US1698668A - Demodulator and method op demodulation - Google Patents

Description

JAN. 8, 1929. 1.698.668 S. BALLANTINE ET AL DEMODULATOKR AND METHOD 0F DEMQDULATION Filed June 8. 1923 @ffl 4 Sheets-Sheet 1 lef/'ned S27/kga GHG: new.

Jams, 1929. 1,698,668 -S. BALLANTINE ET AL DEMODULATOR AND METHOD OF DEMODULATION Filed June 8.V 1523 4 Sheets-Sheet 2 Jan. s, 1929. 1,698,668

S. BALLANTINE ET AL DEMODULATOR -AND METHOD OF DEMODULATION Filed June 8. 1.923

. 4 Sheets-Sheet 5 Jan. 8, 1929. 1,698,668 i s. BLLANTINE ET AL Y DEMODULATOR AND' METHOD oF DEMODULATION Filed June 8. 1923 4 Sheets sheet 4 33?? www( 1M Patented Jan. e; 1929.-

UNITED sr-Aras PATENT oFFIcE.

` STUART BALL'ANTINE AND LEWIS u. HULL,

or BooNToN, :mw mam, AssIGNoas '120l RADIO FREQUENCY LABORATORIES, ITNCORIORA'JED,v OF BOONTON, NEW .TER- SEY; A CORPORATION' OF NEW-JERSEY.

'- DEMonULAToa AND METHOD or nnMonULATIoN l Bussum Appucaon mea :une s', l192s. serial No. 344,215.

lThis invention relates to demodulators for use in radio receiving circuits, particularly thoseused for radio telephony, and to methods of demodulation.

An object ofthe invention is-to provide a demodulator which is free from distortion.

A further'object of the invention is to `provide a method of demodulation by which the audio frequency output current mafy be made proportional to the rst power o the radio frequency amplitude.- A further object is to provide a method of demodulation-by l which the stimulus-response -characteristic of the demodulator may be adjusted to 've a true and undistorted copy of the modu ating currents. operating in the' transmitter, and to adjust the tonal characteristics of the sounds produced.

The usual system of demodulation is based circuit element or device,

upon the use of -a usually called a detector, which possesses an asymmetrically conducting property. The essential property` of the detector, in prior art, is not a true unilateral conductivity, resulting in simple rectification, but the property of asymmetrical conductivity corresponding to the curvature of its currentvoltage characteristic. It has been recognized that the audio-frequencyl current o tained in prior devices is not a true 'copy of the original modulation of the signal and it can be demonstrated that thedetected audiofrequency current is not of a lower order than the second power of the impressed signal voltage.

According to the present invention, however, the old types of detectors which rely solely upon asymmetrical conductivity are abandoned and we employ instead a detecting element having a certain operating range in which the response is directly-proportional to the amplitude of the impressed voltage stimulus. Thus by means of our invention we are able to obtain in response to arnodulated radio frequency signal voltage an audio frequency current whose amplitude is directly `proportional to the amplitude of said radio frequency voltage.

' tained between the stimulus impressed upon our demodulating quency response stands in contrast with the square-law relation` characteristics of ionic tube and crystal detectors as commonly employed and results in a marked improvement voltage over This linear relation obsubstance, device and the audio fre-.-

' ly used in the detection of signals.

Fig. 2 is a diagrammatic representation y of static, current-voltage characteristics of a detector element having substantially constant conductivity over a wide range.

Fig..- 3 is a diagrammatic representation of experimental response characteristics for adetector element having current-voltage characterlstics as shown in Fig. 2.

Fig; 4 is a diagrammatic representation of the variation of the exponent in the law of response for a detector element having a response characteristic such as shown 1n Fig. 3; and

Figs. 5,'and 6 are diagrams of apparatus embodying our invention.

-.The experimental data which is represented in the curves of Fig. 1 was obtained by measurements taken on an asymmetrical conductor formed by a light metallic pointcontact upon a crystal of refined silicon. The characteristic curves were obtained for three different points by impressing a.A steady but reversible voltage u n the" detector and measuring the resulting current in the two directions for direct and reversed voltage. When operated 'by a radio-frequency si al a small range on one .of t ese curves the direct current and audio-frequency current components are greater2 the greater the curvature of thepcharacteristic. Accordingly the characteristic marked 3 represents an operatingcondition more favorable for detection than that marked 2; 2 represents a more favorable condition than 1, where the detector is practically a simple conductor, without appreciable de properties. The curvature and general crm. of the current-voltage characteristics depend first upon the com 'tion of the crystalline and second, upon the location and pressure of the contact' point. Locations upon the crystalsurface at which the current-voltage curvecxlitbly asymmetrical are commonly charac as sensitlve spots in the crystal and canbe located by connecting the crystal series with a telephone recelver and with a source of modulated radiorequency voltage and exploring the crystal while listening for the sound which indicates a detection of the modulated wave. It was by this method that the spots whose characteristics are shown at 2 and 3 were located.

