Low-Loss Millimeter-Wave Isolators for Cryogenic Systems

Ferrite based isolators are useful for controlling standing waves in a wide variety of millimeter-wave (MMW), and terahertz (THz) systems. A good example of their usefulness is found in high frequency local oscillator systems. These systems typically comprise a lower frequency oscillator driving a cascade of frequency multipliers. Standing waves arise due to impedance mismatches between the highly tuned components. This in turn gives rise to dips or even nulls in the output of the multiplier chain. The standing waves are often mitigated with complicated impedance matching techniques that are implemented on a case-by-case basis at a great cost in time and money. By using suitable isolators, the standing waves could be suppressed at a fraction of the time and cost.The high insertion loss of these components renders them unsuitable for use in most MMW systems. The typical insertion loss of a WR-6.5 isolator (110-170 GHz) is more than 3 dB. The problem is worse at higher bands. Micro Harmonics Corporation has recently developed a revolutionary new line of isolators with significantly reduced insertion loss. Our WR-8 and WR-6.5 isolators have a measured insertion loss of less than 1 dB over most of the waveguide band. By April of this year, a full line of low-loss isolators covering bands from WR-12 (60-90 GHz) through WR-3.4 (220-325 GHz) will be available. Many NASA instruments are designed for use at cryogenic temperatures. However, cryogenically rated isolators are currently unavailable at frequencies above 40 GHz. Isolators designed for room temperature operation have poor isolation at cryogenic temperatures due to changes in the ferrite properties. The isolators must be fundamentally redesigned for cryogenic use. We propose to develop high-frequency isolators optimized for cryogenic temperatures that exhibit significantly reduced loss and improved isolation and bandwidth making them useful for many of the instruments now being developed for NASA missions.