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The design and preliminary results of an experimental fundamental 300 GHz mixer are reported. The mixer has a modular construction consisting of three major parts (see Fig. 1). They are the waveguide connector with claw flange, the diode mount unit with IF-connector (see Fig. 2) and the backshort unit with micrometer drive. This separable building-up enables on the one hand a simple replacing of the diode mount in case of damage by another mount held on hand. And of course the procedure of contacting itself is facilitated by observing the whisker through the waveguide under a microscope. And on the other hand separated experimental studies of capabilities of the single units independent of another are made possible. So for example different types of backshorts can be tested. Furtheron one limitation of broadband mixer design, i.e. the coaxial mounting structure /2/, is experimentally investigated. Therefore a special IF-coupling is realized at the diode mount. The waveguide for 300 GHz has the dimensions 0.864 x 0.432 mm2 and is not height reduced because of a practical fabrication limit of the whisker length of 0.35 mm. The honeycomb Schottky diode is glued on a turned bush fixed on the center conductor of the glass bead of a K-connector (see Fig. 3). So a one step lowpass filter, which provides a short circuit for 300 GHz, and a good mechanical stability are achieved. Furtheron the extremely short way for the IF provides a very small dependence of the IF-impedance versus frequency. The IF-impedance at a bias current of 1 mA shows (see Fig. 4) in the bandwidth 0.04 - 10 GHz a mismatch loss of less than 0.09 dB and even for the bandwidth 0.04 - 15 GHz less than 0.7 dB related to 50 Ohm. The IF-impedance is investigated with no LO-power applied. Nevertheless the results are transferable. For low level of LO-power (less than 1 mW) there is no significant deviation of IF-impedance. And even for high LO-power the IF-reflection coefficient shows a similar variation of phase versus frequency but at a higher amount of reflection. And the small dependence of phase variation is decisive for a broad-band power and noise matching. In connection with a backshort (the same as used in /3/) in form of a slotted hollow cylinder (made of 0.05 mm thick CuBe sheet) the mixer shows as video detector in the frequency range 290-300 GHz a responsitivity of 280 ± 50 V/W at a bias current of 3 microamperes and a chopping frequency of 1 kHz. Results in mixer operation will be reported in the future.
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