The audio-frequency current which is -.passed by an asymmetrical conductor such as described is afunction of the voltage and maybe expressed as =f(re). The absolute strenvth o the signal. is small and the device may be polarized by means of an auxiliary steady E. M. F. so adjusted that operation takes place about the point Eo on the characteristic curve 3 of Fig. 1. Then expanding z' in a power series about this point, denoting the difference between E and E,l by e, we have:

where the symbols f', f, indicate the first, second and third derivatives, respectively, of i with respect to e, taken at the point o. Now a radio telephone signal is impressed upon the detector and for simplicity this signal will be taken as of the following generic type:

Fra-)Sin a: (2') where w represents the radio-frequency angularvelocity, and FU) the function, called the modulation function, which describes the variationin amplitude of the radiofrequency wave. The variation in the current thru the detector consequent to the application of (2) can be calculated from (l), neglecting for simplicity, any ordinary linear impedances which may be in the circuit. Thus The production of a current of the type of the modulation function FU) is thus seen to depend upon the presence of derivatives of the second and higher even orders. The important thing to be noticed is that the relation etween the detected audio-frequency current and the impressed signal voltage is not of a lower order than the second, and as an important practical result of this, the signal is distorted in the process of detection. This second-order detection, resulting in a squarelaw response and distortion of the signal is an` inherent result of operation upon an asymmetrical, continuously curved characteristic. If the characteristic were not continuously curved the above power series would not be a legitimate expansion of z' as a function of e. In order that the audio current shall be a faithful copy of the origmodulation process must be linear; otherwise the various component vibrations in the spectrum of the complex sound will not be dealt with in a proportionate fashion. As we have just shown, such a characteristic is impossible of attainment using the property of asymmetrical conductivity of the detector -as is the practice of the present day.

It must be emphasized that although a silicon crystal having a continuously curved characteristic has been taken herev as an example of the usual square-law detector, the same considerations and the same mathematical reasoning apply without .modification to the ionic-tube detector, wherein the detec- 9" tion of modulated signals is brought about by the curvature (second and higher derivatives) of the current-voltage characteristic of some conducting branchof the tube. It is a fact familiarto most experimenters that the audio-frequency response of all ionic-tube detectors is proportional to the square of the modulated impressed voltage over wide ranges of operation.

It can be shown mathematically that the 90 current-voltage characteristic of the ideal demodulator would consist of two straight lines` meeting at an angle at some definite point, which point should be used as. the operating point. With aconductor of this eccentric nature the audio-frequency currents flowing as a result of the impressionaof a modulated radio-frequency voltage would be directly proportional to the voltage amplitude. Experience has indicated, however, that no con- NIU ductors of this ideal characterexist and no person has been able to produce one by mcchanical or electrical combinations Continuous curvature over a finite voltage range hasV been exhibited by the conduction characteristics 'of all such combinations which -do not follow Ohms law. l

We have found that light metallic contacts upon a commercial ferro-silicon alloy containlng about 70% to 80% silicon and 110 about 20% to 30% iron, in -the crystalline form in which this alloy comes from the electric furnace, ield current voltage characteristics of whic 1 those shown in-Fig. 2 at 1 and 2 are typical. These curves indi- '115 cate that for impressed voltages in one direction (arbitrarily taken as positive upon the diagram) the resistance is practically constant, giving a linear current-voltage characteristic; that the regions of curvature of the characteristics are, in general, 'very limited, extending over not more than 0.3 volt 'y about the origin; and finally, that for voltages in the reverse direction (negative) the ing straight lines. With ferro-silicon 'having more than 30% iron the inclination to each other of the two branches of the curves gradually increases without increasing the range of curvature, until at about the 50% point all contacts lose their rectifying qualities and become simple straight-line c onductors. With ferro-silicons containing 20% iron or less the region of curvature increases and the characteristics of sensitive contacts change gradually with lincreasing proportions of silicon into the quasi-cubic form shown for the silicon. It will be understood that the foregoing statements are based on the examination of a limited number of samples, and it is not excluded that ferro-silicon crystals vhaving a chemical composition outside of the preferred range indicated may be found well adapted 'for the purposes of this invention. Hence the above statements as to the preferred constitution of the ferro-silicon are not to be regarded as restrictive of the inven- `tion. It will also be lunderstood that the ferro-silicon may contain minor quantities of'metals or elements other than iron and silicon.

