US2248753A - Frequency control system - Google Patents

Description

July 8, 1941. A. L. GREEN FREQUENCY CONTROL SYSTEM Filed April 6, 1939 3 Sheets-Sheet l INVENTOR. gFRED L. GREEN M A E .5 Q n" n :1 M" V \v %+A .v Q Q a a m I u mm mm: u .000 G \QN A u w 5 m r n" T N T w m WWW: P w WWW Q m in -m, m Q u n w m N NMA m N Q N ATTORNEY. V

July 8, 1941., A. L. GREEN 2,248,753

FREQUENCY CONTROL SYSTEM 460 INTERMEDIATE FREOUENC Y l .Zfldl/IO 80.1 WWW/8.7.570 ENTOR' 4L FRED L. GREEN ATTORNEY.

3 Sheets-Sheet 3 A. L. GREEN GREEN R m m m ALFRED L. BY

ATTORNEY.

FREQUENCY CONTROL SYSTEM Filed April 6, 1939 A l A A July 8, 1-941.

Patented July 8, 1941 ATENT EFICE FREQUENCY CONTROL SYSTEM Application April 6, 1939, Serial No. 266,400 In Australia April 8, 1938 11 Claims.

This invention relates to frequency control systems and more particularly to automatic frequency control (AFC) systems suitable for automatically securing accuracy of tuning of radio and like receivers.

AFC action in a superheterodyne receiver can be obtained by employing a discriminator net- Work for producing a direct current voltage from intermediate frequency (I. F.) signal energy, this voltage depending for magnitude and polarity upon the sign and amount of frequency departure of the I. F. energy from thepredetermined I. F. of the receiver, and utilizing this direct current voltage to control the gain of a control tube which is connected to the oscillator tank in such a Way as to produce an effective correcting reactive effect across the latter. Variation of the gain of the control tube then results in a change in frequency of the oscillator tank circuit.

As the reactive effect of the control tube is dependent upon the amplitude and polarity of the control bias applied to its grid from the discriminator network, it is very desirable that the amplitude of the control potentials supplied to the control valve from the discriminator be substantially uniform for equal variations of the I. F. carrier frequency from the predetermined assigned frequency at all levels of carrier intensity.

In order that this effect may be achieved, automatic volume control (AVC) means are provided for maintaining the carrier intensity substantially constant at the input of the discriminator network. For economical reasons, however, it is generally desirable to incorporate the AFC feature into a receiver with the minimum number of additional components, the arrangement generally being such that the efiiciency of the AVC action is relatively poor. This results in a different controlling reactive effect for strong and weak signals.

With most of the popular circuit arrangements for obtaining AVC potentials in a receiver adapted for AFC, it is generally true that the control potentials from the discriminator are proportional to the AVC potentials. That is to say, if the AVC potential is high the frequency control potential is high and conversely when the AVC potential is low the control potential is low and the consequent reactive effect of the frequency control valve on the oscillator tuned circuit is small. At medium frequencies there is the possibility that a strong signal from a local station may provide more AFC than the adjacent weak signal which it is desired to receive, the eflfect being that the oscillator is pulled towards the strong signal and that the Weaker one is not receivable.

In short-Wave reception the fading carrier problem is of great importance. Two main difficulties arise, one that the AFC device may fail to regain control if the oscillator has drifted outside the control range in the interval between onedeep fade and another. In another type of undesirable effect the natural fading of the carrier is actually accentuated by the AFC action. It is easy to see how this effect may occur, supposing f-or the moment that the uncontrolled value of the signal frequency lies towards one of the outer limits of the pass-band of the receiver. When the received carrier is strong the AFC is effective in centering the I. F., but as the signal fades the AFC action becomes weaker and signal tends to drift outside the pass-band of the I. F. amplifier, thus being further attenuated, due to the selectivity of the receiver.

These disadvantages have up till now restricted the application of the AFC feature in broadcast receivers to local stations having carriers of relatively constant intensity. The principal object of this invention is, therefore, to provide an improved AFC arrangement in which the strength of the AFC action may be made very nearly independent of the intensity of the aerial signal so that in effect the AFC is improved for weak signals.

A further object of the invention is to provide an AFC system in which means are provided for correcting for asymmetrical AFC action, when one of the discriminator diodes is used simultaneously for producing the AVC biases applied to the carrier frequency stages.

