US2266658A

US2266658A - Electrical frequency-selective system

Info

Publication number: US2266658A
Application number: US232194A
Authority: US
Inventors: Robinson James
Original assignee: Individual
Current assignee: Individual
Priority date: 1937-10-06
Filing date: 1938-09-28
Publication date: 1941-12-16
Anticipated expiration: 1958-12-16
Also published as: GB515640A; GB509040A; GB509235A; FR844373A; GB515739A

Description

Dec. 16, 1941. .1.v ROBINSON 2,266,658

ELECTRICAL FREQUENCY-SELECTIVE SYSTEM Filed Sept. 28, 1938 2 Sheets-Sheet 1 I f. Z1 Z0 In 511 1? 53 1 jTj Dec. 16,1941. J. ROBINSON 2,266,653

ELECTRICAL FREQUENCY-SELECTIVE SYSTEM Filed Sept. 28, 1958 2 Sheets-Sheet 2 I v a 1 Invg uter- MMYMM;

Patented Dec. 16, 1941 ELECTRICAL FREQUENCY-SELECTIV SYSTEM l James Robinson,- London, England Application September 28, 1938, Serial No. 232,194 In Great Britain October 6, 1937 (or. 17s-44) 16 Claims.

This invention relates to electrical frequencyselective systems and is primarily though not essentially concerned with such systems employing mechanically resonant selective devices or for example piezo-electric devices.

It is one of the objects of the invention to provide a circuit affording a generally uniform response over a given band of frequencies and abruptly reduced or negligible response to frequencies adjacent such band, the frequency response curve being thus characterized by a generally flat top with unusually sharp cut-off at each side. Various modifications of the circuit are contemplated affording variations of the response at different frequencies as hereinafter more fully described.

According to a feature of the present invention there is provided an electrical frequencyselective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of these arms which selective devices have different resonant frequencies, means combining the outputs of the two arms in the opposed sense and an additional conductive arm which is substantially nonresonant in parallel with the others. By means of this system there is provided a filter having a band-pass characteristic with a sharp cut-off at the limits of the band determined by the resonant frequencies of the piezo-electric devices.

Further objects and features of the invention will be apparent from the following description, taken in connection with the accompanying drawings in whichi Figure 1 is a diagram of a selective system according to the invention;

Figure 2 shows curves representing the performance of the system illustrated in Figure 1;

Figure 3 shows a modification of Figure 1 employing retroaction;

Figure 4 shows another modification of Figure 1 employing a selectivedevice as an absorber;

Figure 5 shows a modification of Figure 1 employing highly selective devices coupled in series;

Figure 6 shows a curve representing the operation of the system shown in Figure 5;

Figure 7 shows a further modification of the system; 7 V

Figure 8 shows another selective system according to the invention;

Figure 9 is a curve representing the operation of the system shown in Figure 7;

Figure 10 is a curve showing the operation of the system shown in Figure 8;

, II coupled thereto.

I Figure 11 shows another modification of the system shown in Figure 8; and

Figure 12 shows a modification of the system shown in Figure 11.

Like references indicate like parts in the several figures of the drawings.

Referring to Figure 1, signals are applied across inductance ill from an input inductance The inductance I0 is preferably tuned by a variable condenser 12 and the ends of the inductance are connected respectively to one terminal of each of two piezo-electric devices l3 and M. The other terminals of these piezo-electric devices are connected together at I! and to one side of a tuned circuit I5, the other side of the tuned circuit being connected to a centre tapping I8 on the inductance It). With this arrangement, the inherent capacities of the piezo-electric devices l3, M are normally equal so as to beelectrically balanced. In parallel with the conductive arms of the circuit constituted by the piezo-electric devices aforesaid there is the third conductive arm consisting of a variable condenser l6 connected in parallel with the piezo-electric device l4 and thus unbalancing the system.

