Oct. 2, 1951 WEN YUAN PAN 2,569,687 SIGNAL CIRCUIT Filed Dec. l, 1948

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UNITED STATES PATENT OFFICE 2,569,687 SIGNAL DETECTOR, CIRCUIT Wen Yuan Pan, Collingswood, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application December 1, 1948, Serial No. 62,808 Claim. (C. 250-20) 1. 2 The invention relates to signal transformer 5, and ground 7, all connected in detector circuits, more particularly to de series relationship. A crystal detector 9, pref tector circuits. - erably of the type (Sylvania 1N34) An object of the invention is to provide an im is connected to a tap S on the inductor as a step proved non-amplifying detector circuit having down auto transformer of such ratio that the im substantially increased selectivity and/or eff pedance match is optimum, for maximum trans ciency not heretofore obtainable in the absence fer of signal energy. The primary of the trans of amplification. former may be designated by points PG and the Another object of the invention is to provide an secondary by points SG. The output of the de improved crystal detector circuit having substan O tector is connected to conventional earphones 3, tially enhanced selectivity without decreased of about 2000 ohms impedance, by-passed in the sensitivity or efficiency. usual manner with a 5, of the order It is well known in the art that crystal de of .005 mifd., to ground 7. The circuit was de tector circuits are characterized by poor selec signed to operate at 1500 kc., and the inductor tivity and sensitivity. Attempts to increase se s 5 had sufficient winding turns that with a tuning lectivity have resulted in further decreases in capacity of fifty micromicrofarads the circuit was sensitivity. resonant to this frequency. In accordance with the invention the Sensi Measurements showed that the operating Q, tivity or efficiency, and the selectivity of a de or factor of merit of the series resonant primary tector circuit, employing a detector such as a 20 circuit, was about twenty-five with the secondary crystal that consumes no power for its operation, tap point S about one-third the distance along have been substantially improved by means of a the inductance from the ground end, using 7/41 effect resulting from feed litz wire wound on a coil form one inch in di back of radio frequency components from the ameter and about two and one-half inches long. rectified output or audio frequency circuit into With this arrangement, better than normal re the radio freqeuncy input circuit. While feed sults, heretofore realized, were obtained in re back has been well known in regenerative circuits gard to sensitivity and selectivity. using three electrode vacuum tubes with gains Referring to the modification shown in Fig. 2, in excess of unity, there has never been any rea a feed-back inductor f7 was inserted in series be son to expect that improved results would follow 30 tween the detector 9 and the phones 3 of Fig. 1, from using feed-back with crystal detector or and closely coupled to the main inductor or auto detector circuits where the gain is less than transformer 5 at the lower end thereof. In the unity. output circuit comprising the secondary of trans This invention will be better understood by former 5, detector 9, inductor f7, and bypass reference to the following detailed description, 35 capa?itor 5, there are audio frequency signal taken in connection with the accompanying currents resulting from detection of the incoming drawing which renresents the preferred embodi modulated radio frequency carrier, as well as ment of the invention. radio-frequency components or unidirectional Referring to the drawing: 40 pulses resulting from rectifying the carrier. The Fig. 1 is a circuit diagram of a crystal detector radio frequency components traverse the bypass receiving circuit used, as a basis of comparison, capacitor, the inductor f7 and are fed back into to derive one of the curves shown in Fig. 4; the main inductor or transformer primary. It is Fig. 2 is a schematic diagram of a radio fre believed that the unusual results are caused by quency receiving circuit embodying the inven 45 the introduction of a certain amount of negative tion; resistance tending to neutralize the resistance re Fig. 3 is an isometric view in elevation of the flected into the tuned transformer circuit by the apparatus embodying the invention; load. It was found that the operating Q, hence Fig. 4 shows a plurality of characteristic curves Selectivity, was increased as much as five times based upon the circuits of Figs. 1 and 2; and 50 and the sensitivity was greatly improved, as Fig. 5 is a Schematic diagram of a modified form shown in Fig. 4, hereinafter