HSMS-285C-TR1 datasheet(6/13 Pages) HP

Part #	HSMS-285C-TR1

Description	Surface Mount Zero Bias Schottky Detector Diodes

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6

RS is perhaps the easiest to

measure accurately. The V-I curve

is measured for the diode under

forward bias, and the slope of the

curve is taken at some relatively

high value of current (such as

5 mA). This slope is converted

into a resistance Rd.

0.026

RS = Rd – ––––––

If

RV and CJ are very difficult to

measure. Consider the impedance

of CJ = 0.16 pF when measured at

1 MHz — it is approximately

1M

Ω. For a well designed zero

bias Schottky, RV is in the range of

5 to 25 K

Ω, and it shorts out the

junction capacitance. Moving up

to a higher frequency enables the

measurement of the capacitance,

but it then shorts out the video

resistance. The best measurement

technique is to mount the diode in

series in a 50

Ω microstrip test

circuit and measure its insertion

loss at low power levels (around

-20 dBm) using an HP8753C

network analyzer. The resulting

display will appear as shown in

Figure 7.

3

-40

FREQUENCY (MHz)

-10

-25

3000

-20

10

1000

100

-35

-30

-15

50

Ω

50

Ω

0.16 pF

50

Ω

50

Ω 9 KΩ

Figure 7. Measuring C

J

and R

V

.

At frequencies below 10 MHz, the

video resistance dominates the

loss and can easily be calculated

from it. At frequencies above

300 MHz, the junction capacitance

sets the loss, which plots out as a

straight line when frequency is

plotted on a log scale. Again,

calculation is straightforward.

LP and CP are best measured on

the HP8753C, with the diode

terminating a 50

Ω line on the

input port. The resulting tabula-

tion of S11 can be put into a

microwave linear analysis

program having the five element

equivalent circuit with RV, CJ and

RS fixed. The optimizer can then

adjust the values of LP and CP

until the calculated S11 matches

the measured values. Note that

extreme care must be taken to

de-embed the parasitics of the

50

Ω test fixture.

Detector Circuits

When DC bias is available,

Schottky diode detector circuits

can be used to create low cost RF

and microwave receivers with a

sensitivity of -55 dBm to

-57 dBm.[1] These circuits can take

a variety of forms, but in the most

simple case they appear as shown

in Figure 8. This is the basic

detector circuit used with the

HSMS-285x family of diodes.

In the design of such detector

circuits, the starting point is the

equivalent circuit of the diode, as

shown in Figure 6.

Of interest in the design of the

video portion of the circuit is the

diode’s video impedance — the

other four elements of the equiv-

alent circuit disappear at all

reasonable video frequencies. In

general, the lower the diode’s

video impedance, the better the

design.

VIDEO

OUT

RF

IN

Z-MATCH

NETWORK

VIDEO

OUT

Z-MATCH

NETWORK

RF

IN

The situation is somewhat more

complicated in the design of the

RF impedance matching network,

which includes the package

inductance and capacitance

(which can be tuned out), the

series resistance, the junction

capacitance and the video

resistance. Of these five elements

of the diode’s equivalent circuit,

the four parasitics are constants

and the video resistance is a

function of the current flowing

through the diode.

26,000

RV

≈ ––––––

IS + Ib

where

IS = diode saturation current

in

µA

Ib = bias current in

µA

Saturation current is a function of

the diode’s design,[2] and it is a

constant at a given temperature.

For the HSMS-285x series, it is

typically 3 to 5

µA at 25°C.

Saturation current sets the detec-

tion sensitivity, video resistance

and input RF impedance of the

zero bias Schottky detector diode.

[1] Agilent Application Note 923, Schottky Barrier Diode Video Detectors.

[2] Agilent Application Note 969, An Optimum Zero Bias Schottky Detector Diode.

Figure 8. Basic Detector Circuits.

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HSMS-285C-TR1 Datasheet(PDF) 6 Page - Agilent(Hewlett-Packard)

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