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HSMS-286B Datasheet(PDF) 10 Page - Broadcom Corporation. |
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HSMS-286B Datasheet(HTML) 10 Page - Broadcom Corporation. |
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10 / 18 page  10 As was the case at 2.45 GHz, the circuit is entirely dis‑ tributed element, both low cost and compact. Input impedance for this network is given in Figure 22. 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. Such a circuit can easily be realized using the two series diodes in the HSMS‑286C. The “Virtual Battery” The voltage doubler can be used as a virtual battery, to provide power for the operation of an I.C. or a tran‑ sistor oscillator in a tag. Illuminated by the CW signal from a reader or interrogator, the Schottky circuit will produce power sufficient to operate an I.C. or to charge up a capacitor for a burst transmission from an oscilla‑ tor. Where such virtual batteries are employed, the bulk, cost, and limited lifetime of a battery are eliminated. Temperature Compensation The compression of the detector’s transfer curve is beyond the scope of this data sheet, but some general comments can be made. As was given earlier, the diode’s video resistance is given by 8.33 x 10‑5 nT RV = IS + Ib where T is the diode’s temperature in °K. As can be seen, temperature has a strong effect upon RV, and this will in turn affect video bandwidth and input RF impedance. A glance at Figure 6 suggests that the proper choice of bias current in the HSMS‑286x series can minimize variation over temperature. The detector circuits described earlier were tested over temperature. The 915 MHz voltage doubler using the HSMS‑286C series produced the output voltages as shown in Figure 25. The use of 3 µA of bias resulted in the highest voltage sensitivity, but at the cost of a wide variation over temperature. Dropping the bias to 1 µA produced a detector with much less temperature variation. A similar experiment was conducted with the HSMS‑ 286B series in the 5.8 GHz detector. Once again, reducing the bias to some level under 3 µA stabilized the output of the detector over a wide temperature range. It should be noted that curves such as those given in Figures 25 and 26 are highly dependent upon the exact design of the input impedance matching network. The designer will have to experiment with bias current using his specific design. HSMS-0005 fig 26 was 23 FREQUENCY (GHz): 5.6-6.0 HSMS-285X fig 27 was 24 5.6 -20 FREQUENCY (GHz) 5.8 0 -10 -15 6.0 -5 5.9 5.7 HSMS-285X fig 11 was 7 VIDEO OUT Z-MATCH NETWORK RF IN Figure 22. Input Impedance. Input return loss, shown in Figure 23, exhibits wideband match. Figure 23. Input Return Loss. Voltage Doublers To this point, we have restricted our discussion to single diode detectors. A glance at Figure 9, however, will lead to the suggestion that the two types of single diode detectors be combined into a two diode voltage doubler[4] (known also as a full wave rectifier). Such a detector is shown in Figure 24. Figure 24. Voltage Doubler Circuit. |
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