US2467308A - Interference reducing radio pulse receiver - Google Patents

Description

April 12, 1949.

C, W. HAN SELL INTERFERENCE REDUCING RADIO PULSE RECEIVER Filed March 17, 1945 Patented Apr. 12, 1949 INTERFERENCE REDUCING RADIO PULSE RECEIVER Clarence W. Hansell, Port Jefferson, N. Y., assgnor to Radio Corporation of America, a corporation of Delaware Application March 17, 1945, Serial No. 583,239

(Cl. Z50-20) Claims.

1 This invention relates to pulse radio signalling systems, and more particularly to an improved .pulse radio receiver for improving the reception of pulses of radio frequency energy.

An object of the present invention is to provide a pulse radio receiver which is characterized by an improved signal-to-noise ratio.

Briey stated, the pulse receiver of the invention utilizes a degree of selectivity before the final detector so great as to lose or distort the original Wave form of the signal pulse and then by suitable apparatus correct the modulation frequency characteristic after detection so as to restore the original wave form of the signal pulse so far as may be necessary. This is accomplished by employing a superheterodyne receiver in which the inter-- mediate frequency circuits are designed to be too selective to pass pulses undistorted, and by passing the rectied pulses through an equalizer or compensator having a characteristic which is opposite to that of the highly selective intermediate frequency circuits within the band of frequencies which it is desired that the receiver as a Whole be capable of utilizing. The receiver is thus able to operate at lower signal levels before the signal pulses are destroyed by noise; or, putting it in other words, the signal threshold is lowered to a value below which the noise blots out or masks the signal. It should be understood, however, that the receiver of the invention operates with substantially the same overall band width and sensitivity to tuning in the case of good signal-to-noise ratio as presently used on conventional type pulse receivers.

An explanation of the theory underlying the present invention will now be given in order that the invention may be better appreciated. While this theoretical explanation is believed to be correct, it is not of necessity complete, nor does the operation of the invention depend upon its accuracy or otherwise.

The depression of the "improvement threshold by increasing the intermediate frequency selectivity is possible because the signal-to-noise ratio per frequency interval is not constant throughout the frequency band occupied by the pulse, but is a maximum at the center of the pulse frequency spectrum and tapers to a. minimum at the limits of the spectrum. Because of this unequal distribution of signal-to-noise ratio per frequency interval, the total signal-to-noise ratio of current reaching the detector throughout the whole frequency band ls substantially less than the signalto-noise ratio for frequencies at the middle of the band. By increasing the amplitude of cur..

. 2 rents at the middle of the band, Where the signalto-noise ratio is best, with respect to currents in other parts of the band where the signal-to-noise ratio is Worse, I am able to provide at the detector an improved overall signal-to-noise ratio. This improvement in overall signal-to-noise ratio automatically depresses the improvement threshold and enables the receiver to reach down to weaker signals before the signals are destroyed due to masking the signal in the output of the detector by noise in the input to the detector.

By masking is meant the observed destruction of useful signal currents in the output of a rectier due to the presence of strong noise in the input to the detector. It is an experimental fact that the signal-to-noise ratio in the output of the detector may be less than the signal-to-noise ratio in the input to the detector when the input signal is below a so-called improvement threshold" level. This improvement threshold level is determined by the total amount of noise currents in the input to the detector which in turn is dependent on the frequency selectivity of circuits preceding the detector.

The theoretical ultimate in retrieving signals from the noise will be reached when the selectivity is provided by resonant circuits of zero resistance tuned to the frequency of the pulse carrier current. In employing a theoretically zero loss circuit before the detector, a circuit responsive only to rate of change of detector output current must be used to correct the overall pulse response to avoid loss of Wave form. With this theoretically zero loss circuit, the signal-to-noise ratio at the input of the detector will have reached an ultimate limit. This ultimate limit is equal to the signalto-noise ratio per frequency interval at the exact center of the pulse spectrum for a band of zero limiting width. I thus furnish a new definition of the theoretical ultimate lower signal limit of a receiver as one in which the maximum signalto-noise ratio at any place in the band determines the point at which signals are masked by the noise. For radio locating purposes, I consider the limit of low signal strength to be obtained when at that best point in the frequency band, signal and noise have equal R. M, S. values at the input to the detector.

From one point of view, my use in a radio pulse receiver of excessive selectivity ahead of the detector with correction of the frequency characteristic after detection, is a form of, or analogous to, carrier exaltation. The use of excessive frequency selectivity in a broadcast receiver serves to reduce the percentage modulation of the carrier current by the combination of signal side frequencies and noise, and to maintain it below 100% down to lower initial signal-to-noise ratlos. These same principles apply to the reception of any modulated carrier current including pulse modulated carrier currents. Stated in another way, in any communications system where the intelligence is carried by the variations or modulations in carrier current, the possibility of the intermediate frequency currents (including both signals and noise) reaching the detector rectifier exceeding 100% amplitude modulation of the carrier current should be prevented if the intelligence modulations are to be preserved.

