US2128661A

US2128661A - Automatic frequency control system

Info

Publication number: US2128661A
Application number: US151794A
Authority: US
Inventors: Mountjoy Garrard; Dudley E Foster
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-07-03
Filing date: 1937-07-03
Publication date: 1938-08-30
Anticipated expiration: 1955-08-30
Also published as: GB516647A; DE712378C

Description

Aug. 30, 1938. G. MOUNTJOY ET AL AUTOMATIC FREQUENCY CONTROL SYSTEM Filed July 5, 1957 Patented Aug. 30, 1938 UNITED STATE-S PATENT CFFICE AUTOMATIC FREQUENCY CONTROL SYSTEM of Delaware ApplicationJuly 3, 1937, Serial No. 151,794

14 Claims.

Heretofore, automatic frequency control circuits (AFC) have vutilized in the frequency control network a special control tube functioning to produce a reactive effect across the oscillator tank circuit. It has been suggested, for the pur- "pose of simplification of circuit arrangement, that the oscillator tube include auxiliary electrodes functioning to provide the control tube action. However, such earlier circuits require the use of extra tubes, or specially designed oscillator tubes.

15\There are many situations wherein it is not desirable, and often uneconomical, to employ these past arrangements.

. Accordingly it may be stated that it is one of the main objects of our present invention to profvide an automatic frequency control circuit for a superheterodyne receiver, wherein the AFC bias derived from the usual discriminator network is applied to one of the existing local oscillator electrodes to secure a correction frequency adjust- T-ment in the oscillator tank circuit.

In carrying out such an improved and simplied form of AFC circuit it is important, however, to make sure that the controlled oscillator electrode, as for example the oscillator grid, is conl nected to a low impedance direct current voltage source. If the oscillator electrode is connected to the output circuit of the well known types of discriminator networks, undesirable results are secured since such output circuits are usually `of a high impedance. Again, since the AFC bias should be of a suciently large magnitude to eiiect appreciable control of the tank circuit, amplification of the I. F. energy prior to rectification by the discriminator is usually performed. However, where economy of tube usage is a consideration, such amplification can not be secured.

Hence, it may be stated that it is an important object of our invention to utilize one of the existing. amplier tubes, such as the I. F. amplifier, as a direct current ampliiier for the purpose of amplifying the AFC bias derived from the discriminator network; and the amplied bias being applied from the cathode circuit, which has a low impedance, of the I. F. amplifier to one of the existing electrodes of the local oscillator for the purpose of adjusting the oscillator tank circuit frequency when the I. F. energy shifts in frequency from the assigned I. F. value.

Another important object of our present inven-J tion'isto' provide, in general, a novel method of (Cl. Z50-20) varying the frequency of an oscillator circuit supplementally of -a main frequency adjustment device; the method involving the utilization of quadrature elements in at least one of the electrode circuits ofthe oscillator for rotating the phase of the current flowing in the electrode circuit with respect to the alternating voltage applied thereto, thereby to vary the effective reactance of the oscillator tank circuit.

Still another object of our invention is to improve existing AFC systems, commonly employed in superheterodyne receivers, by employing the existing local oscillator electrodes as the frequency control electrodes; and the AFC bias being amplified by one of the I. F. amplifiers prior to impression of the AFC bias on one of the local oscillator electrodes.

The novel features which we believe to be characteristick of our invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference Vto the following description taken in connection with the drawing in which we have indicated diagrammatically a circuit organization whereby our invention may be carried intoeffect.

