US2440653A - Locked-in oscillator circuits - Google Patents

Description

April 27, 194s.

LOCKED- IN OSCILLATOR CIRCUITS M. S. CORRINGTON Filed Nov. 14, 1944 3 Sheets-Sheet 2 IN VEN TOR. IWI/ELAN 5. CORR/NGmN BY )IL-02 M- A TT'ORNEV April 27, 1948. M. s. coRRlNGToN LOCKED-IN OSCILLATOR CIRCUITS Filed Nov. 14, 1944 s sheets-sheet s INVEN TOR MUR/ AN .5'. CORR/mwN B Y //wvJ/V A TTQNE-y Patented Apr. 27, 1948 Murlan S. Corrington, Camden,

Radio Corporation of America,

Delaware N. J., assigner to a corporation of Application November 14, 1944, Serial No. 563,379 7 claims. (c1. 250420) My present invention relates generally to improved locked-in oscillator circuits, and more particularly to a novel methodof, andmeans for, extending the lock-in range of frequency dividers of the locked-in oscillator type.

In my application Serial No. 513,371, filed December 8, 1943, I have disclosed and claimed novel circuits for receiving angle-modulated carrier waves employing a looked-in oscillator system acting as a frequency divider. These circuits were improvements on the locked-in oscillator circuit disclosed and claimed by G. L. Beers in his U. S. Patent No. 2,355,201, granted August 22, 1.944.

The locked-in oscillator operation of the Beers patent was explained in the following way in my aforesaid yappli-cation: The angle modulated waves, of a predetermined mean or center frequency F, are applied to the input control grid of a tube of the pentagrid type. The output or plate electrode circuit is tuned to a desired subharmonic F/n, where n is a small whole number. In the illustrative embodiment described in Fig. 1 of the Beers patent the fifth subharmonic is developed across the plate circuit. The third grid of the pentagrid tube is regenenatively coupled to the plate circuit with the result that oscillations of mean frequency F/n are produced in the plate circuit. These oscillations occur even in the absence of signal voltage on the first grid. Suppose, now, that F=4300 kilocycles (kc.) per second, and a signa-l of that mean frequency is applied to the first grid. Assume, further, that the plate circuit is tuned to 860 kc. In other words, the oscillator section operates to provide subharmonic oscillations at 860 kc. The grid voltage-plate current characteristic of the pentagrid tube is nonlinear; this would be true of any other type of tube. Hence, harmonics of F/n are impressed on the oscillator grid. These harmonies which are present on the third grid from the cathode include the fourth (3440 kc.) and sixth (5160 kc.) harmonics 4of the oscillator current.

Hence, the signals of 4300 kc. Willbeat with these fourth and sixth harmonics to provide a difference frequency the same'as the desired fundamental frequency of 86C! kc. (the fifth subharrnonic of 4390 kc). Now, this new component (which I term the harmonic difference component to distinguish it from the normal oscillator current) will not, in general, have the same phase as the normal oscillator current. It. thus, is equivalent to injecting into the plate4 circuit 'an out-of-phase current. This causes the tube to act in themanner of the well-known reactance tube. j If .the oscillator frequency is not exactly one-fifth of the incoming signal frequency, the natural frequency of the plate circuit is-pulled over until the oscillator frequency is exactlyonefifth. This causes the oscillator section tolock in with the applied signals. j

The maximum amount the natural frequency of the oscillator'can be pulled over, and still be locked-in, occurs when the harmonic difference current is 90 degrees out of phase with respect to the normal oscillator current, since this gives the maximum out-of-phase current. When the injected (harmonic difference) current is leading the normal oscillator current, the oscillator section frequency will be pulled to one side of its natural frequency (860 kc.). When the injected ycurrent is lagging, the oscillator frequency will be pulled to the other side of its natural frequency. It will be recognized that there will thus be developed in the plate circuit a frequency modulated current locked-in with the applied signals. g

There is a limitation on the lock-in range. This larises from the fact that the amount of fourth and/or sixth harmonic on the oscillator third grid is limited. Accordingly, this limits the amount of injected current, and thus it Vfollows that the lock-in range of the oscillator section is limited. The result is that when the deviation of the oscillator frequency exceeds approximately 20 kc., from the oscillator mean frequency (860 ka), the oscillator suddenly breaks out and its frequency returns to the mean frequency value. When the oscillator is `used with a frequency discriminator in a frequency modulation receiver, distortion results by virtue of this phenomenon. There are practical operating 'causes for the break out of the locked-in oscillator.