The invention therefore provides improvements in automatic frequency control (AFC) systems comprising means for receiving signal at radio-frequency (R. F.), means including an ascillator for converting said R. F. signal to energy at intermediate-frequency (I. F.), means for utilizing said I. F. energy, means including a, discriminator for producing an AFC bias potential of a magnitude and polarity depending respectively on the amount and sense of departure of the I. F. from the predetermined mid-frequency of the discriminator, a control valve having two or more control grids, means providing for the use of said control valve for automatically controlling the frequency of said oscillator, said last mentioned means including mean-s providing for the application of said AFC bias potential to a control grid of said control valve, charac-- terized in this, that means are provided for producing an automatic volume control (AVC) bias voltage of a value depending on the intensity of said R. F. signal and that means are provided for the application of said AVC bias voltage to a second control grid in said control valve.

In order that the invention may be more readily understood reference will now be made to the accompanying drawings whereinr Fig. 1 is a typical circuit incorporating AFC and showing one way of carrying out the invention,

Fig. 2 shows curves indicating in a qualitative manner the effect of AVG action in producing an asymmetrical AFC action.

Fig. 3 illustrates a modification of the invention, and

Fig. 4 shows curves relating to the performance of the modification shown in Fig. 3.

Referring nowto Fig. 1, there is shown in schematic manner a superhetcrodyn-e receiver which employs an AFC arrangement embodying the present invention. In general the. receiver may be of the conventional type and embodies the usual signal collector I followed by one or more stages of tunable radio frequency amplification 2. The amplified radio frequency signals are impressed upon the input circuit 3 of the composite first detector-local oscillator valve 1, which may be for example a so-called pentagrid type valve. Although a composite first detectorlocal oscillator valve is shown in the drawings, it is to be understood that separate first detector and local oscillator valves may be used if desired.

The oscillator section of the composite detector oscillator valve 1 comprises a network connected to the first and second grids 12-! l. The network 15 is provided with a tunable tank circuit m and a feedback winding I8. 40

The tank circuit i4 is coupled to the grid l2 through a grid condenser 2i! and associated grid leak 82, and the grid-like anode II is connected through the feedback coil 18 to a termina1 55 of a positive potential supply source-mot shown.

The variable condenser 15 functions to tune the oscillator tank coil through a range of frequencies differing at all times from the signal circuit frequencies by the operating I. F. r

The capacity It of the oscillator tank circuit [4 has its rotors mechanically uni-controlled with the rotors of the variable tuning condensers of the tunable signal circuits feeding the first detector. The mechanical linkage is indicated by the dotted lines 54. 55

The I. F. energy output of the composite detector-oscillator valve 1 is amplified in one or more stages of I. F. amplification. The resonant circuits of the I. F. amplifier 22 will of course be tuned to the operating I. F. and the numeral 23 60 designates the resonant output circuit of the I. F. amplifier and it is fixedly tuned by means of condenser 5'! and coil 58 to the operating 1. F. The coil 58 of the circuit 23 is magnetically coupled to a coil 59 forming part of the resonant 5 input circuit of the discriminator network.

The descriminator comprises a pair of diodes 28-29. The anodes of diodes 28 and 29 are connected to opposite ends of the coil 59. The cashodes of the two diodes are connected in series by a pair of resistors 3233, the junction point D of the resistors being connected to the mid-point of winding 59 through an I. F. choke coil 30.

The circuit is fixedly tuned to the predetermined I. F. and the mid-point of the coil 59 of the circuit 25 is connected to the high alternating potential side of circuit 23 through a. blocking condenser 24.

The condenser 3| is connected across resistors 3233, whereas condenser 34 is connected across resistor 33. The cathode side of resistor 33 is at ground potential. The cathode side of resistor 32 is connected by lead 36, the AFC lead, to a gain control electrode 39 of the frequency control tube 40.

The AFC line includes a filter network 3B-3l for suppressing any pulsating components in the AFC voltage. An AVC lead may be connected to the point D for impressing AVC bias upon the I. F. amplifiers, and, if desired, upon any of the signal transmission stages preceding the amplifier 22. Furthermore, the audio voltage component of the rectified I. F. energy may be taken oil from point D and transmitted through coupling condenser 56 to one or more stages of audio frequency amplification followed by a reproducer.

The theoretical basis for the production of the AFC voltage resides in the following considerations. The potentials at either end of the secondary coil 59 with respect to the center tap are 180 degrees out of phase. Therefore, if the center tap of the coil 59 is connected to the primary circuit 23 two potentials will be realized. One of these potentials maximizes above the center frequency (that is the resonant frequency of both circuits 23-435) the other potential maximizing below this center frequency. Now, if thesepotentials are applied to a pair of rectifiers such as the diodes in Fig. l and the resulting direct current voltages are added in opposition, the sum will be zero. If the frequency of the I. F. energy departs from the predetermined operating I. F., the sum of the rectified outputs of the two diode rectifiers combined in opposition will be some real value whose polarity will depend upon the sign of the frequency departure.