Referring now to the accompanying drawing, there is shown the circuit diagram of a lsuperheterodyne receiver embodying generally a signal collector A; a tunable radio frequency amplifier; a converter; an 1. F. amplifier; a combined second detector-audio amplier; and an AFC discriminator. The collector A may be a grounded antenna circuit;' but it may be a tap to a radio distribution` line,l or it can be a loop antenna, .and it can even be the usual signal pick-up device on mobile structures. The amplifier tube l, any well known type of radio frequency pentode tube, has its signal grid connected to the high alternating potential side of the tunable input circuit 2. The input circuit includes the variable tuning condenser 3 adapted to vary the tuning of the amplier over the receiver signal frequency range. The. latter may comprise a range of 500 to 1500 kc., that is, the broadcast range. .The cathode of tube l is connected to ground through a bias resistor 4; a radio frequency bypass condenser.

being employed to shunt the resistor 4, and a similar bypass condenser connecting the low potentialV side of input circuit 2 to ground.-

The input circuit 2 is magnetically coupled to thev collector A; the plate of amplifier tube I being connected to a source of positive potential. latter-.source is vnot shown, but it is to be understood that a common bleeder circuit can be used for this purpose. The various electrodes of the different tubes will be connected to points of proper potential on the bleeder. The cathode side of resistor 4 is connected to a proper positive point on the bleeder to raise the cathode potential above ground; the potential of the amplifier cathode with respect to ground may be about 7 volts.

The plate circuit of amplifier I is coupled to the signal input circuit 5 of the converter (combined local oscillator-first detector) tube 5. The latter may be a pentagrid converter tube of the well known 6A8 type; the tube comprising a cathode l, an oscillator grid electrode 8, an oscillator anode electrode 9, a signal input grid I and an output plate II. The signal grid |0 is electrostatically shielded by positive screen electrodes; and the plate is connected to a positive potential point through the primary winding I2 of the coupling transformer M. A condenser I3 resonates the coil I2 to an operating intermediate frequency (hereinafter referred to as I. F.); the frequency may be chosen from a frequency range of '75 to 450 kc.

The signal input circuit includes the variable tuning condenser I4 which is adapted to vary the tuning of the circuit over the broadcast range; the high potential side of circuit 5 is connected to signal grid I0, while the low potential side of the circuit is connected to ground by a radio frequency bypass condenser. 'Ihe oscillator anode 9 is connected to the high alternating potential side of the tunable oscillator tank circuit comprising coil I5, variable tuning condenser I6 and padder condenser I 'I. The junction of condensers I6 and I1 is established at ground potential; the low potential side of coil I5 being connected to a point of positive potential through a resistor I8 having a magnitude of approximately 10,000 ohms. The oscillator grid 8 is connected to the high potential side of feedback coil I9 through a pair of condensers and 2| arranged in series.

Condenser 20 may have a value of approximately 10 mmf., and is shunted by resistor 22 having a magnitude of about 50,000 ohms; condenser 2| having a value of about 100 mmf. The coils I9 and I5 are magnetically coupled to provide the local oscillations; coil I9 being connected, at its low potential terminal, to the junction of coil I5 and condenser I'I. The cathode 'I of converter tube 6 is connected to ground through the bias resistor 23; the cathode being raised above ground by connecting the cathode side of resistor 23 to a proper positive potential point of the bleeder. 'I'he converter cathode may be established at a potential of approximately 9.5 volts above ground.

The rotors of the tuning condensers 3, I4 and I6 are arranged for mechanical unicontrol; the dotted line 24 denotes such control mechanism. The tank circuit I 5-I'I-I 6 is tunable over a frequency range which constantly differs from the signal frequency range by the Value of the I. F. The padder condenser I 'I acts, in a manner well known to the art, to maintain the I. F. value constant at all station settings of the tuner 24. The local oscillations beat with the signal energy to produce the I. F. energy. Those skilled in the art are fully aware of the action of conversion which takes place in tube 6. Briefly, the electron stream thereof is modulated by both the signal and oscillation frequencies; electronic coupling, is employed to produce the I. F. energy in circuit |2-I3.

