Rapidly Growing Satelite communications (satcom) markets
pose new demands on designers of ground terminals. these include
smaller and lighter mobile multiband terminals as well as more
highly integrated, fixed ground stations. DC power consumption,
size, weight, and rugedness are critical to achieve these goals. A line of low-power, compact frequency synthesizers fro Elcom, Inc. (Closter, NJ) offers considerable advantages in power cosumption, size, and weight compared to conventional frequency synthesizer designs. The synthesizers, which are available in bands from 1 to 23 GHz, meet the requirements of IESS 308/309, Eutelsat, and MIL-STD-188-164 for fixed and mobile satcom ground stations with large safety margins.

The rugged synthesizer line (Fig. 1) is capable of operating over bands as wide as 25 percent at Ku-band and as mch as 33 percent at C-band. The wide operating ranges are made possible by using wideband frequency multipliers in combination with GaAs field-effect-transistor (FET) amplifiers. Standard units include coverage of 1.50 to 2.50 GHz, 4.35 to 5.25 GHz, 5.15 to 5.75 GHz, 6.60 to 7.20 GHz, 8.6 to 10.70 GHz, 12.60 to 13.40 GHz, and 15.50 to 16.30 GHz. Standard staep sizes are 1 and 125 kHz; in both cases, the phase coherence of the reference frequency is maintained. Synthesizers that feature optional steps sizes from 0.1 Hzto 250 kHz are also available in the company's standard housing.

For those applications requiring phase coherency while tuning different frequencies, these synthesizers offer high-quality output signals while also providing decimal or easily-dividable frequency steps, in comparison to direct-digital-synthesizer (DDS) sources. Such sources, while providing fast switching speeds with phase coherency, generate analog signals in response to digital command words. They operate as a function of some fraction of the clock rate, often producing unconventional frequency steps, such as 0.13 Hz, rather than the 0.1 Hz and coarser resolution possible with the Elcom synthesizers.

All models include a fixed L-band output port for use in a satcom earth station's second-stage frequency conversion. The L-band signals are available over a frequency range of 0.6 to 3.5 GHz with +12dBm output power. The typical spurious content is -95dBc. The phase noise is -100 dBc/Hz offset 1kHz from the same L-band carrier.


BASIC DESIGN

The sarcom synthesizers employ a single-module design, implemented with a complementary-metal-oxide-semiconductor (CMOS) applicaton specific integrated circuit (ASIC), advanced components, and a dedicated microcontroller. Even with the microcontroler, the power consumption is considerably less than in standard synthesizers. For example, the DC power consumption of a typical Ku-band frequency synthesizer with 1 kHz step size and 2 GHz bandwidth is 20 W. A second stage L-band local oscillator (LO) consumes and additional 4W. The new satcom synthesizers, in comparison, consume only 8-W DC power, including generation of the auxiliary L-band output signals. This outstanding 65 percent savings in power consumption translates into lower operating temperature (and higher reliability) and the ability to use a smaller power supply.

The single-module enclosure for these satcom synthesizers measures only 5.5x7.7x0.72 in. (13.97x19.56x18.29 cm). All connections,including frequency -and low- frequency LO outputs, reference input, and a 14 pin control connector , are located on one side of the housing.

The total volume of the enclosure is 30 cubic inches compared to 60 cubic inches or more for conventional designs. The small size and low-power requirements of the synthesizers make them suitable for portable satcom terminals as well as modular VME- and VXI- based test equipment.  LOW-NOISE VCOs In order to ensure stable performance in environments with high noise and vibration level from generators and engines, the satcom synthesizers are implemented with wide-loop bandwidths, low-noise voltage-controlled oscillators (VCOs), and stable coaxial dielectric resonators. Low spurious levels are maintained by using a sharp multi-stage filter at the synthesizer output port to eliminate out-of-band products. In selected bands, such as the intermediate frequency (IF) or the converter transmit band, spurious levels are checked down to -125dBc. The fixed L-band output spurious specification is at least -100 dBc. Further below 200 MHz, the spurious products are better than -120 dBc.

The standard C-band synthesizer surpasses the IESS 308 phase-noise specification by 20 dB (Fig 2), with better than -80 dBc/Hz phase noise offset 100Hz from a C-band carrier and better than -100 dBc/Hz offset 10 kHz from the same carrier (see table). Even lower phase noise is available as an option. The low phase noise is the result of an innovative three-loop design that incorporates fractional-N synthesizer technology for the fine frequency steps. Each synthesizer's output VCO is optimized for low phase noise. At Ku-band, this VCO phase noise is -110 dBc/Hz offset 100kHz from the carrier. The second VCO in each synthesizer employs coaxial dielectric resonators to minimize phase noise and effects of acceleration force and vibration.

Standard synthesizer tuning speed is less than 5 ms for a 100 MHz step and 25 ms across an entire 2GHz bandwidth. This switching time includes the serial frequency-information loading period. Although this time is fast compared to the 100 ms tuning time of competing units, faster frequency-switching speeds are available as an option.


INTERNAL CONTROL

A built-in microcontroller supports the operation, testing, and manufacturing of each synthesizer. During manufacturing, this RISC-based microcontroller helps linearize the synthesizer loops. It does this by providing loop peaking information via a full-duplex RS-232 interface to a GPIB-based personal computer. The required loop attenuation is then automatically programmed into an electronically-erasable, programmable-read-only-memory (EEPROM) table. In normal operation, the microcontroller receives the new frequency command, calculates the division parameters of the three loops, and loads the information serially into the proper shift registers. It performs all of this in a few milliseconds. After loading the data to the loops, the microcontroller monitors the locking conditions of every loop separately.

The microcontroller also helps to test a complete synthesizer.    The syntheszer's built-in-test (BIT) circuitry includes separate lock detection for every loop and automatic frequency control (AFC) for every VCO. The microcontroller uses its analog-to-digital converter (ADC) to monitor AFC voltages and maintain those voltages within specified voltage windows during temperature cycling. All of the BIT information is available to an external computer through the serial interface. During temperature cycling, the synthesizer is programmed to random frequency changes. In each frequency, locking conditions are verified. In this way, every synthesizer is tested over thousands of different frequencies in every temperature. The serial interface makes it possible to connect many synthesizers on the same two RS-485 communication lines, which simplifies testing multiple synthesizers within the same temperature chamber.

The satcom synthesizers are equipped with RS-232, RS-485, or TTL-compatile serial interfaces. The serial communications bus can be configured as a full-duplex asynchronous system (operating at rates to 19.2 Kbaud) or as a half duplex synchronous peripheral device (using clock, data, and enable signals). The interface is implemented in hardware in the internal microcontroller. Each new tuning frequency is loaded in eight binary-coded-decimal (BCD) digits that represent the output frequency in kHz. A serial interface requires much less wiring than the parallel interface. For example, the 1 kHz Ku-band synthesizer requies 30 wires for the parallel interface, while a serial interface requires only three wires. The parallel interface is optional.

All of the components within each synthesizer are rated for operating temperatures from -25 to +85 degrees celsius. The satcom syntesizers feature a standard operating temperature range of -15 to +70 degrees celsius, although wider operating temperature ranges are available for satellite-communications systems that must withstand extreme outdoor conditions.P&A: 30 days.

MICROWAVES & RF * JANUARY 1997