TK Instruments Products Agilent VNA Quasi-Optical Systems
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Quasi-Optical Measurement Circuit for Agilent's VNA'sThomas Keating has long been associated with Quasi-Optical Measurement systems. Here we present a simple free-space measurement system, which when combined with the current generation of Agilent's VNA's and analysis software provides a powerful measurement tool for the determination of complex material properties. QUASI-OPTICAL MEASUREMENT SYSTEMBy Quasi-Optics, we mean optics where the sizes of the optical components is small with respect to the wavelength, and whereby a Gaussian beam waist at the aperture of a corrugated feed horn (Port S1) is refocused by an ellipisolidal mirror to form a beamwaist at the sample position and then is passed, via a second mirror, to the S2 Port, where a second corrugated feed horn feed the beam into the VNA waveguide. Corrugated feed horns are used here because they produce axially symmetrical beams with low side lobes. We can use Gaussian beam mode analysis to predict the form of the beam passing though the circuit: At 94 GHz, the circuit shown below gives a value of just under 25mm for the beamwaist at the sample position (We use the 1/e amplitude level as the definition of beamwaist - the power level has dropped to -8.6 dB). We use mirrors in our Quasi-optical circuits to control the expansions of the Gauissian beams. They are prefered over lenses, having wider bandwidth, very low (basically not measureable) absorption loss. They do generate, however - when used off-axis - higher order and X-polar modes. We choose the focal lengths of mirrors in our QO circuits to keep these higher order and X-polar mode conversions very low. The size of the beamwaist at the sample is chose to be large enough that the sample is being probed by a plane wave. In the case shown below, the waist is 25mm in size and is associated with plane waves over a characteristic spread of angles of about 2.3 degrees. (via the Gaussian Beam omde divergence formula: Theta = Lambda/Pi Wo, Theta being in radians). It is obviously important that the power at the edges of the beam do not suffer from truncation. The circuit is built on a cube system - where the cube size is 125mm per side. At the sample holder, the clear aperture is >100mm and edge truncation levels therefore (because the beam is very close to being a pure Gaussian) at the -35dB level. This rises slightly at the long wavelength end of operation (30mm at 75 GHz) dropping to smaller value (22mm at 110 GHz) at the edge of the upper band. Losses in well designed QO systems can be very low: In this case lossesd in the whole circuit, from S1waveguide to the S2 waveguide port (passing though two corrugated horns, two mirrors and free space inbetween) is aroud the 1 to 2dB level. Gaussian Beam mode analysis of the beam passing through the QO Measurement Sysyem
The beams shown in this picture at the the 1/e amplitude level
Systems can be build for operation in Automotive Radar frequencies -Hear around 78 GH
And a 3-D view of the beams:
which have a very respectable 0.6dB S21 (and S12) transmission loss at centreband. Here are bandsweep measurements taken with Here are S11 and S22 measurements, giving horn losses - by placing a mental reflector across the horn aperture.
Frequency Range of QO Measurement systemsThomas Keating has built QO systems to operate over a very wide range of frequencies. Here is a system operating down to 3 GHz
and here are components of as system for making measurements above 600 GHz The QO system at the IEEE's annual MTT exhibition - here at Long Beach, CA in June 2005 Richard Wylde - Thomas Keating Ltd - This e-mail address is being protected from spambots. You need JavaScript enabled to view it |
