Building a Microwave Frequency Synthesizer — Part 2 : Component Selection

A s discussed in the previous article, a frequency synthesizer can be thought of as a black box containing being properly connected, translate an input reference signal to a number of output frequencies. The synthesizer implementation as well as its ultimate performance depends heavily on characteristics of the individual components used in the design. Although there is no set definition for the term " components " (they can be actually complex conector-ized modules), in this article we will mostly refer them as surface-mount parts, which can be placed on a printed circuit board. The characteristics and behavior of the main synthesizer parts are reviewed from the perspective of their use in practical synthesizer designs. A reference oscillator is one of the most important parts that defines stability and phase noise characteristics of frequency synthesizer. Various reference oscillator schemes are possible as shown in Figure 17. A 10 MHz temperature-compensated crystal oscillator (TCXO) provides low size and cost benefits for low-to moderate-performance applications. Better stability and noise characteristics are achieved by using an oven-compensated crystal oscillator (OCXO), but this is a more expensive and bulky part with a higher power consumption. It is worth mentioning, that using a higher frequency OCXO (e.g., 100 MHz instead of 10 MHz) can potentially result in a better synthesizer noise. There is a comparable noise floor for both parts, but the high frequency reference requires a significantly lower overall multiplication factor. Even better phase noise performance at higher frequency offsets (100 kHz and above) can be obtained with additional low-noise oscillators (e.g., SAW, CRO, or DRO) locked to the main OCXO. The chain of oscillators (which can include two or even more parts) provides the lowest phase noise profile at any frequency offset and can be used in high-end synthesizer designs. The high-frequency oscil-lators are usually purchased parts, although, they can be built as a part of the synthesizer design to minimize the overall size and cost. However, building a low-noise microwave oscillator (e.g. a DRO) with adequate phase noise performance is not a trivial task; it requires careful design and optimization. The phase noise behavior of a microwave oscillator (shown conceptually in Figure 18)