For a more complete description of the construction and method of operation of discriminator networks of this type attention is directed to U. S. Patent No. 2,121,103 of S. W. Seeley, granted June 21, 1938.

The AFC voltage developed across the diode load resistors of the discriminator network is ap plied, as already stated, through the AFC line 35 to a gain control electrode of the control tube 40 which, in accordance with this invention, is a multi-grid valve having a cathode M, an anode A2, a first control grid 45, a screen grid 43, a second control grid 39, a further screen grid 43 and a suppressor grid 4. The two screen grids 43 are joined together within the valve.

A potentiometer $1, 53, 52 is shunted across the B+ and B- terminals of the main high tension supply, and the screen grid 43 is connected to the junction point of resistors 52 and 53; The cathode Al is connected to the junction point of resistors 41 and 53, and cathode by-pass condenser 45 is connected across resistance 41. The anode 42 of control valve 40 is connected to the oscillator tank circuit lead 60, the point of connection having a high radio-frequency potential with respect to earth. Feed resistor and feed condenser 51 are connected together in series and shunted across the tank circuit l4, one terminal of condenser 5| being earthed and one terminal of resistance 50 being connected to lead GD. Control grid 39 of control valve 40 is connected, through blocking condenser 49, to the junction point of the feed components comprising resistance 53 and condenser Control grid 39 is also connected through lead 36 and through filter resistance 3'! to the discriminator network comprising diode load resistances 32 and 33.

It is therefore apparent that a signal at oscillator frequency is developed across feed condenser 5i and applied between control grid 39 and cathode 4| of control valve 33. Afteramplification in control valve 43 this oscillator signal is passed back through lead 63 to the oscillator tank circuit Hi and, as is well known to those skilled in the art, the control valve 40 thereby acts as a virtual inductance shunted across the oscillator tank circuit [4. The magnitude of this virtual inductance is known to be proportional to the product of the capacity of feed condenser 5! and to the resistance of feed component 50 and to be inversely proportional to the trans-conductance of control valve 43, said transconductance being measured with respect to anode 42 and control grid 39. Since, however, the transconductance of control valve 43 is dependent on the mean d. 0. (direct current) bias of control grid 39 to which AFC lead 35 is connected, it follows that variations in discriminator bias developed in the diode loads 32 and 33 are translated into variations of transconductance of control valve 43 and thereby into variations of the virtual inductance shunted across the oscillator tank circuit 14. Using the arrangement of parts illustrated in Fig. 1 it is known that the sense of the AFC of the oscillator frequency is such that the oscillator frequency decreases when the transconductance of control valve 43 decreases, that is to say when the AFC bias potential of control grid 39 becomes more negative. Similarly a net positive discriminator bias developed across diode loads 32 and 33 makes AFC lead 36 positive to earth po tential, whereby control grid 33 tends to become more positive with respect to cathode 4| and the corresponding increase in transconductance of control valve All has the effect of increasing the frequency of the oscillator tank circuit I4.

Assuming now that a signal at radio frequency is being received in the aerial l and that the unicontrol mechanism 53 is adjusted approximately to tune the receiver to this signal, it may happen that the frequency of the I. F. energy in amplifier 22' is lower than the predetermined frequency to which the discriminator input oscillatory circuit 2325 is fixedly tuned. It follows that the sum of the rectified output voltages in discriminator diode loads 32 and 33 is not zero and, using the arrangement of parts illustrated by way of example in Figure 1, the sense of the rectified voltage unbalance is such that the cathode of discriminator diode 28 becomes positive in potential with respect to the cathode of discriminator diode 23, whereby the AFC lead 36 becomes positive with respect to earth potential. A positive AFC bias voltage is thereby applied to grid 39 in control valve 40, the transconductance of control valve 40 increases and thereby the reactive effect of control valve 43 on the oscillator tank circuit I4 is changed. This change in reactive effect is equivalent to a decrease in the virtual inductance shunted across the oscillator tank circuit Hi and is therefore accompanied by an increase in the frequency of the oscillator network l5. As is well known to those skilled in the art, the above-mentioned increase in oscillator frequency due to discriminator action is a desired AFC effect whereby the approximate setting of the uni-control mechanism 54 is electronically adjusted for fine tuning of the receiver to the incoming aerial signal.

