US2338527A - Frequency variation response network - Google Patents

Description

Jan. 4, 1944. J. E. MAYNARD FREQUENCY VARIATION RESPONSE NETWORK Original Filed Jan. 17, l94l RADIO TRANSMITTER Inventor: John E. Maynard,

by W 5.

Hi5 Attorney.

Patented Jan. 4, 1944 PATENT OFFICE FREQUENCY VARIATION RESPONSE NETWORK John E. Maynard, Schenectady, N. Y., assignor to General Electric Company, a corporation of Newv York Original application January 17, 1941, Serial No.

374,906. Divided and this application December 23,1941, Serial No. 424,153

I 1' Claims. (01. 178-44) My invention relates to frequency variation response networks, and more particularly to an improved type of network which may be'easily constructed and adjusted. v

This application is a division of my former application, Serial No. 374,906, filed January 17, 1941, for a Frequency variation response network, and assigned to the same assigneeasthe present application. A 'I v i It is frequently desirable to utilize a frequency variation response network, also known as a frequency discrimination network, to transform a In Fig. 1 an electron discharge amplifier I is illustrated as connected to impress high frequency 1 small deviation of frequency of a wave into a correspondingly large variation of voltage, the mag-. nitude'and phase of the voltage corresponding respectively to the amount and direction of frequency deviation of the wave from a predetermined frequency. Such a network/may, for example, include a piezoelectric vibrator, whose impedance changes very' rapidly with changes in frequency over a smallrange of frequency. Such networks are useful for purposes of frequency control and discrimination where the frequency.

does not deviate beyond the small range within which the piezoelectric vibrator changes impedance more or less linearly.

It is an object of my invention to provide such a network including apiezoelectric vibrator in which the rate of variation of reactance with variation of frequency, while high, is lower than the rate produced by such a vibrator alone.

It is also an object of my-invention to provide such a frequency variation response network in which the rate of variation of reactance with variation of frequency is substantially linear over a wider range of frequencies than in anetwork including only a piezoelectric vibrator.

It is another object of my invention to pro-V- vide an improved and simplifiedfrequency variation response network including a piezoelectric vibrator, which network is easily constructed and simply adjusted and is especially well adapted for use in combination with rectifying means to form a frequency control circuit. Y 0 The features of my invention which I believ to be novel are set forth with particularity in the appended claims. My invention itself, however,

both as to its organization and method of operation, together with further objects and advantages thereof may best be understood by reference to-the following description taken in connection with the accompanying drawing'in'whi'ch Fig. 1 illustrates one embodiment of my invention and Fig. 2 is a modification thereof.

waves. The high frequency wave across the tuned circuit I I is transmitted through a coupling condenser l2 to an intermediate point of a tuned circuit l3 through a pair of condensers l4 and V I5, serially connected across the tuned circuit l 3.

One terminal of the tuned circuit I3 is connected through the anode-to-cathode path of a diode rectifier Hi to ground. The other terminal of the tuned circuit I3 is connected through the anodeto-cathode pathof a second diode rectifier l1, and through a pair of load resistances l8 and I9 in series to ground. The rectifiers l6 and II are so poled that they transmit rectified current through the load resistances l8. and 19 in opposite directions.

A condenser 20 is connected in shunt to the re- I ristances l8 and I9, and a second condenser 2| is connected in shunt to the resistance l 9. These two condensers are of low impedance at the frequency to which the circuit I3 is tuned, and, therefore, by-pass current of such frequency around the respective resistances.

paratus and impresses the voltage across resistances l8 and 19 upon such apparatus. The apparatus may, for example, be a signal reproducing system or may be indicating means to indicate the frequency deviations of the Wave from the tuned circuit II from a predetermined frequency. a

The frequency variation response netwofls including the tuned circuit l3, condensers l4 and I5, and rectifiers IG'and I1 is arranged as a bridge circuit in which two adjacent arms are formed by the condensers HI .and [5, the two opposite arms are formed by the impedances presented by the rectifiers l6 and il, an a diagonal isformed by the tuned circuit l3. Since the condenser 20 offers low impedance to the wave across the tuned circuit II the cathodes of the diode rectifiers l6 and I! are effectively connected together for high frequencies to complete the bridge circuit. Without provision of means to unbalance this bridge and assuming that rectifiers l6 and II are alike and that condensers i4 and I 5 are of approximately equal capacity, the bridge would be balanced and the tuned circuit l3 could never be excited from device l0.