If the receiver is mistuned slightly, as is readily done in the case of frequency modulation receivers, the "break out of the locked-in oscillator will occur on one end of the frequency swing, even for normal '15`kc..swings or deviations. if

' the frequency modulation (FM hereinafter for brevity)V transmitter over-modulates, the"break out may occur on both ends of the swing. Like'- wise, gradual drifts in frequency of the oscillator or discriminator can occur due to temperature or other changes during operation, thus causing mistuning and consequent break out.

In my aforesaid application there was provided 4a general method of extending the lock-in range of avlocked-in oscillator thereby to overcome the undesirable effects due to break out of the oscillator caused by insufficient harmonic on the oscillator grid. Specifically, there was provided a highly practical method of increasing the lockin range of a locked-in oscillator of the type shown in the |aforesaid Beers patent; the method comprising tuning the oscillator `plate circuit sufciently to one side of F/n so that it will readily lock in on. one end of the frequency swing, and tuning the oscillator grid circuit suiiiciently on the other side of the desired harmonic (4F/n or (iF/n) to help lock in the .oscillator on the opposite end of the swing.

It is an important object of my present invention to provide a novel method-distinct fromthose disclosed in my aforesaid application, of increasing the lock-in range of a locked-in oscillator by adding a passive electrical network; in parallel with the oscillator tuned output circuit,

and adjusting the added electrical net-work so that the equivalent input capacity falls oil at the proper rate to just keepv the oscillator in tune throughout m-ostof the operating range. A passive electrical network is one which, in contradistinction to an active network, is not a source of electrical energy, and is` free of amplier tubes.

Another desirable object of my present invention is to provideV a novel method of locking in a locked-in oscillator over a greater range for a given amount of injected current.

A.. further object of my present invention is to provide a passive electrical network across the tank circuit of a locked-in oscillator, the equivaient seriescapacity of the network being chosen to vary-with frequency at just theproper'rate-to keep theoscillator in tune for a portion ofthe lock-in range; the added electrical networkbeing the discriminator in an FM receiver. if desired.

Stillother objects ofmy invention are to improve generally the efficiency, reliability and range .of locked-inoscillators, and more especially to provide locked-in oscillators of extended range which are economical in construction and assembly.

Still other features of my. invention willbest be understood by reference to the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In .the drawings:

Fig. 1 shows the basic locked-in oscillator circuit-ofthe aforesaid Beers patent;

Figs. la, lb and 1c are employed to analyze the-.circuit action;

Figy 2 shows one embodiment of my invention;

Fig; Eishows amore generalized embodiment of my invention;

Fig. la shows the'resistance and-reactance components of the added passive network in Fig. 2;

Fig. 4b shows the equivalent input capacity of the latter;

Fig, ashows the. oscillator capacity required to keeptheoscillator in tune asthe,` frequency is changed;

Fig. 5b shows, howto 'match the added circuit to theoscillator;

Figs, 6ta-,61), 6c and 6d vshow several circuits that can` be used for passi-ve network 35 of Fig- 3;

Fig. 'lishows a modification, wherein-the added network V'of Fig., 2 acts as an FM receiver cliscrimnator; and

Fig. 8 isa circuit diagram of an FM, receiver embodying a further modification of the lockedingoscillator.

Referring now to the accompanying drawings, wherein like reference numerals in the different iigures designate similar circuit elements, reference is made to the circuit shown in Fig. 1 for an explanation ci the fundamental mode of operation of my present invention. In Fig. 1 I have shown a locked-in oscillator :which is of the general type shown in the aforesaid Beers patent. The circuit comprises a tube l which may be of the pentagrid type. My invention is not limited to such a tube. It will operate with any tube normally used as an oscillator. Between the input grid 2 and the cathode 3 there is impressed high` frequency energy of a predetermined frequency F. The plate d has connected in circuit therewith a resonant circuit 5 which is tuned to a subharmonc F/n. of F, the symbol n denoting a small integer. By way of illustration, the iifth subharmonic may be employed. The plate `li is, of course, established at a positive potential with respect tothe grounded cathode. The second and fourth grids of the turbe are connected in common to a source of positive potential, and thesegrids function as a positive screen grid for the intermediate grid 6.

The latterv grid 6 is regeneratively coupled, as at 'Lto the plate circuit 5. The fifth grid of tube l is :connected back to the cathode,- and this grid, therefore, functions as a suppressor grid. The cathode 3, grid 6 and plate @provide the oscillator section of the circuit. This oscillator section produces oscillations of the subharmonic fre'- quency F/n. These oscillations are continuously produced even in the absence of input energy at grid 2; The oscillations developed lacross circuit 5 may be transferred to any utilization network.

