US2316909A - Selective tuned circuits - Google Patents

Description

April 20, 1943. R. A. WEAGANT 2,316,909

SELECTIVE TUNED CIRCUIT I Filed May 15, 1941 Usual flararzazwe IN'VENTOR EoyJZWBay ATTORNEY Patented Apr. 20, 1943 2,316,909 SELECTIVE TUNED cmcm'rs Roy A. Weagant, Douglaston, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 15, 1941, Serial No. 393,532

10 Claims.

My present invention relates to high frequency tuned circuits of high discrimination between voltages of different frequencies.

One of the main objects of my present invention is to increase the effect originally obtained with tuned circuits, and to produce discrimination effects which cannot be obtained through the employment of solely coupled tuned circuits.

Another important object of my invention is to utilize a coupling network, between cascaded high frequency circuits, arranged to transfer the high frequency energy of one circuit to the other in such a manner that a phase shift is produced when frequencies other than the resonant frequency of the system are impressed upon the network.

Another object of the invention may be said to reside in the provision of a tuned network which is constructed so that the current flow in a particular part of the network is reduced due to frequencies oif resonance with the resonant frequency of the network, and the reduction being to a low value which is much less than that which corresponds to the action of resonance alone.

Another object of the invention is to provide a network capable of distinguishing between the effects of voltages of difierent frequencies; currents being produced to correspond to the frequencies, and the effects of the'currents being combined in a common circuit in a manner which causes the amplitude of the current in the circuit to vary in accordance with a phase angle between the aforesaid currents.

Still other objects of my invention are to improve generally the efficiency and selectivity of tuned signal circuits, and to provide signal selector networks which are reliable, durable and economically manufactured and assembled-in radio receivers.

The novel features which I believe to be char.- acteristic of my invention are set forth in par.- ticularity 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 I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a circuit diagram of a receiver embodying the invention,

Fig. 2 illustrates the angular relations of the coupling coils of the invention,

Fig. 3 shows a vector-ial explanation of the in: ent n,

Fig. 4 illustrates a resonance curve of a net: work employing the invention,

Fig. 5 shows graphically the improvement due to the invention.

Referring now to the accompanying drawing, and particularly to Fig. 1, there is shown a superheterodyne receiving circuit which embodies the invention. It is not believed necessary to describe the various circuit details of such a receiver, since those skilled in the art are fully aware of such constructional details. The converter I may be of any desired type, and, for example, may comprise a pentagrid tube which functions as a combined oscillator and first detector circuit. Of course, separate first detector and local oscillator tubes may be utilized. The signal collector may be of any desired type, and may be coupled to the tunable input circuit 2 of the converter through one or more stages of radio frequency amplification. The tunable local oscillator tank circuit 3 may also be provided in the usual manner. The dotted line 4 designates the customary tuning control mechanism employed between the rotors of the variable reactance elements of the signal circuits and local oscillator tank circuit. Assuming that the receiver is used, by way of example, in the broadcast band of 550 to 170.0 kilocycles (kc.), the local oscillator will be operated over a frequency range such that at each setting of the tuning control mechanism there will be produced a constant intermediate frequency value such as, for example, 465 kc.

In the output of the converter I there is arranged a resonant circuit comprising coils II and I3 which are serially arranged between the plate of the first detector tube and the positive terminal of the direct current source which energizes the positive electrodes of the converter. The condenser I0 is shunted across coils II and I3. The network Hie-l le-l3 is tuned fixedly to the operating intermediate frequency (I. F.) value. As many I. 'F. amplifier tubes as is desired may be employed. By way of simplicity only one such tube 20 is shown. This tube may be of the pentode type, although any other type may be utilized. The control grid 2| thereof is connected to ground for alternating currents through a path which comprises coil 3| and con-.- denser 3!. The condenser 32 is connected in shunt with coil 3|, and fixedly resonates the latter to the operating 1. F. value. The cathode of tube 20 may be connected to ground through l3 and M.

to the signal grids of the various I. F. amplifier tubes. This is exemplified by the connection of lead 56 to one end of coil 3| through a filter resistor 58. The lead 56 may be connected to any of the prior signal grids in the usual manner. It will, therefore, be seen that the grid return for grid 2| is through lead 56, resistor 55 and load. resistor 53 to ground.

The audio voltage developed across load resistor 53 is taken off by an adjustable potentiometer 6|) which is connected to ground through a se- The circuit 33-|4|6 is tuned to the operating I I. F. value, and the circuit I5 is also tuned to the I. F. value. It will now be appreciated that signal voltages are transmitted to coil 3| through the path including coil l3, and through a second path between coils II and I4.