The coil I2 is coupled to coil 3| in the input circuit of the I. F. amplifier |02; the input circuit is resonated to the operating I. F. The cathode of tube |02 is connected to ground through a path including resistors |03 and |04 in series. The resistor |04 is shunted by I. F. bypass condenser 32; resistor |03 may have a magnitude of 400 ohms, While resistor |04 has a value of 500 ohms. The cathode side of resistor |03 is connected to the oscillator grid B by a path including lead 30, resistor 25 and coil 24. The resistor 25 acts as a filter element, while the coil 24 functions in a manner to be described at a later point. Condenser 33 is connected between the AFC lead 30 and ground, and acts as an I. F. bypass. The condenser establishes the low potential side of input circuit 3 |-3 I at ground potential for alternating currents; switch 35 is an on-off control element for the AFC line. The switch 35 has its xed contact connected to the low potential side of coil 3|, while the adjustable element thereof is connected to the junction of resistors H13-|04.

The plate of the I. F. amplifier |02 is connected to a source of positive potential through coil 35; the latter being resonated to the operating I. F. by the condenser 3l. The coil 35 is magnetically coupled, as at M1, to the coil 38 of the discriminator input circuit. Coil 30 has its midpoint c'onnected to the plate side of coil 35 by the condenser 39; the latter may have a magnitude of approximately 100 mmf. The coil 38 is resonated to the operating I. F. or center frequency by its shunt condenser. The discriminator employs a double diode tube, known as a SHS type tube; the anode II of one diede section being connected to one side of coil 33, and the anode l2 of the second diode section being connected to the other side of coil 38. Cathode 4| is connected by lead I3 to the junction of resistors |03 and |041; while cathode 42 is connected by lead 44, which includes resistor |00 therein, to the low potential Side of coil 3l.

The resistor I 06 may have a magnitude of approximately one megohm; and the resistors |00 and IDI, connected in series between cathodes 0I and 02', may have a Value of approximately one megohm. The junction of resistors |00 and IBI is connected by lead i5 to the midpoint of coil 38; the I. F. bypass condenser i0 being connected in shunt across the resistors |00-IOI- The point 09 of resistor IIII, to which lead i4 is connected, has a variable direct current potential with respect to point 99' when the I. F. energy shifts in frequency from the assigned I. F. value. The potential is amplified by the tube |02, and appears in amplified form across resistors I {i3-|00; the AFC lead 30 applies the amplified direct current potential to the oscillator grid 8 for oscillator frequency adjustment. The I. F. energy is demodulated for securing audio and AVC voltages by tube 50; the latter may be a diode-triode tube of the 6Q'7G type. 'Ihe cathode of the tube is connected to ground through a path including resistors |07 and |08 in series; the resistor I 0l may have a magnitude of approximately 200 ohms. The cathode side of resistor I0'I is connected to a proper positive potential point on the bleeder; the cathode is at about 4.5 volts above ground.

The diode anode 5| of tube 50 is connected to the cathode of the latter through a path including coil 52, link coupling coil 53, resistor 54 and resistor 55. The coil 53 is of but a few turns, and is magnetically coupled to coil 33 at the midpoint of the latter. Resistors 54-55 are shunted by the I. F. bypass condenser 55, and the resistors are of about 50,000 ohms and 250,000 ohms respectively.; The condenser 'I resonates the coils 52=53 to the assigned I. F. value; AVC bias is taken off from the junctionfof resistors 54 and 55.1; The AVC lead 60, including the resistor-ca- .'.pacity filter network 6i, is connected from the junction of resistors 54 and 55 to the low potentialsides of input circuits 2 and 5. The AVC connections'to the signal grid circuits of tubes I and 6 .include appropriatefilter resistors 6I. The audio component ofthe detected I. F. energy is impressed on the control grid of the tube 56 byV April 3, 1936, Patent No. 2,120,974, dated June 2.1,

1938 by D. E. Foster. For this reason it is not believed necessary to describe Athe operation of thesenetworks in detail; a general explanation will now be given.