As shown, the coil 'l' has it lower end connected to ground through the resistor 9, and the latter is by-passed for high frequency currents by the condenser l0. This is substantially similar to the locked-in oscillator circuit shown in the aforesaid Beers patent. The current from the tube isnot sinusoidal, but comes through as a series of pulses.` These cause harmonics of the subharmonic frequency F/n to be developed in the plate circuit 5. These harmonics will be applied to the grid 5 because of the -coupling l. Furthermore, the grid 6 is operating withselfbias, and draws grid current during the positive swings of voltage. These pulses, also, contain harmonics of F/n.

Let us assume .by way of illustration that F has a value of 4300 kc. This frequency will beat with the fourth (3440 kc.) and/or sixth (5160 kc.) harmonics of the resonant frequency of the plate circuit thereby to provide theV desired subharmonic of 860 kc. In this case it is assumed that the'letter n is equal to 5, When the fourth and sixth harmonics are present simultaneously, it can be shown that the result is a single injected current of variable amplitude and phase. The process is similar to ,that when only one harmonic is present. Usually the fourth and sixth harmonies will be of unequal amplitude, andthe eifect of the weaker one is to 'produce relatively small variations in the other. When the frequency of the incoming signal is exactly five times the natural frequency of the oscillator, the har- Y monicdiierence component will bein phase with the current in the oscillating plate circuit. The circuit becomes stable in this condition, and the injected current will lock in the incoming 4300 kc. signal with the 860 kc, current in the plate circuit. Since `the injected current has the same phase and frequency as the normal current in the plate circuit it is equivalent to an increased output from the tube.

Now assume for the moment that the incoming frequency is higher than 4300 kc. and is within the lock-in range, but that the oscillator has not yet locked in. The eiect of the fourth harmonic will be to inject a current of slightly greater frequency than 860 kc. into the tank circuit. In Fig. la., OA is a vector assumed to be rotating 860,000 times per second, and represents the normal current in the oscillator tank circuit. Let AB be a vector representing the instantaneous injected current of frequency slightly greater than 860 kc. resulting from the fourth harmonic of the oscillator voltage beating with the incoming signal voltage. This vectorwill rotate slightly faster than 860,000 times per second, and thus will have an angular velocity relative to OA equal to the difference of the two angular velocities.

Now consider further the instantaneous condition shown in Fig. la. The injected current AB has a component AC, shown dotted, in phase with OA and another component AD (shown dotted) 90 out of phase with respect to OA. The vector OB represents the resultant of OA and AB. Let this current OB be applied to a tuned circuit LC as shown in Fig. lb. Since the circuit LC is tuned to 860 kc., it will be at resonance with respect to the current OC, which is also 860 kc., and equals ifi-Hb. The quadrature current AD is a leading current at the instant shown, and the result is the same as though an additional condenser C' is in the circuit LC. The eect is to decrease the natural frequency of the tuned circuit LC.

Now consider the condition at a later instant,

'as shown by Fig. 1c, the incoming frequency being the same as in the preceding discussion of Fig. 1a.. The oscillator has not yet locked in. The vector AB has rotated to the new position as shown. The injected current AB now has an in-phase component AC as before, but the cornponent AD is now lagging instead of leading. If the current OB is now impressed on the circuit of Fig. lb, the lagging component AD will cancel part of the leading current through capacity C, and this will be equivalent to reducing the capacity C since it is now drawing a smaller leading current. This will raise the resonant frequency of the tuned circuit LC.

' It is now evident that the circuit of Fig. l behaves like a reactance tube circuit. Itis easy to see that if the frequency of the incoming signal is approximately ve times that of the tuned circuit 5, a point will be reached when the frequency of the tuned circuit becomes exactly one-fth 'of the incoming signal frequency. When this happens the oscillator will lock in with the incoming signal. This means that the amplitude and phase of the plate current now remain xed with respect to the incoming signal.

If the incoming signal is exactly five times the frequency of the tuned plate circuit, the vector AB will be in phase with OA. As the incoming signal frequency is decreased, the oscillator remaining locked in with it, the vector AB rotates to some position such as that shown in Fig. 1a..

A further decrease in frequency will rotate the vector until it is 90 out of phase with respect to OA. Since this position gives the maximum amount of quadrature current it corresponds to the maximum amount that the oscillator frequency can be pulled over, and thus gives the passive network are so 6 lower limit of the lock-in range. If the incoming signal frequency becomes greater than ve times the plate circuit frequency, the conditions will be similar except that the vector AB will be lagging, as shown by Fig. lc, instead of leading. The upper limit of the lock-in range is reached when the injected current lags by The lagging current tends to reduce the effective capacity of the circuit, and thus `raises the frequency.

