US2121735A - Automatic frequency control circuit - Google Patents

Description

June 21, 193s. D. E. FOSTER UAL 2,121,735

AUTOMATIC FREQUENCY CONTROL CIRCUIT Pilebec'. 21. 193e LOCAL OSC/LLA T09 AFC MMAM

ATTORNEY Patented `lune 21, 1938 UNITED STATES PATENT OFFICE Dudley E. Foster, Morristown, N. J., and Garrard Mountjoy, Bayside, N. Y., assignors to Radio Corporation of America, a` corporation of Delaware Application December 21, 1936, Serial No. 116,882

6 Claims.

Our present invention relates to automatic frequency control circuits for superheterodyne receivers, and more particularly to an improved type of frequency control arrangement for the local oscillator tank circuit.

There has been disclosed by S. W. Seeley, in application Serial No. 45,413 led October 17, 1935, an automatic frequency control circuit which employs a discriminator network forderiving a direct current voltage from I. F. signal energy, the voltage depending in magnitude and polarity upon the sign and amount of frequency departure of the I. F. energy from its assigned operating value. The derived direct current voltage is used to control the gain of a control tube; the plate and grid circuits of the control tube being connected to the oscillator tank circuit to produce an effective inductive effect across the latter. Variation of the gain of the control tube then results in a change in frequency of the oscillator tank circuit. It has been found highly desirable to maintain a substantially uniform inductive effect across the oscillator tank circuit throughout the signal frequency range of the receiver, since the lack of such uniformity results in a much greater frequency correction of the tank circuit at the high frequency end of the tuning range.

It may, therefore, be stated to be one of the main objects of our invention to provide'an automatic frequency control network for a superheterodyne receiver, which control network includes a -discriminator network fed by I. F. energy, and providing a direct current voltage varying in magnitude and polarity with frequency departure of the I. F. energy fro-m its assigned predetermined frequency value; and the direct current voltage being employed to vary the gain of a control tube whose plate and input grid circuits are connected across the local oscillator tank circuit in such a manner as to provide an effective inductance across the tank circuit; the connection between the control tube grid and the tank circuit being carried out so that the H frequency correction of the oscillator tank cirtube being connected to a point intermediate to a resistor and condenser arranged in the oscillator tank circuit; any frequency shift produced in the oscillator tank circuit by variation of the gain of the frequency control tube being substantially the same throughout the tuning range of the tank circuit.

Another object of our invention is to provide a control tube, coupled through a resistor and condenser quadrature element across the padder of a superheterodyne oscillator, with the alternating plate current of said control tube flowing through a primary coil coupled to the oscillator and through the oscillator padder network in series, thus producing a capacity effect from the high side of said primary to the low side of the padder circuit, which capacity effect is controllable in magnitude by the discriminator voltage applied to the control grid of the control tube, with a resulting improvement in AFC systems, since the use of a control tube and attending circuit components in this manner does not substantially decrease the range of tuning of the superheterodyne oscillator over the range which said oscillator would have if the circuit were uncoupled from the oscillator circuit, and further improvement resulting from the substantially equal degrees o-f shift at all points of the tuning spectrum.

Still other objects of the invention are generally to improve the efficiency and reliability of automatic frequency control arrangements for superheterodyne receivers, and more specifically to provide improved frequency control circuits in a manner such that they can be economically manufactured and assembled in receivers o-f the superheterodyne type.

The novel features which we believe to be characteristic 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 to the following description taken in connection with the drawing in which we have indicated diagrammatically a circuit organization whereby our invention may be carried into effect.

