Dec. 31, 1946. 2,413,543 W. L. CARLSON ' REDUCTION OF SELECTIVE FADING D_ISTORTION Filed Sept. 22, 1943 My. 1 2 Sheets-Sheet 1 /4 /’ Moat/‘470R v SOURCE 0; ‘ (Halli-470R I I ’2 ——-—> _ ‘AUDIO . MODULATION /5 r0 RAD/470A’ _ /2 MODUl/JTOR r ' OSCILLATOR ' - F2 70 RAD/470R 1/119. 2 ' E . z; 077/ k ' f“ , I -é'A F,’ NO I 0 2' FIRST A ‘ v 1 A “60w AMPl/F/ER DETECTOR _ 70 4E NETWORK /8 ,1 ' F2 l I I'm/(cal II II I.' ' " 0£7ECTOR v , "" ZOC’AL osc/zuro/z ‘ INVENTOR. . WENDELL L BY C'ARLSO/V wgzm I AUQRNEY ,1’ 2,413,543 Patented Dec. 31, .1946 UNITED STATE 2,413,543 - e REDUCTION OF sELEcnvE DISTORTIQN _ _1 " . - Wendell L. Carlson, Princeton, N. J ., assignor_ to 1’ Radio Corporation of America, a corporation of Delaware I Application September 22, 1943; Serial No. 503,465 5 Claims. (01. 250-20) 1 ' ~ . - 2 . , , diatefrequency networks thereof, and, which commonv path has a 'pass band which is suffi My present invention relates generally to sys tems for reducing selective fading distortion. In areas fairly distant from standard broadcast ciently wide to pass each intermediate frequency stations ' (530—1700 kilocycle broadcast band) with its modulation side bands. distortion occurs by virtue of the well-known phenomenon known as “selective fading.” Dis tortion of this type is actually a distortion of audio frequency modulation due to interference of sky and ground waves at a particular signal wave collector of a receiver‘. Selective fading 10 - distortion is also encountered on the frequency ‘ Still other ‘objects of my invention are to im prove generally the efficiency and reliability of‘ systems for reducing distortion accompanying: selectivefading, and more especially to provide: methods ,of reducing selective fading distortion whichv are economical so far as apparatus is concerned. - The novel features which I believe to be char» bands higher than the broadcast frequency range. acteristic of my invention are set forth with. One important object of my invention is to particularityin the appended claims; the invenprovide means for automatically rejecting a dis torted signal and to select an undistorted signal. 15 tionitself, however, as to both its organization. and method of operation will best be understood‘, Another important object of this invention is by'reference to the following description taken: to provide a novel method of reducing selective in connection with the drawings in which I have fading distortion in receivers, wherein vcommon indicated diagrammatically circuit organizations. audio frequency modulation signals are trans mitted on‘ adjacent channel carriers, the recep 20 whereby my invention may be carried into ef tion of such adjacent channel carriersbeing car fect. ried out in a system having a pass band wide - ' ~In the drawings: ' , Fig. 1 schematically shows'a transmitter sys enough topass both carriers up to the demod tem adapted to radiate a pair of adjacent chan ulator whereby when one carrier fades substan tially below the strength of the other carrier, 25 nel carriers of common modulation, ‘ Fig.- 2 shows in a pictorial manner the mod then the efficiency of recti?cation is reduced for ulated carriers radiated by the system of Fig. 1, the ‘fading carrier and the audio frequency out Fig. 3 schematically shows a receiver adapted put from the fading signal is abnormally reduced. to receive the'radiated waves from the system of Still another important object of my invention ' . is to provide a method of reducing distortion due 30 Fig. 1, > Fig. 4 shows the pass band curve‘ of a receiver to selective fading which is based on the ob constructed in accordance with my invention, served phenomenon that at any given reception Fig. 5 schematically‘ illustrates a modi?ed em point where skyand ground waves from a given bodiment of the invention wherein a single chan transmitter are of the same order of magnitude, v ' ' combining these waves in, two different phase 35 nel carrier is received. Referring now to the accompanying drawings, relations will» cause one of the resulting waves wherein like reference‘ characters in the di?er_ to be undistorted and strong while the other is ent'?gures designate similar circuit elements, I distorted and weak’. The method employed in volves conversion of the two waves into two dif have shown in Fig. 1 in purely schematic man ferent intermediate frequency waves, and passing 40 ner a~transmission system adapted to be em ployed in one method of reducing distortion them through a wide transmission path to a de caused by selective fading. In this transmitter tector, or to employ separate detectors for the two waves with means for increasing the dif system a pair of master oscillators I and 2 is ference in amplitudes of the signals impressed utilized. >These oscillators are constructed and on the detectors. ' 45 designed to'produce oscillations