Four frequency measuring process for coupled-dual resonator crystals

Abstract: A third case or manner of obtaining the four critical frequencies useful in accurately measuring coupled-dual resonator crystals, wherein both ports of the crystal structure are monitored, rather than a single port in the previously known cases. Here the B port, for example, is monitored with the A-side open-circuited or with a capacitor in parallel with it, and the A port is monitored with the B-side short-circuited.

Description: This application is generally related to monolithic coupled-dual resonator crystals and in particular is directed to an improved four frequency measuring process which is useful in accurately measuring three important crystal parameters such as the two resonator frequencies and the synchronous peak separation frequency at any step in the crystal fabrication process after the resonators are formed and up to and including the final test operation. A typical coupled-dual resonator crystal includes a plate of piezoelectric material having an input electrode and output electrode on one face of the plate and a common electrode on the opposite face of the plate with a first resonator formed by the input electrode and common electrode, and a second resonator formed by the output and common electrodes. Such dual resonator crystal filters are used extensively in electronic applications including IF filters for mobile and cellular radios, for example. Although such devices are mass produced using automated techniques, control of parameters such as electrode locations, thicknesses and spacings are critical, particularly where high center frequencies are required. Such manufacturing techniques require real time accurate measurements of key coupled-dual crystal filter characteristics during the fabrication process as well as at the final test operation. As indicated in U.S. Pat. No. 4,093,914 to Peppiatt and Roberts, accurate determination of key characteristics such as the resonant frequency of each of the dual resonators is desirable. Moreover, determination of "synchronous peak separation frequency" (SPSF) is indicated to be of particular significance since it provides a common reference value at a particular point in a process of manufacturing a specific crystal design such that each crystal in the group may be properly evaluated by being mathematically related to standard conditions. These key crystal parameters including synchronous peak separation frequency are calculated based upon four critical frequencies determined by either of two disclosed methods. Both of the disclosed methods involve obtaining four critical frequencies by monitoring one of the two crystal ports while either shorting or effectively open circuiting the second port. Although the processes disclosed in this patent offer the advantage of requiring the monitoring of only a single port, we have discovered an additional process for obtaining a new set of four critical frequencies which can be used in the calculation of the noted key crystal parameters where both ports must be monitored while producing similar accurate and consistent results. In brief summary, we have discovered that two of the above noted critical frequencies may be obtained by monitoring a first port of a high frequency monolithic crystal filter with the second port open circuited or with a capacitor connected in parallel therein; whereas, the second two critical frequencies may be obtained by monitoring the opposite or second port with the first port short circuited. Although the latter two frequencies obtained are basically the same as those taught in the aforementioned commonly assigned patent, the first two frequencies measured may be somewhat different than those previously obtained and usually are different unless the coupled-dual crystal is symmetrical, which is not generally the case. Nevertheless, the frequencies obtained would produce correct calculated results for the above noted key crystal parameters in accordance with contemporary measuring standards. Such contemporary measuring standards are discussed in "The four-frequency process for coupled duals using error-corrected S-parameter measurements", by G.E. Roberts, IEEE Trans. on Ultrasonics, Ferroelectrics & Frequency Control , Vol. 35, No. 3, May 1988, pp. 306-314. This document discloses a method in accordance with the preamble of claim 1. According to the present invention, there is provided a method of accurately determining selected parameters of coupled-dual resonator crystals comprising the steps of: applying a plurality of swept frequencies to the input of one of the resonators with the output of the other resonator effectively open-circuited; monitoring the output of said one resonator for zero phase output signals; and determining the frequencies (F1,F3) which cause the maximum output voltages to occur near the zero phase output; characterised by the steps of: applying a plurality of swept frequencies to the input of said other resonator with the output of said one resonator short-circuited; monitoring the output of said other resonator for zero phase output signals; determining the frequencies (F2',F4') which cause the maximum output voltages to occur near the zero phase output; and determining the selected parameters based on the frequencies so determined. Since our process can be implemented in a convenient and practical manner using the illustrated structure or known vector network analyzers while producing the noted advantages, substantial improvements in crystal measurements throughout the crystal manufacturing process are possible. BRIEF DESCRIPTION OF THE DRAWINGS These as well as other objects and advantages of this invention will be more completely appreciated by carefully studying the following detailed description of a presently preferred exemplary embodiment of this invention in