Schottky Diode Mixers

Ainong the first TFIz mixers to be constructed were those based on room-temperature Schottky diodes [11]. Over the past decade, new mixers based on superconducting tunnel junctions have been developed that have effective noise levels only a few tunes the quantum limit of [12]. Flowever, certain conditions... 

While heterodyne detection is typically the most sensitive, it is problematic to extend its use to FPA systems. Each detector element in the FPA requires LO power, on the order of 1 mW for Schottky diode-based mixers. For large FPAs operating in the millimeter-wave or terahertz bands, this level of power is currently impractical. Various components including LNAs and mixers are available as MMICs at up to about 140 GHz and may eventually extend to 220 GHz or possibly higher [49], Schottky diode-based mixers are available at frequencies extending well into the terahertz range, to 2.5 THz and possibly higher, and can be used wherever a suitable LO source can be obtained [53],... 

The Schottky diode is the most practically useful and affordable solid state detector at present and can be obtained for frequencies up to 325 GHz. In terms of frequency range and noise performance the nearest contender, the Si point-contact diode mixer, has a noise figure of 11-14 dB over the range 60-80 GHz, its maximum operating frequency. Compared with 5-6 dB for a Schottky device... 

Difference frequencies up to 900 GHz between two visible dye lasers have been measured with Schottky diodes (Fig. 4.98) by mixing the differenee frequency with harmonics of 90-GHz microwave radiation which was also focused onto the diode [4.115]. Meanwhile, Sehottky-barrier mixer diodes have been developed that cover the frequency range 1—lOTHz [4.115],... 

The ability to make ever smaller solid-state devices by improved lithography techniques has led to the development of so-called beam lead Schottky-barrier diode detectors and mixers in which diodes are fabricated by the same techniques used to make integrated circuits, and for this reason, they can be included in these circuits. Figure 7 shows such a beam-lead detector/mixer made by Virginia Diodes of Charlottesville, VA [15]. This same configuration is used in fabricating the varactor devices used for frequency multiplication discussed in the preceding section. 

Cryogenically cooled detectors employ the low-noise GaAs Schottky barrier Mott diodes. Between 140 and 220 GHz they exhibit 400 K noise equivalent temperature at a lower limit junction temperature of 20 K, below which the performance degrades. The noise temperature is around 1000 K at 300 K junction temperature. Sensitivity of a Schottky barrier mixer diode ranges from about 2.75 VmW" to 1 VmW over the range 90-325 GHz.In comparison the helium-cooled InSb bolometer used by the present authors (Section 3.4.1) can provide double sideband noise temperatures of 200-300 K in the region 100-300 GHz and sensitivity of 5-6 VmW . ... 

Vaks et alf have designed a novel spectrometric approach for gas analysis based on a BWO and low-Q absorption cell. The BWO spectral output was split between a reference cell and aim waveguide measurement cell. The gas in the 0.2 m pathlength waveguide reference cell was used to lock the BWO frequency to the spectral line at the 304 GHz OCS spectral line and the frequency modulated spectral source directed to the analytical cell. Detection was with a Schottky barrier diode mixer. 

The peak absorption coefficient of OCS, 10 m", occurs at 462 GHz. This is by no means, however, the optimum working frequency due to the non-ideal behaviour of most MMW detectors. Commercial Schottky barrier mixer diode detectors show a quadratic roll-off in sensitivity at frequencies >100 GHz. If this is factored into Equation 6.1, the peak sample sensitivity occurs around 300 GHz, and the response is so flat that even at 100 GHz it has only fallen off by a factor of two. What is common to both curves is the dramatic fall-off in sample sensitivity at frequencies <100 GHz, reinforcing the point that the band 26-40 GHz is ill suited to high-sensitivity analytical spectroscopy. 

The Gunn oscillator frequency-lock became progressively more unstable as its slewing rate increased, whether caused by rapid frequency stepping or by the application of FM. We found ourselves limited in practice to a stepping time of 50-100 ms and an FM deviation of 10 MHz at a rate of 1 kHz. This was considered reasonably satisfactory in that a complete scan could be completed in a few seconds and that our signal detection rate 2 kHz was close to the frequency at which manufacturers, e.g. Millitech, specified their detector performance. The same FM rate was used whether our detector was a Schottky barrier mixer diode or the helium-cooled bolometer. 

T.W. Crowe GaAs Schottky barrier mixer diodes for the frequency range from 1 —lOTHz. Int. J. IR and Millimeter Waves 10, 765 (1989) ... 

H.V. Daniel, B. Maurer, M. Steiner A broadband Schottky point contact mixer for visible laser light and microwave harmonics. J. Appl. Phys. B 30, 189 (1983) H.P. Roeser, R.V. Titz, G.W. Schwaab, M.F. Kimmit Current-frequency characteristics of submicron GaAs Schottky barrier diodes with femtofarad capacitances. J. Appl. Phys. 72, 3194 (1992)... 

The development of metal-insulator-metal (MIM) point contact diodes, and of point contact and planar Schottky barrier mixers... 

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