Referring again to the discriminator, the process of connecting AVC lead 35 to the junction point D of discriminator diode load resistances 32 and 33 and thereby using discriminator diode 29 simultaneously for the production of AVG bias voltage, as illustrated in Figure 1, introduces noticeable lack of symmetry into both AFC and AVC actions, there being differences depending on the method of approach of the signal frequency to the predetermined mid-frequency of amplifier 22 and of discriminator fixedly tuned circuits 23 and 25.

A qualitative idea of the liability to asymmetry in AVC and AFC actions, due to using one of the discriminator diodes to produce AVC bias, may be gained from Fig. 2. In this diagram curves I and 2 illustrate the magnitudes of the discriminator biases produced respectively in diode loads 33 and 32 of Fig. 1, when the AVG lead 35 is disconnected from junction point D. Curves l and 2 of Fig. 2 are similar but in opposite senses, due to the differential connection of the discriminator diode loads 33 and 32. When, however, the AVC lead 35 is connected to junction point D, the discriminator diode 23 furnishes AVC bias to the I. F. amplifier 22 according to curve 3 'in Fig. 2. Thus curves l and 2 no longer represent the magnitudes of the discriminator biases but, with AVC lead 35 connected to junction point D, curve 4 in Fig. 2 illustrates the magnitude of the discriminator bias voltage developed by diode 23 while curve 3 respresents the bias developed by diode 29, said last mentioned bias acting simultaneously as AVC and AFC bias. Curve 5 in Fig. 2, representing the algebraic combination of curves 3 and 4, therefore illustrates the magnitude and sign of the differential bias developed across diode loads 32 and 33 in combination, that is to say the net AFC bias available in AFC lead 33 to control valve 43.

It is apparent in Fig. 2 that the magnitude of the available AVC bias, illustrated by curve 3, is markedly greater when the receiver is being tuned such that I. F. energy has a frequency higher than the predetermined mid-frequency of the I. F. amplifier, for example higher than 465 kilocycles, than when the I. F. energy has a frequency lower than 465 kc. Thus marked asymmetry is present in AVC action, the controlling effect being greater when the mid-frequency is approached from 475 kc. than when it is approached from 455 kc. As to AFC action, it is apparent in Fig. 2that the higher value of AVG bias developed at 470 kc. acts to reduce the output from the I. F. amplifier 22 and hence to reduce the magnitude of the AFC bias illustrated by curve 5. 0n the other hand at 4:60 kc., where the AVG bias represented by curve 3 has a smaller value, the output from the I. F. amplifier 22 is relatively great and hence the AFC bia represented by curve 5 is simultaneously great. Thus marked asymmetry is introduced also into the AFC action, in the particular case illustrated in Figs. 1 and 2 the AFC being stronger if the initial setting of the uni-control mechanism 53 is such that the I. F. energy has a frequency lower than 465 kc. and the AFC action being weaker if the initial setting of the oscillator network 15 is such as to produce an I. F signal having a frequency higher than the final controlled predetermined value of 465 kc.

A method of securing symmetrical AFC action is illustrated in Fig. 1. In this case the usual high mu high impedance control valve of the prior art is replaced by a control valve having two control grids. A valve well known to the trade as the 6L7, or similar type, possesses the necessary characteristic for carrying out this invention.

Using a 6L? type valve as the control valve AFC bias is applied, as shown in Fig. 1, to the number 3 sharp cut-off control grid 39. The transconductance of this valve is also controlled, in accordance with this invention, by AVC bias applied to the number I control grid through a filter network comprising resistance El and condenser 48. Clearly, the functions of the grids may be reversed.

With this arrangement two important facilities become available. Possibly the more advantageous feature in Fig. 1 is that the strength of the AFC action may be made very nearly independent of the intensity of the aerial signal. This follows since the amount of AFC, for a given change in discriminator bias applied to grid 39, depends on the value of the AVC bias applied to grid @5. Thus, for weak incoming carriers for which the differential discriminator bias is small for a given amount of mistuning, the AVC is simultaneously reduced so that the control valve works at a high level of transconductance. Assuming therefore that the control valve is adjusted to deal with relatively weak incoming carriers, it follows that the AFC action then remains approximately the same independently of the strength of the signal.

In practice, however, the control action in the SL7 type control valve All is rather more complicated than is apparent from the simple explanation given above. It is well known to those skilled in the art that the control valve 19 places in effect a virtual inductance in shunt with the oscillator tank circuit Hi. The value of the shunt inductance is numerically equal to the product of the capacity and resistance of the feed components El-Eil, divided by the transconductance of the SL7 control valve 40. Variations in discriminator bias to grid 39, depending on sign, may either increase or decrease the frequency of the oscillator is from its mean value. On the other hand, the application of AVC bias to grid 45 produces a uni-directional change in oscillator frequency, since a reduction in control valve 48 conductance always lowers the frequency. Thus, in some cases the AVC and the AFC actions combine in the control valve 49 to reduce the oscillator frequency to an extent greater than that obtainable with AFC action alone, while in other cases the AVC and AFC biases are in opposition and the increase in frequency, due to the AFC action, is retarded by AVC through grid 65.