The pulse radio receiver of the invention is useful in radio locators, sometimes called radar or object detection systems, which function by detecting the radio echo or pulse reflected by any object which the radar beam strikes. By narrowing the pass band of the intermediate frequency circuits of the radar receiver, so as to reduce the area under the normal frequency selectivity curve to half, and then by correcting the overall frequency response by circuits following the detector, it is believed that there often will be achieved a lowering of the minimum of useful signal level by about 2:1 in power, which should result in 21A greater range of detection and enable protecting an area 21/2 times larger than conventional radar receivers. Heretofore, the conventional design of a radar receiver would not permit depressing the improvement threshold and thereby gaining an increase in the ultimate range to any substantial degree by lengthening the pulses to increase the transmitted power. The reason for this is that pulses of the present length already rise to maximum amplitude in the input circuits of the rectifying detectors, and making them longer will not raise the peak amplitude any higher with respect to noise. However, by making the intermediate frequency circuits too selective to reproduce the wave form and length of the pulses in accordance with the present invention, then, over a range,

' lengthening the transmitted pulses can be made to increase the amplitude of signal with respect to noise in proportion to the length of the signal i pulse and the improvement threshold will be depressed by 2:1 in amplitude, or 4:1 in power when the pulse length is doubled. From the ordinary point of view, this comes about because the average transmitted power has been doubled and at the same time its frequency band width reduced to one-half.

The invention is also useful in pulse communications systems regardless of the type of modulation employed; that is, whether the radio frequency energy constituting the pulses is itself modulated, or the timing or phase of the pulses is modulated. The most general application of the pulse radio receiver of the invention for communication purposes will be in connection with systems wherein the pulse timing or phase is modulated at the remote transmitter.` This pulse receiver is particularly adapted for use in any of the foregoing systems wherein the transmitted pulsesl are short compared to the time intervals between them, and greater peak power employed at the transmitter compared to the average power.

The following is a more detailed description of the invention in conjunction with drawings, wherein Fig. 1 illustrates one embodiment of the invention; Figs. 2a and 2b illustrate the detalls of different types of pulse form restorer circuits employed in Fig. 1; and Figs. 3a, 3b and 3c are amplitude versus frequency curves given to graphically illustrate the operation of the present invention.

Referring to Fig. 1 in more detail, there is shown, in box form, a pulse radio receiver adapted to receive pulses which are short compared to the time intervals between them. This receiver is of the superheterodyne type and includes a receiving antenna I, a radio frequency amplifier 2, a frequency converter 3 which is supplied both with the output of the amplifier 2 and with energy from a local heterodyning oscillator 4, and intermediate frequency amplier circuit 5 of one or more stages. Amplifier 5 is designed to be too selective to retain the pulse wave form supplied to its input circuit; or, putting it in other Words, it provides excessive selectivity ahead of the pulse rectifier 6 which is supplied with the output from the amplifier 5.

This excessive selectivity in the intermediate frequency amplifier 5 may be provided by suitable frequency selective transformers, or, where desired, by a crystal filter circuit. For greatest benefit, the highest possible selectivity should be furnished by the intermediate frequency amplilier 5, provided this selectivity can be represented by a more or less smooth curve throughout the frequency band occupied by the transmitted pulses. The output of the pulse rectier 6 is in the form of unidirectional current pulses and is supplied to a pulse form restorer 1 which in effect is an equalizer or compensator having a characteristic which within the signal band is substantially opposite to that of the excessively selective intermediate frequency amplifier circuit 5. Output from the pulse form restorer 'l is supplied to leads A and B, and this output may, in turn, be passed to a pulse demodulator 8 followed by an audio amplifier 9 when the system is used for general communication purposes, or, in the alternative, the output from leads A and B may be passed on through an amplifier to a cathode ray oscilloscope system I0 when the system is used for radio locating or object detection purposes.

The most advantageous application of the system of Fig. 1 when used for general communication purposes, is in a system in which the timing or phase of the pulses is modulated by the signal intelligence at the remote transmitter, in which case the demodulator 8 would be a. demodulator of phase modulated pulses.

Where the system of Fig. 1 is used for object detection or radio locating purposes, thecathode ray oscilloscope I0 will be provided with suitable sweep circuits and electron defiection electrodes for indicating on the screen of the tube the distance from the receiving apparatus of the object which is being detected, and in some cases both direction and distance, in accordance with the principles of radar systems now in use.