Referring now to the accompanying drawing, there is shown only that portion of a superheterodyne receiver which embodies the present invention. The receiver is of a conventional type adapted to receive amplitude-modulated carrier waves; for example, those in the broadcast range of 550 to 1500 kc. The superheterodyne receiver shown may comprise the usual signal collector followed by one, or more, stages of tunable radio frequency amplification. The amplified radio frequency signals will be fed to a rst detector l which has a tunable input circuit 2, local oscillations being impressed on the detector i by a local oscillator of the tunable type. The local oscillator tube may be of any desired type, and is generally designated by the numeral 3. It is shown as comprising a triode, of the 6C5 type, which has a tunable tank circuit ll connected between its control grid and cathode. The plate of the oscillator tube may be connected to any desired source of positive potential +B through resistor 5. The tank circuit ll includes a variable tuning condenser S which has its low alternating potential terminal at ground potential, it being noted that the cathode of the oscillator tube 3 is also at ground potential.

The usual direct current blocking condenser 'l is connected between the control grid of tube 3 and the high alternating potential terminal of condenser 6, a grid leak resistor 8 being connectu ed between the control grid and cathode of the oscillator tube. The numeral S denotes a padder condenser which is arranged in series with the tank circuit coil l and the variable condenser 6. The condenser 9 may have means for factory adjustment, and those skilled in the art are fully aware of the fact that the condenser S is employed to properly track the tuning of tank circuit 4 with the tuning of the various signal circuits. The dotted line Il denotes the usual mechanical uni-control tuning adjustment device which is employed simultaneously to vary the rotors of the various variable tuning condensers. It will be understood that the condenser rotors of the tunable radio frequency amplifiers, the first detector and local oscillator will be arranged on a common adjustment shaft. The local oscillations from the local oscillator stage may be impressed upon the rst detector l through a path generally designated by the numeral I2. The manner of impressing these local oscillations on the first detector circuit is well known and need not be shown in detail, except to point out that the local oscillations may be taken oif from the grid side of condenser 1, if

desired. Y

The plate of oscillator tube 3 is reactively coupled to the tunable tank circuit 4 by connecting the plate to the coil I3 through a coupling condenser I. The coil I3 is magnetically coupled to the tank circuit coil il), and the local oscillations are produced by virtue of this reactive coupling between the plate circuit and tank circuit. The tank circuit 4 is adjusted in tuning through a frequency range which differs from the irequency range of the tunable signal circuits by the frequency of the I. F. energy. The output circuit of the first detector I will be resonated to the operating I. F., and the latter may be chosen from a range of 75 to 465 kc. The function of the padder condenser 9 is to maintain the frequency diiference between the tank circuit l5 and the signal circuits substantially constant in frequency value.

The I. F. energy output of the mixer, orrst detector stage may be amplified in one, or more, stages of I. F. amplication l5. The numeral I6 designates the I. F. transformer which couples the amplifier l5 to the second detector. It Will be understood that the I. F. amplifiers are provided with resonant input and output circuits which are xedly tuned to the operating I. F. Subsequent to amplification, the I. F. energy is impressed upon an audio demodulator for detection. The audio demodulation device or second detector, may b-e of any desired type, and may Vceived carrier amplitude increases.

for example be of the diode type. The audio output of the second detector will then be arnpliled in one, or more, stages of audio amplification, and finally will be reproduced by any desired type of reproducer, such as a loudspeaker.

The automatic frequency control circuit for the local oscillator derives its signal energy from any desired point in the I. F. transmission network. For example, I. F. energy may be tapped off from the high alternating potential side of the primary circuit of I. F. transformer it, and the I. F. energy may be impressed upon the signal input grid of an amplifier H. Thus, the signal input grid of amplifier il is connected to the junction of resistor i8 and condenser i9, the condenser I 9 and resistor lli being connected in series to ground from the high alternatingvpotential side of the resonant primary circuit of trans former i6. The tube ll is shown as of the pentode type, and functions as an I. F. ampliiier tube to amplify the I. F. energy prior to impression upon the discriminator network. The cathode circuit of the ampliiier il includes the usual signal grid biasing circuit Ztl, and its plate circuit includes a resonant circuit 2l which is tuned to the operating I. F. The coil 22 of the output circuit 2i is magnetically coupled to the coil 23 of the resonant input circuit 2d of the discriminator network.

