Application Notes – Millimeter-Wave Interferometers
Interferometers are specialized microwave measurement instruments. They are important for studies such as:
- determining plasma density
- making radio astronomy observations
- characterizing material properties
- measuring complex dielectric constant (permitivity)
- monitoring or characterizing processes in real-time
Furthermore, interferometer instruments can fulfill additional objectives. For example, some typical variations or modifications of interferometer instruments include:
Millimeter-wave (MMW) 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. Specifically, this occurs either by splitting the power from a common oscillator or by phase-locking another oscillator to the transmitter source.
To illustrate, figures 1 and 2 show some 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.
The received signal resolves into its I-Q components to characterize the time-variant (transient) amplitude and phase shift characteristics of the medium, material, or phenomenon under observation. Then, the LO is derived from the same source as the transmitter to provide a reference for the data. Finally, standard equipment or boards amplify and digitize the I-Q channel output signals.
In addition, specialized applications of MMW interferometry use passive free-space components such as filters, beam splitters, and reflectors in order to create interferometer configurations. These are analogous to their infrared and optical counterparts. Typical configurations include Fabry-Perot interferometers and Michelson interferometers. In addition, radio astronomical measurements often use a large receiver array in an interferometric configuration.
Operation and Typical Performance Characteristics
The receiver is the most critical component in the MMW interferometer. It produces 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. Generally, it is not necessary to phase lock the master oscillator, but a low phase noise and high stability are key.
Typical Examples and Case Histories
||Description and Features
||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.
||Plasma Diagnostic System
||Uses high-power transmitter source.
|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.
- “Electron Density Profile Measurements on LHD,” Kawahata, et al, IEEE Trans. on Plasma Science, April 2004.
- “Permittivity Measurements of Lossy Liquids at Millimeter-Wave Frequencies,” L. Zanforlin in Microwave Theory and Techniques, IEEE Trans., May 1983.
- “Dielectric Measurement of Millimeter Wave Materials,” M.N. Afsar in Microwave Theory and Techniques, IEEE Trans., Dec. 1984
- Owens Valley Radio Observatory, Caltech Astronomy, www.ovro.caltech.edu.
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: QTM; Power Dividers/Hybrids (Short Slot Coupler, Matched Hybrid Tee, Directional Couplers)