US2476985A - Receiver for electrical pulses - Google Patents

Description

July 26,1949 M. M. LEVY 2,476,985

RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 5 Sheets-Sheet 1 F/ G. /b.

llwenlnr Mnumce Manse LEVY M. M. LEVY 2,476,985

5 Sheets-Sheet 2 F GZa.

A 1 'l l v HKHA I I H U WV vyupw A l 12" W WV Y July 26, 194-9.

RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 July 26, 1949.

M. M. LEVY RECEIVER FOR ELECTRICAL PULSES 5 Sheets-Sheet 3 Filed Jan. 16, 1945 y m Y 7% Wm M n z 1cm A O M M! M y B July 26, 1949. M. M. LEVY 2,476,985

RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 5 Sheets-Sheet 4 5 WWW VVWV W Trace 2O Trace o Ip Zero vemjca/ a Inuenlor NRMWZE MovnE L9H July 26, 1949. M. M. LEVY 2,

RECEIVER FOR ELECTRICAL PULSES Filed Jan. 16, 1945 5 Sheets-Sheet 5 /ST. 'BE/IM QOUBLE Gil/V 056716 OSCOPE KIM/ L #751? INVENTOR. MAURICE MO/SE Aft Y Patented July 26, 1949 RECEIVER FOR ELECTRICAL PULSES Maurice Moi'se Levy, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a

corporation of Delaware Application January 16, 1945, Serial No. 573,101 In Great Britain December 1, 1943 Section 1, Public Law .690, August 8, 1946 Patent expires December 1, 1963 3 Claims.

The present invention relates to receivers for electrical signals, particularly pulses and which utilise a cathode ray oscillograph for giving an indication of the reception of the signals which have a tendency to be masked by noise signals.

Receivers of the type specified are particularly applicable, for example, to radio location equipment for determining the presence of objects, for example, such as aircraft, or for determining distance by the utilisation of the reflection of electrical pulses in the form of direct current pulses or in the form of trains of high frequency Waves which are detected to produce a direct current pulse which is then applied to the cathode ray osci llograph to obtain the required indication. The invention is also applicable to other cases utilising the reception of an electromagnetic signal wave which requires a relative indication rather than an absolute value indication. For instance, in some cases utilising the reflection of electromagnetic waves, it is desired to adjust the frequency of the transmitted continuous wave until the wave received, after reflection at an object, has a specific phase relationship, for example, in phase or anti-phase, to the contemporaneous'ly transmitted Wave. The frequency of the transmitted wave is adjusted until this condition is attained which condition is given by an indication on the oscillograph screen.

'In obtaining these indications the electron beam is caused to scan the screen in one direction, for example, the horizontal or abscissa direction under the control of a time base circuit whose period is equal to the pulse repetition period or other convenient period. The received signal is applied after the necessary treatment, for example, amplification and rectification, to cause the beam to be deflected in a direction perpendicular to the scanning direction, e. the vertical three-- tion in the example given.

These reflected signals, as will be understood by those skilled in the art, are very weak in intensity or energy level, andare difiicult to detect when mixed with noise which appears on the oscillograph screen as an irregular wavy horizontal line which tends to mask the indication of the received signal.

It is the object of this invention to provide arrangements for increasing the signal to noise ratio of the indication on the oscillograph screen and thus rendering the signal indication more clearly distinguishable and prominent against the background noise effect produced on the screen.

The invention: resides in limiting the vertical or ordinate deflection ofthe electron beam to a. predetermined ordinate value irrespective of the instantaneous deflecting force of the received signal above that predetermined ordinate value.

Arrangements according to thisinvention comprise ampl-itude limiting device which limits the amplitude of the recsivedsignal before application to the ordinate deflecting unit of the cathode ray tube. By this means the deflecting force on the electron beam is maintained substantially constant: during the greater period ofreception of the signal, with a consequential brighter indication on the screen during that portion of the period compared to the brightness during the remaining portion-of the period when the ordinate deflecting force is less than or does not persist for so longas the deflecting force due to the signal above the predetermined value.

The invention also includes arrangements for increasing the visibility of indications produced by contrast of the intensity of illumination of an area with respect to the intensity of illumination of surrounding areas comprising means for scanning said areas, for example, a cathode ray oscillograph screen having an indication produced as hereinbefore specified, as regards intensity of illumination and means for producing an amplified indication of the variation ofintensity of light along the direction of scanning.

The principle on which the invention is based is deduced from the'law of distribution of the peak amplitudes in the-noise.

