CA1065420A - Multiple electrode crystal frequency discriminator circuit - Google Patents
Multiple electrode crystal frequency discriminator circuitInfo
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- CA1065420A CA1065420A CA247,663A CA247663A CA1065420A CA 1065420 A CA1065420 A CA 1065420A CA 247663 A CA247663 A CA 247663A CA 1065420 A CA1065420 A CA 1065420A
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03D—DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER
- H03D3/00—Demodulation of angle-, frequency- or phase- modulated oscillations
- H03D3/02—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal
- H03D3/06—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal by combining signals additively or in product demodulators
- H03D3/16—Demodulation of angle-, frequency- or phase- modulated oscillations by detecting phase difference between two signals obtained from input signal by combining signals additively or in product demodulators by means of electromechanical resonators
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- Power Engineering (AREA)
- Piezo-Electric Or Mechanical Vibrators, Or Delay Or Filter Circuits (AREA)
Abstract
IMPROVED MULTIPLE ELECTRODE CRYSTAL
FREQUENCY DISCRIMINATOR CIRCUIT
ABSTRACT
An improved multiple electrode crystal frequency dis-criminator is disclosed which is especially suited for inter-facing with integrated circuits and the attendant high capacitive and relatively low resistive terminations presented. The crystal discriminator includes a central electrode tuned to a reference frequency and serving as the input resonator. Two additional electrode pairs on either side thereof function as output res-onators tuned for resonance at slightly lower and slightly higher frequencies as compared to the reference frequency.
These output signals within the designated passband are suit-ably detected and subtractively combined to recover the desired modulation at audio frequencies. In addition, the electrode pairs of the discriminator of the present invention have selected surface areas to precisely and independently match the impedance of the terminations to which connected. Further, freedom from undesired spurious response is assured by selective adjustment of the aspect ratio of each electrode pair while maintaining the same surface areas for optimized impedance matching.
FREQUENCY DISCRIMINATOR CIRCUIT
ABSTRACT
An improved multiple electrode crystal frequency dis-criminator is disclosed which is especially suited for inter-facing with integrated circuits and the attendant high capacitive and relatively low resistive terminations presented. The crystal discriminator includes a central electrode tuned to a reference frequency and serving as the input resonator. Two additional electrode pairs on either side thereof function as output res-onators tuned for resonance at slightly lower and slightly higher frequencies as compared to the reference frequency.
These output signals within the designated passband are suit-ably detected and subtractively combined to recover the desired modulation at audio frequencies. In addition, the electrode pairs of the discriminator of the present invention have selected surface areas to precisely and independently match the impedance of the terminations to which connected. Further, freedom from undesired spurious response is assured by selective adjustment of the aspect ratio of each electrode pair while maintaining the same surface areas for optimized impedance matching.
Description
Background of the Invention The present invention relates in general to frequency discriminators and more particularly to a three-electrode crystal frequency discriminator of improved operating charac-teristics substantially free of spurious response.
Present day radio communications requirements, and par-ticularly for voice communication, demand narrow-band operation so as to maximize the number of available channels. Conven-tionally, the deviation in frequency modulation voice channels is restricted to 5 kHz or less. At high-frequency carrier . .
: ,'.' '' :: . : , ; , : .. ' ' ~ :
. ". ~' ' ~ , :
' '' '-'' "' " : -. -, -: , . . .
~-~ CM-75539 ~0654Z0 operation, say, 150 mHz or above, the demodulation of such narrow-band signals present significant difficulties, partic-ularly since the more conventional demodulator apparatus or frequency discriminators are relatively complex and broadband in operation, not to mention the additional problems in tuning and alignment customarily required.
One approach to the foregoing has been the utilization of crystal discriminator arrangements which achieve very narrow passbands, without complex circuitry, and which demodulate the received carrier signals directly, or at least the heterodyned intermediate carrier signals of relatively high frequency. The type of crystal discriminator comprises a plurality of electrodes, usually three pairs, on a single crystal blank or wafer. One pair serves as the input resonator while the remaining two func-tion as two output resonators. The input resonator is tuned to the carrier or intermediate frequency, as occasion may be, with the two output resonators tuned to slightly higher and slightly lower frequencies, respectively. By making the various electrodes sufficiently massive to achieve energy trapping significant decoupling occurs between the input and output resonators, and a relatively narrow passband results. More particularly, the energy coupled out of the input resonator to each of the output resonators form respective stagger-tuned passbands, which do not coincide with one another. The high and low band output frequencies may then be suitably detected and combined subtrac-tively to recover the desired modulation at audio frequencies as contained therein.
While such crystal discriminators work satisfactorily in certain applications, there are others where they do not.
