Frequency synthesizers have improved dramatically in recent years. Ten years ago, direct digital synthesis (DDS) was in its infancy; now it is widespread as a primary or secondary technology in many signal sources. Over that time, designers have learned more than a few ways to curb the spurious levels that once plagued the technology. Current DDS designs can routinely achieve spurious levels as good as −60 and −70 dBc without sacrificing the frequency agility that makes DDS technology so attractive.
The technology was often considered frivolous by "serious" synthesizer designs, since the bit resolution of early digital-to-analog converters (DACs) essentially dictated the spurious performance of the end product. A decade earlier, 8-b DACs were the standard at higher clock speeds, essentially limiting high-speed DDS sources to a spurious floor of about 6 dB per bit, or about −48 dBc. Compare that to what is being done with modern DDS designs. As a recent example (see Microwaves & RF, April 2003, p. 94), the ADV-3000S DDS from Advanced Radio Corp. (Reston, VA) tunes from 20 MHz to 3 GHz with 1-Hz resolution. Although somewhat slower than some DDS sources, with better than 5-µs frequency switching speed, the VME module achieves worst-case spurious performance of −60 dBc.
Many other synthesizer suppliers, including Elcom Technologies (Rockleigh, NJ) and Synergy Microwave (Paterson, NJ), have succeeded in blending DDS technology with clever circuit design to overcome traditional spurious limitations. One of the more innovative recent applications of DDS technology was by FEI Communications (Mitchel Field, NY) as a "correction circuit" for oven-controlled crystal oscillators (OCXOs). By employing DDS technology, the company's FE205A, FE-405A, and FE-505A OCXOs can approach the frequency stability of more expensive rubidium atomic clocks.
Of course, any discussion on DDS technology would be incomplete without mention of Sciteq Electronics. The company's synthesizer product lines, which were acquired by Osicom (itself renamed to Sorrento Networks), now can be found as part of the RF production lines of optical-communications-equipment supplier Meret Optical Communications (San Diego, CA). Although small, Sciteq achieved a great many landmarks in DDS development, constantly pushing clock frequencies and introducing the first GaAs-based DDS source.
DDS technology appears to have a promising future in this industry, although advances in more traditional synthesizer technologies, including PLL, fractional-N, and direct-analog technologies, are far from exhausted. In the end, synthesizer specifiers reap the benefits of this push for the ultimate RF synthesizer.