Coils l3, 3| and H are preferably arranged in the manner of a goniometer. This type of coupling unit is well known in the art, and is shown in Fig. 2. Coils-| 3 and H! are arranged at right angles to prevent mutual coupling between them. The 'coil' 3| 'is arranged at approxi mately 45 degrees between the planes of coils Hence, the-signal voltage induced in coil 3| is the resultant of two signal voltages induced by the currents flowing in coils 3 and |4 respectively. It 'will, therefore, beseen that whereas coils |3 and |4 are free of any magnetic coupling, the coil -3| is coupled to each of coils :l3 and H. 'Asis well known in goniometer construction, the coil'3l -may be arranged for adjustment relative'to the planes of coils 3 and 4. It will now be seen that the tuned circuit 3| -32 is fed with I. F. signals through tWo paths. One of these paths is the I. F.-tuned circuit ||J-||-|3.. The second path is the cascaded series of I. F.-tun ed circuits which includes the tuned circuit l5 and the tuned circuit 33--|6-|4. The amplifier has its plate connected to the high potential side of an I. F.-tuned circuit whichcomprises coils and 4| arranged in series in the'same manner as coils II and I3, the shunt condenser 42 resonating the coils 40 and 4| to the operating'I. F. value. The link coupling circuit 43 is constructed in the same manner as coupling circuit l5, and the'circuit 44 is constructed in the. same "manner as tuned circuit 33--|6 +'|4. The coil 45 is magnetically coupled to the coil 45, and coil 4| is also coupled to coil 45. Coils 4|, 46, and 45 are arranged in the manner of a goniometer as in the case of coils |3-4||4. Coil 4 5 is shunted by a resonating condenser 41 which tunes the coil to the operating I. F. value. In other words-the network between the second detector and I. F. amplifier 20 is constructed in the same manner as the network between the converter and the amplifier 20. V Tube may be of the usual duplex diodetriod type. To preserve simplicity of description the duplex diode anode are shown as a single anodewhich is connected to one terminal of condenser 41. The cathode 5| is connected to ground through a selfbiasing resistor 52 shunted by the usual I. F. by-pass condenser. The diode detection circuit is completed through the'load resistor 53 which is shunted by the I. F. by-pass condenser 54. Direct current voltage developed across load resistor 53 is utilized for automatic volume controlpurposes. The AVC bias so developed is applied through the filter resistor 55 to a lead 55, one end of resistor 55 being connected to ground through the filter condenser 51. The bias over lead 56 may be applied ries condenser and resistor. The control grid T0 of tube 50 is connected to any desired point along the potentiometer resistor 6| by an adjustable tap. The triode section of tube 50 functions as an audio amplifier. triode section may be connected to one or more audio frequency amplifiers which may be followed in turn by any desired type of audio reproducer. Since the network following the detector stage is very well known to those skilled in the art, it is not believed necessary to describe it in any further detail.

For reasons which will be gone into in detail at a later point, the voltages induced in each of coils 3| and 45 by their associated coils l3 and I4 and 4| and 45 respectively, will have their phase relations altered as the input frequencies depart from resonance with the result that'their vector sum in each case will decrease with departure from resonance. This means that the input to the detector stage will fall oif more rapidly than would be the case if only the ordinary phenomena of resonance was employed. At resonance, that is at the operating I. F. value, the voltages induced in coil 3| by each of coils l3 and M are to be equal. This is also true in the case of coils 4|, 46 and 45.

Confining attention to the network between converter and I. F. amplifier 20, the following explanation is given of the action which takes place in the system. It will be understood that the same explanation applies to the coupling network between the amplifier 20 and the second detector stage. When circuits |0| 3 and 3 |--32 are loosely coupled and in resonance with the intermediate frequency value, the current in circuit 3|--32 will be in quadrature with the cur rent in circuit |0| ||3, or will lag or lead as may be desired. Similarly, under the same conditions, the current in circuit l5 will be in quadrature with the current in the circuit ||l-| |-|3. A similar shift in phase will take place between circuit [5 and the circuit 33-|6|4. If these two shifts are in the same direction, then the current in coil 3| will be shifted degrees with respect to the current in coil 3. The direction of connection of coil I4 is so chosen that the voltage which it induces in coil 3| is in the same direction as that induced by the current in coil l3. This means that at resonance with the I. F. value the two coupling effects will add in so'far as coil 3| is concerned.

When the applied frequency departs from the I. F. value, this condition no longer obtains. On the contrary, there occurs a shift in the relative phase relations of the voltages induced in coil 3| by the two circuits coupled thereto, and an explanation of the action which then takes place is as follows: Assume that the frequency departs from resonance in either direction, then the current in circuit |0| |-|3 will shift relative to its phase at resonance. The voltage which is induced in circuit 3l-32 will shift by the same amount, while the current in this latter circuit will also The plate of the.

shift relative to the induced voltage by the same amount.