The inductance 22 forms means to unbalance the bridge and allow excitation of the tuned circuit Hi. When the bridge is so unbalanced, the wave from the tuned circuit II, which wave is impressed across that diagonal of the bridge ly ing between ground and the point between the condensers H and I5, excites the tuned circuit l3 which forms the other diagonal of the bridge. The tuned circuit I3 is resonant at the predetermined frequency from which frequency deviations of the wave from the tuned circuit H are to be measured.

The inductance 22 performs another function besides producing unbalance of the bridge. The capacitance of the diodes l6 and i1 is small, so that their reactance is high at the frequency of the wave from the tuned circuit ll, whereby a substantial voltage appears across each of-the diodes. The inductance 22 is so adjusted as to resonate with thecapacity of the diode l I at a frequency somewhere near the frequency of the wave from the tuned circuit II, and correspondingly the effective impedance through the diode I! is even higher than the reactance through the diode l6, whereby even higher voltages may be developed across the diode I! for more efficient rectification.

The tuned circuit I3 is reactively coupled to the tuned circuit II by reason of the unbalance of the bridge constituted by condensers ll and I5 and. diode rectifiers l6 and I1. A tuned circuit reactively coupled to a source of voltage, whose frequency is that to which the circuit is tuned, has a voltage across its terminals which is at a 90 degree phase with respect to the voltage of the source. The tuned circuit I 3 is coupled to the diode rectifiers l6 and I! in balanced or pushpull relation. When a wave excites the net work and is of the same frequency as that to which the circuit I 3 is tuned, the circuit l3 oscillates and applies a voltage across diode l6 which is 90 de grees out of phase with the voltage across the tuned circuit II, and applies a second voltage across diode II, also 90 degrees out of phase with the voltage across "the tuned circuit II, and of opposite phase to the voltage applied to diode IS.

The condenser and the diode l6 act as a capacitive voltage divider and apply across the diode It a voltage in phase with the voltage across the tuned circuit 1 I. Similarly, the condenser l5 and the diode I! act as a capacitive voltage divider and apply voltage across diode II in phase with the input voltage. Thus, when the wave from the circuit H is of the same frequency as that to which the circuit I3 is resonant, two voltages in quarter phase relationship are applied to each of the diodes IBand I].

The diode I6 is provided with a load resistance It and the diode I! is provided with a separate load resistance l8, across which each pair of these quarter phase voltage is respectively rectified. At the mid-frequency, that is, at the frequencyto which the circuit I3 is resonant, the

resultants of these two pairs of voltages across the respective diodes l6 and I! are equal, and

the rectified voltages across the load resistances resultant voltage appearing across the one diode,-

. therefore, becomes larger than the resultant voltage across the other diode. The rectified voltages across the load resistances l8 and it are consequently unequal, and a voltage of one polarity or the other, depending on the direction of deviation of the frequency from the tuned circuit II from the mid-frequency, appears on the output conductor 23.

Viewed in another way, the unbalance of the bridge effectively divides the tuned input circuit into two unequal parts, so that one of the parts is resonant above a mid-frequency of operation of the network, and the other part is resonant below that mid-frequency. The difference in characteristics of the two parts is such as to present different types 01' reactance to oscillations from the input, and hence such oscillations reach the respective diode rectifiers l6 and I! in diflerent phase.

The inductance 22 may be so adjusted, while maintaining its proper relationship with the re- -mainder of the network as described previously, that there is a linear relation over a wide range between frequency deviation of the wave from the tuned circuit H from the mid-frequency and voltage developed on the output conductor 23. The adjustment of linearity by the adjustment of the inductance 22 is associated with adjustment of the unbalance of the bridge by the inducta-nce, which is in effect an adjustment of the reactive coupling between the tuned circuits II and I3.

The frequency variation response network herein disclosed is useful in a receiver designed to detect amplitude variations of a received wave, as well as frequency variations. To this end, a coil 24 is inductively coupled to the tuned circuit 9 l3 and is connected in series with an inductance "125 and a condenser 26. The circuit 24, 25; 26

is tuned to the frequency of an amplitude modulated wave appearing across the tuned circuit II. A diode rectifier 21 is connected through load resistances 28 and 2'9 across the adjustable inductance 25. A condenser 30 is connected in shunt to the resistances 28 and 29, and a condenser 3l is connected in shunt to the resistance 28 to by-pass high frequency current therearound. A voltage appears across the resistance 28 which corresponds to the audio modulation of the wave from the tuned circuit I I. The voltage across the resistance 28 is transmitted to a suitable utilization circuit, such as an audio repro- 5 ducer, through a conductor 32.

