US2471418A - Interference reducing radio impulse receiver - Google Patents

Description

c. w. EARP 2,471,418

INTERFERENCE REDUCING RADIO IMPULSE RECEIVER May 31, 1949.

f Filed sept. 9, 1,942

L f f Planted Mey 31. 1949 2,471,418

UNITED "STATES PATENT oEFicE t .VINTERFERENCE m10 IMPIIILSE i Charles William Earp, London W. C. 2, mi.

assignor, by meme assignments, to Inter'- natlonal Standard Electric Corporation,l New l RESSUD York, N. Y., a corporation of Delaware september o, 1942, seria No. 451,186 Great Brltain January 17, 1941 Section 1, Public Llw 69d, Augult 8, 1946 Application DEC 1 8 1951 Patent expires January 17, 1961 .i

modulation of a. wave modulated by a signal waveform which is substantially repetitive at a predetermined frequency. i

In the art of obstacle detection by radio pulses,

and the signal changed into such formthat it (Cl. Z50-20) occupies the minimum'possible band width consistent with the rate of information which is required of it.

According to one feature of the invention in a system for the demodulation of a wave modulated such pulses are transmitted at a known rate of e by a Signal Wave-form Which iS Substantially re- 1,00o per second, for example, whereby the repetitive at a predetermined frequency. the signal iiected wave from an obstacle also consists o! a Wave and a locally-derived Wave f th Same fretrain of pulses of the known periodicity or 1,000 queney and phase and preferably of the same per second. 10 shape as the signal wave-form are respectively In the case of obstacle detection by frequencyapplied t0 the tWO input Circuits 0f a differential sweep, a transmitted wave is varied cyclically in detector 0r balanced modulaturfrequency, at, for example 60 cycles per Secchi According to another feature of. the invention The reflected wave from the obstacle is used to e in a System for deniodulation of signal Waves 0f interact with a portion of the transmitted wave l5 the type reieri'ed'to above the signal Wave is eli* by detecting them together, when the resuitent plied i0 e mixlng device er modulator to which is wave is, in general, of a complex nature, but is re.. also applied a locally-generated wave constituted peet-,ed 60 times per seeondn by one orboth sidebands of a carrier wave of It is evident that when e signal, of either the frequency F modulated by e wave 0f the same A above types, is simply rectied to give a direct frequency and phase and Preferably 0f the Same current as an indication of the signal, a poor disshape as the signal Wave-form the output from crimination is obtained against noise current even the mixing device being Passed through a ,ltei though these noise currents are random in period having a mean DaSS frequency F. and phase A According to a stillv further feature of the in- An object of the present invention is to provide vention in e' system for demodulatlOn 0f Signal arrangements for the detection of signals sub- Waves of tile type referred to, the signal-medustantially repetitive at a predetermined frequency lated Wave 'is fed over two Paths t0 a modulator, which will discriminate against all noise which one oi` both of such Paths including a further does not conform to the characteristic envelope or modulator for combining tile signal mOdulated phase of the signal. wave with a carrier wave of constant frequency (It may be mentioned here that though the use and meaiis is provided for Producing a Predeteroi' a highly selective receiver could give a certain mined dierence iii transmission delay tluugh measure of discrimination against noise, extreme the two Pathe I selectivity must eventually exclude some c0m In the simplest form of the present invention, pement part or parts of the signaL If every cm the signal wave is demodulated in a differential pement of the Signal can be used Without @ddh detector, in which one input is the signal-wave, tional bandwidth, a better solution can be exand the other input is d locally generated Wave peetedg Y of the same periodicity and phase and preferably The characteristic repetitive nature of the sigof the Same shape This second input Will be nal does not, in itself, carry any intelligence (or 40 known asthe comparison Wave: which is es neel" information) but appears as an unwanted or ly as possible, ofthe same form as the signal, but y spurious modulation of the demodulated informahas-n0 added noise' tion bearing signal, and causes the signal to occu- In, a' puise receiver' the comparison Wave py a frequency band width much greater man consists or a train of locally generated pulses, in that which is strictly required for the transmiswhich the timing is controlled by manual 01 sion of the information of the signal. In the cirautomatic tuning' so that the pulses are coincicuits described herein, for example, for distance dent With the received signal pulsesdetermination of an obstacle in which the signal In the frequency Sweep ieeiVel', the 00111D811- output eohsists of e direct current, in addition son weve isthe Semeshepe as the weve-ferm repto a reduction of the random noises in the output resenting the frequency variation of the carrier e demodulated signal, this unwanted or spurious wave- Figures 1A and 1B of the accompanying modulation can be easily eliminated by filtering drawings ShOW Schemati blOCk diagrams 0f this the output without reducing the signal level, and simple system for the two types of equipment and' furthermore everyvcomponent of the signal is used Figs. 2, 3 and 4 ShOW block diagrams 0f three further systems according to the invention.

