8

915 MHz Detector Circuit

Figure 11 illustrates a simple impedance matching network

for a 915 MHz detector.

The HSMS‑282x family is a better choice for 915 MHz ap‑

plications—the foregoing discussion of a design using

the HSMS‑286B is offered only to illustrate a design

approach for technique.

HSMS-285A/6A fig 14

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.

HSMS-285A/6A fig 15

FREQUENCY (GHz): 0.9-0.93

HSMS-285A/6A fig 16

0.9

-20

FREQUENCY (GHz)

0.915

0

-10

-15

0.93

-5

100 pF

VIDEO

OUT

RF

INPUT

WIDTH = 0.017"

LENGTH = 0.436"

WIDTH = 0.078"

LENGTH = 0.165"

TRANSMISSION LINE

DIMENSIONS ARE FOR

MICROSTRIP ON

0.032" THICK FR-4.

0.030" PLATED THROUGH HOLE,

3 PLACES

0.094" THROUGH, 4 PLACES

FINISHED

BOARD

SIZE IS

1.00" X 1.00".

MATERIAL IS

1/32" FR-4

EPOXY/

FIBERGLASS,

1 OZ. COPPER

BOTH SIDES.

HSMS-2860 fig 15

Figure 11. 915 MHz Matching Network for the HSMS-286x Series at 3 µA 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 simul‑

taneously provides the return circuit for the current

generated in the diode. The impedance of this circuit is

given in Figure 12.

Figure 12. Input Impedance.

The input match, expressed in terms of return loss, is

given in Figure 13.

Figure 13. Input Return Loss.

As can be seen, the band over which a good match is

achieved is more than adequate for 915 MHz RFID ap‑

plications.

Figure 14. 2.45 GHz Matching Network.

Figure 15. Physical Realization.

2.45 GHz Detector Circuit

At 2.45 GHz, the RF impedance is closer to the line of

constant susceptance which passes through the center

of the chart, resulting in a design which is realized

entirely in distributed elements — see Figure 14.

In order to save cost (at the expense of having a larger

circuit), an open circuit shunt stub could be substituted

for the chip capacitor. On the other hand, if space is at a

premium, the long series transmission line at the input

to the diode can be replaced with a lumped inductor. A

possible physical realization of such a network is shown

in Figure 15, a demo board is available from Avago.

CHIP CAPACITOR, 20 TO 100 pF

HSMS-2860

HSMS-285X fig 20 was 17

VIDEO OUT

RF IN

Figure 16. Test Detector.