This asymmetry in oscillator control may, by proper design, be turned to useful account in correcting asymmetry in discrimination which has already been explained in connection with Fig. 2. In the type of circuit shown in Fig. l, in which AVC bias is derived from one of the discriminator diode loads, it will be remembered that the AVC is greater than the differential AFC biassmaller when the signal is mistuned to one side of the mid-frequency rather than to the other. The effect in the SL7 control valve 40 is then as follows: Using the sense of frequency control illustrated in Fig. 2, it is seen that the AVC and AFC biases are additive in effect on the oscillator frequency, both tending to lower the I. F., when both biases are negative. For the same conditions the AFC differential bias is reduced on account of the stronger AVC action on this side of the mid-frequency, so that the combined eifect in the control valve 49 of AVC and AFC biases acting additively is to enhance the AFC action in the desired manner. On the other hand, for I. F. lower than the mid-frequency, the AFC action is naturally stronger on account of the reduction in AVC action, but in this case the AFC bias is positive and the AVC bias negative so that the net effect in the 6L7 control valve 48 is to retard the AFC action again in the desired manner.

Thus, the modification, in accordance with this invention, as illustrated in Fig. 1 is capable of introducing two interesting refinements into AFC systems. On the one hand the application of AFC to weak signals in the medium frequency band and to short wave fading carriers is markedly improved, while in addition there is available, by proper design, the facility for correcting for asymmetrical AFC action when one of the discriminator diodes is used simultaneously for producing the AVC bias applied to the car rier-frequency stages.

That the circuit shown in Fig. 1 may have undesirable limitations is evident from the manner in which detector and AVC potentials are taken from a discriminator diode. The characteristics of the detection diode circuit are represented by curve in Fig. 2, and it is clear that since the circuit resonates at 470 kc. there is sideband cutting occurring at 465 kc. The intermediate frequency stages of the receiver are tuned to 465 kc, and it therefore follows that the fidelity of the receiver is decreased when the set is properly tuned to the incoming signal.

It is the object of a further development of the invention to reduce the distortion introduced into the detection diode circuit when the receiver is properly tuned, at the same time retaining the advantages of the arrangement shown in Fig. 1.

It is a feature of this development that the AVC bias applied to the frequency control valve is not derived from the diodes in the frequency discriminator circuit. It is a further feature that the intermediate frequency is different from the mid-frequency of the discriminator.

This is more clearly seen in Fig. 3 which shows in a schematic manner a superheterodyne receiver embodying an arrangement of the present invention. The radio-frequency amplifier 2, the oscillator-mixer stage 1, the I. F. amplifier 22 and the control valve 4?) are the same as those previously described in connection with Fig. l and the nuemerals represent corresponding components. Signal energy is fed from the output of the I. F. amplifier 22 to the final I. F. transformer 63-64. To the primary winding 53 are connected three blocking condensers 65, I5 and IE, whereby I. F. signals are fed respectively to AVC diode anode l2 and to discriminator diode anodes SI and 82. I. F. Signal is fed to detector diode anode H via the secondary winding 84 of the final I. F. transformer and the audio-frequency component of the detected signal appear-- ing in load resistance 74 is passed via blocking condenser 56 to the audio-frequency amplifier and thence to the reproducer (not shown).

AVC bias voltage is developed along load resistance 66 connected to AVC diode 12, said AVC bias voltage being applied through filter resistance 69 and across filter condenser 79 to control grid 6 of the oscillator-mixer valve 1. The said AVC bias is, of course, also applied in known fashion to amplifying valves in I. F. amplifier 22 by suitable connections (not shown) and, if desired, in a similar manner to radio-frequency amplifier 2. In accordance with this invention a portion of the AVG bias voltage developed along load resistance 66 is tapped off and applied through filter resistance 61, across filter condenser 68 and through grid leak M to control grid 45 of control valve 40. As illustrated in Fig. 3 AVG bias is applied to control grid 45 and AFC bias is applied to control grid 39 in control valve 40, but clearly the relative functions of these two control grids could be reversed Without departing from the spirit of the invention.