Interfacing with integrated circuitry is but one instance of the latter category. With large capacitive terminations presented by such IC circuits, together with the substantially lower impedances involved, impedance matching is a significant problem.
Present day radio communications requirements, and par-ticularly for voice communication, demand narrow-band operation so as to maximize the number of available channels. Conven-tionally, the deviation in frequency modulation voice channels is restricted to 5 kHz or less. At high-frequency carrier . .
: ,'.' '' :: . : , ; , : .. ' ' ~ :
. ". ~' ' ~ , :
' '' '-'' "' " : -. -, -: , . . .
~-~ CM-75539 ~0654Z0 operation, say, 150 mHz or above, the demodulation of such narrow-band signals present significant difficulties, partic-ularly since the more conventional demodulator apparatus or frequency discriminators are relatively complex and broadband in operation, not to mention the additional problems in tuning and alignment customarily required.
One approach to the foregoing has been the utilization of crystal discriminator arrangements which achieve very narrow passbands, without complex circuitry, and which demodulate the received carrier signals directly, or at least the heterodyned intermediate carrier signals of relatively high frequency. The type of crystal discriminator comprises a plurality of electrodes, usually three pairs, on a single crystal blank or wafer. One pair serves as the input resonator while the remaining two func-tion as two output resonators. The input resonator is tuned to the carrier or intermediate frequency, as occasion may be, with the two output resonators tuned to slightly higher and slightly lower frequencies, respectively. By making the various electrodes sufficiently massive to achieve energy trapping significant decoupling occurs between the input and output resonators, and a relatively narrow passband results. More particularly, the energy coupled out of the input resonator to each of the output resonators form respective stagger-tuned passbands, which do not coincide with one another. The high and low band output frequencies may then be suitably detected and combined subtrac-tively to recover the desired modulation at audio frequencies as contained therein.
While such crystal discriminators work satisfactorily in certain applications, there are others where they do not.
Interfacing with integrated circuitry is but one instance of the latter category. With large capacitive terminations presented by such IC circuits, together with the substantially lower impedances involved, impedance matching is a significant problem.
- 2 - , Moreover, spurious response has a serious deleterious effect not easily compensated for and, if not, results in nonlinear operation. For miniaturized electronic apparatus, such as, say, portable, hand-held, two-way radio apparatus, space is an extremely important factor, and may in fact be critical. Tuning and the attendant additional circuit components required simply cannot be tolerated.
Summary of the Invention Accordingly, it is an object of the present invention to provide an improved multiple electrode crystal frequency dis-criminator arrangement which overcomes the foregoing deficiencies.
A more particular object of the present invention is toprovide an improved crystal discriminator of the foregoing type which is especially suited for use in integrated circuit applica-tions to efficiently and effectively accommodate the relatively large capacitive and low resistive terminations as there encoun-tered.
Still another object of the present invention is to provide an improved crystal frequency discriminator of the foregoing type wherein the respective electrodes may be dimensioned so as to effect correct impedance matching to the requirements of any terminations as well as ensuring no spurious response within the passband of interest.
Yet another object of the present invention is to provide an improved crystal frequency discriminator of the foregoing type which requires no external components, particularly induc-tors, and which exhibits reduced size and space requirements.
In practicing the invention, a crystal frequency discrim-inator is provided having a plurality of electrode pairs, normally three such pairs, on a single or monolithic block of piezoelectric material, such as a quartz wafer. The electronic pairs are plated on the quartz material and are sufficiently ' - ~
massive to achieve energy trapping and significantly reduced coupling therebetween. One pair of electrodes serves as the input resonator and is tuned to substantially the carrier or intermediate frequency, as the case may be, while the two remaining electrodes on either side thereof are tuned to a slightly higher and slightly lower frequency so as to form a narrow passband. The high and low frequencies in the passband of frequencies are intended for connection to suitable circuitry for detection and to be combined subtractively so as to recover the desired modulation at audio frequencies.
Where the crystal discriminator is to be used with integrated circuitry which customarily involve substantial capacitive, as well as relatively low resistive, terminations, impedance matching is very important. Nevertheless, impedance matching of the crystal discriminator to the terminations as presented to it can be effectively optimized by the careful dimensioning of the associated electrodes. That is, the sur-face area of each pair of electrodes is selected so as to pro-vide the desired impedance and thus correctly match the impe-dance presented by the termination to which the electrodes are connected. However, this is not to say that proper operational characteristics as a whole are ensured, and in fact, may give rise to undesirable spurs within the response curve of the dis-criminator. The present invention nevertheless contemplates further compensation in terms of selected aspect ratios. That is, the length and width dimensions of the electrode pairs may be suitably adjusted so as to eliminate or, more properly, position any resultant spurs outside its passband range, while at the same time carefully maintaining the overall surface area constant so as to retain the desired impedance match optimiza-tion.
