modulated power amplifier circuit MPA. This is rectified by the power amplifier tube PT, and a current of 1000 cycles is produced, which can be filtered out by an "audio filter" AP, without disturbing the transmission of energy to the power amplifier for purposes of modulation.

The system may be adjusted so that when the interference is so close that beat notes of less than 200 cycles say exist, the transmitter operation may be automatically modified so that beat notes less than 200 cycles do not exist at the receiver.

The audio energy in the plate circuit of tube PT for example may be filtered by filter circuit AF, amplified by amplifier A, and rectified by rectifier R3, and the rectified output used through switch S1 when in the position shown to bias the buffer stage tube BT so as to render the buffer stage BF inoperative or if desired, through switch 82 when in the position shown to make a sudden change in the carrier frequency so that the beat note becomes higher than 200 cycles and increasing, instead of lower than 200 cycles and diminishing. When switch Si is used in the position shown, the radiation will be cut off and the system does not contribute to the signal. This causes the impulse of energy through the rectifier R3, to cease, but the time constant of the circuits is arranged to be sufficiently long so that when the system recovers, the audio beat note is above 200 cycles and increasing. The system can be organized of course, so that the buffer stage is not completely cut off, and the amount of signal merely reduced during the time that interference would be produced.

In the case where switch 82 is used, the buffer stage if desired, need not be modified at all in which case the switch Si is placed in its upper position. Additional wobbler condensers Ci and C2 controlled by commutators CMi and CM2 respectively, are used so that the frequency of the oscillator will make a sudden jump. If the low beat note is produced due to the fan wobbler increasing in capacity, commutators on the wobbler shaft distribute the D. C. output of the rectifier through switch 82 so that additional condenser C2 is cut into circuit, but if the low note results when the fan wobbler is decreasing in capacity, then the other additional condenser C1 is cut out of the circuit. The relays Ri and R2 which control the switching of the condensers are of the quick acting slow release type and return to normal (the positions shown) when the audio control signal ceases, with a suitable delay such that the fan wobbler F has advanced far enough to make a change which is at least equal to the change produced by the relay controlled condensers. It should be understood that in place of electromagnetic relays Ri and R2, it is within the scope of the invention to use electronic frequency shifting devices. However, since the rate of wobble for such systems is of the order of 20 wobbles per second so that several wobbles exist in the duration of a signalling "dot," electromagnetic devices of the type shown can be used.

While in general it is preferable to fail to transmit signals at that portion of the wobble cycle at which interference is produced, it is recognized that the space relations between the transmitter, receiver, and interfering signal may be such that the receiver may be subject to interference while the transmitter is screened 7 from the interference. In Fig. 2, therefore, there is shown an arrangement for stopping the transmission of signals through the receiver when there is an interference so close that disturbances would result. It is to be understood that 7 the signal is integrated from all portions of the wobbled cycle, and that the omission of signals for a small portion of the cycle will not necessarily be noted. The decrement of the receiver intermediate and audio filters may be sufficiently low that the signal is practically steady. It can be arranged, of course, that loss of signal strength due to the portion of time that the signal is throttled can be made up by increased gain of the transmission when the interference does not potentially exist.

In the circuit arrangement of Fig. 2, the usual antenna A, radio ampifier RA and first detector Di are provided as well as a first stage of intermediate amplification IFAi corresponding to the amplitude modulation frequency at the transmitter with the key Ki down. In the output of this amplifier there will be currents of the signalling frequency, and also possibly beat notes of changing pitch due to interferences.

For the signal, a second IF amplifier IFA2 is provided and this feeds into a circuit including a second detector D2 with heterodyne H, and a tuned audio amplifier TAF. It is clear that when the interference matches any of the three transmitted radiations, so that the IF amplifiers are actuated with the frequencies to which they are tuned, which frequency results in part from the presence of the interference, then the audio amplifier will be actuated.

Whenever the extraneous energy exists, there will be currents of two frequencies in the IF input, one associated with the signal (either the key Ki down or up) and one associated with the interference. The difference of the key up and key down frequencies may be of the order of 100 cycles, and the first IF amplifier not sharp enough to discriminate between the key up and key down modulating frequencies.

The subsequent circuits in the signal channel are provided with electronic gain control circuits, as for example to control the signal grid of the second detector. A control circuit comprising a control intermediate frequency amplifier CIFA, is provided for amplifying and detecting the intermediate frequency energy, whereby audio frequency energy of continuously varying pitch in accordance with the transmitter wobble is produced whenever there is potentially an interference upon the system. The control circuit includes amplifier CIFA and detector CDi. The audio frequency is filtered by a resistance-condenser type filter RCF with suitable cut off, so that when the beat note falls below a desired value, say 300 cycles, a rectifier CD2 is energized which develops a D. C. voltage impulse which is impressed upon the control grid of the second detector D2, and blocks the transmission of signals. The circuit recovers when the beat note becomes higher than 300 cycles, and no interference is producable. An additional feature of the invention comprises a tube ST which shunts the controlled tube D2 and is oppositely actuated whereby the total current through the input to the audio circuit does not change, thus eliminating shock excitation.

