Oscillators, Crystal

The behavior of AT-quartz resonators subject to acceleration is studied. For an AT strip resonator with cantilever mounting, piezoelectric theory predicts that excitation of the pure lowest frequency flexural mode of vibration generates no charge. However, experiments show electromechanical resonance corresponding to this mode for sinusoidal support motion normal to the plane of the crystal with no applied electric field. An amplifier with a voltage follower first stage senses charge output from the crystal. Shielding is required to reduce sixty hertz and drive frequency electromagnetic interference. Measurement of resonant frequency and damping factor for the lowest frequency flexural mode is reported for a group of 17.76 MHz oscillator crystals. This mode is responsible for crystal breakage in some portable communications products. Significant variation in sensitivity among crystals was observed. Charge sensitivity of this mode is attributed in part to mechanical coupling to other modes involving torsion and lateral flexure.

This thesis investigates the characteristics of varactors and crystals that affect Q and therefore the frequency stability of a voltage controlled oscillator (vco) when employed in a one MHz to three hundred MHz frequency synthesizer. A low noise VHF crystal oscillator, a low noise common base UHF oscillator, and a low noise test set-up are described, built, and tested.

There is an increasing market demand for quartz crystals
which exhibit minimal activity dip or frequency anomalies
over a predefined temperature range. Characterization of
the frequency vs temperature (f-T) activity dip profiles of
these tight tolerance crystals is best carried out using a
system which has been expressly designed for such a purpose.
Frequency measurement accuracy and reliable temperature
control must be established in the system, along with its
being compatible with a computer controlled production or
engineering enviornment. This thesis will present design
and implementation details for a system with the
aformentioned objective of enabling the generation of
accurate crystal activity dip data. The system will provide
the user with up-to-date facilities for meeting the
temperature testing requirements of tight tolerance
crystals.

Portable, wireless devices have become ubiquitous, and ever-increasing requirements of functionality and reliability are being placed upon these devices. There is a need to accommodate the oft-conflicting requirements of small size, low power consumption, low cost, and extremely accurate control of frequency. To address these RF design requirements, a comprehensive survey of existing frequency-control methods is presented, followed by a novel design which accomplishes a high degree of frequency accuracy through the use of direct digital synthesis and a software seeking algorithm. The proposed design can be implemented pursuant to the requirements of current and future portable wireless devices.