USRE18579E - Demodulator and method op demodulation - Google Patents

Description

s. BALL NTINE l A ET AL Re. 18,579

Aug' 23 DEMODULATOR AND METHOD OF DEMODULATION' 4 Sheets-Sheet l uriginav. Filed June s, 41923 l. Noa-rechi@ "Ihrem/'fige Z Rechbry Gems/713e' l Y Sito: wu.

s. BALLANTINE ET A!- Re. 18,579

DEMODULATOR AND METHOD OF D'MODULATION 4 Sheets-Sheet 2 AugQzs, 1932.

Original Filed Juno 8,- 1925 Ferro-571km, Z6-f5.

3 and fran-Pic' /zsa s. BALLANTINE ET AL Aug. 23, 1932. K DEMODULATOB AND METHOD oF DEMODULATION Re. 18,579

4 Sheets-Sheet 5 original Filed June a, 1925' Varia/'1bn of fxpanen/'f'n Law of ies-MM Ferro-J//fcan Fach' /er JWM SWMTMw/ZL,

Jan. 8,*1929.

S. BALLANTINE ET AL DEMODULATOR AND METHOD 0F DEMODULATION original Filed June 8', 1925 4 Sheets-Shea#I 4 .JWM

@Hoy nur.

. in which the response is directly proportional to the amplitude ofthe im ressed. voltage stimulus.

Reissued Aug. 23, 1.932

UNITED STATES PATENT OFFICEN STUART BALLANTINE AND LEWIS M. HULL, F MOUNTAIN LAKES, NEW JERSEY, AS-

SIGNORS, BY MESNE ASSIGNMENTS, TO RADIO CORPORATION 0F AMERICA, OF NEW YORK,`N. Y., A CORPORATION 0F DELAWARE DEMODLATOB AND. METHOD 0F DEMODULATION Original No. 1,698,668, datedl January 8, 1929, Serial No. 644,21-5, il1ed June 8, 1923. Application for reissue illed October 1, 1930. Serial No. 485,7 85 i `This invention relates to demodulators for use in radio receiving circuits, particularly those used for radio telephony, and to methods of demodulation. I

.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 lcurrent may be made proportional to the first power of the radio frequency amplitude. A further oblect is toprovide a method ofdemodulation y ,which the stimulus-response characteris; ticV of thedemodulator may be adjusted to give a true and undistorted copy of .the

modulating currents operating in the transmitter, and to adjust the tonal characteristics of the sounds produced.

' "The usual system of demodulation is based upon the use 'of a circuit element or device',l usually called a detector, which possesses an asymmetrically conducting property.

vof the original modulation ofthe si 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 current-voltage characteristic. It has been recognized that the audio-frequency current obtanedin prior devices is not a true copy al and it can be demonstrated that the detected audio-frequency current is 4not of a lower order than the second power of the impressed signal voltage. According to the present invention, however,the old types Iof detectors which rely solely upon asymmetrical conductivity are L abandoned and we employ instead a detecting element having a certain operatingvrange Thus by means o our invention we are able to obtain in response to a modulated radio frequency signal voltage an audio frequency current Ywhose amplitude-is di-` frequency response stands in contrast with the. square-law relation characteristics of 1on1c tube vand crystal detectors as commonly employed and results in a marked improve- F ig. 2 is a dia rammatic representation of static current-vo tage characteristics -of a detector element having substantially v constant conductivity over a wide range.

Fig.`3 is a'diagranimatic representation of experimental response characteristics for a detector element aving current-voltage characteristics 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 respcnse characteristic such as shown in Fig.. 3; an Figs. .5 and 6 are diagrams of apparatus embodying our invention.

The experimental data which is represented in the curves ofFig. 1 was obtained by measurements taken on an asymmetrical conductor formed vby a light metallic pointcontact upon a crystal of refined silicon'. The characteristic curves were obtained for three different points byimpressin'g a steady but reversible voltage upon .the detector and measuring the resulting current in the two directions for direct and reversed voltage. When operated by a radio-frequency signal voltage lover a small range on one of these curves the direct current andaudio-frequency current components are greaterhthe greater the curvature of the characteristlc. Accordy ingly'the characteristic marked 3 -represents an operating condition more favorable for detection than that'marked 2; Qrepresents a more .favorable condition than 1, where the detector is practically a simple conductor, without appreciable' .detecting properties.

