Broadband p-i-n diode switch in the 8-mm wave region with low losses for Dicke radiometer

Abstract

Introduction. The paper presents the results of the development of a broadband p-i-n diode switch for the Dicke radiometer of the 8-millimeter wavelength range. The basic requirements of the low losses and high isolation were achieved due to the careful development of the topology of the finline hybrid integrated circuit (HIS). The development was based on the diode parameters, measured directly in the fin-line and the optimization of the branching region and diode supply circuits. The measured parameters of HIS (losses low than 1.5 dB including losses in connecting circuits and isolation more than 29.5 dB) are among the best in such devices.
Development of HIS of the switch. The following main factors were taken into account in the development of HIS. To implement the switch available and inexpensive p-i-n diodes with a sufficiently large inductance were used (measured value of inductance ). Secondly, the 120-degree positioned switch ports, traditionally used for implementation of fin-line SPDT, was considered unacceptable from the constructive point of view and was replaced by 90-degree positioned output ports. In addition, in order to reduce the commutation noise when using as the element of Dicke radiometer, both diode electrodes had to be isolated throw the “ground”. In was shown that optimization of the position of fin-line slot junction point with respect to the point of branching of waveguides makes it possible to increase the isolation of the disconnected arm. The placement of the absorber in the branching region excludes the resonant excitation of this region without increasing losses.
The isolation frequency response correction is achieved by optimizing the sizes of radial stubs. The dimensions of stubs were calculated by electrodynamic analysis to provide the reactance necessary for resonance with diode inductance of the open diode, since it was shown that approximate formulas available for calculation of their input reactance do not allow making correct estimates due to the influence of the high impedance lines connected to stubs to provide the diode supply.
Conclusion. Accurate, step-by-step development of HIS elements, including the measurement of diode parameters in HIS line, optimization of branching region and diode supply circuit made it possible to implement a device that meets the high requirements of small insertion loss and high isolation in 8-millimeter wavelength range. The developed switch was successfully used in the front end of Dicke radiometer with electric beam scanning.