,P~
1~654ZO
More particularly, there is provided:
An improved three-terminal crystal frequency dis-criminator, comprising in combination:
a monolithic wafer body of piezoelectric material;
first electrode means deposited on said wafer to form input resonator means and being of a mass for responding to a modulated input signal of a given reference frequency; and second and third electrode means similarly deposited on said wafer on respective sides of said first electrode means and comprising first and second output resonator means, said second and third electrode means having respective masses for responding to respectively higher and lower frequencies than said given reference frequency and thereby forming with said input resonator means respective passbands of predetermined bandwidths, said first electrode means having a surface area to provide an impedance to substantially match the terminal impedance to which it is adapted to be connected and said second and third electrode means having respective surface areas to provide impedances to substantially match the terminal impedance to which they are adapted to be connected, said first, second and third electrode means having respective aspect ratios which are adjusted to ensure the absence of spurs in said discriminator passband while maintain-ing the surface area constant so as to maintain optimized impedance matching.
Brief Description of the Drawings The features of the present invention which are be- :~
lieved to be novel are set forth with particularity in the appended -~a-I
CM-75539 lO 6 5 420 claims. The invention itself, however, together with further objects and advantages thereof, may be best understood by reference to the following description when taken in conjunction with the accompanying drawings, in which like referenced numerals refer to like elements in the several figures, and in which:
Fig. 1 is a schematic representation of a circuit arrange-ment incorporating a multi-electrode crystal discriminator which has been constructed in accordance with the present invention;
Fig. 2 is a further schematic representation of such dis-criminator interconnected to terminations of circuit valuesin the range that may be anticipated in typical integrated cir-cuit applications;
Fig. 3 is a graphic representation of the crystal discrim-inator constructed in accordance with the present invention ` having three electrode pairs of varying surface area on a mono-lithic quartz wafer;
Fig. 4 is a schematic representation of the equivalent circuit of the quartz crystal discriminator of Fig. 3; and Fig. 5 illustrates the bandpass characteristics of crystal discriminators generally;
Fig. 6 illustrates the response of a monolithic discrim-inator of the type illustrated in the various Figures which includes undesired spurious response in its passband;
Fig. 7 illustrates the preferred response of the mono-lithic discriminator in which the undesired spurs in the pass-band have been removed or at least repositioned.
Detailed Description of the Preferred Embodiment Referring now to the drawings, a frequency discriminator circuit arrangement 10 is shown in Fig. 1 which includes a crystal discriminator 12 constructed in accordance with the present invention. The crystal discriminator 12 includes three pairs of metalized or conductive electrodes, 12a, 12b and 12c, plated or otherwise deposited on a single or monolithic block 12d of piezoelectric material, such as quartz. As indicated, a source 14 of radio frequency or intermediate frequency signal information applies frequency modulated signals to the pair of electrodes 12a deposited on opposing faces of the quartz wafer 12d. Electrodes 12a, together with portions of the quartz crystal body 12d, serve as the input resonator portion for dis-criminator 12. As will be appreciated, the quartz crystal body 12d couples the energy applied to electrodes 12a forming the input resonator to two output resonators formed by electrode pairs 12b and 12c acting in concert with other portions of the quartz crystal body 12d.
As will be more apparent from response characteristic represented in Fig. 5, the masses of the various electrodes 12a, 12b and 12c are such that the input resonator formed by elec-trodes 12a is tuned to a frequency Fs corresponding to the fre-quency of the received carrier or some heterodyned intermediate frequency constituting the source 14. The masses of the other electrode pairs 12b and 12c are such that the output resonator formed by electrode pair 12b is tuned to a frequency -Fl somewhat lower than that of Fs, while the masses of electrode pair 12c is such to tune the formed output resonator to a fre-quency Fh somewhat higher in frequency than that of the source Fs. Accordingly, the energy coupled out of the input resonator 12a to output resonator 12b forms one stagger-tuned passband while the energy coupled from the input resonator 12a to the other output resonator 12c forms still another stagger-tuned passband, not coinciding with the first.
The output signals Fl and Fh from output resonators 12b and 12c appear across loading resistors 16a and 16b connected thereacross and are coupled into a suitable detector arrange-ment, such as that indicated generally at 18. Detector 18 is preferably in integrated circuit form and additionally includes amplification circuitry, as indicated. Signal information at the frequencies Fl and Fh are detected and subtractively combined in the detector arrangement 18 whereby the desired modulation at audio frequencies is recovered and coupled to the output terminal 18a. The detector and demodulation process effected by detector arrangement 18 is entirely conventional in opera-tion such that additional and further operational description should not be required. A more detailed analysis, where desired, can be obtained from any standard reference, such as, for example, Terman's Radio Engineers Handbook, pages 578-588.