The invention will be more clearly understood f-rom-thefollowingdescription taken in conjunction with the accompanying drawings in which Figs. 1 a, 11),- 10, 2a, 2b,- 3, 4 and 5 show curves used in explaining the invention; v

Fig. 6 shows the circuit arrangements of an embodiment of the invention, given by way of example only; 7

Fig. 7 is a further explanatory curve;

Fig.. Sshows diagrammatically an arrangement for improving. the indication obtained by means ofthe present invention;-

Figs. 9' and 91; show the kind of trace obtained by the improved arrangement shown in. Fig. 8-; and

Figs. 10 and 11 show, diagrammatically, arrangements according to the invention.

The noise may be looked upon as the resultant of an infinite number of pulses of very short duration, the amplitudes being-distributed at random. It will: be assumed that these amplitudes follow Gauss law of distribution. If: a curve be traced. the ordinates of which represent the peak amplitudes of the noise pulses and the .abscissae the proportion of pulses occurring in a specified time having a givenpeak amplitude, that is the value of the probability f or a noise pulse to have a given amplitude, a curve suchas the one represented at inFig. 1a is obtained. Pulses having zero amplitude are the most probable and the of the noise amplitude by the relation pulses, this curve may be represented by the can 1 1M2 Now since the movement of the spot of the cathspot is at. a certain amplitude follows the same kind of law, and the distribution of the intensity on the screen produced by the movement of the spot follows also the same law. That is to say; the curveC of Fig. 1a represents also the law of noise. This intensity is maximum on the abscissa line and decreases very quickly at points away from it.

It should be observed that this is really a hypothesis exactly'as Gauss law of distribution. This hypothesis is most probably correct. However, even if the intensity distribution follows a different kind of law, the general principles of the method explained hereinafter can still be applied. For simplicity, and as an example, it will be assumed that the conditions obey the laws referred to.

What happens when the signal pulses aremixed with the noise will now be discussed. Fig. 1c rep- 1 resents the resultant of the noise of Fig.' la and the pulse of Fig. 1b. When a signal pulse appears, its elfect is to increase the noise deflecting force by an amount equal tothe amplitude of the signal pulse. Thishas been shown at B in Fig. 1c. The

intensity distribution curve has been traced for 1 a point A where there is no signal pulse and for l a point B where the signal pulse is present. The

intensity "distribution law is the same, but the pulse raises the curve by an amount equal to the 7 pulse amplitude.

The value of the minimum amplitude of the signal pulse which can be visible can now be found,

that is, the minimum value of the signal noise ratio for which the presence of a signal pulse may be detected by visual observation. Y

, In Fig. 1c the root mean square noise amplitude (standard deviation in statistics) has been represented by the ordinate Aan. The intensity of light for this point is nearly 60% of the valuefor point A. Assuming a variation of 40% is nearly the minimum variation which can be distinguished, one can see that the minimum lift of curve B making this variation In visible is, in

the example shown, equal to Aan. Thus, the minpulses is'approximately one.

imum signal/noise ratio for visibility of the signal A method by which ,much smaller values of signal/noise ratio can be made visible will now b de c ib d I I a 7 One practical means is to insert an amplitude limiter in the receiver, at any point before the cathode ray tube, v, V

Theeifect of this limiter. on the intensity distribution on .the cathode ray tube will now be examined. Assume first no pulse signal is re-' ceived, Without the limiter the intensity distribution curve is assumed to be Gauss curve as where k is a constant connected tothe root mean 7 s re valu qua V e H ode ray tube is random the time during which the g represented at C, in Fig. 1c and Fi 2a. Assumenow the limiter euts all amplitudes above or below a given value, AaL above if AocL is greater than the average intensity on this line is equal to the area of the intensity distribution curve occurring above this line. This average intensity is negligible when AozL is great; and increases gradually when AozL decreases and becomes sharply maximum when AozL=O (see curve D, Fig. 2a) whereas the amplitude of the noise intensity distribution (curve C) varies very slowly for very small values of AaL. If AaL starting from a zero value intensity light will not vary, but if a limiter is used the variation will be marked (compare curves C and D, Fig. 2a). This property is used in carrying out the present invention in order to increase the visibility of indications due to very small pulses. The principle, is to use an amplitude limiter, preferably adjusted so that AaL is small or zero, and to observe on the horizontal scanning line produced by the limiter the difference of intensity between the points or instants where there is onlynoise and the points where the noise is mixed with a signal pulse. This differencein intensity is explained as follows.

Fig. 2b represents the average intensity light on the scanning line for twopoints: one A, where there is no signal pulse, andone,B, where the noiseis mixed with a signal pulse. The effect of the pulseis to lift the noise curve of a heighten equal to the amplitude of the pulse. The average intensity on the scanning line at point A is I0, the maximum amplitude of the curve Ib, and at point B it is Ip. The diflerence Io-Ip is pro portional to a when cc is small because the curve near the cusp is approximately constituted by plained; in connection with Fig. 1c.