The amount of fourth and/or sixth harmonic current which is applied to grid 6 is limited. This limits the lock-in range of the oscillator, since it limits the amount of the injected current. The result is that when the deviation of Y the incoming signal from 4300kc.4 exceeds approximately 30 kc., the oscillator suddenly breaks out and the frequency goes back towards center. This means that the oscillator is no longer locked in; the ratio of the incoming frequency to' the oscillator frequency is no longer a definite small integer. If the oscillator is being used as a secondary frequency standard, to compare two frequencies, to operate a clock. or in Van FM receiver, this break-out will be objectionable.

In an FMreceiver this will cause distortion. The practical operating disadvantage of this break-out effect in an FM receiver resides in the fact that if a receiver is mistuned slightly, the break-out will occur on one end of the frequency swing and even for normal frequency swings. Such mistuning of a receiver is quite possible, since the average operator of a frequencymodulation receiver finds it diflicult to tune the receiver exactly. Again, if the FM transmitter station overmodulates, the break-out can occur on both ends of the frequency swing. In accordance with my present invention, the break-out effect is substantially eliminated by extending the lock-in range of the oscillator. In general, in accordance with my present invention, and contradistinct from the methods of my aforesaid application, there is provided an auxiliary passive electrical' network which is placed in `parallel with the tuned plate circuit of the os- `This makes it possible t-o lock in the oscillator over a greater range for a given amount of injected current. The passive electrical network does nothave any amplifier tubes, and is not `a source of electrical energy. The constants of the chosen that it has an effective capacity vs. frequency characteristic which falls oi at a rate to provide a relatively wide lock-in range for `subharmonic frequency oscillations. In Fig 2 I have shown the present methodof extending the range of a locked-in oscillator. The circuit of Fig. 2 is similar in all respects to that of Fig. l, except for the network now to be described. i

The coil 33, resistor 32 and condenser 3l are connected in shunt relation, while the common ungrounded terminals of the three components are connected by condenser 30 to plated. Thus, the auxiliary passive network 34 is located in a path to ground parallel to circuit 5. Circuit 34 is tuned approximately to F/n, but in practice may be slightly higherin frequency than F/n. In practice, the circuit 34 of Fig. 2 can be-adjusted in several different ways to give the Lde- 'ance lof :the parallel `circuit-34 in Fig.l 2 as a func- .tion fof frequency. The curve is secured by plot- .ting ohms against .frequency. Curve b is the equivalent series reactance. The two curves thus lshow the resistive and reactive components of the .impedance of parallel circuit 3d at each frequency. When series condenser 33 is added, if .its value is properly ch'os'en, the reactance curve is displaced until it is entirely below the axis as shown4 by dashed curve bi. The equivaflent shunt capacity corresponding to the dashed curve b of Fig. 4a "can be determined from the V`reactan'ce and frequency, and is shown in Fig. 4b. The equivalent capacity of the added circuit 34 `ithus decreases with increasing frequency on both sides of the operating frequency F/u of the oscillator.

. When this circuit 34 is placed in parallel with :the `tuned plate circuit 5 yof the oscillator, the tank 4circuit, capacity vcan 4be reduced slightly and the `slope of the capacity-frequency curve (see Eig. 4h) of the added circuit 34 can be adjusted until the lcapacity decreases at just the proper rate to keep the oscillator circuit 5 in tune for a considerable variation in frequency. In order tokeep the oscillator tank circuit in tune over the operating frequency range, the tank circuit capacity r'should decrease with increasing frequency as shown by Fig. 5a. 'I'he slope of this curve is Adetermined by the L/C ratio of the tank circuit.