The theoretical basis for the production of the AFC voltage across resistors Ili-l 6I is explained in the following manner. The potentials at either end of coil 38, with respect to its midpoint, are 180 degrees out of phase. Hence, if the midpoint is Q connected to the high potential side of coil 36, one

potential is realized which maximizes above the resonant frequency of the I. F. value, and a second potential is realized which maximizes below this value. If these two potentials are applied to a 404;.pair of rectiers, such as the diodes of tube 40,

and the resulting direct current voltages are added in opposition, the sum will be equal to zero. Inthe -type of discriminator network shown in the drawing, the primary and

secondary coils

36 and 38 are solconnected that two Vector sum potentials `of the primary and secondary voltages maybe realized.v When the I. F. energy departs in frequency value from the assigned operating I. F.',then there is developed across resistors IMI and. `IIlI ,a direct current voltage. Since point 99 is connected by lead i3-to the junction of resistors |03 and IM, then the point 99 will vary in potential in magnitude and polarity depending upon the-amount yand-.direction of shift of the I. F.

, energy.

Of course, any other type` of discriminator for producing a direct voltage from the I. F. energy, when; the latter shifts in frequency from the assigned I. F. value, may be utilized. Such other networks are well known to those skilled in the art. In general, it may be stated that there may be employed a source of direct current voltage, which voltage is derived from the I. F. energy when the latter departs fromthe assigned I. F.

value; and the direct current voltage being produced in a sense to vprovide `correction bias for the'local oscillator network. The separate diode rectifier; arranged in tube 50, is employed so as to secureadequate selectivity even though but one .stage of I. F. amplification is employed. The diodeof 4tube 56 has its tuned input circuit coupled to the coil 38, and hence it will rbe seen that the input circuit4 of the second detector diode is really a tertiary tuned circuit coupled to the ,secondaryA circuitincluding coil 38. The coupling between coils 38 and 53 is arranged so that coil 53 is coupled only to coil 38, and not to coil 36. This is done, as schematically shown in the drawing, by using a few coupling turns 53 close to coil 38. These turns are shown at the center of coil 38, and this arrangement is physically followed in order to keep the capacity coupling of coil 53 symmetrical with respect to both sides of coil 38.V Inthis wayin spite. of the use of but a single I. F. amplifier, the selectivity preceding the audio demodulator is satisfactory.

The AVC arrangement shown acts, of course, to reduce the gain of amplifier I, as well as to reduce the conversion gain of converter 6, as the signal carrier amplitude at the second detector input circuit increases. In this way the carrier amplitude at the audio demodulator input circuit is maintained substantially uniform in spite of variations in signal amplitude at the signal collector. Of course, the carrier amplitude at the discriminator input circuit also remains substantially uniform in spite of signal amplitude variations at the signal collector.

The AFC bias is applied by lead 30 to the oscillator grid 8. This causes a change in the` frequency, at any setting of the tuner 24, of the oscillator tank circuit. rIhe oscillator strength is fairly constant with changes in bias. The conversion conductance increases with an increase of negative oscillator Voltage. This effect is in part compensated for by the fact that an increase in negative oscillator voltagel is obtained by a decrease in the I. F. amplifier tube current, with an attending decrease in the mutual conductance of the I. F. amplifier |02. There will now be described the mechanism by which the oscillator tank circuit is shifted in frequency as soon as AFC bias is produced. It is to be clearly understood that our explanation involves, in part, theoretical aspects, and we are not restricted to such aspects. The frequency changes occur as described. n

The oscillator produces an oscillatory voltage across the tank circuit I5, I6, I1, which is transferred to coil I9 by mutual inductance. In an oscillator the grid draws current so that there is a finite grid impedance, predominantly resistive, if the grid-cathode static capacity can be neglected. For the purposes of explaining the action this capacity will be neglected and its effect shown later. The voltage in the grid coil I9 causes a current to iiow through resistor 22, capacity 26 and the input grid resistance of grid 8 of tube 6. Capacity 2I is a blocking capacity and does not iniiuence control action. Capacity 20 and resistor 22 and the grid resistance rg of the tube rotate the phase of the current relative to the voltage in that circuit. The current flowing through rg therefore has an in-phase and a quadrature component. The in-phase component serves to maintain oscillation and the quadrature component to vary oscillation frequency. These two components of current owing through 'rg produce the grid voltage at oscillator frequency which, by virtue of the mutual conductance of the tube 6, produces an alternating current in the plate circuit thereof. The direction of the mutual inductance between coils I5 and I9 necessary to maintain oscillation is such that this alternating plate current produces an effect equivalent=to a negative resistance and a negative capacity in shunt to the tank circuit.