Itis rather difficult, under these conditions, when the curves depend upon the location of contact as well as upon the nature of the fundamental substance, to isolate with certainty those qualities which depend only upon the composition. that although the curves for different contacts upon a substance of a given composition may differ Widely in extent and curvature, the general shape or type of curves for all contacts on a given substance is the same; thus y the general form of all the curves for contacts upon commercially pure silicon (Fig. l) is that of a cubic through the origin of coordinates, one branch of which is displaced or distorted from the true form representing a cubic equation, thus providing a certain asymmetryabout the origin. By investigating a large number of contacts upon different sampleslof the ferro-silicon and upon the commercially pure silicon we have determined conclusively that the form shown in Fig. 2 is typical of sensitive spots upon the ferro-silicon, and that the form shown in Fig. l is typical of sensitive spots upon the silicon, and we have never found characteristics of any spots upon the ferro-silicon and the silicon which approached each other in form more closely than curve 3 in Fig. 1 and curve l in Fig. 2.

We have also discovered that a further approach to the ideal characteristic can be accomplished by using, in place of an iron or copper contact point upon the ferro-silicon, v

a point of iron pyrite (FeSZ). A splinter of this substance, when held in .light contact with a sensitive spot on the ferro-silicon,

yields volt-ampere characteristics of which the one shown at 3 in Fig. 2 is typical. Ow-

It should be noted however ing t o some superposition of the surface conducting qualities of the pyrite and the ferrosilicon, both branches of the characteristic curve become so nearlystraight lines that it -1s Unpossible to detect any curvature at points removed from the main bend, at the origin of coordinates. JWe have found, however, that this desirable feature is compensated to some extent, from a practical standpoint by such a decrease in the relative inclination of the two branches of the curve that the rectifying sensitivity of the most sensitive. point that can be located with an ironpyrite contact oint is smaller than that obtainable with t e simple iron or copper contact point.

Further experimental tests with radio-frequency impressed voltages have shown that the slight departures of the characteristics of our rectifiers from the ideal non-distorting form are insignificant for all am litudes of impressed voltage which are su stantially greater than the range of curvature of the as distinguished from the voltage-current' characteristics which show the instantaneous value of current which flows through the rectifier as a function of the instantaneous impressed voltage. Data for these curves were obtained by impressing upon the rectifiers a measurable radio-frequency voltage, at a wave length of approximately 1500 meters (unmodulated) and measuring the resulting direct component of the resulting current flowing through the rectifier. The .voltage amplitudes are plotted as abscissae, and the rectified, or direct current as ordinates; curve l represents the results for an iron point contact on a ferro-silicon crystal (25% Fe), and curve 2, the results for a contact of iron pyrite upon a similar crystal. These curves indicate that the direct current response of our rectifiers to unmodulated voltage is directly proportional to the amplitude of said voltage throughout a range of amplitude extending from 0.3 volt to 3.5 volts.` Within this range, therefore, these rectifiers behave as distortionless demodulators to modulated alternating voltages, since the magnitude of the audio-frequency response to a modulated radiofrequency voltage is directly proportional to the amplitude of the envelope of said voltage, and true demodulation, vor first-power rectification results.