Turning now to the discriminator which may be of any suitable design, the method illustrated by Way of example in Fig. 3 is a modification of the well known side-tuned circuits. I. F. energy is fed through blocking condensers 75 and (6 from the primary winding 63 of the final intermediate-frequency transformer to the anodes 8i and 82 of the discriminator diodes. Sidetuned circuits 1! and 18 are connected to diode anodes 8| and 82 and respectively tuned to frequencies of 470 kc. and 450 kc. Rectified discrimination bias voltages are developed across discriminator diode load resistances l9 and 89 which have a common point of connection. One terminal of load resistance 19 is earthed and one terminal of load resistance 80 is connected, through a suitable filter network, to control grid 39 of control valve 40, so that the differential AFC bias produced in resistances l9 and 80 in combination is available to control the transconductance of control valve 0. Signal at oscillator frequency is also derived from feed components 50 and 51 and applied through blocking condenser 49 to control grid 39 of control valve 48 as previously described in relation to Fig. 1.

Assuming now that a signal at radio frequency is being received in the aerial l and that the unicontrol mechanism 54 is adjusted approximately to tune the receiver to this signal it may happen that the frequency of the I. F. energy in amplifier 22 is lower than the predetermined mid-frequency of the discriminator. Supposing that the frequency of the I. F. energy is 460 kc. it follows that side-tuned circuit 18 is resonant to the I. F. energy whereas side-tuned circuit H, which is fixedly tuned to 4'70 kc. is out of tune with the impressed I. F. energy. It then happens that the rectified bias developed in diode load resistance 2'9, which is associated with the resonant side-tuned circuit 18, is greater than the rectified bias developed in diode load resistance 8!! associated with the out-of-tune side-tuned circuit H. The net AFC bias developed along load resistances T9 and 8% is therefore not zero but such as to make control grid 39 of control valve 40 more positive with respect to cathode 45, whereby the transconductance of control valve 4!! is increased. This eifect in turn decreases the value of the virtual inductance represented by the reactive effect of control valve 49 on the oscillator tank circuit i4, whereby the frequency of the oscillator increases.

As is well known to those skilled in the art the above-mentioned increase in oscillator frequency due to discriminator action is a desired AFC effect whereby the approximate setting of the unicontrol mechanism 54 is electronically adjusted for fine tuning of the incoming aerial signal. In a similar fashion it is easy to see that in a case where the initial setting of the urn-control mechanism 54 is accompanied by the production of I. F. energy having a frequency higher than ances l9 and 8!) in combination is such as to im-' pose a negative bias on control grid 39 whereby the transconductance of control valve 48 decreases and there results a corresponding decrease in oscillator frequency. In this case also it is clear that the abovementioned change in oscillator frequency produced by discriminator action is a desired AFC effect which aids fine tuning.

Referring again to the discriminator illlustrated in Fig. 3, the method of operation of this arrangement of the present invention is further illustrated by the curves in Fig. 4, in which curves 5 and 2 illustrate qualitatively the magnitudes of the discriminator bias voltages developed respectively along load resistances and 79. Thus curve I exhibits a maximum when the I. F. energy has a frequency of 470 kc. whereas the maximum of curve 2 occurs at 460 kc. corresponding to resonance inside-tuned circuit 18. The net AFC bias developed along load resistances i9 and 8B in combination is illustrated in Fig. 4 by curve 4 in which the magnitude of the net bias voltage is obtained by taking the algebraic combination of curves l and 2.

Assuming for the moment that AVC action in control valve 56 is removed, for example by shortcircuiting control grid d5 directly to earth, it follows that AFC action in the receiver has the effect of adjusting the frequency of the I. F. energy in amplifier 22 very closely to the predetermined mid-frequency, i. e. 465 kc. of the discriminator. When this happens it is apparent from Fig. 4 that the biases developed in discriminator diode loads 19 and 80 both assume small values, as shown in curves l and 2, while the net AFC bias assumes a still smaller value as illustrated in curve 4.

According to this invention, however, AVC bias developed in diode load 56 is applied to control grid 45 of control valve 48, One effect of applying AVC and AFC bias voltages to the two control grids 45 and 39 is to improve AFC action for weak and fading carriers in accordance with the invention. Another effect is illustrated in Fig. 4. Since it is known that a decrease in transconductance of control valve 49 is accompanied by a decrease in oscillator frequency, it is clear that the application of negative AVC bias to control grid 45 must have a similar effect on oscillator frequency as a negative AFC bias applied to control grid 39. Thus the final controlled value of the frequency of the I. F. energy is no longer very closely that of the predetermined mid-frequency of the discriminator, i. e. 465 kc., but is at some lower frequency, as illustrated by the point of equilibrium X in Fig. 4. When, therefore, fine tuning has been completed by AFC action, the I. F. energy has a frequency of, for example, 463 kc. whereas the mid-frequency of the discriminator has the predetermined value of 465 kc. This state of affairs is clearly one of equilibrium in which a negative AVC bias applied to control grid 45 of control valve 4i tends to lower the transconductance of valve 48 and to decrease oscillator frequency whereas simultaneously a residual positive AFC bias applied to control grid 39 tends to increase both transconductance and oscillator frequency.

