US2477391A - Radio receiving system - Google Patents

Description

July 26, 1949. J. D:RElQ E.1-AL 2,477,391

RADIO' RECEIVING SYSTEM Filed NOV. 24, 1944 2 Sheets-Sheet 1 sewg s+ T JOHN D INxr/:ENTOR' A 9 .Re ,Ff-'y- 4 Hamas amLGouR ATTORNEY.

July 26,1949. J. D. Ram ETAL' 2,477,391

RADIO RECEIVING SYSTEM Filed NOV. 24, 1944 SPEAKER 2 Sheets-Sheet 2 INVENTOR. JOHN D. Rem

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Zw H #MQW Patented July 26, 1949 UNITED STAT 2.4713911 anoto RECEIVING SYSTEM .rollin4 op nella; Mount Healthy', and charles n; lilgoui?,` Cincinnati, Chio, assgnors', by niesne assignment-'sg to" Aveo Mmuifacturing Corporation, a corporation. of Delaware application November 24, 1944.', seriell No. einen 3' Claims. (C1. Z50-2U) This invention relates to a rad-io receiving system `for receiving frequency` modulated c "rior Waves so that the effect ofl amplitude modi ations is'reducedor` avoided Without using conventional limiting circuits.

As is wellknowri in the art, limiting is erf/.s-loii`i-l plished by overloading ajvacuum tube', via., by operating it under conditions such as after a certain input' voltage is reached, the output rer? constant. This isdisadvantage-ous in that it r tails a loss off selectivity and results in ajlow effective sensitivity; An object' ofthe present iii-- vention is to reduce amplitude' modulations such` as' caused by interference or noise without the use. of the. eqnventenallimitea circuits wier out impairing selectivity. 4r'furiiher objects are to provide amplitude modulationV reducing means that' are more sensitive thanthose that have herem tofore been` used, to simplify the manufacture frequency modulation receivers and to reduce thecost thereof; `The novel features that We consider character# istic o f. our invention are. set forth in the appended claims; the invention itself, however, bothv as to its organization and. method of operation, tiogether with. additional. objects and advantages thereof Will best be understood from. the follow.'-A ing description. of specific embodiments iv" read in conjunction with the accompanying draw ings in Which- Fig. 1. is a bloclr diagramshovving how our in ventionmay loe utilized in a receiver for frequency modulated Waves.

. Fig. 2. is a circuit diagram of a niodication of our invention, showing an amplifier stage used in conjunction therewith. A

Fig. Bis. a circuit diagram of a complete. receiver for standard broadcast, short wave and Fig. 4 is a circuit diagram showing how ampliu tude modulated waves may be amplified and recn tfed for obtaining additional bias.

Briefly described, our invention comprises arnplifying and uniformly rectifylng a frequency modulated radio frequency carrier, deriving a voltage therefrom to bias the control grids of amplifiers toward cutoff according to the am;t tude of the signal envelope, and substantially dependent of the frequency variation of the car rier, and simultaneously applying a portion of the direct current voltage developed during rectiilcation to such grids, thereby preventing overl-oath ing. The audio frequency voltage derived the carrier isapplied to the control grids of' amplifying stages in conjunction with capacita-nce and 2 resistance of suchtime constant tliata rapid controlinay be effected. s

Eig; lis a block diagram of asu-perlieterodyne rec'eiverior frequency modulated waves, showing the stages of' converting the incoming signal to an Aintermediate frequency,` amplifying the internl mediateY frequency; discrimina-ting' the frequency modulation thatis impressed on the incoming carrienamplifying the sameand converting ift to' sound Waves. A. diode ll] isconnected at Sl tween thehigh side of the intermediate frequency outputand; ground hycoupling V.capacitance it. A voltage `is developed across diode lll which cor" responds to the amplitude modulated envelope of the carrier and appears across capacitancf mi Anaudio voltage corresponding to the amp modulation on the carrier is then applied throu audio coupling capacitance lE-to the contro? ofpreceeding ampliiiers at I8, thus autore biasing` them, .infasense tending` to reduce the am" plitude modulation ofthe carrier. Resistors lfi and 'l actasV a voltage divider, dividing the components ofthe rectified voltagdthe resultan-t of which is also applied to the preceding amplifiers through an audio pass filter network as shown in detail in Fig. 3'. This controls the gain of the amplifiers and thus prevents overloading.