In order to illustrate more clearly the existence and boundaries of the region in which we operate our rectifiers as distortionless demodulators, the diagram, Fig. 4 is presented. The data for Fig. 4 were derived by computation from the experimental curves of Fig.

voltage-current characteristic. This is illus- 3. Suppose the radio signal voltage to be demodulated is s y-'mmetrically modulated to 5012i, about its mean value. Then if' this signal voltage be so wea-k that its mean value does not exceed 0.2 volt, its peak value does not pass out of the rcgionof continuous curvature on the rectifier charmteristic (Fig. and the rectifier operates as a square-law detector. the audio responso being proportional to the square of' the amplitude of the radio input. This condition of operation is represented bythe region AB of the Fig.- 4. If the mean amplitude of the modulated input lies between 0.2 volt and 0.6 volt, the input voltage crosses from the region of curvature on to the straight portion of the rectifier characteristic in'every radio-frequency cycle, and the law of response of the rectifier cannot be expressed-gas a single power of the inipressed voltage; this is indicated by the transition region shown on the diagram from B to C, in which the simple exponential relation:

audio A"radio breaks down, and the exponent le is indeterminate. With signal voltages whose mean amplitude exceeds 0.6 volt, however, the audio output is proportional to the first power of the amplitude of the modulated input; and it is in this region that We operate our rectifiers as distortionless demodiilators, represented by C to D on the diagram. Of Course the extent of this range of linear response depends, for a given rectifier characteristic, upon the degree of modulation of the incoming signal; this region always exists, however, and we have chosen the case of 50% modulation merely to illustrate and to define what we mean by the range of linear response with respect to the characteristics of a particular ferro-silicon contact.

Modulated signal voltages of the order of magnitude 0.001 volt, obtained across the reactance elements of a receiving antenna can be detected and heard with existing practical apparatus. In order to render our invention useful in connection with certain weak signal voltages obtained in practice, therefore, we combine our ferro-silicon rectifier l with means for obtaining high radio-frequency amplification of the signal voltage before itis impressed upon the rectifier. The essential feature of this process is the provision that the modulated amplitude of this signal voltage shall be so adjusted before being impressed upon the rectifier that it falls wholly within the above-defined range of linear response of the rectifier. For most rectifiers which fall within the scope of our invention we contemplate amplification of such an extent that the voltage impressed upon the rectifier shall exceed 1 volt and may exceed ten volts. We have discovered no definite upper limit to..the range of linear response with iron contacts on the ferro-silicon, and prefer therefore to employ as' high radio-frequency voltages as are consistent with stable operation of the amplifier in order to minimize the effects of variations in the slope of the operating characteristic of the rectifier with the location of the Contact point upon the crystal surface. Recent improvements in cascaded ionic tube amplifiers invented by Ballantine and described in the copending application (Serial No. 629,702, filed April 3, 1923) which relate to the elimination of the eect of reaction from plate to grid circuit in every tube, make possible the cascading of tubes in any number of stages, so that any desired amplification may be obtained.

The apparatus which is employed for securing the desired amplification and distortioiiless detection of modulated radio frequency waves may take various forms. As shown diagrammatically in Fig. 5, the demodulator includes a multi-stage radio frequency amplifier 10 of any appropriate construction from which the amplified signal wave is passed to a ferro-silicon rectifier 11 having an iron or iron-pyrite contact point. The detected audio-frequency currents pass through the telephone 12 which is preferably shiinted by a by-pass condenser 13. It is usually unnecessary to provide a bias voltage,

but when this is desired the audio-frequency circuit may include an auxiliary voltage divider 14.

In the preferred embodiment of the invention which is illustrated in Fig. 6, the demodula-tor includes a radio-frequency inductive coupling 15 between the multi-stage radiofrequency amplifier 10 and the ferro-silicon rectifier 11.V The radio-frequency coupling preferably comprises a vario-transformer such as describe-d in the copending application of Ballantine, Serialy No. 590,514, filed September 25, 1922, and its purpose is to keep currents inadvertently rectified by the amplifier out ofthe rectifier circuit. The rectifier circuit includes the secondary of the transformer 15, the rectifier 11, and the primary of an audio-frequency transformer 16. The primary of the transformer 16 is preferably shunted by a. condenser 17, and if desired a voltage divider 14 may be included in the rectifier circuit. The audio frequency apparatus 18 which is connectedV across the secondary of the transformer 16 may comprise any suitable arrangement of audio-frequency amplifying units, telephones, loud speaker, ete.

In laboratory tests with the circuit shown in Fig. 6, excellent distortionless demodulation was obtainedwhen the radio-frequency amplifier 10 comprises three UV201A tubes coupled With vario-transformers and the transformer coupling 15 was also a variotransformer.