The negative resistance component is responsible for the oscillation and the negative capacity.Y component varies the frequency to some Value.

other than that due to the constants of the tank circuit alone. Now if the direct current is applied to grid 8 of tube through inductance 24" and resistance 25 the effect on tube 6 is to vary the mutual conductance and the grid resistance rg. If the direct current bias on grid ES` is made more positive the mutual conductance increases but the grid resistance decreases. The effect of these two factors being opposite tends to maintain the oscillatory voltage substantially constant unless the bias on grid 3 is made so negative that cut-off is approached or exceeded, in which case the oscillator ceases to function. If direct current bias on grid 3 is made positive, the effect on the negative capacity due to the quadrature component is chiefly that of the variation of rg. In this case rg decreases and the negative capacity in shunt to the tank circuit increases and the oscillatory frequency increases. An increase in negative capacity in shunt to a tuned circuit has the same effect on frequency as a decrease in a positive inductance in shunt to the tuned circuit. However, a negative capacity has an irnpedance which decreases with increasing frequency whereas a positive inductance has an irnpedance which increases with increase in fre quency. The frequency shift produced by a given change in negative capacity is thus greater at the high frequency end of the tuning spectrum Where the tank circuit capacity is low, whereas the effect of a given change in positive inductance is a substantially constant percentage of 'the frequency over the tuning spectrum, since the tank circuit inductance is a constant. It has been shown that making the direct current potential of grid 8 more positive results in an increase in oscillatory frequency. If the direct current potential of grid 8 is made more negative the frequency decreases. It has been shown, also, that the effect of capacity 2li is to produce a negative capacity in shunt to the tuned circuit. If capacity Z is replaced by an inductance the effect would be as if a negative inductance were shunted across the tuned circuit, in which case the amount of shift would be more constant over the tuning spectrum.

However, there are advantages in using a capacity in the grid circuit, namely it is possible to secure a capacity without inductive effect, but all inductances have associated therewith some distributed capacity which may disturb the desired phase relations at some tuning frequency. Furthermore, there is less change in the amount of frequency shift when a capacity is used than the foregoing description of operation would show, because of the effect of the static capacity of grid 8 to cathode Tl of tube 6. This capacity shunts rg and exhibits a lower reactan'ce at high frequencies than at low frequencies and therefore tends to decrease the variation in frequency shift obtained with variation of frequency over the tuning spectrum.

Inductance 2t acts as a choke coil so that resistor 25 may be made low in value without decreasing the oscillation amplitude. inductance 2li is preferably made of such value as to be resonant with the internal capacity between grid 8 and cathode 'F of tube at or below the lowest tuned frequency, so that the effect of this internal capacity is partially cancelled at the low frequency end of the spectrum to enhance the frequency shift at those tuned frequencies. A value of inductance 213 of 2.4 millihenries has been found suitable when tube 6 is of the 6A7 or GASG type.

Resistor Z5 improves the ease of starting oscillation by giving a small amount of self bias to grid 8 of tube 6. It is preferably made as small as consistent with this requirement in order that any variations in direct grid current of grid 8, due to changes in oscillator `amplitude over the tuning spectrum, will not produce large changes in bias on that grid; as it is required for purposes of frequency shift, that changes in bias of grid 8 be due predominantly to changes of discrimi` nator potential caused by intermediate frequency shift. In an experimental receiver a value of 2500 ohms has been found suitable for resistor 25.