discussed. The tap of the invention. S on the main inductor was adjusted to such a Referring to Fig. 1, there is illustrated a crystal point that the voltage transfer was a maximum, detector circuit comprising an , , a vari that is, the ratio of the antenna, impedance Za able tuning capacitor 3, an inductor or auto 55 to the reflected detector or load impedance Zd 2,569,687 3 4. equals the ratio of the primary inductance Tup the input terminals A and G of the circuit in Fig. to the secondary inductance Is, the inductance 1 through a dummy antenna, and a resulting volt of the part below the tap S. age of 25 volts was measured across inductor 5, between terminals P and G. A corresponding signal of one volt at 1000 kc. was impressed be The voltage wave at point F consists of radio tween terminals A and G of Fig. 2 through a frequency pulses, either positive or negative, de dummy antenna. A voltage of 125 volts was pending upon the polarity of the crystal detector measured across inductor 5 between terminals P connection. The R.-C. time constant was made and G. In the case of Fig. 1, it was determined quite long, so that such radio-frequency pulses O that the Q of inductor 5 was about 125 under retain their original phase relationships. The similar load conditions. The tap positions of S amplitude of the radio-frequency pulses was in Figs. 1 and 2 were adjusted to optimum in both somewhat less than that of the signal input to cases. The effect of the feedback was to neu the detector, but when such radio-frequency tralize a substantial position of the resistance on pulses are fed back to the input circuit they in s the input circuit, and resistance comprising that part to the input circuit a negative resistance. of the tuned circuit and that reflected into in The reactance of the feedback inductance at ductor 5 from the load circuit. This arrange radio frequencies involved should be large com ment may be used in a high frequency high volt pared to the impedance of the R.-C. circuit, but age, low current, power supply circuit, wherein a the reactance at audio frequencies should be Small 20 substantially higher D. C. voltage is obtainable compared to the R.-C. impedance. The relative from rectifying the output of a radio frequency polarity of the inductor 7 should be such that Oscillator. the feedback is positive or regenerative. Other Tests were made with reception in Camden types of feedback meains, e. g. capacity coupling, from the following Stations varying nay be used. 25 in distance from the receiver from two miles to Referring to Fig. 4, curve a was plotted between twenty-five miles, with good selectivity and sen response in microvolts and the frequency in the sitivity: WFIL, 560 kc.; WIP, 610 kc.; WPEN, 950 case of the circuit of Fig. 1, and shows a broad, kc.; WIBG, 990 kc.; KYW, 1060 kc.; WCAU, 1210 unselective characteristic. urve b is a Similar kc.; WCAM, 1310 kc. characteristic curve plotted for the circuit of Fig. : : While in the foregoing cases the invention has 2, and shows the much greater selectivity and been shown in its application to half Wave rec enhanced sensitivity obtained by means of the tification where the half cycles of one polarity are feedback action. fed back in proper phase, or regeneratively, to Fig. 3 shows the structural characteristics of reinforce the corresponding half of the unde the device shown Schematically in Fig. 2. The 35 tected wave in the input circuit, it has equal utility inductor or transformer 5 was found With 7/41 in a full wave rectification, particularly in a volt litz wire a distance of about one inch along a coil age doubler detector. Referring to Fig. 5, the in form having a one-inch outside diameter and Vention has been illustrated in connection with about 2% inches long. The feedback inductor 7, a thermionic dual diode detector comprising closely coupled to the main inductor or trans 40 2 and 22 and 23 and 24, where former 5, was wound adjacent the lower end G a substantial improvement in gain and selectivity of inductor 5 a distance of about 42 inch along are also obtained. In this arrangement, an in the coil forin. The adjustable capacitor 3 was termediate frequency signal of the order of 455 mounted upon terminals A and P at the upper kc. is supplied to the 24 of one diode from end of the coil folia. This device Was designed an I. F. transformer in the intermediate fre for operation at 1500 kc. and for that reason a quency channel of a . trimmer type capacitor having a limited capacity The transformer comprises a primary inductor Variation, adapted for adjustment by means of a 25 and Secondary inductor 26 shunted by a small screw driver, was provided. In case the device capacitor 30. A feedback inductor 