The piezo-electric devices are selected to have resonant frequencies corresponding to the limit of the band of frequencies which the system is required to pass and thus the resonant frequencies of the two piezo-electric devices may, for example, be 5 kilocycles apart. Since these piezoelectric devices have their outputs combined in opposed phase the total response for frequencies lying between the band is augmented, while the response for frequencies outside the band is diminished, because for frequencies within the band the outputs from the two piezo-electric devices will be substantially in phase, Whereas for frequencies above and below the band the outputs Will be opposed in phase and therefore reduced.

Referring to Figure 2, the piezo-electric device l4 has a resonant frequency at f1 and the piezoelectric device l3 has a lower resonant frequency at 1. Since their outputs are in opposed phase the total response for frequencies lying within the band defined by their resonant frequencies is augmented, while the response for other frequencies is diminished, so that the overall characteristic for the system if the two piezo-electric devices l3 and M are balanced is shown by the dotted line 23 in Figure 2. The system is unbalanced by the condenser l6 providing an alternative path for applied signals. This alterna- 6 tive path being capacitative the phase of signals passed thereby is advanced through 90 with respect to the signals passed by the piezo-electric devices at resonance so that at some higher frequency than the frequency f1, the response of the system will have a minimum or zero value, for instance, at f2 and at some frequency lower than the frequency f the response of the system will have a zero or minimum value as indicated at f3. At the same time, the signals passed by the condenser |6 at frequencies between the resonant frequencies of the piezo-electric devices will be in substantial phase agreement with the signals of such frequencies passed by both these piezoelectric devices so that the response of the system for frequencies between the resonant frequencies of the piezo-electric devices is raised. The characteristic of the system may thus have the form indicated by the broken line 24in Figure 3.

By increasing the capacity of the condenser l6 further to unbalance the system, the frequencies at which a minimum or zero response is obtained may be brought closer as indicated at f4 and f5 and the response for frequencies between the resonant frequencies of the piezo-electric devices may be made still more uniform so that the system has a response of the form indicated for example by the full line curve 25 in Figure 2.

It will be seen that for frequencies more remote from the required band than those at which zero or minimum values occur, some response may be obtained, especially in the case where the system is considerably unbalanced by the condenser I6 as shown by the curve 25. In order to overcome any disadvantage arising from this cause the circuit l 0, l2 may be broadly tuned to the mean frequency of the required band so as to raise the response for frequencies within the required band and lower the response for other frequencies. The tuned circuit I5 is also tuned to an intermediate frequency of the required band in order further to assist in obtaining a uniform response throughout the required band and a minimum response outside the band.

Another manner of levelling the response for frequencies within the required band consists in the appropriate selection of the relative impedances of the coupling of the system with associated apparatus. This is exemplified in Figure '1 by the connection of the output line to a tapping l9 on'the inductance of the tuned circuit. The coupling is 50 selected as to favour the transfer of signal frequencies within the required band to associated apparatus.

A further method for correcting the shape of the response curve is shown in Figure 3. In this arrangement the filter of Figure 1 is provided with retroactive means for the purpose of raising the response of the system for frequencies within the required band and also lowering the response for frequencies outside the band. In Figure 3 the terminal I? is connected to a control electrode of a thermionic valve 2'! and the centre tapping IE on the inductance I6 is connected to the cathode of this valve, a leak resistance 28 being provided between the control electrode and the cathode. The anode circuit of the valve 2! includes a coil 29 retroactively coupled with the inductance l0 and phase adjusting means consisting of a variable resistance 30 and variable condenser 3!. The retroactive coupling is thus adjustable as regards phase to ensure the proper phase relationship for building up the response within the required band and this being the case it necessarily follows that the retroactive effect will be in the opposed sense for frequencies lying to either side of the band.

