Sources Shave Phase NoiseFor Millimeter-Wave Applications

Device-level improvements are helping to lower phase noise in high-frequency sourceseven when multiplied to millimeter-wave frequencies.

Phase noise limits the performance of many high-frequency systems from electronic-warfare (EW) to test equipment. But new low-noise transistors are making possible a score of higherfrequency oscillators and synthesizers with outstanding spectral purity to meet the performance requirements of existing and emerging systems.

At Micro Lambda Wireless, for instance, designers are replacing traditional gallium- arsenide (GaAs) field-effect transistors (FETs) with silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) to improve the phase-noise performance of millimeter-wave yttrium-iron-garnet (YIG)-tuned oscillators. According to Vice President of Marketing, Richard Leier, "Using low-noise SiGe HBTs with transition frequency above 60 GHz, we have achieved a 15-dB improvement in the phase-noise performance of the YIG-tuned oscillator."

With GaAs FETs, the phase noise of the YIG-tuned oscillator is 100 dBc/ Hz at 100 kHz from the carrier, states Leier. After replacing the FETs with SiGe transistors, the phase noise is 115 dBc/Hz at 100 kHz offset from the carrier.

To serve applications that call for tunable bandpass filters with a broad frequency range, high rejection in the stopband, and good linearity, proponents like Omniyig and Micro Lambda Wireless have been offering a variety of multistage, multi-octave YIG-tuned filters. Now, Omniyig is developing such a filter for the 2-to-26-GHz and 2-to-40-GHz ranges for integration into a broadband radar's phase-locked circuit. The firm also is investigating magnetic materials for YIG filters beyond 40 GHz and developing a tunable multiplexer that leverages tunable YIG filters.

Meanwhile, Micro Lambda is making its four-stage, millimeter-wave, YIG-tuned bandpass filters available with coaxial connectors for coverage from 7 to 40, 18 to 40, and 3 to 50 GHz. For example, the model MLFP-43050 offers a 30-MHz, 3-dB passband over a frequency range of 3 to 55 GHz. While it exhibits passband insertion loss of 6 dB, the filter offers offresonance isolation above 60 dB.

To reduce the third harmonic of a local-oscillator (LO) chain, SpaceK Labs has combined a lowpass filter with a waveguide-to-coaxial transition in the same housing (see the figure). This design, which is labeled TLP28-K, is available in bands from 18 to 65 GHz. According to COO Dave Greim, two units can be put together to form a coaxial highpass/lowpass filter.

To address the needs of receivers in the 60-to-90-GHz range, SpaceK Labs has developed a receiver that blockconverts a millimeter-wave signal in the 60-to-90-GHz range to an intermediate frequency (IF) of 1 to 31 GHz. The LO frequency of 59 GHz is generated by a Gunn oscillator, which requires a bias of + 5 V at 1 A. Typical conversion loss is 6 dB with a maximum of 13 dB.

To support a wider continuous bandwidth, Narda Microwave East is using 2.92-mm connectors with its directional couplers covering a continuous range of 1 to 60 GHz. This compact coupler offers 10-dB coupling with 2 dB coupling flatness and 8 dB minimum directivity. It can handle 20 W power. Among the other firms advancing the performance of millimeter-wave components are Farran Technology, Merrimac Industries, Millitech, Inc., and Teledyne Microwave.

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