The discriminator comprises a pair of diodes 25 and 2li. The anode of diode 25 is connected to the high alternating potential side of input circuit 24, whereas the anode of diode 26 is connected to the low alternating potential side of the circuit. The cathodes of both diodes are connected through a resistor, and the latter has connected in shunt across it a condenser 28; it being noted that the junction of the cathode of diode Z5 and condenser 28 is at ground potential. The circuit 24 is xedly tuned to the operating I. F., and the midpoint of the coil 23, which point is designated by the numeral 29, is connected t0 the high alternating potential side of circuit 2l through the direct current blocking condenser 3l). The midpoint 2S on coil 23 is connected by lead 3i to the midpoint 32 on the discriminator resistor.

The condenser 28 has a low impedance at the operating I. F., and., in general, it is desirable that it be oflow value ier useful modulation fre quencies. It is desired that the I. F. energy level at the input of amplifier Il be maintained substantially uniform. This may be accomplished by using any well known type of AVC arrangement to properly decrease the gain of I. F. amplifier Il, as well as preceding stages, as the re- For example. the direct current voltage component of the detected I. F. current in the second detector may be used for such AVC purpose. It is not believed necessary to show the AVC arrangement, since Vthose skilled in the art are fully aware of itsI construction and function. It is thought suflicient to point out that it is desirable to maintain the carrier amplitude at the discriminator input circuit substantially uniform in intensity.

The'differential direct current potential for the automatic frequency control (AFC) function is taken from the terminal of the discriminator resistor connected to the cathode of diode 25. The AFC lead lil is connected to the ungrounded terminal of resistor portion 2l through a lter resistor M, and its opposite connection is to the grid i2 of the frequency control tube 3. The AFC connection to the grid l2 of control tube 43 is made through a second filter resistor 44, and the condensers 45 and 45 cooperate with the nlter resistors 44 and 4l respectively to suppress all pulsating components in the AFC bias. The tube 43 may be of the pentode type, and its various potentials are supplied from a direct cur-- rent voltage potentiometer as shown. It will be understood that the voltage supply potentiometer may be a part of the common direct current voltage supply system of the receiver, but it is believed only necessary to show that portion of the voltage supply source used in connection with the control tube 43.

The voltage supply potentiometer comprises a resistor having four sections 49, 48, 43 and 41. The -l-B terminal, which may be at 250 volts, for example, is connected to the junction of sections 48 and 49, the plate of tube 43 being connected to the section 49. The screen grid electrode of tube 43 may be connected to the junction of potentiometer sections 46 and 48 thus establishing the screen grid at a potential of about +115 volts, and the cathode of the tube may be connected to the junction of sections 46 and 41v thereby establishing the cathode at a potential of approximately 8.2 volts. The low potential side of section 41 leads to ground (or the -B side of the voltage supply system). The plate of tube 43 is connected to the high alternating potential side of coil i3 through a condenser 5i); the low alternating potential side of coil I3 bein-g connected to ground through a path which includes the condenser 5! and'= resistor 52 arranged in series. The junction of coil I3 and condenser 5I is connected to the junction of coil l0 and padder condenser 9. The grid 42 of tube 43 is connected to the junction of resistor 52 andcon# denser 5l through the condenser 60, and it Will be noted that the AFC connection to the grid 42 is made to the grid side of condenser.

Considering now the .operation of the AFC system shown in the drawing, it is first pointed Out that the audio voltage component developed in the discriminator network is not employed since an independent second detector is utilized. If desired, the direct current voltage existing between point 32 and ground may be used for the AVC action since this voltage has the proper polarity for lsuch control. However, rthe AFC voltage is derived 'from across the entire resistor `2l-2'l. The theoretical basis for the production of the AFC voltage across the resistor resides ii the following considerations. The potentials at either end yof coil 23 with respect to the center tap 29 are A180 out of phase. Hence, if the center tap 29 is connected to the primary circuit 2 I, one potential is realized` which maximizes above the resonant frequency of circuits 2l and 24, and a second potential is realized which maximizes below this common resonant frequency. If these two potentials are now applied to a pair of rectiers, such as the diodes 25 and 26, and the resulting direct current voltages are added in opposition, then the sum will be equal to zero at resonance. The resistor sections 27 and 27 are the output loadsl ofthe diodes 25 and 26, and these loads are arranged in series relation.

