The invention relates to measuring equivalent circuit parameters of piezoelectric resonators by the so-called transmission method. In this method a variable frequency test signal is applied to a transmission network containing the resonator, and the resonator parameters are determined from measurements of the transmitted signal. The transmission network comprises passive components and may have different forms. For example a widely used network is called Pi network which comprises a number of resistors and places the resonator in series with the transmission path. Further, the test signal is frequently split into two channels, one a reference channel applied directly to a first input of a phase/amplitude meter, the other a test channel connected to a second input of the same phase/amplitude meter via the transmission network.
The transmission method is widely used for parameter measurements of quartz crystal resonators and is documented in many publications, some of which are listed in the attached prior art statement.
The method is implemented in various configurations. The ones known to this author are:
1. The basic system for manual measurement comprises a signal generator, a phase/amplitude detector, a frequency counter, and a transmission network containing the resonator.
2. In a modification of configuration 1, if the signal generator is D.C. controllable and the phase detector output connected to control the generator frequency f, the system can be phase-locked to the resonator frequency.
3. In a modification of configuration 2, the phase detector is a balanced mixer connected as a 90.degree. phase detector. In this case the two signal channels to the phase detector must always have a 90.degree. phase difference. This system is relatively simple but impractical if the measurement frequencies are changed often.
4. In another modification of configuration 2, if the signal generator and phase/amplitude detector are microprocessor controllable, the measurements can be automated by adding a controller and software.
5. In another modification of configuration 1, two programmed synthesizers are used, one producing a test frequency f, the other an offset frequency differing from f by IF, where IF is a constant intermediate frequency. The synthesizers are synchronized by microprocessor control. The test and reference channels are connected to the RF input of two balanced mixers. The offset frequency is applied to the local oscillator inputs of both mixers. The phase and amplitude relationship of the RF mixer inputs is retained in and measured at the IF mixer outputs. The measurement is simplified since it occurs at a constant and convenient frequency.
6. In a "Tracking Servobridge Detector", the test signal of frequency f from the RF generator is applied via a bridge circuit containing the resonator to the RF input of a first balanced mixer; the reference channel is applied to the RF input of a second balanced mixer. The "local oscillator" input of both mixers are connected to an offset frequency generator providing a frequency differing from f by IF, where IF is a constant intermediate frequency. The offset frequency is generated from the RF signal f by a single sideband generator. The phase and amplitude relationships of the RF mixer inputs is retained in the IF outputs, which are applied to the phase detector whose output is connected to control the RF generator frequency f, thereby making automatic phase lock to the resonator frequency possible.
Presently a major application of transmission methods is in the measurement of quartz crystal resonators. Desired features for this application include: accuracy of better than 1 part per million; frequency range of 1-250 MHz; fast automatic frequency lock; fast and wide-range frequency search sweep; lock to fast changing resonator frequencies, such as encountered during resonator frequency adjustment; automatic measurement; automatic parameter evaluation; minimum complexity and cost.
All the described conventional configurations lack one or more of the desired features. Configurations 1, 2 and 3 lack in regard to automatic features. Configuration 3 is inconvenient and problematic for measurements at varying frequencies. Configuration 4 is based on interconnecting various commercially available instruments, where especially the phase/amplitude meter is complex and has a relatively narrow bandwidth that precludes a sweep that is both wide and fast. Configuration 5 is also relatively complex in that it uses two synthesizers. It appears to lack a sweep that is wide, continuous and fast. Configuration 6 has been under development since 1973 and appears unlikely to be used in practice because of its complexity.
The present invention contemplates a new and improved resonator parameter measurement apparatus which overcomes the above referenced problems.