A Low-cost Digital Synthesizer
The conversion from analog to digital technologies, in such fields as wireless communications, music and video, has historically emerged first in the high end of their respective markets. Initial appearances of digital technology are usually seen in those applications requiring leading-edge performance. The same pattern has been emerging, albeit slowly, with respect to broadband microwave synthesizers based on direct digital synthesis (DDS) technology.
DDS-based synthesizers have traditionally led the industry in frequency agility, but they have also led in high recurring costs. These high costs stem from the typically complex nature of DDS-based synthesizers and have resulted in such synthesizers being utilized primarily in military applications where performance considerations have taken precedence over cost.
This cost disadvantage has, in effect, left the market for lower cost synthesizer solutions to those manufacturers utilizing analog technologies. Vendors offering YIG-based products have enjoyed a long history of dominating the market for low-cost microwave synthesizers.
The WaveCor™ Synthesized Local Oscillator (SLO) is the next step in the evolution of digital synthesizers and is designed to dramatically reduce this cost disadvantage, while still offering customers a higher performance solution. This synthesizer provides 50 MHz to 20.48 GHz of usable bandwidth in a very compact package. As with ITT’s other DDS-based synthesizers, the WaveCor SLO provides low phase noise and spurious performance combined with fast tuning.
Where the WaveCor SLO differentiates itself from other digital solutions, though, is in the value proposition it provides. The price of this synthesizer is very much cost competitive with commercially available YIG-based synthesizers.
The price and performance of the WaveCor SLO is now feasible due to the signal purity of ITT’s patent-pending low-spurious direct digital synthesis technology, which provides 2.5 GHz of useable bandwidth directly from the DDS output. This wide bandwidth has spurious free dynamic range of greater than –70 dBc and is combined with the usual DDS benefits of low phase noise, precise frequency control and maximum switching time of 10 μs.
In the past, applying prior generations of DDS technology was limited by high spurious content and narrow output bandwidths, which necessitated “cleanup” circuitry using such techniques as mix/divide to provide usable output. The WaveCor SLO eliminates the need for this complicated and expensive circuitry. With the DDS output of the WaveCor SLO combining wide bandwidth and signal purity, a radical simplification of the synthesizer’s architecture is finally possible. In fact, the high performance of the WaveCor SLO can be realized with simple multiplier blocks. The top-level block diagram is shown in Figure 1.
The 2.5 GHz bandwidth output of the DDS is split, filtered, and then sent to a series of multiplier modules for conversion and filtering. A final multiplier module is used to double the chosen path to the desired output frequency and an output amplification stage is used for power leveling.
The result is the WaveCor SLO, a 50 MHz to 20.48 GHz synthesizer, with a resolution of 1 kHz, 10 GHz phase noise of –126 dBc/Hz at a 10 kHz offset, typical spurious levels of –64 dBc and powered through a single +28 VDC supply. The WaveCor SLO measures 6" x 6" x 2.75" and is controllable through a standard BCD interface utilizing TTL signaling. The WaveCor SLO is a significant step forward in synthesizer design. All aspects of its design are intended to maximize value for the user. With the significant simplification in architecture achieved, it is now economically feasible to select a high performance digital synthesizer in many more applications. More information may be obtained from the company’s web site.
ITT Microwave Systems
RS No. 300