This synthesizer module delivers DDS speed, accuracy, and modulation capability in bandwidths as wide as 25 percent of frequencies from 1 to 40 GHz.

JASON SEIFERT
Vice President of Engineering Spinnaker Microwave, Inc.,
3281 Kifer Rd., Santa Clara, CA 95051;
(408) 732-9828,
Internet: www.spinnakermicrowave.com.

Frequency synthesizers operating at microwave and millimeterwave frequencies that combine excellent noise performance with extremely fast tuning speed and accuracy typically require two things of their owners: deep pockets and a lot of space. However, these performance parameters are precisely those required of many electronic warfare, electronic countermeasures, and fast-hopping communications systems, and microwave test equipment. The SMSDU Series of synthesizers from Spinnaker Microwave (www.spinnakermicrowave.com) satisfies these requirements by exploiting the desirable attributes of direct digital synthesis (DDS), negating their detrimental characteristics, and boosting their output frequencies to as high as 40 GHz in bandwidths as wide as 25 percent, in an enclosure down to the size of a paperback book.

The target audience for the SMS-DU Series synthesizers is designers of frequency-agile radars and communications systems, electronicwarfare (EW) and electronic-countermeasures (ECM) systems and simulators, digital RF memories (DRFMs) for electronic-warfare (EW) systems, and test equipment with fast switching speeds. Designs in this category are typically the domain of prime contractors who for the lack of a suitable alternative have developed their own solutions. The SMS-DU Series provides a "buy" answer to the "buy or build" scenario and adds features such as programming simplicity to make the choice an easy one in many cases.

The unique attributes of DDS have increased its used in high-performance systems over the years as the DDS devices themselves have become more proficient and less expensive. DDS employs digital signal processing to produce a frequency- and phase-adjustable output signal from a fixed input signal generated by a precise clock reference. The DDS approach essentially carves up the clock frequency according to a programmable binary tuning word of 24 to 48 b and produces replicas of the signal. DDS can deliver exceptional tuning resolution of less than 1 Hz and phase resolution of less than 1 deg. Since it is implemented digitally, DDS allows both frequency and phase adjustments to be made easily with high accuracy and at extremely high speed without the errors associated with analog approaches caused by temperature variations or component aging, as well as overshoot and undershoot of the desired values.

A DDS can be combined with additional signal processing to enable frequency-agile clock generators, frequency- agile or phase-agile modulators for FSK, PSK, QPSK, QAM, OFDM, and frequency-swept (chirp) modulators. However, according to Nyquist, the maximum output frequency of a DDS is about 40 percent of its reference frequency. Since the output is digitally generated by using sampling techniques, the benefits of DDS come at the cost of additional spurious components. The output frequency of typical DDS implementations is currently limited to about 1.5 GHz, but spurious signal rejection drops precipitously at the higher end of this range. Instrument-grade arbitrary waveform generators can currently achieve performance similar to a DDS and at much higher sample frequencies, but the result is expensive and quite large in comparison.

The approach taken by Spinnaker in the SMS-DU brings the benefits of the DDShigh speed and resolution, precise control of phase, modulation, and FM chirp capabilityto user-specified bandwidths between 1 and 40 GHz. For some variations, the SMS-DU can be manufactured in an enclosure measuring only 4 x 4.5 x 1 in. (Fig. 1) and employ various techniques to control spurious levels. While the phase-noise levels are not as low as "pure" DDS implementations, the phase-noise performance can exceed instrument-grade arbitrary waveform generators or indirect analog synthesizers at these frequencies costing an order of magnitude more and 10 to 20 times larger.

The SMS-DU operates by integrating a DDS with frequency converters, fixed local oscillators (LOs) and synthesizers, and switched filter banks to achieve the characteristics specified by the customer. The company's design engineers have created a wide array of custom SMS-DU models over the years, and their accumulated knowledge allows them to choose the best DDS, converter, frequency plan, and signal generation components for specific situations.

A typical SMS-DU (Fig. 2) employs the DDS and clock generator, a lownoise integer synthesizer, and multistage upconverters. For outputs through S-band (4 GHz), a single upconversion stage is used while higher frequencies require multiple stages. The programmability of the DDS and internal synthesizers allows LO frequencies to be selected that achieve the best combination of spurious rejection and bandwidth.

Performance of a typical SMS-DU includes a step size of 1 Hz, tuning speed less than 1 s (inside the DDS bandwidth), FM chirp bandwidth up to 1 GHz, RF output power up to +20 dBm 2 dB, harmonic rejection as high as 50 dBc, and spurious rejection as high as 70 dBc. Phase noise at 10 GHz is typically 95 dBc/Hz at 1 kHz from the carrier, 100 dBc/Hz at a 10 kHz offset, and 125 dBc/Hz at a 1 MHz offset (see table). The SMS-DU operates with input reference frequencies from 10 to 200 MHz at levels from 3 to +3 dBm.

Programming a DDS is notoriously difficult, requiring a good deal of knowledge about DDS in general and a specific DDS device in particular. The SMS-DU reduces this task to a near-trivial level thanks to the use of an FPGA that performs all mathematical calculations required to determine DDS frequency as well as to switch bands and program the internal synthesizers. The user programs the SMS-DU through a simple interface such as binary, BCD, or serial formats.

The SMS-DU is invariably a custom design tailored to meet the needs of a specific application. A typical SMS-DU is housed in a rugged, heavily-shielded aluminum enclosure. However, the synthesizer can also be supplied in a case for 19-in. rack mounting or in various form factors for use in embedded systems. The SMS-DU runs on supplies of + 5, +15, and 5 VDC with typical respective current draws of 800, 250, and 150 mA.

Spinnaker Microwave, Inc.,
3281 Kifer Rd., Santa Clara, CA 95051;
(408) 732-9828,
Internet: www.spinnakermicrowave.com.