having frequen-‘ cies on adjacent frequency channels. _Thus, os-. cillator' l operates at'frequency F1 whereas os-. cillator 2 operates at a frequency F2. Assuming that the transmitter is producing modulated car_ their planes at right angles to each other; each of the loops feeding separate converter networks 50 rier waves in the standard broadcast, band, it so as to provide intermediate frequencies locat will be understood that F1 and F2 will be the A more speci?c object of one‘ form of this in-' vention is‘ to provide a broadcast receiver with a pair of loops which are oriented so as to have ed‘on adjacent frequency channels but having common modulation signals thereon, the de-' modulator ofrthe system having a common trans? misslon‘path from the pair ‘of parallel interme‘i carrier frequencies of immediately adjacent channels. These channels are normally spaced 10 kilocyclesikc.) apart. ‘ ' -~ The’ source of audio‘frequency modulation 3 ‘is 2,413,543 4 tion side bands. In designing the receiving sys tem in Fig. 3 it is to be understood that each of the selector transmissionnetworks from the sig nal collector 6 to the input terminals of demodu utilized concurrently or synchronously to modu late the oscillations produced by each of oscil lators l and 2. Thus, the modulator 4 is employed to modulate the oscillations of oscillator l in ac cordance with the modulation signals of source 3. The modulator 5 is adapted to modulate the os lator I!) should have a pass band width which will be sufficiently wide to pass a radio frequency cillations of oscillator '2 in accordance with, the modulation signals of source 3. band 20 kc. wide. The separate . The demodulator I 0 may be of any well-known form. modulated carrier energy of oscillators l and 2 That is to say, it can be a diode detector, may then be transmittedto one or more separate: 10 a grid leak recti?cation type of detector, or a plate circuit recti?cation form of detector. It is a well stages of ampli?cation, and ?nally the separate known fact that with such detectors the stronger modulated carrier waves are radiated from sepa of two signals will dominate, and that the weaker rate radiators. There is shown in Fig. 2 in purely - signal will appear in the detector output only as illustrative manner the relation which exists be tween the modulated carrier waves which are 15 frequencies resulting from the heterodyne beat radiated from the separate radiators of the sys tem of Fig. 1. It will be seen that there is a 10 kc. from reaction with the stronger carrier. These beats will in this case all be of frequencies above spacing between the carrier frequencies F1 and F2, whereas the modulation side band components pass band of the audio system employed. As one 5,000 cycles, and, therefore, will be outside the are exactly the same for both carrier frequencies. At the receiver, which is schematically repre sented in Fig. 3, there is shown a signal collector device 6 which may be the usual grounded an tenna circuit. The receiving system is repre sented as being of the superheterodyne type, since 25 signal intensity falls, the detector efficiency falls at a rapid rate. This has the signi?cance that when one Of the signals F1 or F2 falls substan tially below the strength of the other signal, then the e?iciency of recti?cation is reduced for the fading signal and the audio frequency output derived from the fading signal is abnormally re the latter type of receiver is practically uni duced. It will now be appreciated that a simple versally used to receive radio signals in the stand and effective manner .of reducing distortion ard broadcast band. However, it is to be clearly caused by selective fading is readily provided by understood that any other type of receiver, such as one of the tuned radio frequency ampli?er type, 30 my invention. Let it be assumed that the re or the superregenerative type, may be employed. ceiver of Fig. 3 is located at such a distance from the transmitter of Fig. 1 that severe selective fad Assuming that the receiving system is of the su ing occurs in that receiving locality. perheterodyne type, there is employed the usual ?rst detector or converter 1 which has a tunable If the ground wave and sky wave of carrier F1 input circuit so that the receiver may be tuned to frequencies F1 and F2. The usual local oscillator 8 is also tunable, and the locally produced oscil cancel at the position where collector 6 is located, it is most unlikely that the same cancellation will lations of a predetermined frequency are applied to the ?rst detector 1. In the output of the latter occur for carrier F2. Indeed, actual observation demonstrates that simultaneous selective fading of adjacent carriers is most improbable. Hence, there is produced the intermediate frequency (IF) if, for example, carrier F1 should fade sufficiently energy, which