The amount of D. C.` appliedmay he controlled hy selecting appropriate values for resistances Iii and l'l. If resistance l1 is made ten or more-times as large as resistance I6, substantially the full D. C. voltage developed will be applied for control purposes. Conversely, if resistance It is made one-tenth of the value of resistance l, their slightly less than one-tenth of thedevclopedf' D'. voltage Will be applied for control' purposes. if no D. C. is desired to be applied for control purposes, resistor I6 may be omitted and resistor ill shown in dotted lines may be connected across capacitance I3 as indicated in Fig. l.

A choke coil lf2 is inserted between the diode platea-nd thehigh sideof capacitance i3 for the purpose of completing the D. C. circuit without appreciably reducing the intermediate frequecy voltages.applicdv to the diode plate-and for the pu pose of coupling the diode tothe l. without appreciable selectivity over ti e i band so as to obtain uniform recti-cstion ol trie FM modulated carrier. r

Fig. z-dshows a modification incorporating an additional stage `of amplification between the signal channel and our control means. An ampliiierZB is connected to the signal channel at 9a through coupling capacitance lla. The output of the amplifier is transformer coupled to our control device by a double permeability tuned transformer 22 having primary 2| and secondary 23, and a pass band substantially the same as the I. F. amplifier. The radio frequency output of this transformer is connected between the cathode of diode lila and ground as shown. Between the plate of the diodeandground is included a load resistance 4a, shunted by intermediate frequency bypass capacitance |311.Y The desired audio control voltage corresponding to the amplitude modulation on the carrier is developed across the load resistor |4a. This audio control voltage together with the full D. C. component is applied to the control grids of the amplifiers desired to b-e controlled. Of course, any part of this control voltage jmay be used, different amounts may be used on different tubes, the control bias may be applied to tubes preceding or following point 9a in the signal channel, and as described in connection with Fig. 1, circuits may be used to obtain different ratios of D. C. to audio voltage.

In Fig. 3 is a circuit diagram of a combined standard broadcast, short wave and FM receiver. On the standard broadcast and short wave band, the receiver functions as a triple detector or double superheterodyne, but on the FM band it operates as a single superheterodyne. The rst I. F. stage operates at approximately 8.25 megacycles andV the second I. F. stage operates at either 8.25 megacycles or 245 kc. As shown in Fig. 3, the receiver is connected for FM reception. The tube 40 relating to the present invention is a duplex diode triode of the 6SQ'1 type, the diode plate |01) being connected to the signal channel at 9b through capacitance ilb. Connection 9b is in the output of the last I. F. amplifier designated as 8b.

In Fig. 3 the elements corresponding to Fig. l are shown with the letter suffixes b.- In addition, other components connecting to the control Vgrids of the I. F. amplifiers and first detector are indicated by suitable reference characters. As an illustrative embodiment, these components may conveniently have the following values:

I lla-'10 micromicrofarads |2b-100 microhenries |3b-100 micromicrofarads Mln- 100,000 ohms lh- .5 microfarad lh-1 megohm |1b-2 megohms liz-10,000 ohms 43-.001 microfarad v41--10,000 ohms 55-.001 microfarad 5|-10,000 ohms 52-.001 microfarad 53-2 megohms Control voltages of the type previously described are communicated from the point |81); to the control grid 46 of I. F. amplifying tube 8b through resistor 42 and the secondary 44 of the 245 kc. I. F. transformer and the secondary 45 of the 8.25 megacycles I. F. transformer. Capacitance 43 completes the intermediate frequency circuit to the cathode of tube 8b.

Control voltages are also communicated through point |81) to control grid 49 of I. F. amplifier 1, through resistor 41, and through the secondary .48 of the 8.25 megacycle I. F. transformer. Capacitance 5D is connected to the cathode of tube 1 for the purpose of completing the I. F. circuit. These control voltages are also applied to the control grid 54 of the rst detector 5 through resistors 5| and 53. Capacitance 52 is connected between the cathode of the rst detector and the junction point of resistors 5| and 53 for the purpose of bypassing this junction to ground with respect to R. F. and I. F. potentials. 4

