AN1: Millimeter-Wave Interferometers

Applications:
Interferometers are specialized microwave measurement instruments that are used to determine:

  • Plasma Density
  • Radio Astronomy Observations
  • Material Properties Characterization
  • Complex Dielectric Constant (permitivity) measurement
  • Real-time Monitoring/Characterization of Processes

Description:
Millimeter wave interferometers consist of a transmitter source, a receiver and a reference channel. The local oscillator (LO) for the receiver is derived from the same source as the transmitter either by splitting the power from a common oscillator, or by phase-locking another oscillator to the transmitter source. Figures 1 and 2 show some of the most commonly used architectures for MMW Interferometers. In most applications, the transmitter and receiver have appropriate antennas or optical elements to transmit and collect the power through the medium or object under study.

Some typical variations or modifications of Interferometer Instruments are:

  • Millimeter wave Reflectometers
  • Millimeter wave Spectrometers
  • Scattering Parameter Measurement Equipment


Figure 1

Figure 2

Received signal is resolved into its In-Phase and Quadrature (I-Q) components to characterize the time-variant (transient) amplitude and phase shift characteristics of the medium (such as plasma) or material or phenomenon under observation. Local oscillator is derived from the same source as the transmitter to provide a reference or baseline for the data.

The I-Q channel output signals are amplified and digitized using standard commercially available equipment or boards.

Operation and Typical Performance Characteristics:

The most critical component in the MMW interferometer is the receiver capable of producing two output signals, the in-phase and quadrature phase (I-Q channels) using a pair of balanced mixers and hybrid power dividers with accurate phase relationship. The master source for the interferometer provides the transmitter power as well as the local oscillator signal. It is typically not necessary to phase lock the master oscillator, but a low phase noise and high stability are desired.

Other special applications of millimeter wave interferometry utilize passive free-space components, such as filters, beam splitters, reflectors, etc. to create interferometer configurations analogous to their infrared and optical counterparts. Typical configurations include Fabry-Perot interferometers and Michelson interferometers. In addition, radio astronomical measurements often utilize a large receiver array in an interferometric configuration.

Typical Examples and Case Histories
Table and Figure

Frequency, GHz Application Description and Features
105 GHz Plasma Density Measurement System Uses 105 GHz Gunn diode oscillator as master oscillator for transmitter and as LO for I-Q Phase Bridge. Uses integrated I-Q detector module
60 GHz Plasma Diagnostic System High power transmitter source is used
30-300 GHz (over any available frequency interval) Dielectric Constant and Loss Tangent Measurement Create Fabry-Perot interferometer over broad range of frequencies to obtain dielectric properties.

Technical References and Resources:

  1. “Electron Density Profile Measurements on LHD”, Kawahata, et al, IEEE Trans. on Plasma Science, April 2004.
  2. “Permittivity Measurements of Lossy Liquids at Millimeter-Wave Frequencies,” L. Zanforlin in Microwave Theory and Techniques, IEEE Trans., May 1983.
  3. “Dielectric Measurement of Millimeter Wave Materials,” M.N. Afsar in Microwave Theory and Techniques, IEEE Trans., Dec. 1984
  4. Owens Valley Radio Observatory, Caltech Astronomy, www.ovro.caltech.edu.

QuinStar Components and Products Used:
Amplifiers (Low Noise, Power) Detectors Frequency Multipliers High Power IMPATT oscillators at 34-36, 43-47, 58-62, 92-97 GHz)) I-Q Mixer Oscillators (Gunn Diode Oscillator, series QTM from 18-150 GHz Power Dividers/Hybrids (Short Slot Coupler, Matched Hybrid Tee, Directional Couplers)