A still further method for correcting the shape of the response curve of the filter of Figure 1 is shown in Figure 4 in which two additional piezo-

electric devices

33 and 34 are connected between the junction point I! and the centre tapping on the inductance III, that is to say, across the output of the system. These

piezoelectric devices

33 and 34 have substantially the same resonant frequencies as the piezo-electric devices l3 and I4 and function as absorbers in the system compared with the piezo-electric devices l3 and [4 operating as acceptors. A variable resistance 35 is preferably connected in series with each of the piezo-

electric devices

33 and 34.

By means of the piezo-electric devices 33 and 34'the two peaks in the output of the system due to the pieZo-electric devices l3 and I4 are reduced and may be controlled to have any desired value with respect to theresponse of the system for frequencies within the band by adjustment of the resistances 35. The piezo-

electric devices

33 and 34 operating as absorbers reduce the response of the system at the resonant frequencies of the pie'zo-electric devices [3 and I4 and immediately adjacent frequencies, but have substantially no effect on other frequencies more widely removed, with the result that the final output of the system more closely approaches a uniform response for frequencies within the band without effecting the sharpness of the cut-off at the limits of the band.

It will be appreciated that the impedance matching method described with reference to Figure 1, the retroaction method of Figure 3 and the absorption method of Figure 4 provide alternative methods of adjusting the characteristic of the filter to any desired shape. They are particularly useful when it is desired to limit the action of the condenser IS in order to prevent a too great rise in the transmission outside the desired pass band. In some cases it' may be desirable to use two or more of these methods simultaneously.

It will also be appreciated that although the above described arrangements all relate to band pass filters, the same configurations may be used to obtain band absorption filters. Thus if the piezo-electric device l4 across which the condenser IB is connected has the lower frequency, the response above f1 and below will be increased while the response between f1 and I will be decreased. The methods of Figures 1, 3, and 4 may be similarly applied to obtain any desired shape of characteristic.

Figure 5 shows a filter network in which two of the filters of Figure 1 are connected in series to provide an improved response curve. The output of the fist filter which comprises inductance l0, condenser l2, two piezo-electric devices l3 and I4, and-the condenser H3 in shunt with the device I4 is developed across the inductance 4.4 which is coupled with inductance 45. This inductance 45 together with condenser 52, piezo-electric devices 46 and 41 and the condenser 49 comprise the second filter, the output from which is developed across the inductance 5E].

The piezo-electric device 46 has substantially the same resonant frequency as the piezo-e1ec tric device l3, and the resonant frequencies of the piezo-elec'tric devices l4 and 41 are also substantially the same. By adjustment of the condenser 49 the second filter comprising the devices.

piezo-electric device 46 and 41 is unbalanced to a different degree from that of the first filter comprising the piezo-electric devices [3 and M. For example the first filter may have a response as represented by curve of Figure 2 while the second filter may have a response as represented by curve 25 of Figure 2. In this manner the overall response of the system of Figure will be of the form indicated by the curve 5| in Figure 6, in which there are minimum or zero values is and is at one side of the required band and at f2 and ii at the other side, thus further decreasing the response for frequencies outside the required band. The inductance 45 is tuned by a condenser 52 to the mean frequency of the required band and the response of the system for frequencies within the band may be made still more uniform by employing any of the measures for this purpose hereinbefore described.

Instead of employing piezo-electric devices 46 and 41 having substantially the same resonant frequencies as the piezo-electric devices l3 and I4 respectively, they may have different frequencies, for example, such that one piezo-electric device of each series pair i3, 45 and i4, 41 has a resonant frequency corresponding to the minimum or zero value of the response of the other.

In another form of frequency selective system according to this invention comprising two conductive arms, as aforesaid, a plurality of selective devices is employed in each arm, these selective devices having different resonant frequencies and being arranged alternately in the two arms as regards the order of their resonant frequencies.