may be ampli?ed by one or more so that detection thereof at the demodulator ID would cause audio distortion, with my invention such audio distortion will be greatly reduced and even substantially eliminated. This follows from the fact that when the carrier F1 fades severely relative to its modulation side bands, the recti?ca stages of IF ampli?cation 9. The ampli?ed IF energy may then be applied to a second detector or demodulator ID whose input is tuned to the operating IF value. The modulation output of the second detector It) may then be transmitted through one or more stages of audio frequency ampli?cation, and the ampli?ed audio energy is then reproduced in any desiredlform of repro ducer such as a loudspeaker. > _ It will be recognized that the aforedescribed networks of the receiver of Fig. 3 are purely con tion ef?ciency of demodulator It! will drop very rapidly and the audio output due to the carrier F1 will substantially disappear. However, since the network which feeds the demodulator is su?i cientlybroad to pass the carrier F2, it follows that the audio output reaching the loudspeaker ventional. Indeed, my invention is readily ap plied‘ to any conventional receiver of the standard broadcast type. The only change'that need be will be sufficient because the audio output due to F2 is the same audio output which would have been derived from F1. It is emphasized that since made in the conventional superheterodyne re F2 is not fading relative to its modulation side ceiver is that the pass band characteristic of the bands, the detector efiiciencylwith respect to, the receiver up to the input terminals of the second detector 10 be suf?ciently wide so as to pass the modulated carrier wave F2 will be at a high value and una?ected by the reduction of the detector energy of carriers F1 and F2 and their associated 60 efficiency with respect to’the wave F1. In the system of Fig. 5 there is employed a modulation side bands. In Fig. 4 I have shown ~ anillustrative representation of the type of trans mission characteristic which the receiving system of Fig. 3 should have. Thefullline curve, H__de.-. singlev carrier transmission channel in contrast to the dual channel transmission of ‘Figs. 1 and 3. There‘ isradiated from the broadcast transmitter notesgthe ideal transmission, characteristic of the 65 the usual and standard modulated carrier wave receiver up to the input terminals’ of the second having a channel width of ‘about 10 kc. 1 At the receiverv there are located a pair of loops which detector. It will be noted that the pass ‘band width is 20 kc. This means that the radiated care riers F1 and F2 and their modulation sidebands may readily be passed through the various net works from the signal collector. $.to the input terminals of the demodulator ill. ‘The dotted lines l2 in Fig. 4 show the positions of the car riers F1 and F2 and the 10. kc. spacing between each. ‘ii the terriers. F1 emits‘ and their module are oriented 90.". in respect to each other for re ception of the broadcast band, It will be un derstood that the'loops i3» and IA can here 179'te§1'a<1‘~Q‘lnd 9' Vgrtical axis, shown 913 9e, ‘wilted line, and at any angle ‘in respect to each other. One ,of these, loops will collect the “modulated carrier wave energywvhich appears as a ground wave and. also, as a sky.‘ wave, while the other 2,413,543 5 at least two directional signal collectors arranged loop will collect solely the sky wave energy. Let it be assumed that there is being received the signals from a transmitter operating at 1000 kc. One of the loops can be pointed toward the transmitting station, while the other loop is in predetermined relation, means coupled to one collector to reduce modulated carrier Waves to a ?rst lower carrier frequency, means coupled to the second collector to reduce modulated waves of the same carrier frequency to a second lower turned broadside to the station. With this‘condi tion the receiving channel connected to the loop carrier frequency different from the first lower pointing towards the station would have the frequency, means amplifying said waves of both highest average signal and less average distor lower carrier frequencies in a common amplij tion. Observation has shown that during periods 10 ?er stage, and a common demodulator coupled when the transmitted signal faded and distor to the latter stage. I tion occurred on this channel, the other channel 2. In a receiver of modulated carrier waves, at with the loop broadside usually had strong non least two directional signal collectors arranged distorted signals. It has been observed that the signal received by one of the loops which has its 15 in predetermined angular relation, converter means coupled to one collector to reduce modu lated carrier waves to a ?rst lower carrier fre quency, a second converter means coupled to the