'In the type of discriminator network shown in the drawing the primary and secondary circuits 2l and 24 are so connected that two vector sum potentials of the primary and secondary voltage may be realized. If the frequency of the I. F. Waves applied to circuit 2| departs from resonance, that is, the operating I. F., the sum of the rectified outputs of the two diode circuits combined in opposition will be some real value whose polarity will depend upon .the sign of the frequency departure. Since this has been clearly explained in the aforementioned Seeley application, it is not believed necessary to go into detail concerning the theoretical aspects underlying the productionof the AFC voltage. It is believed suficient to explain that at `the operating I. F. of the primary and secondary circuits 2| and 24 the differential voltage produced across 21h-21 is zero, whereas for frequencies differing from the I. F. the differential voltage increases in `magnitude and `its polarity is dependent upon the sign of the frequency departure. The magnitude and the polarity of the AFC voltage determines the gain, or mutual conductance, of the control tube 43. The tube 43 is so connected with respect to the tank circuit 4 of the oscillator 3 that aY reactance of predetermined sign is reflected, or simulated, across the tank circuit 4 by tube 43. If the AFC voltage applied to grid 42 is positive, thereby overcoming some of the initial lnegative bias applied to the tube, its mutual conductance is increased. y

'I'he'tube 43 is connected to the tank circuit 4 so as to produce an effective inductive reactance across the tank circuit. The'resistor 52 and condenser 5| are effectively in series across the oscillator padder 9. The tank circuit current divides between padding condenser 9, and the circuit comprising 5| and 52. Resistor 52 is small compared with the reactance of the tank circuit comprising inductance l and padder 9, Yso that the current passing through the resistor 52 produces a voltage thereacross which lags the voltage across the tank circuit by substantially 90. 'Ihis voltage is applied to the grid 42 of control tube 43. It will then be seen that the plate current flowing through the plate circuit of tube 43 is substantially 90 ahead of the voltage across the tank circuit. The current through the tuning condenser of the oscillator tank circuit lags the voltage across that circuit about 90. Thus, any plate current flowing in the plate circuit of tube 43 acts as though the current flowing through the inductance I0 had been changed in amount.

In other Words the tube 43 produces an eective inductive eiiect across the oscillator tank circuit; the magnitude of this inductance is a function of the mutual conductance of tube 43. If the AFC' voltage applied to grid 42 becomes positive the mutual conductance of tube 43 is increased, and the amount of current flowing in the plate circuit of the tube is increased. This acts as though the current flowing through the tuning condenser 9 has been decreased. This in turn acts vas if inductance l0 had been increased in value thereby vcausing the tuned frequency to decrease.

The combination of condenser l and resistor v52 connected in shunt to padding condenser 9 is electrically equivalent to a fictitious resistor in series with condenser 9, i. e., in the tank circuit. I'he magnitude of this fictitious resistor varies with frequency, in general decreasing with fre quency. The manner in which this ctitious tank circuit resistor changes with frequency can be varied by adjustment of thevalue of condenser 5I and resistor 52, since the proportion of tank circuit current which flows through the circuit 5I, 52 vdepends upon the impedance of that circuit relative to the reactance of condenser 9.` If the plate of .tube 43 is connected, through condenser 50, to the high potential side of padder 9 so that the virtual reactance of tube 43, due to quadrature voltage impressed on its grid circuit, is across padder 9 only, insufficient frequency shift is secured at the high frequency end of the tuning range. Connection of the plate of control tube 43, through condenser 50, to the feedback Winding I3, increases the shift at the high frequency end of the tuning range. By means of the connection shown in the accompanying drawing, the effect of the control tube on the oscillatory frequency is chiefly through mutual inductance between coils I0 and I3 at higher frequencies, and by variation of effective reactance across padder 9 at-low frequencies. The direction of winding of I0 and I3 which` is correct for producing oscillations is, also, the proper polarity for the frequency shift due to mutual inductance and padder to enhance each other.

