US1997991A - Superheterodyne receiver - Google Patents

Description

April 16, 1935.

w. s. BARDEN SUPERHETERODYNE RECEIVER Filed Oct. 26, 19315 INVENTOR Y WILLAM BARDEN ATTORNEY Patented Apr. 16, 1935 Unirse STAT-Es V 1,997,991 sUPEnnE'rnnoDrNr; eEoltlvER William Stoddard Bardcn, GrasmereZ lStaten Island, N. Y.,assignor to 'Radio Corporation of America, a corporation of Delaware r 12 Claims.

lMy. present invention relates to superhetero- 7 95 ing of the localoscillator.

While composite rst detector-local oscillator networks of the type disclosed by me in my copending application Serial No. 592,461, led February 12th, 1932 function in a satisfactory manduringnorrnal superheterodyne reception, there exist certain abnormall situations when the funcV` tioning of such networks are poor. `For examkw ple, when a receiver embodying such a network has been tuned to a desired signal frequency and the cutoff switch of the setehasY been closed, actuation of the switch to open position results in an inoperative condition ofthe receiver. This is particularly noticeable when the frequencyto which the receiver is tuned corresponds toastrong local station. Againfsudden tuning of such a superheterodynel receiver through a signal frequency havingY a strong e'ld intensity causes the aforementioned inoperative condition. It may also be pointed out that this abnormal. condition may arise when such a re,- ceiver is employedl in an automobile during a l.period when the automobile moves out of an shielding zone, such'as a bridge and. permits the signal collector to collect signals of extremely strong iield intensity. v 'Without resorting to involved. theoretical con siderations, it Vis believed sufficient to point out that thesev aforementioned abnormal conditions are usually caused by blocking of the local oscillatorlportion of the composite network. A superheterodyne receiver utilizing, for example, a radio frequency ampliiier stage of a high or a moderategain v(30 or 40, by way of illustration), a pre-selector having a low gain (say l or 2), normal negative bias on the control grid of the radio frequency stage, such normal bias being about -3 volts, and a composite rst detector-local `oscillator network of the aforedescribed type may be rendered inoperative when very strong antenna signals food the composite network. lt may be stated that such strong signals usually have a magnitude of upwards of 1/4 of a volt. In order to restore normal operating condition when this abnormality occurs,`it is vnecessary to adjust the tuning device to a frequency differentV from the desired signal frequency. Such a requirement necessarily aggravates a condition which by itself is suiciently confusing to theaverage broadcast receiver operator. l

Application October 26, l1933,. *SerialNor 695,251

(Cl. Z50-20.).

Accordingly, Iit may be stated that, it is onejof the main objects of lmy rpresent invention toprovide a; 'superheterodyne receiver utilizing a; cornfposite first ydetector-local` oscillatorv'vliereinl` the impression of vextremely strong" signals "oni the I signal collector `of. the receiver has substantially no effecten the functioning of, the'4 'composite network.

Another important object of the invention may be said to reside in the provision cfa super',- l0 heterodyne receiver which isfadapted; to'foperate under allv conditions of variationin'the recel ed signal intensity, the essential feature lof the re;- ceiver consisting in impressing little Vor no'nor'- Amal negativebias on the signal grid ofY the radio l5 frequency amplier." f, f

Another object of the invention is t0.V pIOvide a method of receiving broadcast ysignals by the superhe'tero'dyne method, extremely stronglsignais being employed to produce a directcurreiit 2p component for reducing the ygain of thej'radio frequency amplifier of thereceiver to a point such that iiooding, ory blocking, of, the local oscillvl'ator of the receiver is prevented, thelofcall os'- c illato-r being embodied iny a compositcnetwpik including a first detector. f

` Still otherobjects .of the invention are to; mprove generally the efficiencyV of superheterodyiie receivers utilizing autodyne Vfirst[detectorY networks, and especially to providea receiver ofthis type which is not'only reliable in operation, but which is economically manufactured."

The novel features'whichlibelieve to hecharacteristic of my invention. are set. forth in particularity Yinthe appended claims. y"Ilie invenl tion itself, however, both as to its organization and' method of 'operation will best be understood f by reference to thefollowng de'scriptioi'i,l taken in connection with the drawing, in which .I here indicated diagrammatioally a circuit vorganizaf 40 tion whereby my invention may becarried'into effect. u o t Referring now to the .accompanying drawing whichshows in diagrammatic manner a conventional and well known type/of superheterodyne vi5 receiver, the signal collector is designated bythe reference numeral I. The signal collector is the well known grounded antenna circuit, but `itis to .be clearly understood 'that a loop antenna or anV automobile radio receiver signal collector may l be utilized in place of the grounded antenna circuit. The transformer'Z couplesthe signal collector to the Yinput electrodes of the rstradio frequency amplifier tube 3,' which tube'is preferably of the lscreen grid type. The variable v5.5

Lsecond detector I4.

tuning condenser 4 is connected across the secondary 5 of the transformer 2, and the direct current blocking condenser 6 is interposed between the ground side of tuning condenser 4 and the low potential side of coil 5.

