9

Two SMA connectors (E.F. Johnson 142‑0701‑631 or

equivalent), a high‑Q capacitor (ATC 100A101MCA50 or

equivalent), miscellaneous hardware and an HSMS‑286B

are added to create the test circuit shown in Figure 16.

The calculated input impedance for this network is

shown in Figure 17.

Figure 19. Input Impedance. Modified 2.45 GHz Circuit.

This does indeed result in a very good match at midband,

as shown in Figure 20.

HSMS-0005 fig 21 was 18

FREQUENCY (GHz): 2.3-2.6

HSMS-285X fig 22 was 19

2.3

-20

FREQUENCY (GHz)

2.45

0

-10

-15

2.6

-5

HSMS-0005 fig 23 was 20

FREQUENCY (GHz): 2.3-2.6

2.45 GHz

HSMS-285X fig 24 was 21

2.3

-20

FREQUENCY (GHz)

2.45

0

-10

-15

2.6

-5

Figure 17. Input Impedance, 3 µA Bias.

The corresponding input match is shown in Figure 18. As

was the case with the lower frequency design, bandwidth

is more than adequate for the intended RFID application.

Figure 18. Input Return Loss, 3 µA Bias.

A word of caution to the designer is in order. A glance

at Figure 17 will reveal the fact that the circuit does

not provide the optimum impedance to the diode at

2.45 GHz. The temptation will be to adjust the circuit

elements to achieve an ideal single frequency match, as

illustrated in Figure 19.

Figure 20. Input Return Loss. Modified 2.45 GHz Circuit.

However, bandwidth is narrower and the designer runs

the risk of a shift in the midband frequency of his circuit

if there is any small deviation in circuit board or diode

characteristics due to lot‑to‑lot variation or change in

temper‑ature. The matching technique illustrated in

Figure 17 is much less sensitive to changes in diode and

circuit board processing.

5.8 GHz Detector Circuit

A possible design for a 5.8 GHz detector is given in Figure

21.

20 pF

VIDEO

OUT

RF

INPUT

WIDTH = 0.016"

LENGTH = 0.037"

WIDTH = 0.045"

LENGTH = 0.073"

TRANSMISSION LINE

DIMENSIONS ARE FOR

MICROSTRIP ON

0.032" THICK FR-4.

Figure 21. 5.8 GHz Matching Network for the HSMS-286x Series at 3 µA Bias.