As indicated in Fig. 2, the crystal discriminator 12 is intended to function with relatively high capacitive but low resistive terminations that are customarily encountered in integrated circuit applications. The source 14 and detector and amplifier 18 have been replaced with capacitive and resistive termination components of values that would be expected in such instances. The source 14 may be represented by resistor 14a and capacitor 14b, while the detector 18 at its two inputs may be represented by resistors 18b and 18d and capacitors 18c and 18e, respectively.
In operation, source 14 supplies energy to the electrodes 12a forming the input resonator at or near the thickness sheer mode or twist mode fundamental frequency of the quartz crystal body 12b. Accordingly, crystal 12b is preferably provided in an AT-cut configuration. The extent the piezoelectrically induced vibrations in the quartz body 12d between electrodes 12a couples through to the other associated electrodes 12b and 12c depends upon the masses of the electrodes themselves and the distances between the respective resonators that they form.
Moreover, the masses of the various electrodes must be of a magnitude to create or achieve energy trapping. This mass loading of the electrodes makes the amplitude of vibration in the quartz body 12d between the resonators drop off exponentially with distance. Hence, the edges of body 12d have little if any effect on overall operation.
As mentioned previously, the resonators form three con-secutive values for effecting two subtracting passbands. The fractional or percentage lowering of the resonant frequency from the fundamental thickness sheer or twist mode of the blank wafer by means of mass loading is referred to as "plateback".
The combination of mass loading of electrodes to tune them to the appropriate frequencies and create the conditions for reducing the coupling, together with the spacing of the resonators to match degree of mass loading, essentially determines the passbands between the input resonator and each of the output resonators.
This forms response as depicted by the S-curve shown in solid line in Fig. 5.
The equivalent circuit for the discriminator 12 shown in Figs. 1 and 3 is illustrated in schematic form in Fig. 4. Each ~-of the electrode pairs 12a,12b and 12c form a series resonant circuit comprised of an inductance, a capacitance and a resistance, the latter of course determining the Q of the circuit. An addi-tional pair of inductances 12f and 12g represent the coupling between the respective electrodes. The same terminal impedances are shown as in the circuit arrangement of Fig. 2.
With the foregoing structural considerations being observed, the crystal frequency discriminator 12 will in fact provide discriminator action. However, there is no assurance that it will operate effectively and efficiently. This is particularly so with respect to the relatively high capacitive and low resis-tive terminals that it may expect to be interfaced with, as indicated in the circuit arrangement of Fig. 2. For optimized operation, the crystal discriminator 12 should substantially match the impedance of the terminations to which its respective input and two output terminals are connected. Moreover, there CM-75539 ~065420 should be an absence of significant spurious response within its passband range.
This is effectively accomplished in the present invention by the selective dimensioning of the parameters for the electrode pairs 12a, 12b and 12c. It has been found that the impedance the discriminator presents at its input and output terminals can be effectively controlled by the surface area of the respec-tive electrode pairs as deposited on the quartz crystal body 12d, and as shown more clearly in Fig. 3. In the embodiment as therein depicted, the surface area of the center electrode pair 12a is greater than electrodes 12b and 12c so as to accommodate a somewhat lower impedance value than electrode pairs 12b and 12c working into a slightly higher impedance termination. The capacitance values 14b, 18c and 18e represent the stray capa-citance formed by the metal electrode pairs as well as that of the interfacing integrated circuits (not shown) to which the discriminator is connected. In any event, it will be appre-ciated that optimized performance will occur wherever correct impedance matching is effected and that the passband of the discriminator 12 approaches its lowest available minimum at any ; frequency where the impedance of the discriminator matches the terminating impedance.
Adjusting the surface area of the respective electrode pairs to optimize impedance matching however, still does not ensure fully optimized performance. This is because the adjust-ment to the electrode surface area, together with the spacing dimensions and the like, may well result in undesired spurious response within the discriminator's passband. This is depicted in Fig. 6 showing the conventional passband S-curve, but where spurs are present at its high frequency end. As will be appre-ciated, the desired passband response is illustrated by the response characteristic as shown in Fig. 7.
The present invention contemplates the elimination of any undesired spurious response from the passband by the selective arrangement of the aspect ratio for each of the electrode pairs.
The aspect ratio refers to the length and width dimensions of the surface area of the electrode pairs. By changing the width dimension with respect to its length, or vice versa, any spurs that may be present will be shifted in position within the fre-quency spectrum to one degree or another. Accordingly, the aspect ratio is selectively adjusted until there are no such undesired spurs within the passband of the discriminator to pro-vide the response characteristic as set forth in Fig. 7. This adjustment of course is effected without changing the overall surface area of the electrode pairs so as to maintain the other-wise optimum impedance matching previously obtained.