The cathode of tube 3 is connected to ground,

and it is to be clearly understood that it is onev of the essential features of the present invention that the usual grid biasing network employed in the cathode circuit be designed so that little or no negative bias is imposed on the signal grid of the tube. For this reason the usual biasing resistor 1, shunted by the radio frequency by-pass condenser 8, should have a magnitude vsuch that the signal grid of tube 3 is only vslightly negative with respect to the cathode. It may even be preferable in certain situations to dispense with the network 'I and 8, and normally maintain the signal grid at the same direct current potential as the cathode.

The anode of tube 3 is connected to a source of positive potential, whichl source is omitted, through a primary of the coupling transformer 9. The reference numeral I0 designates the compositel first detector-local oscillator which follows the radio frequency amplifier. It is not believed necessary to show the circuit connections of this composite network, since it is fully disclosed and claimed in my aforementioned copending application. It is merely necessary to point out that the composite network employs a pentode tube, and has as one of its chief advantages the fact that the signal grid thereof never draws grid current. The reference numeral II denotes the variable tuning condenser of the first detector portion of the network I0, the variable tuning means of the local oscillator portion being omitted to preserve simplicity of disclosure. It is to be clearly understood that the variable tuning condensers 4 and I I, as well as the tuning condenser of the local yoscillator portion which is not shown, are mechanically uni-controlled in any well known fashion, the dotted line I2 representing such uni-control mechanism.

The intermediate frequency energy output of the network I0 istransmitted to an intermediate frequency amplifier I3, and the amplified output of the latter is impressed upon vthe succeeding As is Well known to those skilled in the art, the detected output of the network I4 is impressed upon an audio frequency amplifier and finally reproduced in any well known manner, as bya loud speaker.

While the'functioning of the present invention is independent of the presence or absence of an automatic volume control arrangement in the receiver, such an arrangement has been shown to illustrate. t-he point that even the presence of sucha gain control arrangement is of little valuev in solving the problem which has heretofore been discussed.

The automatic volume control arrangement shown in the drawing is of a purelyconventional type and is merely shown by way of illustrating one form of the arrangement. Since the gain of the radio frequency amplifier 3 is to be varied in response to variations in received carrier amplitude, a device, such as a diode rectifier I5, is utilized to derive a direct current control potential from received carrier energy. The diode anode is customarily connected to one sideof the input circuit of the second detector I4 through a signal transmission condenser I6, while the cathode of the diode is grounded so as to provide a signal return path to the grounded side of the 'merals I9, 20.

detector input circuit. Between the diode electrodes there is usually connected an impedance I1, which may be a resistor, and a suitable selected point of negative direct current potential of the impedance is connected by a direct current connection I8 to the grid circuit of the radio frequency amplifier. This direct current connection has also been designated AVC" and the usual radio frequency filter network of the automatic volume control path is designated by the nu- Of course, gain control connections may be made to the network I0 and the intermediate frequency amplifier network I3, but these are omitted since they are well known to those skilled in the art.

As stated heretofore, whereas it is usual to employ a negative bias about -3 Volts on the signal grid of the radio frequency amplifier, in the present case there is only a slight negative bias thereon, as for example not over -1/2 volt. Because of this potential relation strong signals at the signal collector I, in resonance with the tuned circuit, are better'able to cause a flow of grid current in the grid circuit of the radio frequency v amplifier. This grid current fiows through a diode rectifier circuit wherein the diode comprises the grid and cathode of the amplifier tube 3. The

diode circuit may be traced from the grid of tube 3 through the coil 5, through the resistor I9, the lead I8, the resistor Il, ground, to the ground of the cathode of tube 3, and the cathode of the latter. The diode action causes a rnegative grid bias to be developed across resistor I9 and whatever other-resistance is included in this diode circuit. This negative bias decreases the gain of the radio frequency amplifier stage. and tends to prevent flooding of the composite network I0.

Itl will, therefore. be seen that when the tuning condensers 4 and I I are adjusted to receive a signal frequency having a high field intensity, up-

wards of 1A of a volt for example, and such strong signals are suddenly impressed upon the receiver, the gain of the radio frequency amplifier will be reduced by the positive signal swing sufiiciently to prevent blocking of the local oscillator portion of the composite network I0. As pointed out above, such sudden impression of strong signals occurs when switching the receiver to operative condition while tuned to the frequency of the strongsignals, suddenly tuning through such a strong signal frequency, or suddenly moving the receiver into the strong field. In any of these cases the grid of the amplifier tube 3 is swung positive, and the resultant flow of grid current through the diode circuit including the grid and cathode of tube 3 creates a negative grid bias potential which is sufiicientto reduce the gain of tube 3 below the oscillator blocking point.

