Frequency generation usually comes at a cost: a certain amount of phase noise. Even for the crystal oscillators typically used as time and frequency references in RF/microwave systems, some phase noise must be accepted and overcome as part of the system design. However, as part of the work that won the 2016 International Microwave Symposium (IMS) Student Design Competition in the category focused on a 100-MHz crystal oscillator, Anisha Apte and Matthias Rudolph of Germany’s Bradenburg University explored the various noise limitations of quartz-crystal-resonator-based signal sources. They designed a circuit to optimize the phase-noise performance of a 100-MHz source.
The winners were aided by several co-authors knowledgeable in the design of oscillators, and frequency synthesizers, including Ulrich Rohde (chairman of Synergy Microwave Corp.) and Ajay Poddar (Synergy’s chief scientist). The student researchers compared the design and performance of 100-MHz crystal oscillators based on different thicknesses of AT-cut quartz crystal material. They then developed several equivalent-circuit diagrams for the crystal oscillators based on various well-accepted circuit models, such as the Van Dyke model. By modeling variations of Colpitts oscillator circuits, the researchers accounted for fundamental and harmonic operating modes and mode-coupling effects.
Using commercial computer simulation software, they predicted the phase noise for a 100-MHz fifth-overtone crystal oscillator circuit with small size and excellent spectral purity. They were able to enhance the dynamic loaded quality factor (Q) and minimize phase noise through the use of dynamic phase-injection and mode-coupling techniques. Their design approach optimizes the noise factor, startup characteristics, and resonator frequency-drive sensitivity for voltage-controlled crystal oscillators (VCXOs) and was put to the test by fabricating a prototype 100-MHz VCXO.
To verify the design approach, measurements were made on the 100-MHz oscillator using a test system capable of a -193 dBm/Hz noise floor. Initial measurements revealed phase noise of −141 dBc/Hz offset 100 Hz from the carrier. Following optimization using phase-injection, mode-coupling, and noise-filtering techniques, the measured phase noise improved to −144 dBc/Hz offset 100 Hz from the carrier, compared to computer simulations of −142 dBc/Hz at the same offset frequency. As the researchers noted, these phase-injection and mode-locking techniques can be applied to other variations of oscillator circuits.
See “Optimizing Phase-Noise Performance,” IEEE Microwave Magazine, Vol. 18, No. 4, June 2017, p. 108.