In radio locating systems of which the -system of Fig. 1 may constitute an essential part, the receiver and its transmitter are usually located together. It is customary to transmit extremely short pulses of ultra high frequency energy of the order of ve microseconds down to. a small fraction of a mlcrosecond, depending upon the type of systemk involved, and to employ pulse repetition rates ranging from to several thousand pulses per second. The receiver is customarily blocked by apparatus (not shown) so as to be non-responsive during the intervals in which pulses are actually transmitted, and the receiver isfdesigned to be responsive or receptive during .the time intervals between the transmitted pulses.

The same antenna is preferably used both for transmitting and for receiving purposes, and this antenna, which is shown diagrammatically only in Fig. 1, may be of the lobe switching or scanning .type in which pulses are transmitted synchronously with the lobe switching, while the antenna may be made to rotate during the hunting for objects. Obviously, for radio locating purposes (radar), the antenna is directive in nature, and

-where the system is employed to show both direction and distance of any number of distant objects by means of the oscilloscope presentation, the antenna may be rotated at a constant rate during each rotation of which many pulses may be transmitted. One suitable form of radar System in which the present invention may be employed is illustrated and described in copending application Serial No, 454,661, led by Nils E. Lindenblad on August 13, 1942.

Where the system of Fig. 1 is used for general communication purposes, such as for ordinary narrow band telephone, a pulse rate of 10,000'per second may be sucient for a single channel system, whereas for broadcasting purposes a pulse rate of 30,000 to 50,000 per second should be aqequate.

Figs. 2a and 2b illustrate, by way of example only, two diiferent types of pulse form restorer circuits which can be used in box I of Fig. 1. Vari- 'ous combinations of these two circuits and other known types of equalizers or compensators can also be employed in place of either one of the circuits of Figs. 2a or 2b. In fact, there is a vast developed art in the field of equalizers for obtaining almost any desired overall response charac'- teristics in electrical circuits.

Fig. 3a illustrates, by way of example only, the operation of the highly selective circuit of the intermediate frequency amplifier 5. In this iigure, the rectangular curve 20 illustrates generally the band width and the amplitude versus frequency response characteristic necessary to pass, without distortion of the Wave form, pulses of the length transmitted from the remote transmitter. The curve 2| illustrates the form of frequency versus amplitude curve of the highly selective circuit 5, as a result of which the central portion of the frequency band has the amplitude of its currents greatly increased with respect to the outer portions.

Fig. 3b illustrates one form of typical amplitude Versus frequency curve which may be characteristic of the pulse form restorer l in order to restore the pulse passed by the high frequency amplier 5 to its original form. It should be noted that. the characteristic of the pulse form restorer 1 is such that the amplitude increases with increase in frequency, so as to be opposite in its effect upon modulation to that of the highly selective intermediate frequency circuit on one side of the central or mean frequency. The phase response, or time delay, are also made to compensate.

Fig. 3c illustrates the overall frequency response of the receiver up to the output of the pulse form restorer 1. It should be noted that the response shown in Fig. 3c is similar in shape to one-half the size of the pulse 20 of Fig. 3a because (after rectication) the frequency response begins at zero frequency corresponding to the carrier frequency.

r The frequency response curves have their corresponding efects upon the wave form of the signal pulses. The circuits with the frequency response 2| of Fig. v3a tend to stretch out the pulses in time so that they are broader than the transmitted pulses. After detection, the circuits with the response shown in Fig. 3b shorten the pulses again in order to restore them to shapes substantially like those of the transmitted wave forms, as though the selectivity had been that of 20 of Fig. 3a.

10 However, the altered relation of signal-tonoise, in the input to the pulse rectifier of Fig. l, improves the average signal-to-noise ratio and allows successful detection of the pulses down to a lower power level.

15 The arrangement of the invention is particularly effective against continuous wave interference from an interfering current within one edge of the intermediate frequency pass band.

l What is claimed is:

20. 1l. A radio receiver of the superheterodyne type` if for. receiving pulses of radio carrier current which are repeated and which are of short time duration compared to the time intervals between them, comprising an intermediate frequenc amplifier, a rectifier for convertingarl'urrent pu ses in the output of the intermediate frequency amplier into direct current pulses, and a pulse wave form restorer following said rectier,said intermediate frequency amplier being so designed and adjusted as to greatly emphasize or increase the amplitudes of components of current having frequencies near the center of the frequency band occupied by the carrier current pulses with respect to the amplitude of components of current having frequencies on either side of the center of said band, and said pulse restorer having an amplitude versus frequency response characteristic which increases higher frequency components of the rectified direct current pulses with respect to lower frequency components, within the modulation frequency band occupied by the pulses, thereby restoring to the rectified direct current pulses the same approximate wave form as that of the envelope of the carrier current pulses.