The system here shown is also applicable to reduction of disturbances such as upon the two phase signalling system of U. S. application r0 316,428 of John Hays Hammond, Jr., or to any system in which frequency modulation of a carrier is used to reduce the amount of time that interferences can be produced by interfering signals.

'5 Although only a few of the various forms in which this invention may be embodied have been shown herein, it is to be understood that the invention is not limited to any specific construction but might be embodied in various forms without departing from the spirit of the invention or the scope of the appended claims.

I claim: 1. In a system of communication of the type wherein the transmitter is frequency modulated to thereby reduce the amount of time that a continuous wave transmitter can match the outgoing radiation and harmfully disturb reception, means at the transmitter acting automatically when such interfering energy is intercepted by the transmitter for modifying the transmission characteristics of the system whereby the transmission is reduced when a small frequency difference exists between an interferring radiation and a frequency modulated constitutent of the radiated energy and said means including an 21 output stage of the transmitter.

2. In a system of communication in which the carrier frequency energy is wobbled between certain frequencies to thereby reduce the amount of time that interfering energy of predetermined 2 frequency can match the outgoing radiation and thereby harmfully disturb reception, the method of substantially reducing interference with the transmitted signal when the carrier frequency is at such a value that the frequency difference 3 between the interfering energy and one of the constituents of the radiated signal energy constitutes a lower order of frequencies, which comprises combining at a point in the system prior to radiation of the transmitted signal the inter- 3 fering energy and said constituent of the radiated energy, producing from said combination a direct current component and utilizing said direct current component to substantially reduce the amount of transmission for an appreciable period of time to thereby allow the carrier frequency to change substantially from said value.

3. In a system of communication wherein carrier frequency energy is wobbled between certain frequencies to thereby reduce the amount of time that interfering energy of predetermined frequency can match the outgoing radiation and thereby harmfully disturb reception, the method of substantially reducing interference with the transmitted signal when the carrier frequency is at such a value that the frequency different between the interfering energy and one of the constituents of the radiated signal energy constitutes an audio frequency which comprises combining at a point in the system prior to radiation of the transmitted signal the interfering energy and said constituent of the signal energy and producing therefrom direct current component and utilizing said produced direct current component to substantially reduce the amount of transmission for an appreciable period of time sufficient to allow the carrier frequency to change substantially from said value.

4. In a system of communication wherein carrier frequency energy is wobbled between certain frequencies to thereby reduce the amount of time that interfering energy of predetermined frequency can match the outgoing radiation and thereby harmfully disturb reception, the method of substantially reducing interference with the transmitted signal when the carrier frequency is at such a value that the frequency difference between the interfering energy and one of the constituents of the radiated signal energy constitutes an audio-frequency, which comprises combining at a point in the system prior to radiation of the transmitted signal the interfering energy and said constituent of the signal energy and producing therefrom a direct current component and utilizing said direct current component to stop the transmission of said carrier frequency for an appreciable period of time sufficient to allow the carrier frequency to change substantially from said value.

5. In a system of communication wherein the carrier frequency energy is wobbled between certain frequencies to thereby reduce the amount of time that interfering energy of predetermined frequency can match the outgoing radiation and thereby harmfully disturb reception, the method of substantially reducing interference with the transmitted signal when the carrier frequency is 0 at such a value that the frequency difference between the interfering energy and one of the constituents of the radiated signal energy constitutes an audio frequency which comprises combining at a point in the system prior to radiation of the transmitted signal the interfering energy and said constituent of the signal energy and producing therefrom direct current component and utilizing said direct current component to shift the frequency of the carrier frequency energy to '0 a different value.

6. In a system of communication wherein the carrier frequency energy is wobbled back and forth between certain frequencies to thereby reduce the amount of time that interfering energy i5 of predetermined frequency can match the outgoing radiation and thereby harmfully disturb reception, the method of substantially reducing interference with the transmitted signal when the carrier frequency is at such a value that the 40 frequency difference between the interfering energy and one of the constituents of the radiated signal energy constitutes an audio frequency which comprises combining at a point in the system prior to radiation of the transmitted sig45 nal the interfering energy and said constituent of the signal energy and producing therefrom direct current component and utilizing said produced direct current component to shift the frequency of the carrier frequency energy upward 50 when the wobbler is operating in a direction which increases the frequency of the carrier frequency energy and downward when the wobbler is operating in a direction which decreases the frequency of the carrier frequency energy. 55 7. In a signalling system of the type wherein carrier energy modulated in accordance with signal energy desired to be transmitted is radiated and received at a receiving station, the method of substantially reducing interference with the 60 transmitted signal energy when the difference in frequency between the interfering energy and a component of the radiated energy constitutes a disturbing frequency which comprises combining at a point in the system prior to radiation of 65 the transmitted signal the interfering energy and said component of the radiated energy and producing from said combination a control energy and utilizing the control energy to change the frequency of the carrier energy to such a value 70 that said difference ceases to constitute a disturbing frequency.

ELLISON S. PURINGTON.