Since the final controlled value of the frequency of the I. F. energy tends towards that indicated by the point X in Fig. 4, i. e. to 463 kc., it is clear that, in the interests of fidelity, the tuned circuits in the I. F. amplifier 22 and in particular the output transformer 63-64 should be tuned to 463 kc. rather than to the predetermined mid-frequency of the discriminator, i. e. 465 kc. This is described qualitatively in Fig. 4 in which curve 3 indicates the rectified voltages developed by AVC diode l2 and applied as AVC bias to control grid 45 of control valve 40. At the point X of equilibrium, the biases applied to the control valve 49 are respectively the positive AFC bias represented by curve 4 and the negative AVC bias represented by curve 3. Curve 3, however, may also be used to illustrate the magnitudes of the rectified components of the signal at the detector diode H since both AVC and detector diodes are fed from the output of the I. F. amplifier 22 which is fixedly tuned to 463 kc., corresponding to the point X in Fig. 4. It is then apparent that undesirable detector distortion, due to sideband cutting in the tuned circuits which precede the detector diode, is eliminated since the final equilibrium frequency of I. F. energy coincides with the predetermined fixed frequency of the I. F. amplifier.

It is important to notice that variations in signal strength of the incoming aerial signal are not accompanied by appreciable variations in the equilibrium position of the point X in Fig. 4. This follows since, for example, an increase in signal strength produces corresponding increases in both AVC and AFC biases so that the net effect in the control valve 0 is substantially unchanged, an increase in transconductance due to an increase in positive residual AFC bias being effectively balanced by a decrease in transconductance due to the greater AVG bias. follows that the I. F. amplifier 22 may be fixedly tuned to a frequency corresponding to the point X and that the discriminator side-tuned circuits: may be fixedly tuned to frequencies respectively greater and less than a predetermined discriminator mid-frequency, independently of the strength of the incoming aerial signal. At this stage it is also convenient to notice that, since the mid-frequency corresponding to the point X is in dependence on the degree of AVG action in the control valve 49, it is feasible to adjust the final controlled frequency of the I. F. energy in correspondence with the predetermined mid-frequency of the I. F. amplifier by the prmess of adjusting the amount of AVG bias applied to control grid 45, that is to say by adjusting the tapping point on AVC diode load 56.

This modification of the invention thus introduces better fidelity into the output of the automatically controlled receiver, and makes more uniform the extent of the control exercised on the receiver by a fading carrier. Further, since the control exercised by strong carrier is not markedly different from that of an adjacent weak carrier, there is no great tendency for the strong carrier to capture the control at the expense of the weaker one, even during periods of fading.

It is apparent that the feature, illustrated in Figs. 3 and 4, which involves the provision of a mid-frequency for the I. F. amplifier differing from the mid-frequency of the discriminator circuits, is not limited to the type of receiver in which the AVG bias is derived from a independent diode 12. Similar advantages, in relation to fidelity and to symmetrical AFC action, accrue when the feature is included in a receiver of the type illustrated in Figs. 1 and 2.

What is claimed is:

1. Improvements in automatic frequency control systems for superheterodyne receivers comprising means for receiving signals at radio-frequency, means including an oscillator for converting said radio-frequency signal to energy at intermediate-frequency, means for utilizing said intermediate-frequency energy, means including a discriminator for producing an automatic frequency control bias potential of a magnitude and polarity depending respectively on the amount and sense of departure of the intermediate-frequency from the predetermined mid-frequency of the discriminator, a control valve and means providing for the use of said control valve for automatically controlling the frequency of said oscillator, said last mentioned means including means providing for the application of said automatic frequency control bias potential to a control grid of said control valve, the said improvements being characterized in this, that the said control valve has at least two control grids and that means are provided for producing an automatic volume control bias voltage of a value depending on the intensity of said radio-frequency signal and that means are provided for the application of said automatic volume control bias voltage to a second control grid of said control valve.

2. Improvements in automatic frequency control systems as claimed in claim 1, characterized in this, that the said predetermined mid-frequency of the said discriminator is different from the predetermined mid-frequency of the intermediate-frequency amplifier of said receiver.

3. Improvements in automatic frequency control systems as claimed in claim 1, characterized in this, that the polarity of said automatic volume control bias voltage applied to said second grid of said control valve is independent of the frequency of the said radio-frequency signal.