As disclosed above the values of condenser 5I and resistor 52 may be adjusted to change the manner in which quadrature voltage across resistor 52 varies with frequency; therefore, these values can be so chosen that with a given value of mutual inductance between coils l0 and I3, and a given Value of padder 9 as may be required by the oscillator circuit for proper oscillation and tracking, the oscillator frequency shift brought about by variation of the directY current potential of the grid of control tube i3 may be made substantially constant with frequency. Furthermore, by taking quadrature grid voltage by connecting to circuit 5I and 52 in shunt to padder 9, instead of from a resistor directly in the tank circuit, there is less effect on the strength of oscillation. Again, the electrostatic capacity of the plate of tube 43 has less effect on the tuning range in the connection shown, than if it were connected directly across the whole tank circuit.

Assume, now, that a signal impressed on primary circuit 2I is approaching an I. F. value of 465 kc., but is greater than the latter, and that the ungrounded side of the discriminator resistor will have a positive potential with respect to ground. The polarity of the AFC Voltage with respect to ground depends on the phase of the coupling between coils 22 and 23. By way of example, it is pointed out that the coupling may be phased so that the .AFC voltage becomes positive with respect to ground when the applied signal is higher than the desired I. F. The frequency departure causes the grid 42 `to become positive, and increases thc gain of tube 43. This causes a change in the simulated inductance across tank circuit 4 such that the frequency of the tank circuit will decrease. Thus, the .frequency .difference between the signal and oscillator circuits will automatically be made to decrease and approach towards the desired I. F. Value. The reverse is true of the case Where the signal energy applied to circuit 2l is departing from I. F., and becoming lesser in magnitude.

By virtue of the circuit arrangement employed between the control tube 123 and the tank circuit 4, such frequency adjustment of the oscillator tank circuit i will be substantially uniform no matter how the tuning control means II is adjusted. This circuit attempts to solve the problem of a simple AFC circuit which will produce adequate shift; shift which is uniform over the broadcast band; and, which does not add appreciably to minimum circuit capacities. This control circuit can be applied to existing receivers with minimum change in oscillator circuit constants and characteristics. It may be pointed out that in the absence of an arrangement such as disclosed herein, there may be secured as much as double the frequency shift at the high frequency end of the tuning rangethan at the low frequency end of the range. Such non-uniformity of frequency adjustment is overcome by employing the series condenser and resistor path in circuit with coil I3, and connecting the grid 42 of tube 143 to the junction of condenser 5I and resistor 52. It will be noted that the padder condenser 9, additionally, functions to provide some capacity coupling between the plate and grid circuits of oscillator tube 3 thereby aiding oscillations.

The following specific illustrations are given of circuit constants which may be employed in the receiving system, but it is to be clearly understood that these values are not given in any restrictive fashion, but are merely provided to enable thcse skilled in the art readily to practice While we have indicated and described a systern for 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 depart- I ing from the scope of our invention, as set forth in the appended claims.

What we claim is:

l. In a superheterodyne receiver of the type including a local oscillator having a tunable tank l circuit, the tank circuit including reactive elements of opposite sign, an auxiliary condenser in series with the reactive elements of the tank circuit, a series combination of a resistor and condenser connected in shunt to the auxiliary con- A denser, a frequency control tube provided with an input control electrode and an output electrode, a radio frequency connection between the input electrode and the junction of said series resistor and condenser combination, and a radio frequency connection from the output electrode of the control tube to the tank circuit.

