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HSMS-2850 Datasheet(PDF) 6 Page - Broadcom Corporation. |
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HSMS-2850 Datasheet(HTML) 6 Page - Broadcom Corporation. |
6 / 13 page 6 RS is perhaps the easiest to measure accurately. The V-I curve is measured for the diode under forward bias, and the slope of the curve is taken at some relatively high value of current (such as 5 mA). This slope is converted into a resistance Rd. Detector Circuits When DC bias is available, Schottky diode detec- tor circuits can be used to create low cost RF and mi- crowave receivers with a sensitivity of -55 dBm to -57 dBm.[1] These circuits can take a variety of forms, but in the most simple case they appear as shown in Figure 8. This is the basic detector circuit used with the HSMS-285x family of diodes. In the design of such detector circuits, the starting point is the equivalent circuit of the diode, as shown in Figure 6. Of interest in the design of the video portion of the circuit is the diode’s video impedance—the other four elements of the equivalent circuit disappear at all reasonable video frequencies. In general, the lower the diode’s video impedance, the better the design. [1] Avago Application Note 923, Schottky Barrier Diode Video Detectors. HSMS-285A/6A fig 10 3 -40 FREQUENCY (MHz) -10 -25 3000 -20 10 1000 100 -35 -30 -15 50 Ω 50 Ω 0.16 pF 50 Ω 50 Ω 9 KΩ VIDEO OUT RF IN Z-MATCH NETWORK VIDEO OUT Z-MATCH NETWORK RF IN Figure 7. Measuring CJ and RV. At frequencies below 10 MHz, the video resistance dom- inates the loss and can easily be calculated from it. At frequencies above 300 MHz, the junction capacitance sets the loss, which plots out as a straight line when frequency is plotted on a log scale. Again, calculation is straightforward. LP and CP are best measured on the HP8753C, with the diode terminating a 50 Ω line on the input port. The re- sulting tabulation of S11 can be put into a microwave linear analysis program having the five element equiv- alent circuit with RV, CJ and RS fixed. The optimizer can then adjust the values of LP and CP until the calculated S11 matches the measured values. Note that extreme care must be taken to de-embed the parasitics of the 50 Ω test fixture. Figure 8. Basic Detector Circuits. The situation is somewhat more complicated in the design of the RF impedance matching network, which includes the package inductance and capacitance (which can be tuned out), the series resistance, the junc- tion capacitance and the video resistance. Of these five elements of the diode’s equivalent circuit, the four para- sitics are constants and the video resistance is a function of the current flowing through the diode. RV and CJ are very difficult to measure. Consider the impedance of CJ = 0.16 pF when measured at 1 MHz — it is approximately 1 MΩ. For a well designed zero bias Schottky, RV is in the range of 5 to 25 KΩ, and it shorts out the junction capacitance. Moving up to a higher fre- quency enables the measurement of the capacitance, but it then shorts out the video resistance. The best mea- surement technique is to mount the diode in series in a 50 Ω microstrip test circuit and measure its insertion loss at low power levels (around -20 dBm) using an HP8753C network analyzer. The resulting display will appear as shown in Figure 7. where IS = diode saturation current in µA Ib = bias current in µA Saturationcurrentisafunctionofthediode’sdesign,[2]and it is a constant at a given temperature. For the HSMS-285x series, it is typically 3 to 5 µA at 25°C. Saturation current sets the detection sensitivity, video re- sistance and input RF impedance of the zero bias Schottky detector diode. Since no external bias is used with the HSMS-285x series, a single transfer curve at any given fre- quency is obtained, as shown in Figure 2. V - IR S I = I S (exp ( )-1) 0.026 0.026 I f 26,000 R ≈ V I S + Ib 8.33 X 10-5 n T R j = V– Rs I S + I b 0.026 = at 25°C I S + Ib = R R S = Rd – V - IR S I = I S (exp ( )-1) 0.026 0.026 I f 26,000 R ≈ V I S + Ib 8.33 X 10-5 n T R j = V– Rs I S + I b 0.026 = at 25°C I S + Ib = R R S = Rd – |
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