RF and Microwave Power Standards: Extending beyond 110 GHz John Howes National Physical Laboratory April 2008
We now wish to extend above 110 GHz Why now? Previous indecisions about transmission lines, we are using rectangular metallic waveguide Test equipment and components available To meet a real, current need
NMS Support With the backing of the mm wave User Group we have secured funding to: - Extend our work on dielectric waveguides - Rectangular waveguide IEEE Standard for Rectangular Waveguides and Flanges for Rectangular Waveguides for use at Millimeter and Sub-Millimeter Wavelengths. - Rectangular waveguide complex s-parameter standard facilities within the range 90 220 GHz - Conduct a feasibility study for power standards above 110 GHz
Power Standards above 110 GHz Where do we begin our feasibility study? Start with the NPL Mission Statement:
NPL vision To deliver the highest economic and social impact as a leading National Measurement Institute through excellent responsive science and knowledge services.
Where should we begin? Do we need power traceability to SI units immediately? What power sensors are available to us Are they suitable as traceable standards Should we investigate new sensors Do we need a reliable method of comparing power sensors A fresh approach to a primary facility, Or extend existing methods
Technical Limitations Loss in metallic waveguide increases with frequency Availability of mm wave sources Actions must form part of a coherent, longer term plan Do we need other traceable facilities (eg impedance, attenuation )
Things to Consider Must have connectability with components that are being used Bandwidth of our systems Should we build our systems in other transmission lines (quasi-optic, dielectric line) and adapt to rectangular waveguide etc
Power Sensors What is available? Desirable properties: - stability (inherent or repeatable against a reference) - sensitivity - response time - good port match - an inherent standard - suitable for micro calorimetric calibration - response to non CW input
Power Sensors Thermistors sensors are not readily available over 110 GHz Should we develop a range of sensors with metrology in mind? Designed to be calibrated in a micro calorimeter Designed to be calculable Could we evaluate existing devices to be used as (poor) standards
Power Transfer Systems Primary power systems tend to be slow to operate Even if you have one We would like to compare the efficiency of sensors against one another with minimal increase in uncertainty We can imagine several possible techniques
Primary Power Facilities We could extend the frequency coverage of the existing micro calorimeters Needs RF / DC substitution sensors thermistor sensors are ideal are thermistor sensors available? to obtain sufficient sensitivity need 10+ mw possibly not scaleable above 200 300 GHz Risk of water damage!
Alternative Primary Standards Consider alternative primary standards Could we build a calculable primary standard sensor? Could we evaluate an existing product? Could we build a very wide bandwidth standard?
Photo Acoustic power meter An alternative approach at measuring free space power Golay Cell beam absorbed by a resonant structure / absorbing film inside a gas cell. The power is detected by the expansion of gas in a closed cell Microwave heating compared to the expansion caused by LF resistive heating of the same film Compared with CW power standards 15%
Other Alternatives Optical radiometer - Radiation absorbed by lining of a over-moded cavity - A DC / RF substitution instrument - Cryogenic operation increases sensitivity, but adds complexity
Optical Radiometer
Optical Radiometer
Other Possibilities Radiation Pressure Instruments Torque Vane Wattmeter A MEMS device Electron Beam Standards Microwave E-field velocity modulates an electron beam and power is determined by the modulation depth Sensitivity can be increased by passing the beam through a resonant cavity Construction is similar to a reflex klystron, the modulation depth is determined from the reflector potential needed to bring most energetic electrons to rest
Primary Power Facilities Quantum Standards Measure RF current in terms of magnetic flux quanta need impedance standards to obtain power cryogenic Frequency limitation, a probe signal, at higher frequency than the power being measured is required
Concluding thoughts Where should we start? what would have the greatest impact on your activities What are your ideas? Would you like to collaborate?