A further embodiment of our invention is shown in Fig. 4, in which amplitude modulation components are amplified, rectiiied and applied in order to provide additional D. C. control bias in excess of the audio frequency control bias. This is of advantage in order to prevent the controlled tubes vfrom drawing grid current. This is accomplished by providing a duplex diode-triode si) such as a standard 6SQ'1, and connecting into the signal channel at 9c, in the manner previously described through a capacity |c. Components corresponding to those indicated in the other figures are denoted by the suix "c. One plate of the diode section of the tube 3B is connected through inductance |2c to load resistance |40 shunted by capacitance lc. Voltage components appear across load resistor |40 as described in connection with the other figures. An audio voltage corresponding to the amplitude modulation is applied through capacitance |5c to one or more of the control grids in preceding amplifier stages through connection |80. The D. C. component is not utilized in this embodiment.` In order to obtain additional bias corresponding to the intensity of the amplitude modulation the audio signal is also applied to the grid 2,9 ofthe triode section of tube 30. This amplified voltage appears across the load resistor 3| and whatever voltage thus appears is connected through capacitance 32 to the other diode of the tube 30 and again rectied. The rectified voltage appears across capacitor 3Q. The voltage appearing across this capacitor is applied through filter network 345, 35, 3% and 3l, to the control grids of the amplifiers by the connection shown at |8c. The constants of this low pass lter network are such that substantially all audio components of the rectified voltage are removed.

It will be observed that with circuits constructed in accordance with the foregoing einbodiments it is possible to utilize the voltage derived from the amplitude modulations on the carrier to bias the control grids of the amplifiers in a sense tending to reduce the effect ofsaid. amplitude modulations. Furthermore, it is possible to utilize all or part of a D. C. voltage corresponding to the signal strength to exercise a further control upon the amplifiers so as to preventoverloading with its attendant loss of sensitivity.

It should also be noted that the amplitude modulation reducing means is effective and will operate at relatively low levels of signal at the discriminator. One or two volts of intermediate rrequency signal at the input of the discriminator will be effective when applied through the described amplitude modulation reducing means. With conventional frequency modulation receivers embodying a limiter, one or two volts of intermediate frequency are required at the grid of the limiter, which is the last I. F. ampliiier tube, before any amplitude modulation reduction is 0btained. Accordingly, the effective sensitivity (the sensitivity at which amplitude modulation reduction is effective) of Our system is increased by the gain of the last intermediate frequency amplifier which may be 50 orV more. This obviates the necessity of an additional amplifying stage, siinpliiies receiver design and reduces cost.

Having thus described our invention, we claim:

1. In a receiver for angular velocity modulated waves comprising an amplifier for amplifying the modulated carrier and a discriminator for detecting the desired modulations, an auxiliary detector connected therebetween and coupled thereto by means no more selective than the amplifier, said auxiliary detector comprising a diode and a load circuit connected thereto, whereby it may rectify both the signal carrier and undesired amplitude modulations thereof, and selective means comprising an audio pass filter network connecting said load circuit to said amplifier, said filter i network including a direct current voltage divider whereby substantially all the audio voltage derived from said auxiliary detector and a portion of the direct current voltage derived therefrom is applied to said amplifier.

2. In a receiver for angular velocity modulated Waves comprising an amplifier for amplifying the modulated carrier and a discriminator for detecting the desired modulations, an auxiliary detector connected therebetween and coupled thereto by means no more selective than the ampliner, said auxiliary detector comprising a diode and a load circuit connected thereto whereby it may rectify both the signal carrier and undesired amplitude modulations thereof, and selective means comprising an audio pass filter network connecting said load circuit to said amplifier, said filter network including a direct current voltage divider whereby selected portions of audio and direct current voltages are applied to said amplier.

3. In a receiver for frequency modulated waves having amplifiers a diode for uniformly rectifying the FM carrier, means including a load circuit for deriving a voltage therefrom' corresponding to the amplitude modulation on said carrier, means comprising a coupling capacitance and an audio pass filter network for applying a portion thereof to the control grid of said amplifier, additional means comprising an amplifier and a rectifier for deriving an amplified direct current voltage proportionately to the average value of said amplitude modulations and a low pass filter network for applying a portion of said direct current voltage to said first amplifier.

JOHN D. REID. CHARLES E. KILGOUR.

REFERENCES CITED The follolwing referenlces are of record in the file of this patent:

UNITED STATES PATENTS

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