Accordingly, for the impedance termination as presented in the circuit arrangement of Fig. 2, a crystal discriminator as shown in Fig. 3 which includes the following structural dimensions for the respective electrode pairs has been found to provide satisfactory performance:
20 Electrode Surface Area Aspect Ratio Pair tin sq. mils) (in mils) 12a 8400 width 70; length 120 12b 4000 width 40; length 100 12c 4000 width 40; length 100 The distance between electrode pairs was a nominal 20 mils.
Accordingly, an improved crystal discriminator arrangement has been set forth and described herein which requires no external inductors or other circuit components and as a con-sequence is especially suited for integrated circuit applica-tions where large capacitive and low resistive terminations may be expected. Effective impedance matching is quickly and readily accomplished free of any undesired spurious response in the discrimlnator passband. A linear action is maintained at I
all times. The discriminator requires no tunable components and is smaller in size as compared to the more conventional prior art structures.
While a particular embodiment of the present invention has been shown and described, it will of course be obvious to those s~illed in the art that various changes and modifications may be made without departing from the invention in its broader aspects. Aceordingly, the appended claims are intended to cover all sueh modifieations and alternative constructions that may fall within the true scope and spirit of the invention.
Summary of the Invention Accordingly, it is an object of the present invention to provide an improved multiple electrode crystal frequency dis-criminator arrangement which overcomes the foregoing deficiencies.
A more particular object of the present invention is toprovide an improved crystal discriminator of the foregoing type which is especially suited for use in integrated circuit applica-tions to efficiently and effectively accommodate the relatively large capacitive and low resistive terminations as there encoun-tered.
Still another object of the present invention is to provide an improved crystal frequency discriminator of the foregoing type wherein the respective electrodes may be dimensioned so as to effect correct impedance matching to the requirements of any terminations as well as ensuring no spurious response within the passband of interest.
Yet another object of the present invention is to provide an improved crystal frequency discriminator of the foregoing type which requires no external components, particularly induc-tors, and which exhibits reduced size and space requirements.
In practicing the invention, a crystal frequency discrim-inator is provided having a plurality of electrode pairs, normally three such pairs, on a single or monolithic block of piezoelectric material, such as a quartz wafer. The electronic pairs are plated on the quartz material and are sufficiently ' - ~
massive to achieve energy trapping and significantly reduced coupling therebetween. One pair of electrodes serves as the input resonator and is tuned to substantially the carrier or intermediate frequency, as the case may be, while the two remaining electrodes on either side thereof are tuned to a slightly higher and slightly lower frequency so as to form a narrow passband. The high and low frequencies in the passband of frequencies are intended for connection to suitable circuitry for detection and to be combined subtractively so as to recover the desired modulation at audio frequencies.
Where the crystal discriminator is to be used with integrated circuitry which customarily involve substantial capacitive, as well as relatively low resistive, terminations, impedance matching is very important. Nevertheless, impedance matching of the crystal discriminator to the terminations as presented to it can be effectively optimized by the careful dimensioning of the associated electrodes. That is, the sur-face area of each pair of electrodes is selected so as to pro-vide the desired impedance and thus correctly match the impe-dance presented by the termination to which the electrodes are connected. However, this is not to say that proper operational characteristics as a whole are ensured, and in fact, may give rise to undesirable spurs within the response curve of the dis-criminator. The present invention nevertheless contemplates further compensation in terms of selected aspect ratios. That is, the length and width dimensions of the electrode pairs may be suitably adjusted so as to eliminate or, more properly, position any resultant spurs outside its passband range, while at the same time carefully maintaining the overall surface area constant so as to retain the desired impedance match optimiza-tion.
,P~
1~654ZO
More particularly, there is provided:
An improved three-terminal crystal frequency dis-criminator, comprising in combination:
a monolithic wafer body of piezoelectric material;
first electrode means deposited on said wafer to form input resonator means and being of a mass for responding to a modulated input signal of a given reference frequency; and second and third electrode means similarly deposited on said wafer on respective sides of said first electrode means and comprising first and second output resonator means, said second and third electrode means having respective masses for responding to respectively higher and lower frequencies than said given reference frequency and thereby forming with said input resonator means respective passbands of predetermined bandwidths, said first electrode means having a surface area to provide an impedance to substantially match the terminal impedance to which it is adapted to be connected and said second and third electrode means having respective surface areas to provide impedances to substantially match the terminal impedance to which they are adapted to be connected, said first, second and third electrode means having respective aspect ratios which are adjusted to ensure the absence of spurs in said discriminator passband while maintain-ing the surface area constant so as to maintain optimized impedance matching.