The curvature and general form ofthe current-voltage characteristics depend first upon the composition of the crystalline substance, and second', upon the location and pressure of the contact point. Locations upon the crystal surface, at which the current-voltage 4curve is highly asymmetrical are commonly characterized as sensitive spots in the crystal and can be located by connecting the crystal in series with a telephone receiver and with a source of modulated radio-frequency voltage and exploring the crystal while listening for the sound which indicates, a detection of the modulated wave. It was by thisl 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 Aas described is a function of the voltage and may be expressed as =f(e). The absolute strength of the signal is small and the device may be polarized by means of an auxiliary steady E. M. F. so adjusted that operation takesplace about the point E on the characteristic curve 3 of Fig. l. Then expanding vf 25 in a power series about this point, denoting the difference between E and E@ by e, we have:

where the symbols f', f, f .indicate the first, second and third derivatives, respectively, ofi with respect to e, taken at the point E0. Now a radio telephone signal is impressed upon lthe detector and for simplicity this signal will be taken as of the following generic type:

-mt) sin et (2)" Thev production ofa current of the type of the modulation function FU) is thus seen to The variation in the current CAfl 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 lloperation upon an asymmetrical, continuously ycurved characteristic.' If the characteristic were not continuously curved the above power series vwould not be a legitimate expansion of i as a function of e. In order that the audio current shall be a faithful-copy of the original modulation of the signal 'the stimulus-response characteristic of the detection, or demodulation process mustv 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.

lt must be emphasized that although. a silicon crystal having a continuously curved characteristic has been taken here as an example of the usual square-law detector, the same4 considerations and the same mathematical reasoning apply without modification to the ionic-tube detector, wherein the detection of modulated signals is brought about by the curvature (second and higher derivatives) lof the current-voltage characteristic of some conducting branch of the tube. It is a vfact familiar to most experimenters that the audio-frequency response of all ionictube detectors is proportional to the square of the modulated impressed voltage over wide 10 ranges of operation.

It can be shown mathematically that the current-voltage characteristicgof ther ideal demodulator would consists of two straight lines meeting at an angle at some definite point, which point should bev used as the operating point. With a conductor of this eccentric nature the audio-frequency currents owing as a result of the impression of a mod- 'ulated radio-frequency voltage would be directly proportional to the voltage amplitude. Experience has indicated, however, that no conductors of this ideal character exist and no person has been able tov produce one by mechanical or electrical combinations. Continuous curvature over a finite voltage range `has been exhibited by the conduction characteristics of all such combinations whichv do not follow Ohms law.

We have found that light metallic contacts upon a commercial ferro-silicon alloy containing about to 80% silicon and about 20% to 30% iron, in the crystalline form in which this alloy comes from the electric furnace, yield currentv voltage characteristics of which those shown in Fig. 2 at 1 and 2 are typical. T hese curves indicate 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 thecharacteristics are, in general, very limited, extending over not more than 0.3 volt 'about the origin; and finally, that for voltages in the reverse direction (negative) the resistance is practically constantup to 1 volt. and for higher voltages the curvature is relatively very small. We have found that contacts upon alloys of the above-mentioned compositions yield curves having the nearest approach to vthe ideal shape of two intersecting straight lines. With ferro-silicon having more than iron the inclination to each other of the two branches of the curves gradually increases without increasix the range of curvature, until at aboutv the point all contacts lose their rectifyl ing qualities and become simple straight-line conductors. With ferro-siliconscontaining 20% iron or less the reglon of curvature 1ncreases and the, characteristics of sensltive contacts change gradually` with increasing' proportions of silicon into the quasi-'cubic form shown for the silicon. Itwill be understood that the foregoing statements are based on the examination of a limited number of samples, and it is not excluded that ferrosilicon crystals having a chemicalcomposition outside/of the preferred range indicated may be found well adaptedy for the purposes of this invention. Hence the above statements as to the preferred constitution of the lferro-silicon are not to be regarded asnre'- strictive of the invention. It will also be understood that the ferro-silicon may contain minor quantities of metals or elements other than iron and silicon. f

Itis rather diilicult, 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. It should be noted however` 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 yfor all contacts on a given substance is the same; thus the general form of all curves for contacts upon commercially pure silicon (Fig. l) isv that of a cubic through the origin of co- `ordinates, one branch of which is displaced f or distorted from the true form representing a cubic equation, thus providing. a certain asymmetry about the origin. By investigating a large number of contacts upon different samples-of vthe ferro-silicon and upon the commercially purefsilicon we have determined conclusivelythat the form shown in Fig. 2 is typical of sensitive spots upon the ferro-silicon, and that the form shown in Fig.

