This unique and compact oscillator uses optical techniques to generate extremely stable microwave frequencies with low harmonics and low phase noise.
When evaluating choices for a microwave source with low phase noise, designers are generally limited to a phaselocked- loop (PLL) multipliedcrystal, yttrium-iron-garnet (YIG), dielectric-resonator (DRO), surface-acoustic-wave (SAW)-based, or sapphireloaded- cavity oscillator. To fulfill applications in which space savings and portability are required, however, a new option emerged in the form of an opto-electronic oscillator (OEO) from OEWaves, Inc. It is a compact spinoff of the firm's Advanced OEO, notable for its phase noise of -145 dBc/ Hz at 10 kHz (see figure).
The oscillator also stands out for its low vibration and acceleration sensitivity, which are rated at 5 x 10-11/g. It delivers +10 dBm output power with harmonics down to -40 dBc. The oscillator provides timing jitter of 8 fs RMS. It boasts short-term stability of 2 x 10-11 for 1 s with thermal stability of 5 ppm. The OEO tunes across an overall range of 4 kHz. The key to this oscillator is a highpower light source like a laser, which drives an optical modulator and a fiber delay or storage element. The light source, which operates at 1550 nm, is detected at the fiber output and then amplified, phase corrected, and filtered before being applied back to the modulator in electrical form. With sufficient gain, the circuit will oscillate precisely as determined by the filter characteristics. In addition, the fiber cavity will self-lase and then initiate and sustain oscillation. Although the oscillator is initially designed for operation from 10 to 12 GHz (1.3 MHz), higher frequencies can be supported with optional highbandwidth components. In addition, the output frequency is generated directly rather than multiplied, upconverted, or otherwise translated.
Among this oscillator's options is an optical output port with -3 dBm power on an LC/PC connector. An internal PLL is available as well. The variable loop bandwidth ranges from 10 to 200 Hz. Note that the PLL may degrade the phase-noise performance within the locking bandwidth depending on the phase noise of the external reference source used.
The Compact OEO measures 4.5 x 5.9 x 0.94 in. Connectors include S-SMA female for RF input and output and a 26-pin DB connector for DC and control. The oscillator requires DC power of +23, 15, +8.4, and +5 VDC and consumes 20 W. An internal oven stabilizes the fiber. The OEO is well suited for radar systems, test equipment, and satellite communications. The firm also makes a high-performance automated phase-noise measurement system. It uses a homodyne cross-correlation method from 1 to 48 GHz. The system's absolute phase-noise floor is -174 dBc/Hz. OEwaves, Inc., 2555 E.
Colorado Blvd., Pasadena, CA 91107; (626) 449-5000, FAX: (626) 449-1215, Internet: www.oewaves.com.