2. A pulse radio receiver adapted for use with a remote transmitter radiating pulses of radio frequency energy which are modulated in timing by the intelligence to be carried and which are of short duration compared to the time intervals between them, comprising an energy collector, a

radio frequency amplier coupled to said coll lector, a frequency converter coupled to the output of said radio frequency amplifier, a local generator of oscillations coupled to said converter, a highly selective intermediate frequency amplifier coupled to the output of said converter, a pulse rectier for the output of said intermediate frequency amplifier, and a pulse form restorer circuit following said rectier, said intermediate frequency amplier being so designed and adjusted as to greatly emphasize or increase the amplitudes of components of current having frequencies near the center of the frequency band occupied by the carrier current pulses with respect to the amplitude of components of current having frequencies on either side of the center of said band, and said pulse restorer having an amplitude versus frequency response characteristic which increases higher frequency components of the rectified direct current pulses with respect to lower frequency components, within the modu.- lation frequency band occupied by the pulses. a demodulator of phase modulated pulses coupled to the output of said pulse form restorer, and an 7 audio frequency translator circuit coupled to the output of said demodulator.

3. In a radio object detection system which radiates pulses of ultra short wave energy which are short in duration compared to the time intervals between them, a superheterodyne receiver including an intermediate frequency so designed and adjusted as to greatly emphasize or increase the amplitudes of components of current having frequencies near the center of the frequency band occupied by the carrier current pulses with respect to the amplitude of components having frequencies on either side of the center of said band, a rectifier coupled to the output of said intermediate frequency amplifier, and a pulse wave form restorer coupled to the output of said rectiiier for substantially restoring to the rectified direct current pulses the same approximate wave form as that of the envelope of the carrier current pulses, said restorer having an amplitude versus frequency response characteristic which increases higher frequency components of the rectified direct current pulses With respect to lower frequency components, and a cathode ray tube indicator circuit coupled to said pulse Wave form restorer.

4. In a system wherein there are transmitted signal pulses of radio frequency energy which are of short duration compared to the time intervals between them, a superheterodyne receiver including an intermediate frequency ampliiier so designed and adjusted as to greatly emphasize or increase the amplitudes of components of current having frequencies near the center of the frequency lband occupied by the carrier current pulses with respect to the amplitude of components having frequencies on either side of the center of said band, said intermediate frequency amplier having suflicient response to pass twice the band o ccupied by the modulation frequencies,

. rent signal pulses having an envelope of approximately rectangular shape by noise in pulse communications receivers, comprising circuits coupled to the input and output of a detector for converting carrier current pulses to direct current pulses, the circuits coupled to the input of the detector being so selective as to emphasize the amplitude of components of currents in the center of the frequency band occupied by the carrier current pulses with respect to those at the sides of the band, and the circuits coupled to the output of the detector having an amplitude versus frequency response characteristic which increases the amplitude of higher frequency components with respect to lower frequency components, thereby toA correct for distortions in wave form and length ofV the-detected pulses caused by the selectivity of the circuits coupled to the input of the detector.

CLARENCE W. HANSELL.

REFERENCES CITED The following references are of record in the iile-"of this patent:

UNITED STATES PATENTS Number Name Date 1,922,139 Nyquist Aug. 15, 1933 1,936,153 Burton Nov. 21, 1933 1,961,334 Burton June 5, 1934 2,113,214 Luck Apr. 5, 1938 2,122,990 Poeh July 5, 1938 2,184,978 Nyquist Dec. 26, 1939 2,281,996 Randall et al. May 5, 1942 2,281,997 Randall May 5, 1942 2,337,196 Hollingsworth Dec. 21, 1943 2,416,328 Labin Feb. 25, 1947 OTHER REFERENCES Selective Circuits and. Static Interference, byk

J. R. Carson, Transactions of A. I. E. E.. June 1924, pp. 789-796.

Multichannel Communication Systems, by F. F. Roberts and J. C. Simmonds, Wireless Engineer, Nov. 1945, pp. 538-549.

Band-Width Requirements for Pulse-Type Transmissions, by W. W. Hansen, QST, Feb. 1945, pp. 11-13.

Pulse-Time Modulation, by E. M. Deloraine and E. Labin, Electronics, Jan. 1945, pp 10D-105..

Pulse Position Modulation Technic, Electronic Industries, Dec. 1945, pp. 82-87 and 180- 190.

Images