Brief Description of the Drawings The features of the present invention which are be- :~
lieved to be novel are set forth with particularity in the appended -~a-I
CM-75539 lO 6 5 420 claims. The invention itself, however, together with further objects and advantages thereof, may be best understood by reference to the following description when taken in conjunction with the accompanying drawings, in which like referenced numerals refer to like elements in the several figures, and in which:
Fig. 1 is a schematic representation of a circuit arrange-ment incorporating a multi-electrode crystal discriminator which has been constructed in accordance with the present invention;
Fig. 2 is a further schematic representation of such dis-criminator interconnected to terminations of circuit valuesin the range that may be anticipated in typical integrated cir-cuit applications;
Fig. 3 is a graphic representation of the crystal discrim-inator constructed in accordance with the present invention ` having three electrode pairs of varying surface area on a mono-lithic quartz wafer;
Fig. 4 is a schematic representation of the equivalent circuit of the quartz crystal discriminator of Fig. 3; and Fig. 5 illustrates the bandpass characteristics of crystal discriminators generally;
Fig. 6 illustrates the response of a monolithic discrim-inator of the type illustrated in the various Figures which includes undesired spurious response in its passband;
Fig. 7 illustrates the preferred response of the mono-lithic discriminator in which the undesired spurs in the pass-band have been removed or at least repositioned.
Detailed Description of the Preferred Embodiment Referring now to the drawings, a frequency discriminator circuit arrangement 10 is shown in Fig. 1 which includes a crystal discriminator 12 constructed in accordance with the present invention. The crystal discriminator 12 includes three pairs of metalized or conductive electrodes, 12a, 12b and 12c, plated or otherwise deposited on a single or monolithic block 12d of piezoelectric material, such as quartz. As indicated, a source 14 of radio frequency or intermediate frequency signal information applies frequency modulated signals to the pair of electrodes 12a deposited on opposing faces of the quartz wafer 12d. Electrodes 12a, together with portions of the quartz crystal body 12d, serve as the input resonator portion for dis-criminator 12. As will be appreciated, the quartz crystal body 12d couples the energy applied to electrodes 12a forming the input resonator to two output resonators formed by electrode pairs 12b and 12c acting in concert with other portions of the quartz crystal body 12d.
As will be more apparent from response characteristic represented in Fig. 5, the masses of the various electrodes 12a, 12b and 12c are such that the input resonator formed by elec-trodes 12a is tuned to a frequency Fs corresponding to the fre-quency of the received carrier or some heterodyned intermediate frequency constituting the source 14. The masses of the other electrode pairs 12b and 12c are such that the output resonator formed by electrode pair 12b is tuned to a frequency -Fl somewhat lower than that of Fs, while the masses of electrode pair 12c is such to tune the formed output resonator to a fre-quency Fh somewhat higher in frequency than that of the source Fs. Accordingly, the energy coupled out of the input resonator 12a to output resonator 12b forms one stagger-tuned passband while the energy coupled from the input resonator 12a to the other output resonator 12c forms still another stagger-tuned passband, not coinciding with the first.
The output signals Fl and Fh from output resonators 12b and 12c appear across loading resistors 16a and 16b connected thereacross and are coupled into a suitable detector arrange-ment, such as that indicated generally at 18. Detector 18 is preferably in integrated circuit form and additionally includes amplification circuitry, as indicated. Signal information at the frequencies Fl and Fh are detected and subtractively combined in the detector arrangement 18 whereby the desired modulation at audio frequencies is recovered and coupled to the output terminal 18a. The detector and demodulation process effected by detector arrangement 18 is entirely conventional in opera-tion such that additional and further operational description should not be required. A more detailed analysis, where desired, can be obtained from any standard reference, such as, for example, Terman's Radio Engineers Handbook, pages 578-588.
As indicated in Fig. 2, the crystal discriminator 12 is intended to function with relatively high capacitive but low resistive terminations that are customarily encountered in integrated circuit applications. The source 14 and detector and amplifier 18 have been replaced with capacitive and resistive termination components of values that would be expected in such instances. The source 14 may be represented by resistor 14a and capacitor 14b, while the detector 18 at its two inputs may be represented by resistors 18b and 18d and capacitors 18c and 18e, respectively.
In operation, source 14 supplies energy to the electrodes 12a forming the input resonator at or near the thickness sheer mode or twist mode fundamental frequency of the quartz crystal body 12b. Accordingly, crystal 12b is preferably provided in an AT-cut configuration. The extent the piezoelectrically induced vibrations in the quartz body 12d between electrodes 12a couples through to the other associated electrodes 12b and 12c depends upon the masses of the electrodes themselves and the distances between the respective resonators that they form.