In designing the receiver embodying the present invention it is desirable to have a high stepup ratio between lthe antenna .circuit I and the signal grid circuit of the radio frequency amplifier in order to obtain the diode action without having to make the antenna signal excessively strong. For a given and prerequisite amount of radio frequency gain from the antenna. to the gridof the first detector it is in the best interests of the diode action to have a great amount of that gain ahead of the radio frequency grid, and a smallamount of that gain in the radio frequency amplifier stage. The high step-up ratio between the antenna and the first radio frequency amplifier, and the corresponding reduction in the required radio frequency gain in the radio frequency stage itself are favorable to the maintenance of a low negative bias on the grid of the radio frequency amplifier tube. In this way the erratic performance of the receiver which has been described heretofore is effectively prevented.

While I have indicated and described several 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 particular organization shown and described, but that many modications may be made without departing from the scope of my invention, as set forth in the appended claims,

What I claim isz- 1. A method of receiving signal energy of a selected frequency which consists in amplifying collected signals of a selected frequency, impressing the amplified signals upon a composite first detector-local oscillator, utilizing the resultant intermediate frequency energy, deriving prior to said impression from extremely strong signals of said frequency a direct current component, and decreasing the amplification of said collected signals when said extremely strong signals are suddenly collected.

2. In a superheterodyne receiver, a tunable radio frequency amplifier, an autodyne first detector, means for utilizing the output of said detector, the signal grid of said amplifier being slightly negative with respect to the cathode thereof, and means connected between the signal grid and cathode of said amplifier for developing a grid biasing direct current potential from extremely strong signals suddenly impressed upon said amplifier.

3. In a superheterodyne receiver, a tunable radio frequency amplifier, an autodyne first detector, means for utilizing the output of said detector, the signal grid of said amplifier being slightly negative with respect to the cathode thereof, and means including a direct current connection connected between the signal grid. and cathode of said amplifier for developing a grid biasing direct current potential from extremely strong signals suddenly impressed upon said amplifier.

4. In a superheterodyne receiver, a tunable radio frequency amplifier, an autodyne first detector, means for utilizing the output of said detector, the signal grid of said amplifier being slightly negative with respect to the cathode thereof, and means including an impedance connected between the signal grid and cathode of said amplifier for developing a grid biasing direct current potential from extremely strong signals suddenly impressed upon said amplifier.

5. In a superheterodyne receiver, a tunable radio frequency amplifier, an autodyne first detector, means for utilizing the output of said detector, the signal grid of said amplifier being slightly negative with respect to the cathode thereof, and means including a resistive path connected between the signal grid and cathode of said amplifier for developing a grid biasing direct current potential from extremely strong signals suddenly impressed upon said amplifier.

6. In a superheterodyne receiver, a tunable radio frequency amplifier, an autodyne first detector, means for utilizing the output of said detector, the signal grid of said amplifier being slightly negative with respect tothe cathode thereof, means connected between the signal grid and cathode of said amplifier for developing a direct current potential from extremely strong signals suddenly impressed upon said amplifier, and additional means including said developing means as a part thereof for automatically regulating the gain of said amplifier in response to variations in received carrier amplitude.

'7. In a superheterodyne receiver, a radio frequency amplifier having a tunable input circuit including a variable tuning condenser, a composite first detector-local oscillator having a signal input circuit including a variable tuning condenser coupled to the output circuit of said amplifier, yan intermediate frequency amplifier coupled to the intermediate frequency output circuit of said composite network, and means for utilizing the amplified intermediate frequency energy, a direct current path between the signal grid and cathode of said radio frequency amplifier, said path including a resistive impedance, the signal grid of said radio frequency amplifier normally being maintained slightly negative with respect to the cathode, said resistive impedance developing a direct current potential thereacross of a magnitude sufficient to decrease the gain of said radio frequency amplifier to a point such that blocking of the local oscillator of said composite vnetworkis prevented when extremely strong signals of the frequency to which said variable tuning condensers are adjusted is impressed on the input circuit of said radio frequency amplifier.

8. In a receiver of the type defined in claim 7, an automatic volume control arrangement including said resistive impedance as a part thereof connected between the output of said intermediate frequency amplifier and the signal grid circuit of said radio frequency amplifier.

9. An improvement in a superheterodyne receiver of the type including a composite local oscillator-first detector circuit preceded by a tunable radio frequency amplifier, and the local oscillator network of the composite circuit tending to become inoperative when strong signals of a selected frequency are suddenly impressed on said amplifier, said improvement comprising means for maintaining the signal grid of thev amplifier at a direct current potential which at no time becomes positive with respect to the amplifier cathode potential, a direct current path between the amplifier signal grid and cathode including an impedance, said impedance having a value sufiicient to develop a direct current voltage for negatively biasing said signal grid to reduce the vamplifier gain to a level for preventing said inoperativeness when said strong signals are received.

10. In a receiver of the type defined in claim 9,

a signal collector, and a signal voltage step-up device between the collector and amplifier.

11. In a receiver as defined in claim 9, said maintaining means comprising an impedance in the space current path of the amplifier adapted to bias the signal grid negatively.

12,. In areceiver as defined in claim 9, an automatic volume control circuit connected between the composite circuit output and the signal grid of the amplifier and including the said path as a part thereof.

WILLIAM STODDARD BARDEN.

Images