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HSMS-2865 Datasheet(PDF) 7 Page - AVAGO TECHNOLOGIES LIMITED |
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HSMS-2865 Datasheet(HTML) 7 Page - AVAGO TECHNOLOGIES LIMITED |
7 / 18 page 7 The characterization of the surface mount package is too complex to describe here — linear equivalent circuits can be found in AN1124. Detector Circuits (small signal) When DC bias is available, Schottky diode detector circuits can be used to create low cost RF and microwave receivers with a sensitivity of ‑55 dBm to ‑57 dBm.[1] Moreover, since external DC bias sets the video impedance of such circuits, they display classic square law response over a wide range of input power levels[2,3]. These circuits can take a variety of forms, but in the most simple case they appear as shown in Figure 9. This is the basic detector circuit used with the HSMS‑ 286x family of diodes. Output voltage can be virtually doubled and input impedance (normally very high) can be halved through the use of the voltage doubler circuit[4]. In the design of such detector circuits, the starting point is the equivalent circuit of the diode. Of interest in the design of the video portion of the circuit is the diode’s video impedance — the other elements of the equiv‑ alent circuit disappear at all reasonable video frequen‑ cies. In general, the lower the diode’s video impedance, the better the design. 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 junction capacitance and the video resistance. Of the elements of the diode’s equivalent circuit, the parasitics are constants and the video resistance is a function of the current flowing through the diode. [1] Avago Application Note 923, Schottky Barrier Diode Video Detectors. [2] Avago Application Note 986, Square Law and Linear Detection. [3] Avago Application Note 956‑5, Dynamic Range Extension of Schottky Detectors. [4] Avago Application Note 956‑4, Schottky Diode Voltage Doubler. [5] Avago Application Note 963, Impedance Matching Techniques for Mixers and Detectors. HSMS-285A/6A fig 12 VIDEO OUT RF IN Z-MATCH NETWORK L1 DC BIAS VIDEO OUT Z-MATCH NETWORK L1 DC BIAS RF IN Figure 9. Basic Detector Circuits. HSMS-285A/6A fig 13 1 GHz 2 3 4 5 6 0.2 0.6 1 2 5 Figure 10. RF Impedance of the Diode. R V = R j + R S 0.026 R S = R d - If The sum of saturation current and bias current sets the detection sensitivity, video resistance and input RF impedance of the Schottky detector diode. Where bias current is used, some tradeoff in sensitivity and square law dynamic range is seen, as shown in Figure 5 and described in reference [3]. The most difficult part of the design of a detector circuit is the input impedance matching network. For very broadband detectors, a shunt 60 Ω resistor will give good input match, but at the expense of detection sensitivity. When maximum sensitivity is required over a narrow band of frequencies, a reactive matching network is optimum. Such networks can be realized in either lumped or distributed elements, depending upon frequency, size constraints and cost limitations, but certain general design principals exist for all types.[5] Design work begins with the RF impedance of the HSMS‑286x series when bias current is set to 3 µA. See Figure 10. |
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