Moreover, the masses of the various electrodes must be of a magnitude to create or achieve energy trapping. This mass loading of the electrodes makes the amplitude of vibration in the quartz body 12d between the resonators drop off exponentially with distance. Hence, the edges of body 12d have little if any effect on overall operation.
As mentioned previously, the resonators form three con-secutive values for effecting two subtracting passbands. The fractional or percentage lowering of the resonant frequency from the fundamental thickness sheer or twist mode of the blank wafer by means of mass loading is referred to as "plateback".
The combination of mass loading of electrodes to tune them to the appropriate frequencies and create the conditions for reducing the coupling, together with the spacing of the resonators to match degree of mass loading, essentially determines the passbands between the input resonator and each of the output resonators.
This forms response as depicted by the S-curve shown in solid line in Fig. 5.
The equivalent circuit for the discriminator 12 shown in Figs. 1 and 3 is illustrated in schematic form in Fig. 4. Each ~-of the electrode pairs 12a,12b and 12c form a series resonant circuit comprised of an inductance, a capacitance and a resistance, the latter of course determining the Q of the circuit. An addi-tional pair of inductances 12f and 12g represent the coupling between the respective electrodes. The same terminal impedances are shown as in the circuit arrangement of Fig. 2.
With the foregoing structural considerations being observed, the crystal frequency discriminator 12 will in fact provide discriminator action. However, there is no assurance that it will operate effectively and efficiently. This is particularly so with respect to the relatively high capacitive and low resis-tive terminals that it may expect to be interfaced with, as indicated in the circuit arrangement of Fig. 2. For optimized operation, the crystal discriminator 12 should substantially match the impedance of the terminations to which its respective input and two output terminals are connected. Moreover, there CM-75539 ~065420 should be an absence of significant spurious response within its passband range.
This is effectively accomplished in the present invention by the selective dimensioning of the parameters for the electrode pairs 12a, 12b and 12c. It has been found that the impedance the discriminator presents at its input and output terminals can be effectively controlled by the surface area of the respec-tive electrode pairs as deposited on the quartz crystal body 12d, and as shown more clearly in Fig. 3. In the embodiment as therein depicted, the surface area of the center electrode pair 12a is greater than electrodes 12b and 12c so as to accommodate a somewhat lower impedance value than electrode pairs 12b and 12c working into a slightly higher impedance termination. The capacitance values 14b, 18c and 18e represent the stray capa-citance formed by the metal electrode pairs as well as that of the interfacing integrated circuits (not shown) to which the discriminator is connected. In any event, it will be appre-ciated that optimized performance will occur wherever correct impedance matching is effected and that the passband of the discriminator 12 approaches its lowest available minimum at any ; frequency where the impedance of the discriminator matches the terminating impedance.
Adjusting the surface area of the respective electrode pairs to optimize impedance matching however, still does not ensure fully optimized performance. This is because the adjust-ment to the electrode surface area, together with the spacing dimensions and the like, may well result in undesired spurious response within the discriminator's passband. This is depicted in Fig. 6 showing the conventional passband S-curve, but where spurs are present at its high frequency end. As will be appre-ciated, the desired passband response is illustrated by the response characteristic as shown in Fig. 7.
The present invention contemplates the elimination of any undesired spurious response from the passband by the selective arrangement of the aspect ratio for each of the electrode pairs.
The aspect ratio refers to the length and width dimensions of the surface area of the electrode pairs. By changing the width dimension with respect to its length, or vice versa, any spurs that may be present will be shifted in position within the fre-quency spectrum to one degree or another. Accordingly, the aspect ratio is selectively adjusted until there are no such undesired spurs within the passband of the discriminator to pro-vide the response characteristic as set forth in Fig. 7. This adjustment of course is effected without changing the overall surface area of the electrode pairs so as to maintain the other-wise optimum impedance matching previously obtained.
Accordingly, for the impedance termination as presented in the circuit arrangement of Fig. 2, a crystal discriminator as shown in Fig. 3 which includes the following structural dimensions for the respective electrode pairs has been found to provide satisfactory performance:
20 Electrode Surface Area Aspect Ratio Pair tin sq. mils) (in mils) 12a 8400 width 70; length 120 12b 4000 width 40; length 100 12c 4000 width 40; length 100 The distance between electrode pairs was a nominal 20 mils.
Accordingly, an improved crystal discriminator arrangement has been set forth and described herein which requires no external inductors or other circuit components and as a con-sequence is especially suited for integrated circuit applica-tions where large capacitive and low resistive terminations may be expected. Effective impedance matching is quickly and readily accomplished free of any undesired spurious response in the discrimlnator passband. A linear action is maintained at I
all times. The discriminator requires no tunable components and is smaller in size as compared to the more conventional prior art structures.