`1 is typical of sensitive spots upon the silicon.

and we have never found characteristics of any spots upon thel ferro-silicon and the silicon which approachedeach other in form complished by using, in place of an iron or copper Contact point upon the ferro-silicon, a point of iron pyritc (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. Owing to some superposition of the surface conducting qualities of the pyrite and the ferrosilicon, both branches of the characteristic curve become so nearly straight lines thatit is impossible tol detect any curvature at points removed from the main bend, at the origin of coordinates.- We 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 iron-pyrite Contact point smaller than that obtainable with the simple iron or copper contact point.

Further experimental tests wlth radio-frequency impressed voltages have 'shown that the slight departures of the characteristics of our rectiliers from the ideal non-distorting curves of Fig. 3. Curves showing the direct current response as a function of the amplitude of an impressed radio frequency voltage are called stimulus-response characteristics as distinguished from the voltage-current characteristics which show the instantaneous rvalue 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 1900 meters (unmodulated) and measuring the resulting direct component' of the resulting current flowing through the rectifier.v The voltage amplitudes are plotted as abscissae, and the rectified, or direct current as ordinates; curve 1 represents the results for an iron point Acontact on avferro-sili-con crystal (-25% Fe), and curve 2, the results for a contact of iron pyrite upon asimilar 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 volts to 3.5 volts. range, therefore, these rectiiiers behave as distortionless deinodulators to modulated al- Within this .Y to the square of the amplitude of the radio s the audio-frequency response to a modulated radio-frequency voltage is directly proportional to the amplitude of the envelope of said voltage, and true demodulation, or first` power rectification results.

In order to illustrate more clearly the existence and boundaries of the regionin 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. 3. Suppose the radio signal voltage to be demodulated is symmetrically modulated to 50% about-its mean value. Then if this signal voltage be so weak that its meanv value does not exceed 0.2 volt, its peak value does not pass out of the region of continuous curvature on the rectifier characteristic (Fig. 3)

' and the rectifier operates as a square-law detector, the' audio response being proportional input. This condition of operation is-represented by the region AB of the Fig. 4. If

the mean amplitude of the modulated input Vliesbetween 0.2 volt and 0.6 volt, the input voltage crosses from the region of curvature on to the straight Aportion of the rectifier characteristic in every radio-frequency cycle, and the law of response of the rectifier cannot be expressed as a single power of'the4 impressed voltage; this is indicated by the transition region shown on the diagram from B to C, in which the simple exponential relation:V I

1 @mam-:Aetnaxo breaks down, and the exponent k 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 thisv region that we operate our rectifiers as distortionless demodulators, represented by C to D on the diagram; Of course the. extent of ,this range of linear response depends, for agiven rectifier characteristic, upon the degree of modulation of the incoming signal; this regionalways exists, however, and we have chosen the easev of 50% l'modulation merely to illustrate and to dee 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 ex- 'tent that the voltage impressed upon the rectifier shall exceed l 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 operarelate to the elimination ofthe effect of re-4 action 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 whichisemployed for securing the desired amplification and distortionless detection of modulated radio fre-V quency waves may take-various forms. As shown diagrammatically in Fig. 5, the deasf modulator 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 havin an iron or iron-pyrite contact point. Thej etected audio-frequency currents pass through the telephone `12 which is preferably shunted by a buv-pass condenser 13. It is y usually unnecessary to provide a bias voltage,

lbut when this is desired the audio-frequency circuit may include an auxiliaryyoltage divider 14.

.. In the preferred embodiment of the inven-` tion which is illustrated in Fig. 6, the demodulator includes a radio-frequencyinductive coupling 15 between the' multi-stage radiofrequency amplifier 10 and Vthe ferro-silicon rectifier .11E- The radio-frequency coupling preferably comprises" a vario-transformer such as de scribedin the copendingapplication of Ballantine, Serial No. 590,514, filed September 25, 1922, and its purpose is to kee currents inadvertently rectified by the amplifier out of the rectifier circuit. n The rectifier circuit includes the secondary of the trans'- former 15, the rectifier 11, and theprim'ary 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 connected across the secondary of the transformer 16 may comprise any suitable Aarrangement of audio-frequency amplifying units, telephones, loud speaker, etc.