The condensers II and I5 may, if desired, be utilized to insulate tuned-circuit l3 and the rectifiers Ill and I! from the anode voltage 01' the device III. In that case the coupling condenser I2 can be eliminated. The amount of coupling if condenser I2 is used is dependent on its size, and in any case the coupling depends on the sizes of the condensers l4 and I 5, which may be made unequal to unbalance the bridge and increase coupling. Similarly, diodes 'l6 and I 1 may be is resonant at 455 kilocycles. 'sponds very rapidly to frequency shift within made to have different impedances by means other than inductance 22, such, for example, asby using diodes of different size.

In Fig. 2 therev is illustrated an embodiment of my invention in which certain elements correspond to similar elements'in Fig. 1 and are given like reference characters. A piezo-electric,

or quartz crystal, vibrator 40 is connected in shunt to the condensers l4 and in place of the tuned circuit l3. The coupling condenser'il2 is ill omitted and-insulation from the anode voltage of the device It) is provided by the,condensers l4 and I5. "In this modification the adjustable inductance 22 provides a path for rectified current only for the'diode H. A second adjustable inductance II is connected from a point between the rectifier I6 and the condenser I4 to a point between the resistances l8 and I9 to provide a path for rectified current flowing in the diode rectifier. l6.

The adjustment of the frequency variation response network including the vibrator "is similar to the adjustment of the network including the tuned circuit I3. Unbalance of the bridge formed by the condensers l4 and I5 and the diodes l6 and I1 is affected by adjustment of one or both of the inductances 22 and 4|, The theory of operation of the device is, in general, similar to that explained in connection with the device illustrated by Fig. 1. I

It is usual practice to operate a -piezo-'electric vibrator at such a frequency thatit exhibits parallelresonance characteristics. Under such conand I9.

ditions of operation of the vibrator 40, the net- I work can be made to respond linearly to fre'- quency shift of the wave impressed thereon only over arelativelylsmall range. The range may, for example, be 20 cycles when the vibrator 40 The network rether. increasedthe reactance becomes increasingly inductive. Y

It is by properly combining such characteristics of these two devices, by placing them inshunt to each other, that I provide a desirable form of frequency variation response network. It is, of course, within the scope of my invention to utilize a piez'o-electric vibrator operated in the range whereit exhibits parallel characteristics, and to combine with it a series resonant circuit. In such vcase the overall reactance of the network varies frequency from the To adjust the network, the vibrator is first removed from its holder and the inductances 22 and II are adjusted, as explained previously in connectionwith the adjustment of inductance 22 in the diagram of Fig. 1, to produce proper discriminator action of the rietwork.,Thisdiscriminator action :is linear over a relatively broad range of frequencies, and linearity, as well as other characteristics, may be adjusted by the inductances 22 and 41, or by other circuit changes to unbalance the bridge. formed by the network. I The network is preferably adjusted to provide this limited range, and provides an output voltage highly sensitive to frequency change of the input wave.

The inductances 22 and 4| may be replaced by resistances, if desired, to provide paths for the .rectified current from the diodes l6 and 11.

It is preferred to use the inductances-in most cases and to adjust them near resonance .with the internal capacity respectively-of the diodes l6 and I1, when they form a parallel tuned circuit.

The vibrator 40 may alternatively be operated I at such a frequency that it exhibits series resonant characteristics. By operation of the vibrator M1 at series resonance, it is meant that the vibrator is operated at such a frequency (usuallynear the freqeuncy at which it operates at paral-- lel resonance) that its reactance over a certain range of frequencies varies oppositely to the reactance of a parallel tuned circuit in that range.

The reactance of a parallel tuned circuit decreases as the frequency of impressed oscillations .increases from an inductive value to zero at the frequency at which resonance occurs. As the frequency of the oscillations increases further beyond the resonant frequency of the circuit, the reactance of the circuit becomes increasingly capacitive.

A piezo-electric vibrator operating in the range such discriminator action over a range within which the output voltage is zero at the same frequency as the frequency at which the impedance of the vibrator 40 is a minimum.

When-the vibrator 40 is replaced in its holder the discriminator action is reversed. That is,

-for a frequency below midfrequency, or the frequency at which zero voltage appears on the output, the outputvoltage will beopposite in polarity to that obtained before'the vibrator. 40 was reinserted in its holder. The same effect may be observed at frequencies higher than this midfrequency.-

Since the network exhibits a characteristic relation between output voltage and frequency of the input oscillation which is the resultant of where it exhibits series resonant characteristics the action of the network without thevibrator 40 and the action of the vibrator itself, adjustment of the various circuit elements of the network may be utilized to control the over-all discriminator action of the network. For example, the width of the frequency band over which the network operates linearly, and to some extent the rapidity of change of output voltage with change in frequencyof input oscillations, may be controlled by adjustment of the discriminator action of the network separately from the vibrator 40.