conjunction with the accompanying drawings of which: Figure 1 is a perspective view of a coupled-dual resonator crystal; Figures 2A and 2B show the active and ground sides of a mounted crystal; Figure 3 is an equivalent circuit of a coupled-dual crystal; Figure 4 shows a fixture into which the crystal is inserted before the measurement of critical frequencies; Figures 5A, 5B and 6 are illustrated waveforms helpful to an understanding of the disclosed process; Figure 7 is a block diagram of a computer controlled network analyzer connected to a coupled-dual resonator crystal for the measurement of critical frequencies and the determination of crystal characteristics; and FIGURE 8 is a generalized flow chart of the steps followed by the controlled network analyzer in determining critical frequencies and crystal characteristics. DETAILED DESCRIPTION OF THE DRAWINGS In accordance with the Peppiatt and Roberts patent as well as the publication "A four-frequency process for accurately measuring coupled-dual resonator crystals" by G. E. Roberts, Proc. 33rd Ann. Symp. on Freq. Contr. , May, 1979, pp. 159-165, a typical coupled dual resonator crystal is constructed in a manner generally shown in Figures 1, 2A and 2B. In its simplest form the structure includes an input (A-side) and an output (B-side) resonator each formed by a pair of thin film electrodes (14 and 22, 16 and 24) deposited on opposite sides of a piezoelectric wafer (12). The electrodes are typically rectangular, circular or semi-circular in shape. Such crystal structure is known to have an equivalent circuit of the nature illustrated in Figure 3 and as aforementioned is also known to have key parameters such as resonator frequencies and synchronous peak separation frequency. The measurement of such key parameters is necessary for manufacturing such crystal structures in a precise manner. The Peppiatt and Roberts process is the only process known to us from which the critical frequencies can be obtained for the calculation of such key crystal parameters at any step during the fabrication process. The Peppiatt and Roberts patent discloses two manners or cases which allow the determination of four critical frequencies useful in calculation of key crystal parameters. In a first instance, as explained in the cited 1979 Roberts publication, two frequencies as illustrated in our Figure 5A are obtained from the positive non-zero zeros of the numerator polynomial of the A-side driving-point impedance with the B-side short-circuited; whereas, the other two frequencies (Figure 5B) are obtained from the two positive non-zero zeros of the numerator polynomial of the A-side driving-point impedance with the B-side open-circuited or with a capacitor in parallel with capacitor C 2 . In the second case as illustrated in our Figure 6, two frequencies are obtained from the two positive non-zero zeros of the numerator polynomial of the A-side driving-point impedance with the B-side short-circuited, and the other two frequencies are obtained from the two positive non-zero zeros of the denominator polynomial of the A-side driving-point impedance with the B-side short-circuited. As a practical matter, the frequencies may be measured using a structure of the nature illustrated in Figure 4 wherein a frequency synthesizer would be connected at J1, and the A and B probes of a vector voltmeter would be connected to J2 and J3, respectively. In the first instance illustrated in our Figure 5A the frequencies F1 and F3 are determined by sweeping the frequency synthesizer over a range of interest with the switch S1 closed. The frequencies of interest; namely, F1 and F3, will occur when the voltage amplitudes are at or near the two specific maximum amplitudes illustrated and at zero phase. Similarly, the frequencies F2 and F4, as illustrated in Figure 5B, occur when the synthesizer is swept over the frequency range of interest with switch S1 open and are determined at zero phase in the vicinity of the two specific maximum voltage amplitudes illustrated. As to the second manner of measuring the critical frequencies as disclosed in the cited patent, the four frequencies illustrated in Figure 6 may be determined with the frequency synthesizer and vector voltmeter connected as before and the synthesizer caused to sweep over the frequency range of interest with switch S1 closed. Frequencies F1 and F3 are detected at the two relative maximum voltage amplitudes; whereas, frequencies F2 and F4 occur at two specific minimum voltage amplitudes. Although the values of F2 and F4 in the first measuring instance may or may not have the same values as F2 and F4 obtained in the second instance, as noted in the Peppiatt and Roberts patent, the resulting crystal parameters will be the same. Additionally, it is to be noted that in each measuring process only a single port is monitored. That is to say, as illustrated in Figure 4, the A-side is monitored with the B-side either short circuited or effectively open circuited with C 3 placed in parallel with C 2 . Subsequent to a determination of the four frequencies, the key crystal parameters may be calculated in the manner disclosed by commonly-assigned Peppiatt and Roberts patent which is hereby incorporated by reference. As previously noted, both methods of obtaining frequencies F1 through F4 for use in calculating the key crystal parameters monitor only one of the two crystal ports such as the A-side as illustrated in Figure 4. Such processes offer the advantages of simple and easily calibrated test fixtures wherein only one port need be monitored, and the test procedure is easily automated. We have discovered, however, a third manner or case of obtaining frequencies F1 through F4 wherein, although both ports require monitoring, this new case produces similar accurate and consistent results to those in the Peppiatt and Roberts patent. In this