millimeter wave imaging
V. R. Zakamov, S. E. Kuzmin, V. N. Radzikhovsky, V. B. Khaikin
It is noted in the “Introduction”, that the advent of UWB LNA millimeter chips made possible the construction of non-cooled ultra-wideband receivers of direct amplification with a sensitivity higher than 5 mK c1/2. Creation of a fairly simple broadband radiometric modules is needed for the construction of new generation tightly-packed receiving matrices, in which the number of receiving elements may reach 102-104. In order to build these modules, UWB detectors in strip performance are required. Low-barrier millimiter zero bias diodes give an opportunity to considerably reduce the gain of high sensitivity radiometers — from 50-60 dB to 30-35 dB, and use the total power scheme widely, because the stability of the receiver with a less gain increases, while abnormal own diodes’ noise 1/f caused by bias currents is absent. Matching of diodes in such a frequency band is a difficult technical challenge, as one must take into account all components of the equivalent circuit of the diode, the physical dimensions of the stripline and the size of beam diodes.
The section «Equivalent circuit and design features of the matching circuit” is devoted to the particularities of detector diode matching with a 50 Оhm strip line. The conductivity of the diode has both the active (1/800 Ohm-1) and the capacitive (1/140-1/170 Ohm–1) components. To match the diode with a strip line a matching device is needed, which transforms 50 Ohm into inductive impedance. The possibility of realization of the matching circuit consisting of segments of narrow and wide lines is considered, on substrates with high permittivity, as well as on substrates with low permittivity. High precision implementation of the size of strip lines on dielectrics with high dielectric constant is required, while the strip lines are too narrow. It is known that the matching circuit on such substrates were formed only using the monolithic diode. Broader strip line can be formed in dielectric layers with low dielectric constant, although there are restrictions on the width of the line related to the thickness of metal foil. Since using the beam lead diodes imply the formation of contact pads, matching circuit should be build on substrates with low permittivity only, where the strip lines are wider than on substrates with high permittivity.
The section «Results of modelling and optimization of the matching circuit” is dedicated to choosing the type of the matching circuit and modelling it using the Microwave Office package. A stepped line and circuit with a tree structure are considered as the basis for a matching circuit. Since the step-lines as matching circuits with a capacitive load of R and C type received less attention than the LPF circuits and the circuits with the tree structure, their analysis was performed and the possibility of their physical realization was studied. The analysis has shown that simple step-lines are not physically realizable for broadband matching in the range of 80-100 GHz. However, one can modify two-step-line in such a way that it become physically realizable. Such a modification is in an inclusion the additional elements into a circuit, and there is no significant change in frequency response in the passband. Comparison of frequency response spectra of the modified two-step line and the circuit with a tree structure shows that the developed matching circuit is easier to produce for dielectrics with low dielectric constant.
The section “Features of the detector section and radiometric module” is devoted to radiometric measurements of the characteristics of the developed radiometric module and the detector section. Radiometric module consisted of a waveguide LNA in the frequency band 84-99 GHz with a gain of at least 30 dB and a noise temperature no higher than 450 K and the waveguide detector section based on a low-barrier detector diode. Measurements showed that the Volt-Watt sensitivity of the detector is 3–5 ∙ 103 mV/mW. The temperature sensitivity of the module in a “total power” mode is 15 mK/Hz1/2. Theoretical temperature sensitivity of the module in a “total power” mode is 5 mK/ Hz1/2, this can be achieved by reducing the contribution of the detector noise, improving the parameters of the matching circuit, as well as further matching of the LNA and detector frequency bands.
In the “Conclusion”, conclusions are given and the achievements of this work are highlighted. The main conclusion is, that a highly sensitive detector section is developed, which allowed to realize radiometric direct amplification module using a single InP chip LNA with a gain of about 30 dB. The suggested matching circuit based on the modified two-step strip line exceeds the known analogues in its parameters. One can compare the proposed matching plate with HRL monolithic circuit (chip detector); its matching circuit is designed for the same band for the point-junction tunnel diode with a much lower capacity of 6 fF, instead of used in this paper diode with a capacity of 14 pF.