4. In a superheteroydne receiver of the type including a local oscillator tank circuit operatively associated with a reactance-simulation tube provided with at least two control electrodes and means responsive to receiver detuning for producing a frequency correction voltage; the improvement which comprises means for applying said correction voltage to one of said control electrodes, and means responsive to signal carrier amplitude variation for controlling the potential of the second control electrode.

5. In a superheterodyne receiver of the type including a local oscillator tank circuit operatively associated with a reactance-simulation tube provided with at least two control electrodes and means responsive to receiver detuning for producing a frequency correction voltage; the improvement which comprises means for applying said correction voltage to one of said control electrodes, and means responsive to signal carrier amplitude variation for controlling the potential of the second control electrode, said correction voltage means and carrier amplitude responsive means both being included in a common network.

6. In a superheterodyne receiver including a local oscillation tank circuit, an intermediate frequency energy utilization network, a frequency control tube operatively associated with the tank circuit, means producing a frequency correction voltage whose magnitude and polarity are respectively dependent on the amount and sense of departure of the intermediate frequency energy from an assigned frequency value, and means deriving a unidirectionol voltage from signal energy whose magnitude is a function of the signal carrier intensity; the improvement which comprises means applying said frequency correction voltage to said control tube in a sense to maintain said assigned frequency value, and additional means applying said unidiretional voltage to said control tube to control the transconductance thereof.

7. In a superheterodyne receiver including a local oscillation tank circuit, an intermediate frequency energy utilization network, a frequency control tube operatively associated with the tank circuit, means producing a frequency correction voltage whose magnitude and polarity are respectively dependent on the amount and sense of departure of the intermediate frequency energy from an assigned frequency Value, and means deriving a unidirectional voltage from signal energy whose magnitude is a function of the signal carrier intensity; the improvement which comprises means applying said frequency correction voltage to said control tube in a sense to maintain said assigned frequency value, additional means applying said unidirectional voltage to said control tube to control the transconductance thereof, and in a polarity sense such that the unidirectional voltage acts to control the tank circuit frequency in a supplemental manner with respect to the action of said correction Voltage.

8. In a superheterodyne receiver including a local oscillation tank circuit, an intermediate frequency energy utiliation network, a frequency control tube operatively associated with the tank circuit, means producing a-frequency correction voltage whose magnitude and polarity are respectively dependent on the amount and sense of departure of the intermediate frequency energy from an assigned frequency value, and means deriving a unidirectional voltage from signal energy whose magnitude is a function of the signal carrier intensity; the improvement which comprises means applying said frequency correction voltage to said control tube in a sense to maintain said assigned frequency value, additional means applying said unidirectional voltage to said control tube to control the transconductance thereof, and said frequency correction voltage producing means comprising a pair of rectifiers coupled to said intermediate frequency utilization network and resonated to said assigned frequency value.

9. In a superheterodyne receiver including a local oscillation tank circuit, an intermediate frequency energy utilization network, a frequency control tube operatively associated with the tank circuit, means producing a frequency correction voltage whose magnitude and polarity are respectively dependent on the amount and sense of departure of the intermediate frequency energy from an assigned frequency value, and means deriving a unidirectional voltage from signal energy Whose magnitude is a function of the signal carrier intensity; the improvement which comprises means applying said frequency correction voltage to said control tube in a sense to maintain said assigned frequency value, additional means applying said unidirectional voltage to said control tube to control the transconductance thereof, and said correction voltage producing means comprising a pair of rectifiers having separate input circuits coupled to said intermediate frequency network, and said input circuits being oppositely mistuned with respect to said assigned frequency value.

10. In a superheterodyne receiver including a local oscillation tank circuit, an intermediate frequency energy utilization network, a frequency control tube operatively associated with the tank circuit, means producing a frequency correction voltage whose magnitude and polarity are respectively dependent on the amount and sense of departure of the intermediate frequency energy from an assigned frequency value, and means deriving a unidirectional voltage from signal energy whose magnitude is a function of the signal carrier intensity; the improvement which comprises means applying said frequency correction voltage to said control tube in a sense to maintain said assigned frequency value, additional means applying said unidirectional voltage to said control tube to control the transconductance thereof, and said unidirectional voltage deriving means comprising a rectifier coupled to said intermediate network, and said correction voltage producing means comprising a rectifier circuit coupled to said intermediate network and being independent of said first rectifier.

11.111 a modulated signal carrier receiving system of the type including a signal selector circuit having operatively associated therewith a voltage responsive means adapted to correct ALFRED LEONARD GREEN.

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