' pliication will be necessary to satisfy with any rectiier having constant conductivity over asubstantial range of the impressed voltage, which conductivity is different in the two directions about the operating point, by

:adjusting the amplitude of the modulated radio-frequency wave to bring the entire envelop of this wave within the previously defined range of linear response. It will be apparent that little or,no radio frequency amplification will be required when strong signals are to be received or when the rec-,

tiiier has such properties that its current-voltage characteristic has a very short curved portion lying between the two ranges of substantially uniform conductivity, i. e., the linear portions of the characteristic. When the strength of the signals is low or ,when the rectifier has such properties that the linear portions of the characteristic arey joined by a relatively long curve, more amthe requirement that the envelo of the modulated wave must fall within t e range of linear res onse.

lf e claim:

1. Method of operating a radio receiving system suitable for the rece tion of modulated carrier-wave signals an of the type including a demodulator characterized b the fact that over a substantial range o impressed voltages the relation between impressed voltage and output current is substantially linear, which comprises amplifying received signal voltages to values within said range of linear response, and impressing said amplied voltagesupon said demodulator.

2. Method of operating a radio receiving system suitable forthe reception of carrierwave signals and of the type including a demodulator characterized by the fact that over a substantial range of impressed volta es in excess of approximately 1.0 volt the re ation between impressed voltage and output current is substantially linear, which comprises amplifying received signal voltages to values substantially in excess of 1.0 volt, and impressin said amplified voltages upon said demodu ator.

3. A radio receiving system suitable for the values within said range of linear respo nse and means for impressing said amplified yltages of said values upon said demodua or.

4. A radiol receiving system suitable for the reception of modulated carrier-wave signals, and comprising, in combination, a demodulator characterized by the fact that over a` substantial range of impressed volta es in excess of approximately 1.0 volt, the re ation between impressed voltage and output current is substantially linear; means for amplitying received signal voltages to values substantially in excess of l volt; and means for impressing said amplified voltages of said values upon said demodulator.

5. A radio receiving system suitable for the reception 'of modulated carrier-wave signals and comprising, in combination, ademodulator characterized by the fact that'over a substantial range of` impressed positive voltages the relation between applied voltage and output. currentl is substantially linear, over a substantial range of impressed negative voltages the relation between applied voltage and output current is substantially linear, and over a small intermediate range of impressed positive and negative voltages the current-voltage characteristic is curved, the slopes of said ilinear portions of the current-voltage characteristic being different so that said linear portions are mutual- ,1y inclined; -means for amlifying received signal voltages to values su stantially within the region represented by said linear branches; and means for impressing said amplified voltages of said values upon said demodulator.

6. A radio receiving system suitable for the reception of modulated carrier-Wave signals, and comprising, in combination, a demodulator characterized by the fact that over a substantial range of impressed positive voltages in excess o approximately 1.0 volt the relation between applied voltage and output current is substantially linear, over a substantial range of impressed negative voltages in excess of approximately 1.0 volt the relation between applied voltage and output current is substantially linear, and over a small intermediate range of impressed positive and negative voltages lying between approximately 1.0 volt positive and 1.0 volt negative the current-voltage characteristic is curved, the slopes of said linear portions of the current-voltage characteristic being diiferent so that said linear portions are mutually inclined; means Jfor amplifying received signal voltages to values substantially in excess of 1 volt; and means for impressing saidv amplified voltages of said values upon said demodulator.

` 7. A radio receiving system suitable for the reception of modulated carrier-wave signals and comprising, in combination, a detector crystal composed of ferro-silicon, and characterized by the fact that its voltage-current characteristic comprises two mutually inclined substantially linear branches joined by a relatively short curved portion; a radio frequency amplifier .for amplifying incoming signals to values substantially within' the region represented by said linear branches; and means for impressing said amplified voltages of said values upon saidA detector crystal.

8. In combination, a detector comprising ferro-silicon and characterized by the fact that its voltage-current characteristic comrises two mutually inclined substantially linear branches joined by a short curved portion; and means for so adjusting the voltage of the incoming radio frequency waves that it will substantlally exceed that critical value above which the rectified current is lineally proportional to the impressed radio frequenc voltage.

9. detector for radio frequency oscillations, composed of an alloy of iron and silicon between the limits of 20 percent iron- 80 percent silicon, and v30 percent iron-70 percentJ silicon, said alloy being in crystalline form, and characterized by a voltage-current characteristic curve having two mutually inclined substantially linear branches.

10. In a demodulator, a rectifying device comprising a crystal of ferro-silicon and an iron-pyrite contact. v

In testimony whereof, we aiiix our signatures.

STUART BALLANTINE. LEWIS M. HULL.

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