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One of the most attractive application of the Josephson effect is the realization of precision voltage standards. Present voltage standards are based on series arrays of Josephson junctions (JJA). Accuracy and stability of the quantum step voltage generated in the array of Josephson junctions are limited by the accuracy and stability of the irradiated millimeter-wave signal. Especially for this application the Jülicher SQUID GmbH (JSQ) has developed a low phase noise compact and cost-effective miniature millimeter-wave synthesizer (MMWS), based on new miniature microwave integrated circuits (MMICs), optimized for the operation of advanced precision Josephson voltage standards.
The new MMWS was successfully used for voltage standards made of niobium Josephson junction arrays (JJA), produced by the Physikalisch-Technische Bundesanstalt (PTB Braunschweig), Germany, and the Institute for Physical High Technology (IPHT Jena), Germany. Stable 10V-steps were measured at PTB, IPHT Jena, Mendeleyev Institute for Metrology (VNIIM), St. Petersburg, Russia and at the Bureau international des poids et mesures (BIPM), Sèvres Cedex, France.The Miniature Millimeter Wave Synthesizer (MMWS) unit includes a microcontroller, which allows a precise remote control of frequency and output power level, both being easily adjusted from any PC via an RS-232 interface. Internally the MMWS uses a compact programmable synthesizer and an integrated frequency multiplier (8x). The synthesizer is phase-locked to an external 10 MHz standard reference frequency, which provides the stability of the system. This 10 MHz reference input may be taken from laboratory instruments, if available, or from external sources (GPS, e.g.). The output power spectrum of the MMWS is adjusted by internal feedback control parameters, which have been optimized by JSQ especially for JJA operation.