From this it is evident that the current flowing in a circuit which is loosely coupled to a ,driving circuit changes in phase twice the angle of lead or lag of the current in the driving circuit. Continuing this analysis to circuit l5, we have at frequencies off resonance a current flowing which lags or leads the voltage impressed on circuit Ill-I l--I3 by an angle which is twice the phase angle between current and voltage in this latter circuit. Therefore, the voltage induced in circuit 33+-.l6l,4 differs by this same angle from its phase at the resonant frequency. The resultant current flow in circuit 33I 6-? will difier from the induced voltage by the original phase angle between current and voltage in circuit lll-l l-..l 3, The voltage which this current will induce in coil 3! is shifted from its resonant angle by this same amount. This amount will be three times the phase angle between current and voltage in circuit H1--Il |3.

Since the direction of phase shift in circuits Ill-l I-|3 and in circuit 33-,I6-.-l4 is the same, while the amount of the latter is three times the former, the relative phase of the currents in these two circuits has shifted by an amount which .is

- three minus one .or two times the phase angle between current and voltage in circuit |-l |-l3.

This action takes places for frequencies on either side of the resonance frequency. The inducing voltages supplied from coils l3 and I4 become more and more out of phase as the applied frequency departs from the resonance 1. F. value, and the resultant amplitude of the current flowing in circuit 3 I-3 2 decreases.

In Fig. 3 there is shown a method for determining the amplitude of the resultant voltage induced in the circuit following the coupling network, and in this case that would be amplifier 20. In the diagram A represents the phase angle between the voltages induced in coil 31, while ER represents the resultant voltage. The following expression may be written:

The vectors E and E1 represent the two induced voltages assumed to be equal in magnitude; A being the angle between them. Since the phase angle between E and E1 is twice the phase angle in circuit l!l--l l-l3, the angle 13 which the resultant Ea makes with E is the aforementioned phase angle.

' Since the phase angle B may be expressed in terms of the circuit constant as follows I the angle A may be expressed as 21rFL R equals the /3 and ER equals E or E1. If we assume an intermediate frequency of 465 kc., and

all the circuits have an inductance of one thousand microhenries and a resistance of fifteen ohms, then it can be shown that the change .of reactance per kc. of frequency is 12.18 ohms. To produce an angle B of Fig. 3 of 60 degrees a dee parture from resonance of 2.13 kc. is all that is necessary, and it indicates that the amplitude of the voltage delivered to the following circuit is cut in half at frequencies approximately 2 kc. each side of the resonance point. This statement is to be interpreted as m aning that the phase action herein described reduces the value of the detector voltage to approximately half of what it would be due to resonance alone.

It further indicates that in a radio receiver having an overall resonance curve of conventional form the selectivity is tremendously increased, It indicates, further, that the required selectivity for this type of reception is obtainable with an overall resonance curve which is much broader than that customarily employed. The consequenceof this is that by means of the invention described herein the required selectivity for any service may be obtained with a smaller number of tuned circuits, or with circuits which are inherently less efficient than is customary.

The phase selectivity action taking place between amplifier 20 and the second detector further increases the selective action. It will be understood that by the expression phase selectivity is meant the enhancement of the usual selective action of coupled resonant circuits by virtue of the phase additive arrangement shown in this application. In Fig. 5 there are shown three curves which illustrate in a purely qualitative manner the efiect of phase selectivity according to the present invention. The curve designed Usual resonance curve is the usual type of single peaked resonance curve which would be obtained with loosely coupled cascaded tuned circuits, and would result, for example, if circuits l5 and 33-16-44 were omitted from the network between the converter and amplifier 20. The second curve designated Result of phase action shows the type of selectivity curve that is secured by virtue of the introduction of circuits l5, and 33l 6,l4. This is the type of curve which is secured according to the method outlined in Figs. 3 and .4.

The innermost curve Resultant response curve is the product of the first two curves. This shows the extent to which the usual resonance phenomenon has been enhanced by the present phase selectivity method. Of course, the curves of Fig. 5 are purely qualitative, and are merely used to show the nature of the increase in selectivity secured by means of the present invention. It is pointed out that one cf the important advantages of the present arrangement is that the persistence of the tuned circuits is not increased by this method. That is, even though the selectivity is greatly increased as shown in Fig. 5, the persistence of the cascaded tuned circuits is in no way increased. This is due to the fact that the voltages are induced in coil 3!, as well as in coil 46, iromsources which are additively related. That is, the present invention does not affect the R, C or L constants ofthe tuned circuits. Another way of looking at this advantage is that the time constant of the network is not increased, but remains that of a single tuned circuit. This is of particular advantage in reception of frequency, or phase, modulated carrier signals since circuits of minimum time constant value are required in such cases.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and..described, but that many I network varies in accordance with the relative phase relations of the currents in the said two circuits and the reactive couplings being so relatively phased that currents of said desired frequency are additively induced in said tuned network.