`utilises every signal of the signal. The comparison wave is obtained l by known means. 'For example, the timing of the wave is achieved by the phase control in a network PS of a sine-wave xof pulse periodicity generated by a source S. Amplitude limiting of this wave by a limiter L produces a square wave form. Subjection to a high-pass lter F produces a `series of positive and negative pulses, which are applied to a half-wave rectier R leaving positive D. C. pulses of odicity, and timing. These pulses are passed through a transformer Tl or any other high-pass iilter, whichremoves the D. C. component, but

conserves the envelope. L. P. is a D. C. and low-A pass ter connected to the output of the diilerential detector D andlters out the unwanted spurious modulation of repetition frequency.

If we now make a Fourier analysis of the signal wave, it will be found to be of the form e1 sin (2m-H1) +62 sin 41rft+e2 +e3 sin (emma) +de.

plus noise components which are random in frequency and phase. (f is the pulse periodicity.)

As the comparison wave is of the same general shape, this may be analysed as:

Ic(e1 sin (21rft-i-01) +ez sin (41rft-l-02) -i-etc.)

`The directcurrent component of the output from the differential detector D is due only to identical frequency components in the two inputs, the value and sign depending upon the relative phase of the two components.

In the case of linear detectors, in which the comparison wave dominates, the total D. C. output is proportional to ei-l-ez-i-ea-I-etc.

plus random positive and negative components `due to noise.

Thus, we have a demodulatlng device which component of the original wave, adding them together arithmetically. Noise components, however, are added up as vectors, and may, for certain types of noise, tend to cancel each other out altogether.

Fig. 1B shows the equivalent circuit-for the frequency-sweep equipment. 'I'he signal wave and the comparison wave are applied to a differential detector as before. In this case, the practical benefit is not so great, owing to the fact that the signal envelope is not so well dened, and that some of the noise is not completely random in form. If, for example, the typical signal envelope is derived from amplitude modulation caused by selective high frequency circuits, then the rectified noise tends to be synchronous with the transmitter frequency. It will be described later, how this disadvantage may be entirely eliminated.

In Fig. lB the modulating wave from a modulator M is applied to the transmitter T to frequency modulate a carrier wave and a portion of the output T is applied to a first detector Det I in the receiver together with the received waves after reflection fromthe obstacle giving the difference between the received direct and reiiected waves. Ll is an intermediate frequency amplier and Det 2 a second detector. 'Ihe Signal the correct shape, periwave output is from the secondary of the transformer T. The comparison wave is derived from the modulating wav yafter passing if desired through a shaping network N. The signal wave and the comparison wave are applied to a differential detector as in Fig. 1A.