latter method, two frequencies are obtained from the two positive non-zero zeros of the numerator polynomial of the B-side driving-point impedance with the A-side open-circuited or with a capacitor connected in parallel with C 1 . The other two frequencies are obtained from the two positive non-zero zeros of the numerator polynomial of the A-side driving-point impedance with the B-side short-circuited. The first two frequencies are obtained in the manner schematically illustrated in Figure 4 except that the crystal is inserted in reverse to that which is illustrated; namely, with the A port connected to switch S1. To distinguish these two frequencies obtained by the third case or two-port measuring process, they will hereinafter be designated as F2' and F4'. Again the frequency synthesizer is swept over the frequency range of interest, and the frequencies corresponding to F2 and F4 obtained by the one port measuring process of case 1, for example, occur when the B probe of the vector voltmeter indicates zero phase in the vicinity of the two specific maximum voltage amplitudes as illustrated in Figure 5B. Although the specific values of F2' and F4' in the absence of symmetrical crystals will differ from F2 and F4 obtained previously in the first or second test methods, such values will nevertheless produce the correct key crystal parameters. After measuring the values of F2' and F4', the crystal is removed and inserted as illustrated in Figure 4 where frequencies F1 and F3 are obtained using the first measuring technique discussed to obtain that which is illustrated in Figure 5A. Although this third manner or case of obtaining four zero phase frequencies requires the monitoring of two ports of the crystal structure, we have discovered that it can be conveniently implemented using a computer controlled vector network analyzer such as a Hewlett-Packard 3577A where twelve term vector error correction can be used in accordance with the previously cited Roberts paper to calibrate out systematic errors of the nature discussed in the publication. As indicated in the above-noted Roberts 1988 publication, such analyzers have been applied for the purpose of measuring crystal parameters and also have been applied to the first case measuring process disclosed in the commonly-assigned Peppiatt and Roberts patent. However, since such analyzers are two port measuring instruments, our third case two port measuring process is more easily and conveniently implemented and produces improved accuracy in evaluating crystal structure parameters. As indicated schematically in Figure 7, for example, a coupled-dual resonator with input/output ports A and B may be coupled to a computer controlled vector network analyzer, such as the above referenced HP-3577A, wherein the two port measuring instrument measures all four S-parameters. As indicated in the flow chart of Figure 8, the apparatus is controlled to produce error corrected S-parameters over the appropriate frequency range. The error-corrected S-parameters are converted to Y-parameters and y 11 is converted to 1/y 11 , which is, for example, the B port (as illustrated in Figure 7), short-circuit driving point impedance which yields F1 and F3 as determined by the zero phase points. However, as distinguished from the aforementioned prior art analyzer apparatus, the analyzer here is configured for determination of critical frequencies F2' and F4' from the opposite or A port (as illustrated in Figure 7) open-circuit driving point impedance through detection of the zero phase points, rather than determining all four frequencies from one port. Once frequencies F1 to F4' have been determined, key characteristics of the crystal structure may be determined. In this manner a third case or process is set forth for accurately measuring key crystal parameters at any stage of fabrication after the resonator electrodes are formed. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: A method of accurately determining selected parameters of coupled-dual resonator crystals comprising the steps of: applying a plurality of swept frequencies to the input (14,22) of one of the resonators with the output (16,24) of the other resonator effectively open-circuited; monitoring the output of said one resonator for zero phase output signals; and determining the frequencies (F1,F3) which cause the maximum output voltages to occur near the zero phase output; characterised by the steps of: applying a plurality of swept frequencies to the input of said other resonator with the output of said one resonator short-circuited; monitoring the output of said other resonator for zero phase output signals; determining the frequencies (F2',F4') which cause the maximum output voltages to occur near the zero phase output; and determining the selected parameters based on the frequencies so determined. The method of claim 1 further comprising the step of: determining key crystal characteristics including the resonant frequency of each of the resonators and the synchronous peak separation frequency from said frequencies causing the maximum output voltages; said determining steps occurring at any time in a crystal fabrication process after the formation of said resonators. The method of claim 1 further comprising the step of: determining the resonant frequency of each resonator from the frequencies causing said maximum output voltages. The method of claim 3 further comprising the step of: determining the synchronous peak separation frequency from the resonant frequencies and the frequencies causing said maximum voltage outputs.

Inventors: DANIELS JOHN UPDYKE JR, ROBERTS GERALD EWING, TOLIVER SAMUEL, DANIELS, JOHN UPDYKE, JR., ROBERTS, GERALD EWING, TOLIVER, SAMUEL
Applicants: ERICSSON GE MOBILE COMMUNICAT, ERICSSON GE MOBILE COMMUNICATIONS INC.
Application year: 1991
Main classification: G01R29
IPC codes: G01R27, G01R29