6

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

Detector Circuits

When DC bias is available, Schottky diode detec-

tor circuits can be used to create low cost RF and mi-

crowave receivers with a sensitivity of -55 dBm to

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 equivalent circuit disappear at all

reasonable video frequencies. In general, the lower the

diode’s video impedance, the better the design.

[1]

Avago Application Note 923, Schottky Barrier Diode Video Detectors.

HSMS-285A/6A fig 10

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

VIDEO

OUT

RF

IN

Z-MATCH

NETWORK

VIDEO

OUT

Z-MATCH

NETWORK

RF

IN

At frequencies below 10 MHz, the video resistance dom-

inates 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.

diode terminating a 50 Ω line on the input port. The re-

linear analysis program having the five element equiv-

care must be taken to de-embed the parasitics of the

50 Ω test fixture.

Figure 8. Basic Detector Circuits.

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 junc-

tion capacitance and the video resistance. Of these five

elements of the diode’s equivalent circuit, the four para-

sitics are constants and the video resistance is a function

of the current flowing through the diode.

is approximately 1 MΩ. For a well designed zero bias

out the junction capacitance. Moving up to a higher fre-

quency enables the measurement of the capacitance,

but it then shorts out the video resistance. The best mea-

surement 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.

where

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 detection sensitivity, video re-

sistance and input RF impedance of the zero bias Schottky

detector diode. Since no external bias is used with the

HSMS-285x series, a single transfer curve at any given fre-

quency is obtained, as shown in Figure 2.

V - IR

S

I = I

0.026

0.026

I

f

26,000

R

≈

V

I

R

I

0.026

=

at 25°C

I

= R

R

V - IR

S

I = I

0.026

0.026

I

f

26,000

R

≈

V

I

R

I

0.026

=

at 25°C

I

= R

R