Referring again to the systems depicted in Figures 1A and 1B, the signal wave and the comparison wave interact to produce a direct cur-` lponents of the wave being rent output. If an alternating current output is desired, this may be easily achieved by the system shown in Fig. 2. A

The comparison wave is modulated by a source S of frequency F in modulator M1, and the upper and/or lower sidebands of F are passed to modulator Mz -to demodulate the signal wave. this case the `output from M2 is and bears a constant` phase relationship to the original supply at frequency F. Output due to noise currents is also at frequency F, but this is composed of components which are random in phase. In order to avoid the difficulty that the noise envelope may not be entirely random, advantage must be taken of the fact that the phase of the noise wave is random compared with the phase of the signal wave before detection. Unfortunately, both in pulse systems and frequency ysweep systems, the phase of the received wave changes rapidly according to the position of the obstacle. For example, if the obstacle movesone quarter of a wavelength nearer equal to onel (or an exact multiple) period ofl repetition of the signal. 'I'his delayed wave may now be utilised as the "comparison wave for demodulation of the waves in the non-delayed path in the balanced detector D.

vThe signal waves in the two paths are now identical and simultaneous. but the noise comnon-repetitive are random in phase and frequency. On demodulation, which may be achieved by beating together the signal waves in the two paths, the signal components produce output signal currents i which add together in similar sense or phase,

whereas outputs due to noise currents are random in sense or phase.

The underlying principle of operation of this method for demodulation is described in Patent No. 2,233,384, issued February 25, 1941. It is shown, in this specification, that by the use of a critical band width for the signal transmission, noise currents which are'uniformly distributed over the frequency spectrum exactly cancel themselves out.

' In the above mentioned application, the method was used for the demodulation of a single carrier wave, so that the delay network `was arranged to be a function of the band width of the transmission circuits. In the present invention, however, We are concerned only with the demodulation of a signal which is repetitive at a substantially constant frequency f. Such a repetitionv wave may, of course, be subjected to a Fourier analysis, when the various components will be In` at frequency F,

of the found to be a number of constant frequency carrier waves which are spaced in frequency by exact intervals of f cycles/second. The delay network produces a linear phase distortion of the signals, this distortion being equivalent to a 5' phase rotation of 2r radians per f cycles of band width. v

Referring now to the system of Fig. 3, the signal wave may be represented as:

e1 sin (21m1ft+01)+ en sin (21rn2ft-l-02) es sin (21rnaft+03)+ etc. (where n is an integer) Noise components of indeterminate phase and frequency, which can be written as EN(Sin ivi-P95) After subjecting this wr ve to a delay of f seconds, the resulting signal currents, which are rotated in phase by exact: multiples of 21r, are mathematically unchanged The noise compo- 25 nents, however, must be written down as (1) Signa1 components beating with equivalent signal components.

(2) Noise components beating with equivalent noise components.

(3) Signal components beating with noise com- 45 ponents of identical frequency.

Let us now consider group l components. Here the total output at zero frequency may be written down as Y assuming a, square law detector, or

for any detector law.

In either case, all signal components conspire to give a positive output.

Let us now consider group 2 components. Here the total output at zero frequency may be written down as:l

EMN2 cos 1p1) Here p1 is random, so that addition of the various components is random in amplitude and sense. The total output may, of course, be zero, and is, in fact zero for the case of uniformly distributed noise over a frequency band of any multiple of f cycles, over which band p1 rotates by an exact multiple of 21r radians.

The output at zero frequency in group 3 components is all contained Within:

-ixs'e sin (empre) -N1 sin 21mft+i where N1 corresponds to those particular values 75 of N which are on frequencies identical to signal frequencies. The D. C. component of this series Here, as in group 2, the various components are not all of the same sign, because both a and may be of any value. The summation Vof noise from this cause is therefore inefilcient, and may result in zero output.

In the system shown in Fig. 3, we have assumed that the received wave is composed of frequency components which are exact multiples of frequency f. This assumption is, however, not necessary, the fundamental requirement for the signal being that it is composed of a multiplicity of frequency components separated by f cycles, or

multiples of f cycles.

In this system of Fig. 3, however, it is necessary that the signal components shall arrive at D in similar phase from the two paths. A small displacement in the mean frequency of the signal wave, would of course upset this condition, whereby the two Waves might arrive in phase quadrature, thereby giving zero output. In cases, therefore, where the signal wave may not be accurately defined in absolute frequencyfor example, if the signal wave is derived from the intermediate frequency amplifier of a radio receiver (where frequency shifts occur according to the high frequency'oscillator tuning) it is desirable to make some change.

Referring now to Fig. 4, inthe system there shown this difficulty has been completely avoided.