second collector to» reduce modulated waves of the same carrier frequency to a second lower modulated carrier wave energy subject to se lective fading is most probably subject to can cellation effects due to the combined action of the ground and sky waves. In that case the other loop most likely is collecting sky Wave en 20 carrier frequency different from the ?rst lower ergy, andis providing a substantially uniform frequency, means amplifying said waves of both intensity of desired signals. lower carrier frequences, and a common demodu Each of the loops feeds its collected signal lator for the ampli?ed waves. energy to a respective converter stage. Thus, 3. In a receiving system for modulated car loop I3 is connected to the converter IS. The 25 rier waves subject to selective fading of the car local oscillator I5’ feeds its locally produced oscillations to converter l5. rier relative to the modulation side bands, means The local oscilla— separately effecting at least two collections of said modulated carrier waves in space quadrature of signals of 1000 kc., the local oscillator I5’ is 30 relation, means translating the separate waves to different frequencies, means separately amplify adjusted to frequencies of 1450 kc. while the ing the waves of different frequencies, means oscillator I6’ is adjusted to a frequency of 1470 transmitting the ampli?ed Waves through a kc. The output energy of converter 15 will have common path adapted to pass the separate Waves, an IF‘ value of 450 kc., while the IF Value of and means rectifying the latter waves in a com converter I6 will have a value of 470 kc. Sepa mon detector. rate IF ampli?ers l1 and I8 are utilized to am 4. In a receiver of modulated carrier Waves, at plify the separate modulated IF carrier waves. least two directional signal collectors arranged in Above the ampli?er I‘! there is shown graphical 90 degree relation, means coupled to one col ly the appearance of the pass band of the am pli?er l1, ‘and below the ampli?er box I8 there 40 lector to reduce modulated carrier waves to a ?rst lower carrier frequency, means coupled to is shown the pass band of that ampli?er. the second collector to reduce modulated waves After separate ampli?cation in networks I‘! of the same carrier frequency to a second lower and I8, the combined energies of these two-net carrier frequency different from the ?rst lower works are fed to a common IF ampli?er and frequency, means amplifying said waves of both transmission network H] which has a pass band lower carrier frequencies in a common ampli sufficiently wide to pass each of the IF car ?er stage, said ampli?er stage having a pass band riers of 450 kc. and 470' kc. and the associated width chosen to include both lower frequencies, modulation side bands of each carrier. Above and a common demodulator coupled to the lat the network I9 there is shown in idealized form ter stage. the appearance of the pass band characteristic 5. In a system for receiving a high frequency of the network l9. It will be seen that it is suffi carrier modulated by signals of a predetermined ciently wide so as to pass the output energy of frequency range, a pair of loop collectors ar each of ampli?ers I‘! and I8. Subsequent to ranged on a common axis in quadrature rela the network [9 the demodulator will be similar to the demodulator l0 of Fig. 3. The action is 55 tion whereby one loop collects the sky wave and the other combined ground and sky waves, a precisely the same. If there is selective fading of ?rst converter connected to one loop and adapted the energy collected by loop I4, then the de to convert received carrier frequency to a lower modulator e?iciency for the modulated IF en value, a second converter connected to the sec ergy of 470 kc. will decrease sharply. Hence, ond loop and adapted to convert the received the audio output will be that due to the 450 kc. carrier energy to a lower value differing from energy, which is representative of the sky ‘wave the ?rst lower value by the ‘said modulation fre~ energy which was collected by loop l3. quency range, separate ampli?ers for respec While I have indicated and described several tor I6’ feeds its locally produced oscillations to converter 16. In the assumed case of reception systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the par 65 ampli?ed energies in an ampli?er whose pass band ticular organization shown and described, but that many modi?cations may be made without departing from the scope of my invention. What I claim is: tively and separately amplifying the energies of lower frequency, means combining the separately 1. In a receiver of modulated carrier waves, 70 is at least double said modu1ation frequency range and whose center frequency is a value between said two lower frequencies, and means for demod ulating the combined energies. WENDELL L. CARLSON.