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HSMS-285L-BLKG Datasheet(PDF) 7 Page - Agilent(Hewlett-Packard) |
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HSMS-285L-BLKG Datasheet(HTML) 7 Page - Agilent(Hewlett-Packard) |
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7 / 13 page  7 Since no external bias is used with the HSMS-285x series, a single transfer curve at any given frequency is obtained, as shown in Figure 2. 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 net- works 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.[3] Design work begins with the RF impedance of the HSMS-285x series, which is given in Figure 9. 1 GHz 2 3 4 5 6 0.2 0.6 1 2 5 Figure 9. RF Impedance of the HSMS-285x Series at -40 dBm. 915 MHz Detector Circuit Figure 10 illustrates a simple impedance matching network for a 915 MHz detector. 65nH 100 pF VIDEO OUT RF INPUT WIDTH = 0.050" LENGTH = 0.065" WIDTH = 0.015" LENGTH = 0.600" TRANSMISSION LINE DIMENSIONS ARE FOR MICROSTRIP ON 0.032" THICK FR-4. Figure 10. 915 MHz Matching Network for the HSMS-285x Series at Zero Bias. A 65 nH inductor rotates the impedance of the diode to a point on the Smith Chart where a shunt inductor can pull it up to the center. The short length of 0.065" wide microstrip line is used to mount the lead of the diode’s SOT-323 package. A shorted shunt stub of length < λ/4 provides the necessary shunt inductance and simultaneously provides the return circuit for the current gen- erated in the diode. The imped- ance of this circuit is given in Figure 11. FREQUENCY (GHz): 0.9-0.93 Figure 11. Input Impedance. The input match, expressed in terms of return loss, is given in Figure 12. 0.9 -20 FREQUENCY (GHz) 0.915 0 -10 -15 0.93 -5 Figure 12. Input Return Loss. As can be seen, the band over which a good match is achieved is more than adequate for 915 MHz RFID applications. Voltage Doublers To this point, we have restricted our discussion to single diode detectors. A glance at Figure 8, however, will lead to the sugges- tion that the two types of single diode detectors be combined into a two diode voltage doubler[4] (known also as a full wave recti- fier). Such a detector is shown in Figure 13. VIDEO OUT Z-MATCH NETWORK RF IN Figure 13. Voltage Doubler Circuit. Such a circuit offers several advantages. First the voltage outputs of two diodes are added in series, increasing the overall value of voltage sensitivity for the network (compared to a single diode detector). Second, the RF impedances of the two diodes are added in parallel, making the job of reactive matching a bit easier. [3] Agilent Application Note 963, Impedance Matching Techniques for Mixers and Detectors. [4] Agilent Application Note 956-4, Schottky Diode Voltage Doubler. [5] Agilent Application Note 965-3, Flicker Noise in Schottky Diodes. |
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