In laboratory tests with the circuit shown in Fig. 6, excellent distortionless demodulation was obtained when the radio-frequency amplifier comprises three UV 201A tubes coupled with vario-.transformers and thel transformer couplingl was also a variotransformer.

Although we have found that certain ferrosilicon compositions are well adapted for use in carrying out our method of dis'tortionless demodulation, particularly when such compositions are used wi'th iron-pyrite contacts, it will be understood that our invention is not limited to rectifiers of this composition.

Our method of demodulation may be used with any rectifier having constant conductivcharacteristic has avery short curved por.

tion lying between the two ranges of substan-.

' tially uniform conductivity, i. e., the linear portions of the characteristic. WVhen the strength of the signals'is low oriwhen the rectifier has such properties that the linear portions ofthe characteristic are joined by a relatively longI curve, more amplification will be necessary to satisfy the requirement that the envelop of the modulated wave must fall within the range of linear response.

We claim: l. Method of operating a radio'receiving system suitable -for the reception of modu- -vlated carrier-wave signals and ofthe type including a de'modulator characterized by the fact that over a substantial range of 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 aid amplified voltages upon said demoduator.

2. Method of operating a radio receiving system suitable for the reception of carrier- 'wave signals and of the type including a demodulator characterized by the fact that over a substantial range of impressed voltages in excess of approximately 1.0 volt the relation between impressed voltage and output current n is substantially linear, which comprises amplifying received signalvoltages to values substantially in excess of 1.0 volt, and impressing said amplified voltages upon said demodulator. f

the reception of modulated carrier-wave sigg na-ls and comprising, in combination, a demodulator characterized by the fact that over a substantial range of impressed voltages the relationbetween impressed voltage and output current is substantially linear; means for amplifying received signal voltages to values within said range of linea-r response; and means for impressing said amplified voltages of said values upon said demodulator.

4. 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 voltages in excess of approximately 1.0 volt, the relation between impressed voltage .and output lcur-` rent is. substantially linear; means for amplifyingreceived signalv voltages to values substantially in excess of l volt; and means for impressing said amplified voltages of said values upon said demodulator.

3. A radio receiving system suitable forr 5. 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 the relation between lapplied voltage and output current 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 linear portions of the current-voltage .characteristic 'beingfdifferent so that said linear portions are mutually inclined; means for .amplifying received signal voltages to values substantially within the region represented by said linear branches; and means for impressing said .amplified voltages of said values upon said demodulator. Y

n 6. A radio receiving system suitable for th reception of modulated carrier-wave signals, and comprising, in combinatioma demodulator characterized by .the fact that over a substantial range of impressed positive voltage in excess of 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 approxi#v curved, the slopes of sald linear portionsof the current-voltage characteristic being diff ferent so that said linear portions are mutually inclined; means for amplifying received 4signal voltages to values substantially in excess of l volt; and means for impressing said amplified voltages of said values upon said 'demodulator 7. A radio receiving system suitable for f 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 voltagecurrent characteristic comprises two mutu- 10 ally inclined ysubstantially linear branches joined by a relatively short curved portion; a radio frequency amplifier for amplifying incoming signals to Values substantially with-V in the region represented by said linear i5 branches; and means for'impressing; said amplified voltages of saidvalues upon said detector crystal. v 8. In combination, a detector comprising 'ferro-silicon and characterized by the fact that its voltage-current characteristic com-- prises two vmutually 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 substantially exceed that critical value above which the rectified current is lineally proportional to the impressed radio frequeney voltage.

9. A detector for radio frequency oscillations, composed of an alloy of iron and silicon between the limits of 20 percent iron-8O percent silicon, and 30 percent iron-#7() per- .l

cent silicon, saidy alloy being in crystalline form, and characterized by a voltage-current 36 characteristic curve having two mutually inclined substantially linear branches.

10; Ina demodulator, a rectifying device comprising a crystal of ferro-silicon and an iron-pyrite contact. l

40 11. The methodof operatin a radio receiver including a detector, w ich includes adjusting said detector forits maximum lrectification and applying tosaid detector a modulated carrier wave of such amplitude as to Iproduce a substantially linear ratio between the rectified response and the percentage modulation of said carrier wave. f

In testimony whereof we afiix our signatures.

5o' i STUART BALLANTINE.

` LEWIS M. HULL. f

Images