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HSMS-2855-BLK Datasheet(PDF) 8 Page - Agilent(Hewlett-Packard) |
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HSMS-2855-BLK Datasheet(HTML) 8 Page - Agilent(Hewlett-Packard) |
8 / 13 page 8 Such a circuit can easily be realized using the two series di- odes in the HSMS-285C. Flicker Noise Reference to Figure 5 will show that there is a junction of metal, silicon, and passivation around the rim of the Schottky contact. It is in this three-way junction that flicker noise[5] is generated. This noise can severely reduce the sensitivity of a crystal video receiver utilizing a Schottky detector circuit if the video frequency is below the noise corner. Flicker noise can be substantially reduced by the elimination of passivation, but such diodes cannot be mounted in non-hermetic packages. p-type silicon Schottky diodes have the least flicker noise at a given value of external bias (compared to n-type silicon or GaAs). At zero bias, such diodes can have extremely low values of flicker noise. For the HSMS-285x series, the noise temperature ratio is given in Figure 14. FREQUENCY (Hz) 15 10 5 0 -5 10 100 1000 10000 100000 Figure 14. Typical Noise Temperature Ratio. Noise temperature ratio is the quotient of the diode’s noise power (expressed in dBV/Hz) di- vided by the noise power of an ideal resistor of resistance R = RV. For an ideal resistor R, at 300 °K, the noise voltage can be com- puted from v = 1.287 X 10-10 √R volts/Hz which can be expressed as 20 log10 v dBV/Hz Thus, for a diode with RV = 9 K Ω, the noise voltage is 12.2 nV/Hz or -158 dBV/Hz. On the graph of Figure 14, -158 dBV/Hz would replace the zero on the vertical scale to convert the chart to one of absolute noise voltage vs. frequency. Diode Burnout Any Schottky junction, be it an RF diode or the gate of a MESFET, is relatively delicate and can be burned out with excessive RF power. Many crystal video receiv- ers used in RFID (tag) applica- tions find themselves in poorly controlled environments where high power sources may be present. Examples are the areas around airport and FAA radars, nearby ham radio operators, the vicinity of a broadcast band trans- mitter, etc. In such environments, the Schottky diodes of the receiver can be protected by a de- vice known as a limiter diode.[6] Formerly available only in radar warning receivers and other high cost electronic warfare applica- tions, these diodes have been adapted to commercial and consumer circuits. Agilent offers a complete line of surface mountable PIN limiter diodes. Most notably, our HSMP-4820 (SOT-23) can act as a very fast (nanosecond) power- sensitive switch when placed between the antenna and the Schottky diode, shorting out the RF circuit temporarily and reflecting the excessive RF energy back out the antenna. Assembly Instructions SOT-323 PCB Footprint A recommended PCB pad layout for the miniature SOT-323 (SC-70) package is shown in Figure 15 (dimensions are in inches). This layout provides ample allowance for package placement by auto- mated assembly equipment without adding parasitics that could impair the performance. Figure 16 shows the pad layout for the six-lead SOT-363. 0.026 0.035 0.07 0.016 Figure 15. PCB Pad Layout (dimensions in inches). 0.026 0.075 0.016 0.035 Figure 16. PCB Pad Layout (dimensions in inches). [6] Agilent Application Note 1050, Low Cost, Surface Mount Power Limiters. |
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