Detection and mixing
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Typical E-I curves for both the point-contact and Schottky-barrier diodes are shown in Fig. 2. So far as video detection is concerned, the main difference between the two curves is the point at which forward conduction starts; i.e., “zero” for the point-contact diode and 200 mV for the Schottky diode. The reverse current is assumed to be zero for either diode over the range of the signal input. If, as in Fig. 3, a small rf voltage (40 mV p-p), is applied at α, the resulting detected output is as shown. As noted, the Schottky diode cannot detect at levels below 200 mV. But, if a dc bias is applied to the Schottky diode equal to current cut-off voltage, detected output from the Schottky diode occurs as a result of the signal applied at b. Therefore, if the Schottky diode is properly biased, efficient detection may be realized.
The mixer diode can be treated in somewhat the same manner (see Fig. 4). Conduction for the Schottky diode occurs only after about 300 mV, while the point-contact diode will operate as an efficient mixer, with a bias of only about 200 mV p-p and a signal of 40 mV p-p. Also, if the LO drive is increased to a point where the Schottky diode is driven into forward conduction, it also becomes an efficient mixer. The use of a dc bias, as shown in Fig. 4, permits the Schottky diode to operate with less LO drive. Use of dc bias also enables the point-contact diode to operate with lower drive.
Forward characteristics of point-contact and Schottky diodes are shown in Fig. 5. Comparisons are made between typical S, X, and Ka-band types. From 1-50 mA, the point-contact and Schottky diodes are similar, but below 1 mA they are quite different.
A comparison of the reverse characteristics of the two types is shown in Fig. 6. The point-contact diodes have very low reverse voltage drops for any given reverse current as compared to the Schottky diodes. Although not generally specified, point-contact diodes typically have 3-V PIV’s at 100 μA; whereas, Schottky diodes are generally specified as 10 μA and a 7-V PIV. At 100 μA, the Schottky PIV would be about 12 V or four times that of a point-contact diode.
A comparison of forward characteristics with temperature is shown in Fig. 7. At currents of 1 mA or more, the change vs. temperature is much less than at currents of 100 μA or less. This is because at higher currents, variation dude to internal heating caused by forward conduction I-R drop tends to swamp out variations due to ambient temperature. Although the changes for Schottky diodes are less than those for point-contact units, they still follow the same general pattern.
Comparison of reverse characteristics vs. temperature is shown in Fig. 8. The change in PIV at 150-deg. C is extreme for the point-contact diodes—becoming 1/3 its room temperature value—whereas, the change is only about 10% for the Schottky diode.