In the operation of this circuit it has been found that making the bias of grid 8 of tube G more negative resulted in increased conversion efficiency of that tube, that is, increased intermediate frequency output with constant radio frequency input signal intensity. This is contrary to what might commonly be expected but can be explained by the observation noted above that rg and mutual conductance of grid 8 vary in opposite directions with bias thereon, tending to maintain substantially uniform oscillatory voltage across rg. But if the oscillatory voltage is constant and fg increases with increasing negative bias on grid 8, the current flowing to grid 8 must decrease since the oscillatory voltage on grid 8 is the product of rg and the oscillatory current. The decreased current flow to grid 8 results in increased current flow to the other electrodes and consequently increased conversion efficiency. As has been seen, the bias on grid 8 has a determining effect on the mutual conductance between grid 8 and electrode yhowever, electrode 9 has little effect on the flow of current to anode il of tube but the flow of current to grid E does influence the output from anode Il so that a more negative bias on grid 8 results in increased conversion efficiency and vice versa.

It is desired to point out at this time that the padder condenser il need not be located in the path common to both grid and plate circuits of the oscillator. For example, it can be disposed in series relation with the variable condenser it; and in that case the common junction of coils l5 and l would be by-passed to ground through a radio frequency by-pass condenser. Of course, it is not essential to the operation of this circuit to employ a padder condenser, and the latter may be dispensed with if desired. Moreover, it is to be understood that the oscillator section may include a tube independent of the mixer section. Of course, the quadrature elements 2%-22 are illustrative in nature; they may be replaced by combinations of elements chosen to produce the oscillator frequency shift which is desired. Such combinations of elements may in general include inductive, capacitative and resistive elements. These elements may be used singly in proper cases.

As has been previously explained there is developed across the discriminator output resistors l0@ and l'l a direct current voltage which varies in magnitude and polarity in dependence upon the amount of frequency departure of the I. F. energy from the operating I. F. as well as upon the direction of departure. 'I'here will now be given an explanation of how the Variable direct current voltage is utilized to produce a shiftsofi" ingresistor of tube |02. Resistor |04 is a voltage amplifying resistor in the cathode circuit of tube E02, but serves no function in biasing tube |02,

While the I.F. tube was chosen for this function' inthis specific case, obviously a separate tube might provide the same action, and obviously an R. F. amplifier tube or other tube in the rec eiver might be chosen to provide the same results.

lDiscrirninator volts at center or correct frequency are zero in magnitude. The bias on tube |02 is then only the self bias and the plate current of tube |02 is the result of the operating potential and tube characteristics. This vplate current provides a voltage drop across resistances |03 and |04 in series, which voltage drop is applied to the grid 8 of tube 6 through resistance and coil 24.

The effective xed bias on grid 8` of tube 6 is the voltage across resistors ||03 and |04 plus the voltage across resistor 23 in the cathode circuit of tube 6. The voltage across resistor 23 is produced by the ow of current from cathode 1 and resistor |05 connected to a high potential source B plus. Variations in the voltage across resistors |03 and |04 will vary the bias on grid 8. When an input signal is applied to antenna A of lower frequency than necessary to produce by ,beating with the oscillator frequency an I. F.

of correct frequency or predetermined frequency, an I. F. signal is produced which is higher than the predetermined frequency.

" Signals of this higher frequency are passed through the discriminatorI circuit and a discriminator voltage is developed across resistors |00 and |01, which voltage is negative as automatically applied to the control grid of tube |02. A decrease in cathode current in tube |02 results in a decrease in voltage across resistors |03 and |00 thus changing the bias on grid 8 of tube 6 and producing a more negative bias on grid 8. This more negative bias shifts the frequency of oscillation as previously explained and causes a lower frequency to be produced. This lower oscillator frequency beating with the input signal produces a lowering in the I. F. signal frequency and tends to correct for the assumed condition of I. F. signals higher than the desired I. F.