While a particular embodiment of the present invention has been shown and described, it will of course be obvious to those s~illed in the art that various changes and modifications may be made without departing from the invention in its broader aspects. Aceordingly, the appended claims are intended to cover all sueh modifieations and alternative constructions that may fall within the true scope and spirit of the invention.
Claims (9)
1. An improved three-terminal crystal frequency discrim-inator, comprising in combination:
a monolithic wafer body of piezoelectric material;
first electrode means deposited on said wafer to form input resonator means and being of a mass for responding to a modulated input signal of a given reference frequency; and second and third electrode means similarly deposited on said wafer on respective sides of said first electrode means and comprising first and second output resonator means, said second and third electrode means having respective masses for responding to respectively higher and lower frequencies than said given reference frequency and thereby forming with said input resonator means respective passbands of predetermined bandwidths, said first electrode means having a surface area to provide an impedance to substantially match the terminal impedance to which it is adapted to be connected and said second and third electrode means having respective surface areas to provide impedances to substantially match the terminal impedance to which they are adapted to be connected, said first, second and third electrode means having respective aspect ratios which are adjusted to ensure the absence of spurs in said discriminator passband while maintain-ing the surface area constant so as to maintain optimized impedance matching.
a monolithic wafer body of piezoelectric material;
first electrode means deposited on said wafer to form input resonator means and being of a mass for responding to a modulated input signal of a given reference frequency; and second and third electrode means similarly deposited on said wafer on respective sides of said first electrode means and comprising first and second output resonator means, said second and third electrode means having respective masses for responding to respectively higher and lower frequencies than said given reference frequency and thereby forming with said input resonator means respective passbands of predetermined bandwidths, said first electrode means having a surface area to provide an impedance to substantially match the terminal impedance to which it is adapted to be connected and said second and third electrode means having respective surface areas to provide impedances to substantially match the terminal impedance to which they are adapted to be connected, said first, second and third electrode means having respective aspect ratios which are adjusted to ensure the absence of spurs in said discriminator passband while maintain-ing the surface area constant so as to maintain optimized impedance matching.
2. An improved crystal frequency discriminator in accord-ance with claim 1 wherein said piezoelectric wafer is formed of quartz crystal in an AT-cut configuration.
3. An improved crystal frequency discriminator in accord-ance with claim 1 wherein said wafer body is in a circular con-figuration with a peripheral edge extending thereabout and wherein said electrode means of each of said resonator means are symmetrically spaced with respect to one another and from said peripheral edge.
4. An improved crystal frequency discriminator in accord-ance with claim 1 wherein said electrode means each include two electrodes opposing one another on opposing faces of said wafer body.
5. An improved crystal frequency discriminator in accord-ance with claim 1 wherein said masses of said electrodes as deposited on said wafer body determine the frequency to which said resonator means are tuned.
6. An improved crystal frequency discriminator in accord-ance with claim 4 wherein one electrode of each electrode pair on a common face of said wafer body are electrically intercon-nected with one another.
7. An improved crystal frequency discriminator in accord-ance with claim 1 wherein the electrodes forming said input resonator means have a surface area of approximately 8400 square mils and electrodes forming said output resonator means each have a surface area of approximately 4000 square mils.
8. An improved crystal frequency discriminator in accord-ance with claim 7 wherein said first electrode means has an aspect ratio of approximately 70 mils wide and 120 mils long with said second and third electrodes each having an aspect ratio of approximately 40 mils wide and 100 mils long.
9. An improved crystal frequency discriminator in accord-ance with claim 8 wherein the distance between said electrode means is approximately 20 mils.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US56532575A | 1975-04-07 | 1975-04-07 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1065420A true CA1065420A (en) | 1979-10-30 |
Family
ID=24258119
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA247,663A Expired CA1065420A (en) | 1975-04-07 | 1976-03-11 | Multiple electrode crystal frequency discriminator circuit |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS51123050A (en) |
| CA (1) | CA1065420A (en) |
| DE (1) | DE2614671A1 (en) |
| GB (1) | GB1535222A (en) |
-
1976
- 1976-03-09 GB GB938676A patent/GB1535222A/en not_active Expired
- 1976-03-11 CA CA247,663A patent/CA1065420A/en not_active Expired
- 1976-04-05 DE DE19762614671 patent/DE2614671A1/en not_active Withdrawn
- 1976-04-06 JP JP3788176A patent/JPS51123050A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| GB1535222A (en) | 1978-12-13 |
| JPS51123050A (en) | 1976-10-27 |
| DE2614671A1 (en) | 1976-10-21 |
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