7

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 fre-

quency, size constraints and cost limitations, but certain

work begins with the RF impedance of the HSMS-285x

series, which is given in Figure 9.

[2]

Avago Application Note 969, An Optimum Zero Bias Schottky Detector Diode.

[3]

Avago Application Note 963, Impedance Matching Techniques for Mixers

and Detectors.

HSMS-285A/6A fig 13

1 GHz

2

3

4

5

6

0.2

0.6

1

2

5

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

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.

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 simul-

taneously provides the return circuit for the current

generated in the diode. The impedance of this circuit is

given in Figure 11.

Figure 11. Input Impedance.

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

given in Figure 12.

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 ap-

plications.

Voltage Doublers

To this point, we have restricted our discussion to single

diode detectors. A glance at Figure 8, however, will lead

to the suggestion that the two types of single diode de-

(known also as a full wave rectifier). Such a detector is

shown in Figure 13.

HSMS-285X fig 11 was 7

VIDEO OUT

Z-MATCH

NETWORK

RF IN

Figure 13. Voltage Doubler Circuit.