2. In combination. with a'plurality of cascadecoupled resonant circuits all tuned to a common desired frequency, means for causing the current flow in one of said'circuits to vary in accordance with a phase displacement between the currents of at least two other offsaid circuits and said means comprising a separate reactive coupling between said one circuit and each of the other two circuits, said couplings being related toprovide additive induced'voltage in said one circuit at said desired frequency.

3. In a radio receiving system, a plurality of coupled resonant circuits, means for tuning said circuits to a desired frequency, a detector following said circuits, and means for causing the amplitude of the currents impressed on the detector 'to decrease with departure from resonance in accordance with the variations of a phase angle between two components of the same frequency said means being additionally chosen to cause said impressed currents to be the additive effect of the currents in said plurality of circuits at the said desired frequency.

4. In combination with a pair of electrical circuits which are tuned to a common frequency, means for shifting the relative phases of currents produced by voltages of different frequencies, and

means for combining these currents to produce 'an effect upon a current or voltage indicator whose amplitude "decreases with departure from --said frequency said combining means being constructed to add currents induced from said pair of circuits at said common frequency.

'5. In combinati'on'witli a pair of resonant circuits arranged incasc'ade, each of saidcircuits being tuned to a'coinmondesired frequency, a

coupling network providing a signal transfer path between said circuits which is auxiliary to a first 'couplingpath therebetween, said auxiliary path :"being tuned tosaid common frequency, and signal vvoltages being'induced in the last of the resoinant circuits in additive phase at said common frequency from the first circuit and from the auxiliary path.

6. In combinationwith a tuned circuit reso- 'nant to a desired carrier frequency of a modulated carrier Wave, a utilization circuit tuned to the same carrier frequency, a first coupling path between said resonant circuit and said utilization circuit for providing a desired selectivity characteristic, a second coupling path between said resonant circuit and said utilization circuit, said second coupling path being tuned to said carrier frequency and being so constructed and arranged that signal voltages are induced in said utilization circuit from both said coupling paths in'additive phase at said carrier frequency whereby the selectivity characteristic is made relatively sharper.

7. In combination with a tuned circuit resonant to a desired carrier frequency of a modulated carrier wave, a second resonant circuit tuned to the same carrier frequency, a first-coupling path between said two tuned circuits for providing a desired resonance curve, a second coupling path between said two circuits, said second coupling path being tuned to said carrier frequency and being so constructed and arranged thatsignal voltages are induced in said second circuit from both said coupling paths in additive phase at said carrier frequency whereby the curve is made relatively sharper.

8. In combination with a tuned circuit resonant to a desired carrier frequency of a modulated carrier wave, a second resonant circuit tuned to the same carrier frequency, a firstcoupling path between said two tuned circuits for providing a desired resonance curve, 'a second coupling path between said two circuits, said second coupling path being tuned to said carrier frequency and being so constructedand arranged that signal voltages are induced in said second circuit from both said coupling paths in additive phase at said carrier frequency whereby the-curve is made relatively sharper and said second coupling path comprising at least two resonant circuits arranged in cascade between said two tuned circuits.

9. In combination with a pair of resonant circuits arranged in cascade, each of said circuits being tuned to a common desired frequency, a coupling network'providing a signal transfer path between said circuits which isauxiliary to a first coupling path therebetween, said auxiliary path consisting of at least two tuned circuits each tuned to said common frequency, and signal voltages being induced at said common frequency in the last of the resonant circuits in additive phase from the first circuit and from the auxiliary path.

10. In combination with a first signal transmission tube, a tuned circuit resonant to a desired carrier frequency o'fa modulated carrier wave, a utilization circuit tuned to the same car-.

rier frequency, a second tube having input electrodes coupled to the utilization circuit, a first coupling path between said resonant circuit and said utilization circuit for providing a desired degree of selectivity, a second coupling path between said resonant circuit and said utilization circuit, said second coupling path being tuned to said carrier frequency and being so constructed and arranged that signal voltages are induced in said utilization circuit from both said coupling paths in additive phase at said carrier frequency whereby the selectivity is made relatively sharper.

ROY A. WEAGANT.

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