2. In a superheterodyne receiver of the type including a local oscillator having a tunable tank circuit, the tank circuit including reactive elements of opposite sign, an auxiliary condenser in series with the reactive elements of the tankrcircuit, a series combination of a resistor and condenser connected inqshunt to the auxiliary condenser, a frequency control tube pro-vided with an input control electrode and an output electrode, a radio frequency connection between the input electrode and the junction of said series resistor and condenser combination, a radio frequency connection from the output electrode of the control tube to the tank circuit, means adapted to utilize the oscillator energy to produce signal energy of an assigned I. F. value, and means, responsive to a frequency -departure of the I. F. energy from its assigned frequency value, for controlling the gain of said control tube.

3. In a superheterodyne receiver of the type including a local oscillator having a tunable tank circuit, the tank circuit including reactive elements of opposite sign, an auxiliary condenser in series with the reactive elements of the tank circuit, a series combination of a resistor and condenser connected in shunt to the auxiliary condenser, a frequency control tube provided with an input control electrode and an output electrode, a radio frequency connection between the input electrode and the junction of said series resistor and condenser combination, a radio frequency connection from the output electrode of the control tube to the tank circuit, the radio frequency voltage on the control tube input electrode being substantially in quadrature with the tank circuit voltage, and the output electrode current being in quadrature with the tank voltage.

4. In combination with an oscillator tube provided with a tank circuit comprising a coil and a variable condenser, a fixed condenser in series with said coil and first condenser, a resistor and third condenser in a series connection with the coil and variable condenser and in parallel across said fixed condenser, an electron discharge tube provided with at least an input electrode and an output electrode, a radio frequency connection between the input electrode and the junction of the resistor and third condenser, and a radio frequency connection between the said output electrode and said tank circuit whereby the output current of said second tube is in quadrature with the tank circuit Voltage.

5. In combination with an oscillator tube provided with a tank circuit comprising a coil and a variable condenser, a fixed condenser in series with said coil and first condenser,- a resistor and third condenser in a series connection with the v coil and variable condenser and in parallel across said fixed condenser, an electron discharge tube provided with at least an input electrode and an output electrode, a radio frequency connection between the input electrode and the junction of the resistor and third condenser, a radio frequency connection between th said output electrode and said tank circuit whereby the output current of said second tube is in quadrature with the tank circuit voltage, and means for controlling the gain of the said second tube thereby to regulate the frequency of said tank circuit.

6. In a superheterodyne receiver circuit adapted f for automatic frequency control and including an oscillator tube having input and output electrodes and provided with a tank circuit, the latter comprising a coil, variable tuning condenser and padding condenser arranged in series relation, a feedback path including a winding coupled to said tank coil between the oscillator input and output electrodes, a condenser and resistance connected in series across the oscillator padding condenser, whereby part of the oscillatory current in the tank circuit flows through said padding condenser and part through said series combination of condenser and resistor, a control tube having a control electrode and a plate circuit, a connection to the control electrode of the control tube whereby the voltage across said resistor, which is substantially in quadrature with voltage across the tank circuit, is applied to the control electrode of the control tube, a Variable direct current potential source connected to the control electrode of the control tube, a connection from the plate circuit of said control tube to the feedback winding of the oscillator circuit and a. connection from the opposite end of the feedback winding to the oscillator padding condenser whereby currents caused to flow in the plate circuit of said control tube, by Virtue of quadrature voltage impressed on the control electrode thereof, are impressed on the oscillatory circuit partially through mutual inductance between the feedback winding and oscillatory tank circuit coil, and partially across the said padding condenser, whereby a given variation of direct current potential on the control electrode of said control tube produces substantially uniform shift of frequency of oscillation as the oscillation frequency is varied over the tuning range by manual adjustment of the tuning condenser.

DUDLEY E. FOSTER.

GARRARD MOUNTJOY.

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