With an input signal to the antenna A is higher in frequency than that necessary to produce the correct I. F. by beating with the oscillator frequency, the reversal of the above biasing process takes place and the I. F. frequency is corrected. v The control characteristics of the oscillatorcontrol circuit were experimentally measured o n the experimental receiver from which the schematic diagram was derived. The advisability of van initialV biasing point of about 3.5 volts negative on grid 8 was determined. In consequence values of components were chosen to provide the following D. C. voltages:

Across R23, 9.5 volts Across R103, 2.7 volts Across R104, 3.3 volts Across R4, 7 volts Across R107 and 10s, 5 volts These voltages result in the following biases on the several tubes to be described. Grid 8 in tube 0 is biased 3.5 volts negatively. The control grid of the radio frequency amplifier tube l is biased 2 volts negatively. On grid I0 of tube 6 the bias is 4.5 volts negative. On the control grid of tube |02 the bias is 2.7 volts negative. It is to be understood, of course, that the foregoing values are merely'illustrative in'nature, and are in no way restrictive.

It is -emphasized that the AFC line 30 is connected to a low impedance source of direct current voltage. The magnitude of shift of the oscillator frequency is dependent upon the change in bias existing between grid 8 and cathode of tube E. If a high impedance source of D. C. bias were used the condition of grid current characteristic of oscillator circuits wouldprovide a voltage drop through th-e internal impedanceof the bias source which drop would subtract from the Voltage of the bias source in the form of a loss, and would not provide a bias change effective between grid 8 and cathode. Thus the resistors |03 and |04 provide the double function of permitting the development of the bias voltage, and by virtue of their low impedance minimize the loss in bias voltage actually transferred to the control grid 8.

As explained previously 3.5 volts may be employed forthe oscillator bias at the operating I. F. value. The voltage drop across the I.v F. amplifier cathode impedance will change with 01T- frequency signal plus or minus 3 volts. Experimentally, it was determined that the total effective shift was approximately 14 kc. at a setting of the tuner 24 of 1400 kc.; 11 kc. at 1000 kc. setting; and '7 kc. at 600 kc. setting. ,Since frequency drift and inaccuracies of mechanical tuning devices are larger at the high frequency end of the spectrum, the increased control shift at 1400 kc. is advantageous. v

It will now be seen that we have provided, by virtue of our present invention, a superheterodyne receiver Which'employs a single stage of tunable radio frequency amplification followed by a Pentagrid converter stage and a stage of I. F. amplification; a discriminator network being employed to furnish the AFC bias, and the latter being impressedupon anelectrode of the oscillator section of the converter. A satisfactory AFC circuit is provided by means of the present invention, an existing tube of the receiver being employed to amplify the AFC bias. In the present system of automatic frequency control it is not necessary to utilize a separate frequency control tube; on the contrary, the electrodes of the oscillator are employed for the frequency control function. The

quadrature elementsy

20 and 22, and the choke coil 24', are the only additions to the local oscillator circuit. In general, then, our present invention makes it possible to employ AFC in a superheterodyne receiver of compact construction wherein it is a requirement that the receiver employ a minimum of tubes.

While we have indicated and described a system vfor carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What we claim is:

1. In a superheterodyne receiver of the type employing a local oscillator circuit provided with a tank network having means for tuning it over a desired wide frequency range, said circuit including a tube provided with a cathode, anode and control grid electrode, said anode and grid elec-` trode being reactively coupled, and said tank network being connected between said anode and cathode, means for varying the direct current potential of the oscillator grid electrode, resistance and reactance elements connected in the oscillator grid circuit, and said elements being so chosen as to Vary the frequency of the oscillator tank network in response to variations in said direct current potential.

2. In a receiver as defined in claim 1, said elements being arranged in shunt relation to each other, and the shunted elements being connected in series in the oscillator grid circuit.

3. In a receiver as defined in Claim l, an intermediate frequency transmission network, and said varying means comprising a network responsive to shifts in the frequency value of the intermediate frequency energy.

4. In a receiver as defined in claim 1, an intermediate frequency amplifier, a discriminator network coupled to said amplifier and providing said varying means, a connection between the discriminator output and the input circuit of the amplifier, and an additional connection between the output circuit of said amplier and said oscillator grid.