7 To have an idea of the improvement intro duced by this method some calculations are neces Let In be the noise light, intensity for an ordi-j nate of abscissa Z. The law of distribution intensity of light is given by of the where K is a constant connected to the root mean square value 'of In by the relation 95 Let Ib be the intensity of light on the scanning line Z-, when there is a limiter. Then we have where 7 used in statistics problems. There exists numeri- :"0 "and below is if AczL is less than '0 (Fig. 2a,);

I 3 creases by a very small quantity, the noise inthe vertical distribution of the intensity due to two straight lines. Without the limiter for very small values of there is. no appreciable variation of intensity along the sweep line, as already excal-tables of this integral and "from these tables, the curve shown in :Fig. 3 canbe traced. In this'figure'thecurve representing In is also shown and to make comparison easier, the .two curves are. given the same. maximum amplitudes. The root mean square amplitudeoi the noise is represented the abscissae an as in Fig. 1c. The intensity 112' corresponding to the root mean square amplitude is also represented. It maybe seen that with curve ID. the same amplitude 5. can be. obtained with half the deflection an, that is to say with half the amplitude of the signal pulse which will be necessary with curve In. Thus, the above method improves the signal to noise ratio by about 6' dbs.

It. may be noticed that the adjustment of the limiter may be independent of the amplitude of the noise. and signal pulses and a limiter suppressing either the posiiive or the negative part of the signals may be used. Any amplitude limiter, .for example a biassed. amplifier, can be used. Many such limiters are well known and practically all are very simple. Thus the invention. is very easy to embody, even in any existing cathode ray oscillograph indicating apparatus.

The above. theory is only approximate because of the example of intensity curve. chosen. In practice, the scanning, line produced by the limiter must be adjusted in order to get the best results.

In the description given hereinbefore, the intensity distribution. law has been assumed to be identical to Gauss law. Experiments have shown that. the phenomenon described" does occur in practice. It is interesting to notice, however, that this phenomenon is practically independent of the kind of intensity distribution law so long as thelaw which the intensitydistributiondoes fol low'presents a slow rate of change of intensity with distance fromthe sweep line at the point of maximum intensity (curve C, Fig. 2a,) That is, the-first derivative ortangen-t at the maximum of curve C is zero, and the curve representing the law is convex at its maximum rather than cuspshaped like curve D. A complete analysis shows that even ifcurve-C is cusp-shaped; the phenomenon occurs for AuL=0 providing that the maximum amplitude of curve C is less than the maximum amplitude of curve D.

The case will now be considered of a limiter whose cutoff amplitude varies progressively from u to +11.

Referring to Fig. 2a, it may be seen that the limiter limits all peaks Whose maximum amplitude is greater than AaLI negative peaks are not limited. If AocL has a negative value, the limiter can limit either all peaks above the limiter line (line parallel to the abscissae and. whose ordinates are equal to AaL) or below this line. To trace the symmetric Gauss curve of Fig. 2a, it has been assumed that the peaks below this line are limited. We will examine now the case where all peaks above this line or alternatively all peaks below this line are always limited whatever may be the sign of AocL.

Assume that all peaks below the limiter line are limited. If AaL=a, the intensity along the limiter line is zero. When the limiter amplitude comes nearer zero, the intensity along this line increases following a law represented by Gauss curve (curve C, Fig. 2a, or curve C, Fig. 4). When the limiter amplitude is positive, the intensity still increases, as we assume that all peaks below this line are limited. Finally, at the limit, when the 6 limiter amplitude is +w, the intensityis maximum. The law is represented by curve C; Fig; 4

With this of limiter, Fig. 2b: becomes as represented in Fig. 5 withthe notations of Fig. 2b), except that the limiter as regards Fig. 5 is suppressing the signal: above the limiter'line instead of' below this: l iiie as is the case for'Fig; 2b. The limiter giving the conditions shown in Fig. 5' may be said to give results which arecomplementary to those given by the limiter studied in connection with Figs. 2w and 2b.

All these different kinds of limiters are easy to produce by those skilled in the art. A simple circuit is represented-in Fig; 6-. The noise mixed with signals is appiied at P on the input grid G of an amplifying val ve V. The plate resistance R, is shuntedby a diode E in series with a battery F. The plate of valve V is also connected to plate of diode E. The cathode of diode E is connected to battery F. Valve 'V' is prefierably a pentode so that plate voltage variations do not afiect the plate current of the valve. It is clear that this circuit will act as a limiter oi the type. described above. If the cur-rent of valve V is small and assuming the H. T. voltage is greater than the voltage of the battery F, a currentwill; flow across R and diode E to battery F so that the voltage across diode E is small; let Is. be this current; the voltage across resistance'R will start to vary only when the plate. current oi valve V is greater than Is. This explains the limiter action of the diode. AccL can be ad.- justed' by a convenient choice of the voltage or battery" F.