If the network be properlyadjusted without the vibrator 40 inplace to producezero voltage output at the same frequency as that at which the vibrator 40 exhibits minimum, impedance, the network as a whole produces zero voltage atthis same frequency, and the frequency atwhich the zero voltage output is produced'is substantially independent of circuit changes but depends only on the characteristics of the vibrator l ojitself;

The frequency response network may be used theanode 49 is connected through a tuned circuit 50 and a source 5| of potential to ground. The control grid 52 of the device 44 is connected through a resistance 53 to the cathode 41, and through a condenser 54 to ground. The cathode 41 is coupled through a coupling condenser 55 to an intermediate point of the tuned circuit 59. So connected, the electron dischargedevice 44 maintains oscillations in the tuned circuit 50 "at the frequency. at which it is resonant.

The anode '56 of the device 43 is connected to the anode 49 of the device 44, while the cathode 51 of the device 43 is connected to ground. A condenser 58 is connected between the anode 56 and the control electrode 59 of the device 43, while a resistance 69 is connected between the control electrode 59 and the cathode 51. So connected the device 43 simulates a condenser in shunt to the tuned circuit 50, and accordingly modifies the frequency to which th circuit 50 is resonant.

The microphone 42 in series with a resistance BI is connected in shunt to the resistance 60 to change the bias potential for the device 43. As the bias potential across the resistance 60 is changed, the amount of current drawn by the device 43 changes and the value of the condenser which it simulates correspondingly changes. The oscillations produced by the device 44 therefore have a. frequency which corresponds to the intensity of the signals from the microphone 42.

The stability of the midfrequency of the wave from the antenna 46 depends on the stability of the frequency to which the tuned circuit 50 is resonant, which i in turn dependent upon the possibility of change in the components of the tuned circuit 50 and of the circuits associated with the device 43 The frequency variation response network including the vibrator 40 is useful to produce an average bias voltage across the resistance 60 of such value as to maintain the frequency of oscillation of the device 44 constant over long periods.

To accomplish this control of average frequency, the control electrode 62 of the device I0 is coupled through a coupling condenser 63 to the anode 49 of the device 44. A resistance 84 is connected between the control electrode 62' and ground. The output conductor 23 of the control network is connected serially through a resistance 55 and a filter condenser 66 to ground. The filter formed by the resistance 65 and the condenser 66 has a sufficiently long time constant so that voltage across condenser 66 does not change appreciably during changes of the frequency of the wave from device 44 caused by signals from the microphone 42. A resistance 61 is connected from the control electrode '59 of the device 43 to a point between the condenser 66 and the resistance 65.

When there is no signal from the microphone 42, and assuming that the frequency of oscillation of the device 44 is exactly at the frequency to which the vibrator 40 is resonant, the wave from the device 44 is transmitted through the device It for the control network. Since the vibrator 40 is exactly resonant to the wave, substantially no net voltage is produced across the resistances l8 and I9. No correcting bias voltage is, therefore, applied through resistances 65 and 61 to the control electrode 59 of the device 43. Now if, for some reason, the frequency of the oscillations from the device 44 drifts to ahigher or lower value, the wave from the device 44 applied to the vibrator 40 will be impressed in different values across the diodes I6 and I1, so that a net voltage appears across the resistances l8 and Hi. The polarity and magnitude of the net voltage across the two resistances depends on the direction and amount of frequency deviation of the wave from the device 44 from the mid-frequency, that is, from the frequency to which the vibrator 40 is resonant. The voltage across the resistances I 8 and I9 is applied through the resistances 65 and 51 to the control electrode 59 of the device 43, and correspondingly increases or decreases the current there through in the direction necessary to decrease or increase the frequency of oscillation of the device 44, as is necessary to maintain it constant. If signals are impressed on the device 43 from the microphone 42 the transient frequency variations of the wave from the device 44 applied through the device ill to the vibrator 40 produce transient voltage variations across the resistances l8 and i9. These transient voltage variations are filtered out by the resistance 65 and the condenser 65 and do not influence the device 43 appreciably. The present invention is, of course, not limited to the uses described, since the invention is useful for other purposes. It may be used, as illustrated, to demodulatea frequency modulated wave, or to control the mean frequency of a frequency modulated wave transmitter, as well as for providing automatic frequency control in a receiver, or for the measurement of the frequency of a. wave, or for other uses obvious to those skilled in the art.