The Aslope ofthe curve of Fig. 4b can be matched to that Of Fig. 5a over a major portion vof the `operating range as shown yby Figpb'. The solid line 'could represent 'the falling input capacity of the following discriminator circuit, and the dashed line lis the `same as in Fig. 5a. When these slopes are matched, the lock-in range -will be greatly 'increased vsince a small `amount of -reactive current will vshift the oscillator frequency fa considerable amount. I

In otherwords, the oscillator can Aoscillate at any frequency within the range where the falling -capacity characteristic of the :added circuit 3d matches the rate `of change of the plate circuit capacity with `frequency vrequired to keep the oscillator Vin tune. Since the oscillator is in tune for considerable frequency variation, it is easier to lock it in with the incoming signal. The elect is a considerably increased lock-in range. The slope andthe distance -between the maximum and minimum :points -of the curve in Fig. '2lb can be -changed by :changing the valueV of re- .'sistanc'e 32 in Fig. 2. Any change in capacity of condenser 30 will, also, cause corresponding changes in the slope Aof the curve.' The operating frequency is approximately F/n, and is seslected by vproper choice of the inductance of coil 33 tand `capacity of condenser 3l. It should be noted that any variation in the L/C ratio of Athe .plate circuit 5 will, also, change the rate at which 4the capacity must 4change with frequency to Akeep the oscillator in tune. This means that either the L/C ratio of circuit 5 or the constants fof circuit 34, or both, can vbe adjusted until the 'slopes of the curves (as in Fig. 5b) are the same, yor nearly the same, at the operating point.

In Fig. 3 a generalized passive lter network '35 is shown in place of circuit 34. There are many possible electrical circuits that can be used here, .the requirement being a resultant input capacity variation with frequency as shown in Fig. 4b. My invention is entirely generic in this regard.

Figs. 6a, 612.60 and 6d show respectively :different circuits that fcan be used for network 35 in Fig. 3. It will be understood that the respective networks of Figs. 6a to 6d could vreplace generalized network 35 in Fig. 3 between terminals a :and bof the latter. In Fig. 6a, the series and parallel resonant frequenciesand the circuit Q can be chosen properly so that the equivalent shunt capacity varies in the manner shown in Fig. 4b. In Fig. 6a condenser 36, coil 3l and resistor `38 are connected in series, and condenser 39 shunts the series path. If the capacitor 39 yof Fig. 6a is combined with the tank circuit capacity, the series circuit of Fig. 6b will then produce Athe required conditions. If acoupled circuit, such as 44 in Fig. 6c, is added to the circuit 34 of Fig. 3 it will work properly when the circuits '44 and -34 have a coupling coeiiicient less than or equal to critical, yif the Q ofthe circuit 34 is greater than or equal to thatof circuit 44. In all of these illustrative circuits proposed for use as the shunt auxiliary network, it should be noted that a condenser and resistor in series are substantially equivalent to a condenser and resistor inparallel over a limited frequency range. This `means that a series resistor can be changed to a shunt resistor, and vice versa. Fig. 6d shows a circuit equivalent to the circuit of Fig. 6a if the resistor 33 land 38 are properly chosen.

The circuits 4of either Figs, `2. or 3 can also be combined with the circuits of my aforesaid application to give improved performance. It is, also, possible Ito design the lter network 35 of Fig. :3 to `operate as a frequency discriminator. This permits the detection Iof frequency modulated signals. It will -now be seen that I have provided a novel means for increasing the lock-in range of a locked-in oscillator by adding an electrical network in parallel with the tuned tank circuit, and adjusting the network so that the equivalent input capacity thereof decreases at the proper' rate to keep the oscillator in tune throughout much of the operating range.

-It should be noted that when a network -is designed vso that the oscillator is in tune over a kgiven band of frequencies, the -reactance of the oscillator circuit is zero over the same range. This means that Vthe phase angle characteristic is also zero in this range. Therefore, the functioning of the sh'unt auxiliary network across -tank circuit 5 can be alternatively explained in terms of the addition of ya network so designed that the phase characteristic of the combination is Zero in the desired range.

In Fig. 7 I have shown a method of utilizing the auxiliary network 35 of Fig. 3 for simultaneous discrimination of FM signals and extension of the lock-in range of the oscillator section of a frequency divider. The auxiliary network 34 is still a passive network in this case, since the diodes Il and I8 function as loads on the network. The system shown in Fig. '7 -includes th'e locked-in oscillator acting as a frequency dividing network in a receiving system of the type disclosed in the aforementioned Beers patent. As more fully explained in the Beers patent the locked-in oscillator functions concurrently to ref -megacycles (mc). not limited to any particular frequency band or 9 received FM waves are those which are transmitted in the present assigned FM band of 42-50 Of course, the invention is to the reception of FM waves, since it is generally applicable to angle modulated carrier waves. Those skilled 4in the artare fully aware of the fact that in each channel the rFM waves transmitted in the aforementioned assigned FM band are presently allotted a maximum overall frequency swing of 150 kc, with respect to the mean or center` frequency Fc. The extent of frequency deviation is dependent upon the amplitude of the modulation signals at the transmitter, while the rate of frequency deviation is dependent upon Athe modulation frequency per se.