In Fig. 4 the signal-modulated wave source feeds two separate paths tothe demodulator M2 as before, and a delay network DN is inserted in one path (either path is satisfactory) as before.

In one of these paths, however, the modulator M1 is inserted, in which the signal-modulated wave is modulated by an oscillator S at frequency F cycles. A nlter F (which may also be the delay network) Yselects one of the sidebands of F produced by the signal-modulated wave.

The two inputs to the demodulator are now similar to those of Fig. 3, except that in the lower path all the components of the signal and noise have been raised (or lowered) in frequency by an amount F cycles.

The output is now selected at frequency F by lter FI. 'I'his A. C. output cannot be cancelled by a slight detuning of the signal modulated wave frequency, this detuning of the wave now only causing a phase rotation of vthe output with respect to the oscillator S. If, for example, the mean frequency of the signal-modulated wave is raised by f cycles, the phase of the output rotates through 21r radians, owing to the relative phase shift of the two paths caused by the delay network.

A second modulator, similar to M1, may also be included in the upper path. In this case, the iilter'Fl will have a mean pass frequency equal to the sum or difference frequency of the modulating oscillator S and the similar modulating oscillator of the upper path.

It should be pointed out that the noise suppression depends upon the use of a filter FI at frequency F in the output circuit. Similarly, in

Fig. 3, the D. C. component must be selected by a low-pass filter. The bandwidth of these filters must depend upon the speed of indication of the signal required, this'being designed, for example,

from requirements dictated by antenna commutation for directional indication of an obstacle.

Whatis claimed is:

1. System for the demodulation of a carrier wave present inthe form of the conveyance tion frequency and 'source is energized from further Acomprising `means for adjusting the pulsesoccurring at a given rate of repetition, said pulses being modulated in respect to one of their characteristics for to said pulses, a source for providing al comparison wave having substantially the same repeticarrier wave. a second detector circuit for said signal conveying pulses, means for differentially of intelligence. comprising meansA for detecting the received carrier wave in respect phase as the pulses of said.

. s sum of said signal and comparison wave pulses only and for obtaining the vectorial addition of al1 otherl components not having said repetition frequency. D

' CHARLES WILLIAM EARP. REFERENCES crrnn The following references'are of record inthe `fileofthispaten:

modulating said signal pulses by the pulses' .of

said comparison wave, whereby a resultant pulse wave is obtained having an improved signal to noise ratio and having said n quency, and means for filtering out said repetition frequency for pplication to a utilization circuit. l l

2. A system according. to claim 1 wherein said said nrst detector and phase of the comparison wave pulses into agreement with that of the. signal wave pulses.

3. A system according to claim 1, wherein said second detector comprises an input transformer and a rectiiier in each of the secondary leads thereof, and said means for di'erentially modulating includes an input transformer and means for -balanced application of said comparison Wave pulses to said second detector.

4. A system according to claim 1 wherein saidV second detector and said modulating means comprise means for obtaining the arithmetical given repetitionfre- UNITED STATES' PA'I'EN'rs Number Name Datel c 1,343,308 Carson June 16, 1920 A2,036,022 Conklin- Mar. 31, 1938 2,040,221 Tubbs May 12 1936 2,067,021 Roberts Jan. 5,1937 2,108,117 Gardere et al. v.- Feb. 15,1938 2,159,493 Wright May 23, 1939 2,171,154 Wright-; Aug. 29. 1939 2,183,714 v Franke et al. Dec. 19, 1939 2,225,524 Percival Dec. 17. 1940 2,227,598 Lyman et al Jan. 7,1941 2,231,704 Curtis Feb. 11, 1941 2,233,384 Feldman.- Feb. 25, 1941 2,266,401 Reeves Dec. 16, 1941 2,268,643 Crosby' .J'an. 6, 1942 2,350,702 Ullrich June 6, 1944 2,398,490 Atwood Apr. 16, 1948 2,401,416 Eaton June 4, 1946 2,408,079 Labln Sept. 24, 1946 2,410,223 Percival Oct. 29, 1946

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