8
Such a circuit can easily be
realized using the two series di-
odes in the HSMS-285C.
Flicker Noise
Reference to Figure 5 will show
that there is a junction of metal,
silicon, and passivation around
the rim of the Schottky contact. It
is in this three-way junction that
flicker noise
[5]
is generated. This
noise can severely reduce the
sensitivity of a crystal video
receiver utilizing a Schottky
detector circuit if the video
frequency is below the noise
corner. Flicker noise can be
substantially reduced by the
elimination of passivation, but
such diodes cannot be mounted in
non-hermetic packages. p-type
silicon Schottky diodes have the
least flicker noise at a given value
of external bias (compared to
n-type silicon or GaAs). At zero
bias, such diodes can have
extremely low values of flicker
noise. For the HSMS-285x series,
the noise temperature ratio is
given in Figure 14.
15
For an ideal resistor R, at 300°K,
the noise voltage can be com-
puted from
v = 1.287 X 10
-10
√R
volts/Hz
which can be expressed as
20 log
10
v
dBV/Hz
between the antenna and the
Schottky diode, shorting out the
RF circuit temporarily and
reflecting the excessive RF energy
back out the antenna.
Assembly Instructions
SOT-323 PCB Footprint
A recommended PCB pad layout
for the miniature SOT-323 (SC-70)
package is shown in Figure 15
(dimensions are in inches). This
layout provides ample allowance
for package placement by auto-
mated assembly equipment
without adding parasitics that
could impair the performance.
Figure 16 shows the pad layout
for the six-lead SOT-363.
0.026
Thus, for a diode with R
V
= 9 KΩ,
the noise voltage is 12.2 nV/Hz or
-158 dBV/Hz. On the graph of
Figure 14, -158 dBV/Hz would
replace the zero on the vertical
scale to convert the chart to one
of absolute noise voltage vs.
frequency.
Diode Burnout
Any Schottky junction, be it an RF
diode or the gate of a MESFET, is
relatively delicate and can be
burned out with excessive RF
power. Many crystal video receiv-
ers used in RFID (tag) applica-
tions find themselves in poorly
controlled environments where
high power sources may be
present. Examples are the areas
around airport and FAA radars,
nearby ham radio operators, the
vicinity of a broadcast band trans-
mitter, etc. In such environments,
the Schottky diodes of the
receiver can be protected by a de-
vice known as a limiter diode.
[6]
Formerly available only in radar
warning receivers and other high
cost electronic warfare applica-
tions, these diodes have been
adapted to commercial and
consumer circuits.
Agilent offers a complete line of
surface mountable PIN limiter
diodes. Most notably, our
HSMP-4820 (SOT-23) can act as a
very fast (nanosecond) power-
sensitive switch when placed
0.07
0.035
0.016
Figure 15. PCB Pad Layout
(dimensions in inches).
NOISE TEMPERATURE RATIO (dB)
0.026
10
5
0.075
0
0.035
-5
10
100
1000
10000
100000
FREQUENCY (Hz)
0.016
Figure 14. Typical Noise Temperature
Ratio.
Figure 16. PCB Pad Layout
(dimensions in inches).
Noise temperature ratio is the
quotient of the diode’s noise
power (expressed in dBV/Hz) di-
vided by the noise power of an
ideal resistor of resistance R = R
V
.
[6]
Agilent Application Note 1050,
Low Cost, Surface Mount Power Limiters.
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