The collected FM waves are selected in one or more stages of tunable radio frequency amplication, after which they are combined with locally-produced oscillations at the i'lrst detector network. The output of the first detector or converter is the I. F. (intermediate frequency) energy. In other words, the I. F. energy is the FM wave whose mean frequency has been reduced to a much' lower frequency, but whose frequency deviation is unchanged. After amplification by one or more stages of I. F. amplifiers, the I. F. energy is applied to the locked-in oscillator for concurrent frequency division and frequency deviation reduction. Above the I. F. transformer I3 I have depicted, in a purely graphic manner, an idealized response curve of the coupled primary and secondary tuned circuits Iii and I5. It will be seen that the mean frequency is located at 4.3 mc., whereas the pass band of the network is substantially 15() kc. wide. This signifies that the network I4, I5 is capable of eiciently transmitting the entire frequency swings of the FM wave whose mean frequency has been reduced to the operating I. F. value. While'the value of 4.3 mc. has been assigned as the operating I. F. value, it is to be clearly understood that any other satisfactory value may be employed depending upon the various factors met with in the design of the receiver.

Since the locked-in oscillator circuits in Fig. 7 are exactly the same as shown in Fig. 2, the same reference numerals are employed. The FM signal energy with its mean frequency at the operating I. F, value is applied to the input grid 2.

The input grid 2 is connected to the high' alterhating potential side of the secondary circuit I5. The low potential side of circuit I5 is returned to the grounded cathode 3 through a resistor and condenser network designated by the numeral I6. The function of the network I6 is to provide voltage across the resistor element in response to grid current flow through the input grid circuit. Such grid voltage developed across the network I6 may be utilized for automatic volume control (AVC). 'I'h'e AVC voltage is employed automatically to bias the -control grids of the preceding amplier tubes in a manner wellknown to those skilled in the art.

. Above the plate circuit 5 in Fig. 7 I have graphically represented an idealized response curve of the oscillator section output network. It will be noted that it has a pass band width of 30 kc., while the mean frequency has a value of 860 kc. This is appropriate since the effect of the locked-in oscillator network has been to divide the mean frequency of the FM wave energy by a factor of 5, and the over-all frequency deviation range has also been divided by the same factor.u Additionally, there is secured a substanlso :tialreduction ofi amplitude modulation effects` which may have been created on the FM wave energy in the transmission through space or during the passage of the signal energy through the receiver networks. This elimination of amplitude modulation effects is secured, asexplained in the" aforesaid Beers patent, without the use of any special amplitudelimiter stage.` The advantages of frequency division at this point of the receiving system, have been explained in the Beers patent. The extension of `the lock-in range of the oscil` lator accomplished by this invention as compared with the arrangements shown in the Beers patent will enable reception of waves which are frequency modulated over a wider range and will guard against the distortion which might otherwise occur by reason of the aforementioned break-ou effect.V Since the FM wave energy developed across the plate circuit 5 is modulated in strict accordance with the originally received FM waves,` save that the mean frequency and extent of frequency deviation have been proportionately reduced, `the output of the locked-in oscillator may now be subjected to suitable discrimination and rectification.

The discriminator employed herein feeds a pair of opposed diode rectiers I1 and I8. The discriminator network is generally that disclosed and claimed by J. D. Reid in his U. S. Patent No. 2,341,240, granted February 8, 1944.' In accordance with my present invention, the discriminator network is given a second and-novel Yfunction. It acts to extend the lock-in range of the locked-in oscillator. Further analysis of this new function will be lgiven at a later point. The opposed diodes Hand I8'will be separately fed with signal energy which has the form of amplitude modulated wave energybyvirtue of the action of the discriminator which translates the FM wave energy into corresponding amplitude modulated carrier wave energy.

As will be noted from Fig. '7, `the output load consists of a pair of series-connected resistors I9 and 20. The junction of the resistors is` connected by a return path 2I to the anode of diode I8. The cathode of the diode I8 is established at ground potential. The high potential side of circuit 5 is connected to the anode of diode I'I through the path consisting of condenser I2and resistor I2' in series. Above the discriminator network I have shown the typical characteristic of an FM detector'stage. It will be noted that at the mean frequency Fc the resultant instantaneous output voltage of the detector is zero. The maximum outputoccurs at the opposite peak limiting frequencies F1 and F2.