27 is placed in is intended for operation at any desired frequency 50 Series between the anode 23 of one diode and throughout the broadcast tuning range, a capac ground, and a feedback inductor 29 is in series itor having a larger range of variation would be between 22 of the other diode and the employed. If desired the usual interleaving plate high potential side of the load circuit 3. Both type variable condenser with a tuning dial may inductors are tightly coupled to the secondary of be used. 55 the I. F. transformer. The load circuit in this iine bypass capacitor 5 and detector 9 were case comprises a resistor 3 of the Order of 50,000 Inoll inted in a Similar manner at the same end ohns shunted by 35 and 37 in series Of the coil foln as the capacitor 3. The detector of the order of 470 mm.f. each, and grounded S is preferably of germanium, and is mounted in at 39. The low potential side of the secondary a small cartridge resembling a fuse container, 26 of the I. F. transformer is connected to a point type 1N34 (Sylvania). between capacitors 35 and 37. Experiments Showed that while the above Resistor 3 is provided with a variable tap 32 as dimensions gave good results, still better efficiency a volume control connection for impressing the Was obtained by the use of a transformer of the detected Signals upon audio section, not Order of 1% inches long wound on a tube with an 65 shown, of the receiver. As a result of the inven Outside diameter of 1% inches. Still better re tion the above value of diode load resistance is Sults were obtained by tuning the inductor with lower than normal thereby greatly improving the a ferrite core instead of a capacitor. The partic ratio of the A. C. to D. C. impedance of the diode ular device above described was, however, inex load circuit, hence enhancing the fidelity of re pensive and compact, gave excellent results, and 70 production, without sacrificing selectivity and/or therefore Was chosen for purposes of production. gain. The transformer is shown as tunable by The following tests were made to determine the means of a ferro-magnetic core 28 of a material improvement in selectivity of the device shown in known as "ferrite,' characterized by unusually Figs. 2 and 4. A signal of one volt at 1000 kc., high permeability and low loss. See Phillips derived from an oscillator, was impressed across. 5. Technical Review No. 8 of 1946, Dr. Snoek. 2,569,687 5 6 While a substantial improvement in selectivity denser in series with a grounded step down trans as well as sensitivity is possible, it seems apparent former having primary and secondary Sections, that the greatest utility of the invention is in the said load circuit comprising a crystal de connection with detectors of the crystal type, tector connected to the Secondary section of Said wherein no external source of power is required transformer, a positive feed back inductor con for operation. The circuit of Fig. 2 could ob nected to the output side of Said crystal and in viously be changed to a voltage doubler circuit ductively coupled to said transformer, a by-paSS as in Fig. 5. capacitor connected in parallel with a pair of To one skilled in the art, it would appear Satis earphones, the said earphones being also con factory to obtain a substantial improvement in O nected to the said feedback inductor and to selectivity with a crystal detector circuit with ground, said transformer having a step-down out much sacrifice in sensitivity. One would ratio substantially equal to that between the im consider it quite unusual, however, to obtain such pedance reflected into said input circuit from improvement in selectivity with not only no sac said load circuit. rifice in sensitivity, but with an actual improve s WEN YUAN PAN. ment in sensitivity, such as was obtained in this case in accordance with the invention. In actual REFERENCES CITED practice it may be desirable to use a less ex The following references are of record in the pensive and/or Smaller construction, as by using file of this patent: ordinary wire and/or a smaller and more com 20 pact winding form, thereby sacrificing sensi UNITED STATES PATENTS tivity. In such a case there may be no improve Number Name Date ment in sensitivity over the usual detector cir 1881,032 Staley ------Oct. 4, 1932 cuits known to the art, only an improvement in 1938,657 Hansell ------Dec. 12, 1933 Selectivity. 25 What is claimed is: 2,522,914 Winchel ------Sept. 19, 1950 In a receiving system for modulated radio fre quency signals comprising a tuned radio fre FOREIGN PATENTS quency antenna, input circuit and a load circuit, Number Country Date the said input circuit comprising a variable con- 30 311,524 Great Britain ------May 16, 1929