Thepotentials across R are appliedto the ver--- tical deflecting plates of a cathode ray oscillograph- 0- and a time base circuit provides the horizontal defle cti ng. potentials for 0 In order to make the indication obtained the cathode ray oscillograph screenv more visible,

two limiters may be used, for example, two am plifier valves fed in parallel with the signaland noise and eachhaving a limiter connected in itsontput, for exampleas shown in- Ti-"ig. 6. A cathode ray oscillograph' having a'doublebeam, that is, two beams or one split beam is used and one limiter is arranged to suppress me lower part of the signal and noise pattern, so as to produce the trace l with one beam as shown in Fig. 7, whilst the other limiter is arranged to suppress the upper part of the pattern and produce a trace as shown in trace 2, Fig. 7, with the other beam as hereinbefore described. Trace I is identical to the trace shown in Fig. 2b, and trace 2 is identical with the trace shown in Fig. 5. A pulse is shown as occurring between the times t1 and t2 and it will be seen that the intensity of illumination of trace l during the pulse is weaker whilst in trace 2 the intensity is stronger. The presence and occurrence of the pulse is seen more clearly by the contrast between the two traces.

Fig. 10 represents, diagrammatically, an arrangement incorporating this feature of the invention.

In another alternative arrangement shown diagrammatically in Figs. 8 and 11 for making the indication of a received pulse more visible, an opaque screen [0 covering the face of the oscillograph screen I2 is provided with a horizontal slit ll located so as to expose only the horizontal trace of the oscillograph. This slit is also covered and means are provided for uncovering in a sweeping movement a. small area 7 of the slit. ther disc [3 having slots [4, the disc l3 being rotated by the motor 2|. The disc I3 may be in the form of a Nipkov disc as used to scan a screen in television apparatus or the disc may have a continuous spiral slit in place of the spiral series of holes. By this means only one small or elemental area of the trace of the oscillograph I 5 is visible at a time. The light from this small area is focussed by a lens I6 on to a photoelectric cell IT. The horizontal plates of the oscilloscope l8 are connected to a time base 22 synchronized with disc 13, for example, by a mechanical liaison with motor 20, as shown in the drawing. If the sweeping frequency of this small or elemental area is small compared to the sweeping frequency of the beam of the oscillograph l5, the photoelectric cell current will be proportional to theaverage intensity of the moving elemental area over'the trace. .This current may then be used to produce the trace of a cathode ray beam of a second oscillograph l8 after suitable amplification in an amplifier l9, or to operate a mechanical indicator. In the case of the second oscillograph, the trace 20 will represent the average intensity distribution of light along the trace. on the first oscillograph screen and will appear as shown in Fig. 9a in the case of an'original trace such as I in Fig. 7 in which a pulse is, indicated by a reduction of the intensity or as in Fig. 9b in the case of an original trace such as 2 in Fig. 7 in which a, pulse is indicated by an increase in the intensity.

It will be understood that the arrangements described for increasing the visibility of apulse indication are'not limited in their application to a system of the kind described in relation to Figs. 1 to 5, but are of general application for increasing the visibility of indications produced by contrast of the intensity of an area with respect to the intensity of illumination of surrounding areas.

What is claimed is:

1. Arrangements as claimed in claim 3 comprising means for scanning the trace produced on the oscillograph screen and means for producing an electric current which varies in accordance with the intensity of light along said trace,

Such means may comprise a fur' W M a second cathode ray ioscillograph 'whos'e' beam 2. Arrangementsv as claimed in claim 3 com-' prising means for successively and cyclically exposing elemental areas of the oscillograph trace, a photo-electric cell, means for focussing the light from said elemental areas on to said cell to produce a current which varies in accordance with the intensity of light inthe successive elemental areas exposed, an amplifier for amplifyingsaid current or voltage derived therefromand a second cathode ray oscillograph whose beam is cyclically deflected in one direction in synchronism with the cyclic exposing of the trace,'and in a perpendicular direction by the amplified currents or voltages derived therefrom. 3. A signal indicating system particularlyfor signals whose amplitudes are small compared to the noise amplitude comprising 'meansfor limiting the amplitudes of said signals inthe presence of noise and means to apply clipped signals to an oscilloscope whereby the signals appear on the oscilloscope trace in the form of sections of weaker or stronger illumination, means for scanning said oscilloscopic trace sections and means responsive to said scanning means and including a second oscilloscope for indicating said weaker'or stronger illumination.

MAURICE MOISE LEVY.

REFERENCES CITED The following references are of record in the file of this patent: V NIT STATES PVA'VIENTSV Date Y 2,424,349 Cawein July 22, 1947

Images