It is within the scope of my invention to unbalance the bridge by other means than by inductances 22 and 4| or by condensers l4 and I5, as, for example, by connecting condenser l2v in Fig. 1 to an intermediate point of the inductance oftuned circuit l3 nearer to one diode than the other.

While I have shown and described a particular embodiment of-my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A network comprising a piezoelectric vibrator, a tuned circuit in shunt thereto having reactance which varies with respect to frequency oppositely to the reactance of said vibrator over the band of frequencies in which said network operates said network having portions on either side of an intermediate tap, and means for exciting said portions respectively with alternating voltages of opposite phase and different intensities.

2. A network comprising a piezoelectric vibrator, a parallel resonant circuit in shunt to said vibrator, said circuit being resonant at a frequency at which said vibrator exhibits series resonant characteristics, said resonant circuit having an electrically intermediate tap, asource of -alternating current and a circuit serially including said source and a network comprising the quencies and whose reactance varies over said range, a resonant circuit connected in shunt to said vibrator and having reactance which varies with respect to frequencyoppositely to the reactance of said vibrator over said range of frequencies, whereby the reactance characteristic of the combination of said vibrator and circuit is intermediate the separate reactance characteristicstof said vibrator and said resonant circuit,

said circuit having an intermediate tap, a source of alternating potential, means for connecting a first terminal of said source to said tap and the other terminal of said source to one terminal of said circuit for impressing potential from said source in a predetermined intensity on said vibrator and circuit, and means connecting said first terminal of said source to said tap, and said other terminal of said source to the other terminal of said circuit for impressing potential from said source on said vibrator and circuit in opposite phase and in intensity different from said predetermined intensity, whereby said vibrator and circuit are excited.

4. In combination, in a network whose reactance varies substantially linearly over a range of frequencies, a piezoelectric vibrator which exhibits substantially zero reactance at the center of a predetermined range of frequencies and whose reactance varies substantially linearly over said range, a resonant circuit connected in shunt to said .vibrator and having reactance which varies with respect to frequency substantially linearly over said range and oppositely to the reactance variation of said vibrator over said range, said resonant circuit comprising a pair of serially connected condensers in shunt to said vibrator and a pair of serially connected inductances in shunt to said vibrator and said by the reactancecharacteristic of the combination of said vibrator and resonant circuit is intermediate the separate reactance characteristics of said vibrator and resonant circuit 5. In combination, a, piezoelectric vibrator whose reactance is substantially zero at a predetermined frequency and whose reactance varies substantially linearly with respect to frequency over a narrow range of frequencies including said predetermined frequency, a resonant circuit connected in shunt to said vibrator and having substantially zero reactance at said predetermied frequency, said circuit having a reactance which varies substantially linearly with respect to frequency oppositely to the reactance of said vibrator and over a wider range of frequencies, said vibrator and circuit having an electrically intermediate tap, and means for impressing alternating potential of greater intensity and opposite phase between said tap and one terminal of said circuit than between said tap .and the other terminal of said circuit to excite said circuit and vibrator, whereby the reactance characteristic of the combination of said vibratorand circuit is intermediate the separate reactance characteristics of said vibrator and circuit and is substantially linear with'respect to frequency over a wider range than the linear I reactance variation of said vibrator.

6. In combination, a first resonant element having a reactance which is zero at a predeterminedfrequency and which varies substantially linearly with respect to frequency over a range of frequencies including said predetermined frequency, a second resonant element connected in shunt to said first element and having a reactance which is substantially zero at said predetermined frequency and which varies substan-- tially linearly with respect to frequency oppositely to the reactance of said first element and over a wider range of frequencies including said predetermined frequency, said elements having an electrically intermediate tap, and means for impressing an alternating potential of greater intensity and opposite phase between said tap and one terminal of said elements than between said tap and the other terminal of said elements 'to excite said elements, whereby the reactance characteristic of the combination of said two resonant elements is intermediate-the separate [reactance characteristics of the two elements.

7. In combination, a bridge network comprising four reactive bridge arms forming a tuned circuit, a piezoelectric vibrator connected across one diagonal of said bridge network, said. vibrator being resonant at the resonant-frequency of said tuned circuit, and a source of alternating potential connected across the other diagonal of said bridge network, said bridge being unbalanced whereby said source is effective to excite said piezoelectric vibrator.

JOHN E. MAYNARD.

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