What is in this article?:
- Generating Reliable RF/Microwave Signals
- Comparison Criteria
A variety of different oscillator technologies are employed in RF/microwave systems.
Oscillators are key components in RF/microwave systems, signal sources that must remain stable across a wide range of operating conditions. They come in many shapes and sizes, with either fixed or variable frequencies, in many different package types (even as circuit modules), and from a large number of suppliers. As with choosing any high-frequency component for a design, selecting an RF/microwave oscillator comes down to matching the lowest cost possible while achieving the required performance levels for an application. Knowing which performance parameters are the most critical for that application is equally critical.
There is no “ideal” oscillator for high-frequency use, since the many types of oscillators used for RF/microwave applications offer a variety of functions and performance levels. Perhaps the most common type of oscillator is the fixed-frequency reference or clock oscillator, typically used to stabilize the phase of a higher-frequency tunable-frequency oscillator. Clock oscillators come in many configurations, including those with some form of compensation to correct for the frequency-shifting effects of temperature changes.
Examples of these are temperature-compensated crystal oscillators (TCXOs) and oven-controlled crystal oscillators (OCXOs). Such oscillators can almost universally be found in RF/microwave test equipment, such as signal generators and spectrum analyzers—often as 10-MHz reference sources.
These reference oscillators are noted for their frequency stability. They are characterized in terms of their phase-noise performance, typically at different offset frequencies from a carrier (such as 1 kHz, 10 kHz, and 1 MHz). This phase/frequency stability can serve as a reference for a higher-frequency oscillator, essentially locking its phase characteristics to those of the lower-frequency reference source.
In comparing different crystal oscillators, key performance specifications include frequency stability; aging rate or amount of frequency change with time (which may be given per day, per year, or even per 10 years); operating temperature range; available frequency tuning range; warmup time; power consumption; phase noise; spurious; and harmonic levels. Additional critical performance parameters include frequency pulling (the change in frequency as a result of the load impedance) and frequency pushing (the change in frequency due to variations in the oscillator’s power supply).
At higher frequencies, the number of oscillator choices increases dramatically. They range from oscillators based on traditional voltage-controlled-oscillator (VCO) architectures to more exotic resonator structures, including surface-acoustic-wave (SAW) and yttrium-iron-garnet (YIG) resonators. All of these tunable RF/microwave oscillators exhibit some form of tradeoff in performance. Typically, slower-tuning oscillators, such as YIG oscillators, achieve somewhat better phase-noise characteristics than faster-tuning oscillators, such as VCOs.
In terms of packaging, many of these higher-frequency RF/microwave oscillators have been designed in recent years into compact packages, such as TO-8 housings (along with even smaller options), without sacrificing mechanical-related or temperature-based frequency/phase stability.
Dielectric-resonator oscillators (DROs), for example, are commonly used in communications systems throughout the microwave frequency range (3 through 30 GHz). They are based on a puck of ceramic material with high dielectric constant and high circuit quality factor (Q). The resonant frequency for a particular oscillator design is determined by the physical dimensions of the puck and the dielectric constant of the puck’s material.
The resonant properties of DROs have often been compared to those of circular waveguide at microwave frequencies, with one exception: The electromagnetic (EM) fields within a circular waveguide are contained within that cavity, while the EM fields produced within a dielectric puck continue beyond the puck. This requires a DRO to be enclosed within a conducting cavity for most applications. The presence of the DRO’s housing, or boundary conditions, must be considered when determining the frequency of an oscillator for given mechanical and electrical conditions.