In Figure 7, there is employed an inductance ID to which signals are applied by means of the input coil H coupled thereto. One end of the inductance ID is connected to terminals of two piezo-

electric devices

54 and 56 and the other end of the inductance i0 is connected to terminals of two other piezo-

electric devices

55 and 51 connected in parallel. The other terminals of these piezo-electric devices are connected together at 58 and the output of the system is developed across an impedance 59 connected between the junction point 58 and a centre tapping 60 on the inductance Ill. The piezo-electric devices 54 to 51 have resonant frequencies of progressively increasing value so that the piezoelectric device 54 has the lowest frequency and the piezo-electric device 51 has the highest frequency. Additional piezo-electric devices may be employed if desired, and arranged in the same manner as regards their resonant frequencies so as to be distributed over the required band. A third'non-resonant conductive arm consisting of a condenser Si in parallel with the piezo-

electric devices

54 and 55 may be employed if desired, in which case the signals passing the condenser will be advanced in phase with respect to signals passing each of the piezoelectric devices in resonance therewith.

It will be understood that due to the resonant characteristics of the piezo-electric devices there will be a change of phase of the applied signals passed by these devices which is of the same sign through the range of frequencies between the resonant frequencies of the piezo-electric crystals which are adjacent as regards frequency, and that the change of phase will be of opposite sign for frequencies at the other sides of the resonant frequencies of these piezo-electric Thus, in employing four selective devices, of which the resonant frequencies are indicated at f1, f2, is and f4 in Figure 9, the change of phase for frequencies between ]2 and 1: will be of opposite sign to the change of phase for frequencies between f1 and f2 and frequencies between f3 and it. The signals passed by the condenser Bl, which is connected in parallel with the piezo-

electric devices

54 and 55, will be opposed in phase with regard to signals at frequencies between f1 and f2 and between f3 and f4. Thus, by appropriate adjustment of the condenser 6! a zero response is obtained at a frequency between the frequencies f1 and f2 and at a frequency between the frequencies is and It. By increasing the capacity of the condenser iii a zero response will be obtained at two different frequencies between f1 and f2 and between is and f4 as shown in Figure 9. Thus, there is obtained a sharply defined band-pass characteristic as indicated in Figure 9 with substantial zero response for a range of frequencies immediately each side of the band. The peaks at the frequencies f1 and f4 may be reduced by employing a broadly tuned circuit tuned to the mean frequency of the band defined by the frequencies f2 and is, for example by tuning the inductance ill by means of the parallel condenser 12, by employing a tuned circuit as the impedance 59, or by means of absorbers as described with reference to Figure 4. It will be appreciated that if a pluralityof piezo-electric devices are provided in each parallel arm, this arrangement will enable a plurality of alternating band pass and band absorption channels to be obtained.

The system shown in Figure '7 may be modified to give a single band pass characteristic by connecting the condenser 6| in parallel with the piezo-

electric devices

55 and 51 instead of the piezo-electric devices 54 and 53, as shown in Figure 8. In this case a minimum response is obtained at a frequency lower than the frequency f1 and higher than the frequency ii. The response for frequencies between T1 and f2 and between is and f4 are raised in this case and although the response for frequencies between fz and is is reduced, by selecting the piezo-electric devices to have resonant frequencies f2 and is comparatively close together, the total response may be substantially uniform throughout the range ii to f4. This is illustrated in Figure 10 of the drawing in which the response of the circuit of Figure8 with condenser 5! omitted is shown in full lines, the effect of shunting the condenser Bl about the piezo-

electric devices

55 and 51 being indicated by the dotted line curve.

In Figure 11 there is shown a modification of the system shown in Figure 7 in which the

piezoelectric devices

54 and 51 of lowest and highest frequencies respectively are replaced by series tuned circuits Hi and H. In this case the final characteristic of the system will have the same general form as that shown in Figure 9 except that the peaks at frequencies f1 and f4 will be broader due to the use of the tuned circuits.

It will be appreciated that instead of employing the two piezo-electric devices to define the band, tuned circuits may be tuned for this purpose and the piezo-electric devices may have resonant frequencies, one higher and the other lower than the required band.

