Tracking Oscillator Trends

While oscillator technology makes gains from year to year, the progress is often deliberate—and typically, motivated by the needs of different markets. One recent trend in oscillator design (as noted in this month’s Special Report) is that oscillators are getting smaller and lighter, whether they are fixed crystal oscillators or tunable voltage-controlled oscillators (VCOs). Yet, even as crystal oscillators squeeze into surface-mount packages that are only 5 x 7 mm, they must still deliver high output levels and avoid phase noise.

Oscillator circuits have been refined over the years, with designers taking full advantage of the analysis capabilities of different electromagnetic (EM) software simulation tools. But in terms of an oscillator’s output power and phase noise, the choice of active device within the oscillator has a great deal of influence on those two performance parameters. For many years, higher-frequency oscillator designers, such as builders of VCOs, YIG-tuned oscillators, or dielectric resonator oscillators (DROs), wrestled with the choice between the lower phase noise of silicon bipolar transistors and the higher-frequency operation of GaAs field-effect transistors (FETs).

In recent years, however, device designers have continued to enhance such technologies as GaAs heterojunction bipolar transistors (HBTs) and silicon-germanium (SiGe) BiCMOS transistors, reaching higher frequencies while benefitting from the low-phase-noise characteristics of these device technologies. A number of organizations have sought cost-effective oscillator designs capable of low-phase-noise performance at millimeter-wave frequencies. And they have looked to the promise of SiGe active device technology as a means of achieving such high-frequency operation.

Such advanced transistor technologies allow fundamental-frequency operation well past 100 GHz, depending upon device dimensions, with acceptably low phase noise. To make full use of newer transistors in EM and circuit simulation software, however, computer models of those transistors are necessary, and these are constructed only through laborious scattering-parameter (S-parameter) measurements. Accurate models allow designers to “experiment” in software with different circuit configurations, to better understand the interaction of a resonant inductive-capacitive (LC) circuit with the active circuit represented by the high-frequency transistor. Low phase noise is an often elusive design goal. But for a growing number of millimeter-wave radar or communications systems applications, lower oscillator phase noise is always better given the large number of digital modulation signal formats that are in use. Such modulation formats rely on maintaining the phase integrity of in-phase (I) and quadrature (Q) signal components, which is easier done with a low-phase noise oscillator at any frequency.

In the end, while oscillator designers are to be commended for their progress in increasing frequencies and decreasing phase noise over the years, just as much credit is due to the active device designers.

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