Each of the opposed rectifiers includes in circuit therewith its respective load resistor. At the instant when the applied signal energy has the mean frequency value, the rectified voltages across the resistors I9 and 20 are equal in magnitude and opposite in polarity. Hence, the resultant potential at the upper end of resistor I9 is Zero. On the other hand the potential at the upper end of resistor I9 will vary in magnitude and polarity depending respectively upon the extent anddirection of frequency deviation with respect to the mean frequency. Accordingly, the voltage taken off from the output load of the opposed rectiers corresponds to the original modulation signalV voltage applied to the carrier at the FM transmitter.

The resistor 28 and the pair of parallel condensers 28 to ground provide a lie-emphasis net- 'work whose 'functionis welll knownin the art of frequency modulation reception. Briefly, the de-emphasis network acts to diminish thel response at the. higher audio frequencies, since during transmission of the FM waves such` higher audioy frequency components may have been disproportionately emphasized.

The discriminator circuit inA Fig. 7 consists of theV coil 33', the resistor 32 and condenser 3|' all connected in parallel relation between the anodes of diodes H and t8. The constants of network 33", 32', and.v 31' are chosen so as to extend the lock-in range ofl the` locked-in oscillator; The aforesaid4 Reid patent describes in detail the manner in which the discriminator circuit acts to translate FM signals into correspondingamplitude variable signals. The low potential si'de l0f circuit 34 is connected to ground through the anode to cathode capacity of diode i8, the capacity being indicatedby numeral I3 and being shown dotted.

It will be noted in Fig. 7 that the tank circuit is shunted to ground by a network consisting of series-connected condenser I2 and resistor I2 in series with the circuit consisting of coil 33', resistor 32 and condenser 3l allv in parallel, and that the latter circuit is connected in series to ground by a circuit consisting of condenser I8 shunted by resistance indicated in dotted lines as 29. The function ofcondenser I2 is to couple the discriminator circuit 34' to the tank circuit 5', and also is part of the circuit to extend the lock-in range. Resistor l2' is not always necessary and helps to prevent toomuch interaction between the oscillator and discriminator due to stray coupling., The shunt resistance of circuit 34' not only includes resistance 32', butv should be considered as including approximately one-third of load resistor i9 in parallel therewith. Further, the resistance 20 is equal to approximately onethird of load resistor 20. If resistor 26 and condenser IS are replacedr by an equivalent resistor and condenser in series, and these be respectively combinedv with I2 and l2, the resultant equivalentV circuit would be similar to the auxiliary network 34, tu shown in Fig. 2. The sole difference would be that in Fig. '7 the-condenser I2' would have resistance in series therewith, but, as stated previously, this may be omitted. It Will be seen that the discriminator circuit 34 inV Fig. 7 thereby acts to increase the lock-in range of the locked-in oscillator in the manner previously explained.

The normal functioning of the discriminator` circuit in the manner described by Reid in his aforesaid patent is not interfered with. It isk believed sucient for the purposes of this application to point out thatcircuit 34" normally h as a resonant frequency' slightly higher than F/n (860 kc. in Fig. 7). .For signal frequencies less than 860 kc. circuit 34" appears to have an inductive reactance having series resistance. As resonance is approached from the low frequency side, .the apparent inductance rises very rapidly so that it is possible to obtain series resonance between this inductance and the relatively small capacitance i8 at a frequency close to the Darallel resonance. The resultant voltage across the entire network is due to the voltage curves resulting from the series and parallel resonances, the resulting voltage. curve having the required spaced maximum and minimum peaks. The spacing. of the parallel and 'series' resonances d'eterminethe shape of the resultant curve;

The present invention is not restricted to re- 12 ceptionof frequency modulated signals, butcan, also,- be used at anyv fixed frequency; The: iincreased lock-in range is desirable when operating at a fixedfrequency tc assure that the oscillator will remain locked in even though the'- cifrcuits may become slightly misaligneddue totemperaturef changes, voltage changes, etc.A The use of alocked-in oscillator with a frequency discriminator represents only one application of my-invention. It canbe used to obtainsecondary frequency standards from a given primary stand.- ard such as a crystal oscillator. It canbe used to compare two frequencies; to operate eiectronic clocks; or inV any other case where a definite ratio of frequencies is desired.