In another modification of the system shown in Figure 11, the piezo-electric device 55 may have the same order of frequency as the tuned circuit 10 and similarly the piezo-electric device 56 may have the same order of frequency as the tuned circuit H, in which case a bandpass characteristic is also obtained. A similar modification may be made in the system shown in Figure 7.

Another modification of Figure 11 is shown in Figure 12 in which the piezo-electric devices are joined at 80 and this junction point is connected to the input electrode of a valve 82. The tuned circuits and H are joined at an independent junction point 8! which is connected to the input electrode of a valve 83. The output is derived from the anodes of the valves connected together, the cathodes being connected to the tapping 60 on the inductance H). In this case the condenser 6! providing the additional conductive path of the system is connected in parallel with the piezo-electric device 55 and therefore the tuning of the circuits I0 and H is independent of the capacity of this condenser.

It is convenient in some circumstances to employ a signal response which is higher for the two outer selective devices than for the inner ones and this result may be obtained by discriminate amplification or, for example, in the case of piezo-electric devices, by supplementing these piezo-electric devices with other piezoelectric crystals of the same order of frequency in parallel therewith.

It will be understood that the piezo-electric devices hereinbefore referred to may each consist of a single crystal or a plurality of crystals of substantially the same resonant frequncy connected in series or parallel.

Furthermore the use of one or more broadly tuned circuits tuned to an intermediate frequency of the required band, as described for example with reference to Figure l, or retroaction as described with reference to Figure 3, or absorbers as described with reference to Figure 4, may be applied to any other of the selective systems described herein. Alternatively or in addition one or more absorption circuits may be in the system and tuned to a frequency or frequencies beyond the limits of the required band in order to still further reduce the total response of system for frequencies outside the required band.

Whereas a tapped coil H) has been shown as a convenient means of feeding applied frequencies along two conductive arms for their ultimate combination in opposition, any other means for this purpose may be employed.

It will also be understood that any of the circuits hereinbefore described may be reversed as to their input and output in operation.

I claim:

1. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of said arms, said selective devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and means increasing the output of one of said arms for signals having a frequency lower than the frequency of the associated selective device, and reducing the output of such arm for signals having a frequency higher than the frequency of the associated selective device, said means comprising, in parallel with such arm, an additional conductive arm which is substantially non-resonant.

2. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of said arms, said selective devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and means for altering the relative amplitude of the combined output for signals having frequencies falling within and without the said band, said means comprising an additional conductive arm which is substantially non-resonant connected in parallel with one of said first named arms.

3. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of said arms, said selective devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and means increasing the combined output of said arms for signals having frequencies falling with in the said band, said means comprising an additional conductive arm which is substantially nonresonant connected in parallel with one of said first named arms.

4. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of said arms, said selective devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and a non-resonant conductive device connected in parallel with that one of said selective devices which has th higher resonant frequency for increasing the combined output of said arms for signals having frequencies falling within the said band.

5. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of said arms, said selective devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and capacitative means in parallel with one of said devices for unbalancing the capacitative value of the two arms to alter the relative amplitude of the output for signal frequencies falling within and without the said band.

6. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of said arms, said selective devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, means increasing the output of one of said arms for signals having a frequency lower than the frequency of the associated selective device, and reducing the output of such arms for signals having a frequency higher than the frequency of the associated selective device, said means comprising, in parallel with such arm, an additional conductive arm which is substantially non-resonant, and at least 'one broadly tuned circuit which is tuned to a frequency intermediate the resonant frequency of said selective devices, said last named circuit being associated with said conductive arms and arranged in advance thereof in said system.

7. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device in each of said arms, said selective devices having difierent resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, a third conductive arm which is substantially nonresonant in parallel with one of said first named arms, and phase changing means in said third conductive arm to change the phase of frequencies passing said third conductive arm into opposition with the frequencies passing said selective device and falling outside of the said band of frequencies.

8. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a piezoelectric device in each of said arms, said devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and means increasing the output of one of said arms for signals having a frequency lower than the frequency of the associated piezo-electric device and reducing the output of such arm for signals having a frequency higher than the frequency of the associated piezo-electric device, said means comprising, in parallel with such arm, an additional conductive arm which is substantially nonresonant,

9. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a piezoelectric device in each of said arms, said piezoelectric devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and capaoitative means in parallel with one of said devices for unbalancing the capaoitative value of the two arms to alter the relative amplitude of the output for signal frequencies fallin within and without the said band.

10. An electrical frequency-selective system comprising two conductive arms constituting parallel paths for applied frequencies, a piezoelectric device in each of said arms, said piezoelectric devices having different resonant frequencies within a band of frequencies, means combining the outputs of the two arms in phase opposition, and a non-resonant conductive device connected in parallel with that one of said piezoelectric devices which has the higher resonant frequency for increasing the combined output of said arms for signals having frequencies falling within the said band.

11. 'An electrical frequency selective system comprising two conductive arms constituting parallel paths for applied frequencies, a plurality of piezo-electric devices in each of said arms, said piezo-electric devices having different resonant frequencies and being arranged in the two arms alternately as regards the order of their resonant frequencies, means for combining the outputs of the two arms in opposed phase, and nonresonant phase changing means connected in parallel with one of said conductive arms to increase the output of the system in bands of frequencies defined by alternate pairs of the said piezo-electric devices of adjacent frequency and to reduce the output of the system between the said bands such that a plurality of spaced band pass channels are provided.

12. An electrical frequency selective system comprising two conductive arms constituting parallel paths for applied frequencies, a plurality of piezo-electric devices in each of said arms, said piezo-electric devices having different resonant frequencies and being arranged in the two arms alternately as regards the order of their resonant frequencies, means for combining the outputs of the two arms in opposed phase, and non-resonant phase changing means connected in parallel with one of the said arms to reduce the output of the system for frequencies above the frequency of the piezo-electric device having the highest resonant frequency and below the frequency of the device having the lowest resonant frequency.

13. A frequency selective system comprising two conductive arms constituting parallel paths for applied frequencies, a plurality of selective devices in each of said arms which selective devices have different resonant frequencies and are arranged in the said two arms alternately as regards the order of their resonant frequencies, the selective devices having the highest and the lowest resonant frequency being constituted by piezo-electric crystals, the other of said devices being constituted by series tuned circuits, means for combining the outputs of the two arms in opposed phase, and a non-resonant phase changing device connected in parallel with one of said conductive arms.

14. A frequency selective system comprising two conductive arms constituting parallel paths for applied frequencies, two selective devices consisting of a piezo-electric device and a series tuned circuit in each of said arms, said selective devices having different resonant frequencies and being arranged in the said two arms alternately as regards the order of their resonant frequencies, the devices of highest and lowest frequency being constituted by the piezo-electric devices, means for combining the outputs of the two arms in opposed phase and a condenser, shunting that one of the said arms including the device of highest frequency, of such magnitude as to provide a substantially uniform response over a band of frequencies defined by the resonant frequencies of the piezo-electric devices.

15. A band pass frequency selective system comprising two conductive arms constituting parallel paths for applied frequencies, a selective device consisting of a piezo-electric device and a series tuned circuit having the same resonant frequency in each of said arms, the said selective devices having different resonant frequencies within the pass band, means for combining the outputs of the two arms in opposed phase and a condenser connected in shunt with one of said arms to increase the response of the system for frequencies within the pass band.

16. A frequency selective system comprising two conductive arms constituting parallel paths for applied frequencies, a piezo-electric device in each of said arms, said piezo-electric devices having different resonant frequencies corresponding to the limits of a band of frequencies to be selected, means for combining the outputs of the two arms in opposed phase, non-resonant phase changing means connected in shunt with the piezo-electric device of the higher resonant frequency, and a parallel circuit tuned to a frequency intermediate the limits of the said band of frequencies and connected in shunt with the output of the selective system.

JAMES ROBINSON.