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Comparison of point-contact and Schottky diode performance as a microwave converter, or mixer, is shown in Figs. 12 through 23.
The change in noise figure vs. local-oscillator drive for S-band diodes is shown in Fig. 12, and is similar to the X-band diodes. The Schottky diode has the best performance over the range from +6 to +3 dBm, similar performance from 0 to -6 dBm, and marked degradation beginning around -10 dBm. The point-contact diode gives better noise figure in the -12 dBm range.
In Fig. 13, which shows VSWR vs. LO drive, the two diodes are comparable until the drive drops to -9 dBm, beyond which the Schottky diode deteriorates faster than the point-contact device.
As shown in Fig. 14, the change in i-f impedance vs. LO drive is comparable for both diodes. The use of dc bias will improve the characteristics for both diodes beyond -12 dBm.
The change in noise figure vs. temperature for both diodes at S and X-bands is shown in Fig. 15. Over the range from -55-deg. C to +150-deg. C, the Schottky diode changes 1 dB at S band while the point-contact diode changes about 5 dB. Most of this change occurs over the high temperature range. Somewhat better performance is obtained with both diodes at X-band, as shown.
The VSWR vs. temperature response for S-band diodes, similar to that for X-band diodes, is shown in Fig. 16.
Variation in i-f impedance with temperature for S-band diodes is shown in Fig. 17. The point-contact device varies about 100 ohms. Similar results were obtained on X-band diodes.
The Schottky diode lends itself to broadband applications, as seen from the impedance charts in Fig. 18. It becomes obvious, however, that the Schottky diode cannot be used as a direct replacement for the point-contact type. For example, systems with video detectors using point-contact diodes cannot be replaced by Schottkys without redesigning the system by the addition of bias.
Another concern in mixer performance is low-frequency noise introduced either by LO or dc bias. Noise voltages obtained at various i-f frequencies for Schottky, and point-contact diodes, as compared to a resistor, are shown in Fig. 19. The two curves for each type diode are for two different bias levels. In each case, the Schottky-diode noise voltage at 1 kc is better than the point-contact by a factor of 3, with the two diodes reaching a comparable condition at 1000 kc.
Microphonics is another problem area, often encountered in systems operating at low i-f frequencies. This phenomenon is noise generated by the diode due to mechanical shock. A block diagram of the microphone test set-up is shown in Fig. 20. The diode is shocked by means of a small vibrator driven at 80 cps while under dc bias condition. The noise generated by the diode is then amplified and displayed on an oscilloscope.
The response obtained while shocking typical point-contact diodes is shown in Fig. 21. The ringing effect is caused by the change in pressure exerted on the contact as the whisker vibrates, thus causing a change in the diode’s barrier resistance.
The waveform in Fig. 22 is obtained for a typical point-contact diode which has been filled with a damping medium to reduce the mechanical oscillation of the whisker. Note the drastic reduction in microphonics. Noise from a typical Schottky diode is shown in Fig. 23. The improvement over the point-contact diode is apparent.