Fig. 8 shows a further modiiication of the invention, and differs from the system shown in Fig. 7 in that the functions of lock-in` range extension and frequencyA discrimination are'A separated. The electrical network 35 is shown connected in parallel across tank circuit 5, as in Fig. 3. Thecharacteristic of network 35 is selected to provide the lock-in range extension. The discriminator and detector 4t, schematically represented in Fig. 8, is coupled tothe tank circuit 5 through an isolationamplifier tube 41.' The input electrodes of the isolation tube'V are coupled a'cross the tank circuit 5. The latter is shunted by resistor It?, and the high potential side of circuit 5 is connected to grid t3v of tube 'l by'couplingcondenser dit. Resistor @-2 adjustsl the Q of tank circuit 5. The discriminator-detector it@ may be of any suitable construction. For example, it maybe of the aforesaid Reid' type., or it can be of the type shown by S. W. Seeley in his U. S. Patent No. 2,121,103, grantedv June 21, 1938'. The arrangement of Fig. 8 makes it possibileY to :provide a desired lock-in characteristic, and' also secure a desired discriminatcr characteristic. It is to be understood that the several forms of networks shown in Figs. 6a, 6b, 6c and 6d can be used at network 35 in Fig. 8. These can be designed to give the desired lock-in range andthen adjustedA (without affectingI the discriminator) to give adjustable adjacent channel selectivity;

While I have indicated and described several systems for carrying my invention into effect, it will be apparent' toone skilledV in the' art that my invention is by no means limited' to thejparticular organizations shown and described; but that many modifications maybe made' without departing from the scope of my invention.

What I claim is: Y

1. In combination withl a tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element high frequency current which isi angle modulated and has a desired mean frequency, a resonant tank circuit reactively coupled to said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of said mean frequency, means including a selective network tuned4 substantially to said subharmonic fre'- quency, means coupling. said selective network to said tank circuit; and said selectiveI network having its constants so chosen that it has an effective capacity vs. frequency characteristic whichV falls off at a rate to just keep the subharmonic oscillatorin tune over most of its operating range.

2. In combination with a locked-in oscillator tube provided with at least an electron emitter, a control element, an oscillator grid, andfoscil.- lator output electrode, means for applying to the control element current of a desired frequency, a resonant tank circuit 'regeneratively coupling said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of the input frequency, a network comprising inductance, capacity and resistance all in shunt relation, means coupling said network in parallel across said tank circuit. and the constants of said network I'being chosen to provide a capacity vs. frequency characteristic which decreases as the frequency increases.

3. In combination with a tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element current which is frequency modulated and has a desired mean frequency, a resonant tankk circuit coupled to said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of said mean frequency, an electrical network having a falling capacity vs. frequency characteristic, means connecting said network in parallel across said tank circuit, means for deriving from the frequency modulated current of subharmonic frequency the modulation signals thereof, and an isolation amplifier coupling said tank circuit to said deriving means.

4. In combination with a tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element high frequency current which is frequency modulated and has a desired center frequency, a resonant circuit coupled to said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of said center frequency, a selective network tuned substantially to said subharmonic frequency, means coupling said selective network in parallel to said tank circuit, said selective network having its constants so chosen that it has ak capacity vs. frequency characteristic which falls off at a rate to just keep the subharmonic oscillator in tune over most of its operating range.

5. In combination with a tube provided with at least an electron em'tter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element current which is angle modulated and has a desired mean frequency, a resonant circuit coupled to said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of said mean frequency, an electrical network having a decreasing capacity vs. frequency characteristic, means connecting said network in parallel across said tank circuit, and means for derlving from the angle modulated current of submodulation signal thereharmonic frequency the of. l

6. In combination with a tube provided with at least an electron emitter, a control element, an oscillator grid and oscillator output electrode, means for applying to the control element high frequency current which is frequency modulated and has a desired mean frequency, a resonant tank circuit reactively coupled to said oscillator grid and output electrode and tuned substantially to a desired subharmonic frequency of said mean frequency, means including a selective network tuned substantially to said subharmonic frequency, means coupling said selective network to said tank circuit, said selective network having its constants so chosen that it has an effective capacity vs. frequency characteristic which falls off at a rate to just keep the subharmonic oscillator in tune over most of its operating range, and opposed rectiiiers including said selective network as a common discriminator input circuit whereby thev rectifiers provide an output 'voltage representative of the frequency modulation.

7. In combination with a tube provided with at least an electron emitter, a control element, a grid and output electrode, means for applying to the control element input current of a desired frequency, a resonant tank circuit reactively coupling said grid and output electrode and tuned substantially to a desired subharmonic frequency of the input frequency, means including a passive selective network tuned substantially to said subharmonic frequency, means coupling said selective network to said tank circuit, said selective network having its constants so chosen that it has an effective capacity vs. frequency characteristic which falls off with increasing frequency at the frequency at which said tank circuit is resonant thereby to provide a relatively wide lock-in range for subharmonic frequency oscillations.

MURLAN S. CORRINGTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS

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