5.V In a receiver as defined in claim 1, said varying means including an inductance connected to said oscillator grid electrode, and said inductance having a magnitude such as to resonate the oscillator grid-cathode capacity to a frequency adjacent the low frequency end of said frequency range.

6. In combination with an electron discharge tube of the type including at least a cathode, an anode and a control electrode, a resonant tank circuit connected between the cathode and at least one of the other two electrodes of the tube, means for reactively coupling the second electrode to said tank circuit, means connected to said second of the tube electrodes for rotating the phase of the current flowing in the circuit thereof with respect to the alternating voltage applied thereto thereby Varying the effective reactance of saidtank circuit, and means for varying the magnitude of the current flowing into said tank circuit to control the effectiveness of said phase rotating means.

7. In an oscillator network of the type including a tube provided with a cathode electrode, an anode electrode and a control electrode, means providing a resonant tank circuit between the cathode and at least one of the other two electrodes, impedance means coupling said two electrodes in such a manner as to provide an oscillatory current flow through said tank circuit, means connected to the second of said tube electrodes for rotating the phase of the current iiowing in the circuit thereof with respect to the alternating voltage applied thereto thereby to vary the effective reactance of said tank circuit, and means for varying the magnitude of the current flowing in to said tank circuit to control the effectiveness of said phase rotating means.

8. In an oscillator as defined in claim '7, said varying means comprising a source of variable direct current potential.

9. In an oscillator as defined in claim 6, said phase rotating means comprising at least a reactance.

10. In a superheterodyne receiver, a local oscillator having a resonant tank circuit, said oscillator being of the type including at least a cathode,

a control grid and an anode, said tank circuit being connected between the anode and cathode, said grid being reactively coupled to the tank circuit, a network electrically associate-d with the oscillator for producing energy of an intermediate frequency, means for deriving a direct current voltage from the intermediate frequency energy in response to a frequency departure of the latter from a predetermined frequency value, and means for applying said direct current voltage to said oscillator control grid thereby to adjust the frequency of the tank circuit in a sense to maintain said predetermined frequency value.

l1. In a superheterodyne receiver, a local oscillator having a resonant tank circuit, said oscillator being of the type including at least a cathode, a control grid and an anode, said tank circuit being connected between the anode and cathode, said-grid being reactively coupled to the tank circuit, a network electrically associated with the oscillator for producing energy of an intermediate frequency, an intermediate frequency amplifier for amplifying said intermediate frequency energy, means for deriving a direct current voltage from the amplified intermediate frequency energy in response to a frequency departure of the latter from a predetermined frequency value, means for impressing said direct current voltage upon said amplifier for` amplification of the voltage, and means for applying said amplified direct current voltage to said oscillator control` grid thereby to adjust the frequency of the tank circuit in a sense to maintain said predetermined frequency value.

12. In a receiver as defined in claim l0, means connected to the oscillator control grid for rotating the phase of the current flowing in the control grid circuit with respect to the alternating voltage applied thereto.

13. In a superheterodyne receiver of the type employing a first detector network having a tunable signal input circuit, a local oscillator network having a tank circuit including means for tuning it over an oscillation frequency range, said oscillator network including a tube having a cathode, anode and grid electrode, said tank circuit being connected between the anode and cathode, said grid being reactively coupled to said tank circuit, an intermediate frequency network including at least one amplifier, and a discriminator circuit for deriving a direct current voltage from the amplifier output energy when the latter departs in frequency from a predetermined intermediate frequency Value, the improvement which comprises means electrically connected to the oscillator grid electrode for producing a change in the effective reactance of said oscillator tank circuit, Yand additional means for impressing said discriminator direct current voltage upon the oscillator grid electrode in a sense to control said effective reactance change in that direction which will maintain said predetermined intermediate frequency value.

14. In a receiver as defined in claim 13, said additional means including said one amplifier functioning as a direct current amplifier.

GARRARD MOUN TJ OY. DUDLEY E. FOSTER.