7
WIDTH = 0.050"
LENGTH = 0.065"
100 pF
WIDTH = 0.015"
LENGTH = 0.600"
TRANSMISSION LINE
DIMENSIONS ARE FOR
MICROSTRIP ON
0.032" THICK FR-4.
RETURN LOSS (dB)
Since no external bias is used
with the HSMS-285x series, a
single transfer curve at any given
frequency is obtained, as shown in
Figure 2.
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 net-
works can be realized in either
lumped or distributed elements,
depending upon frequency, size
constraints and cost limitations,
but certain general design
principals exist for all types.
[3]
Design work begins with the RF
impedance of the HSMS-285x
series, which is given in Figure 9.
65nH
RF
INPUT
VIDEO
OUT
0
-5
-10
-15
Figure 10. 915 MHz Matching
Network for the HSMS-285x Series
at Zero Bias.
-20
0.9
0.915
FREQUENCY (GHz)
0.93
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
simultaneously provides the
return circuit for the current gen-
erated in the diode. The imped-
ance of this circuit is given in
Figure 11.
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 applications.
Voltage Doublers
To this point, we have restricted
our discussion to single diode
detectors. A glance at Figure 8,
however, will lead to the sugges-
tion that the two types of single
diode detectors be combined into
a two diode voltage doubler
[4]
(known also as a full wave recti-
fier). Such a detector is shown in
Figure 13.
2
0.2
0.6
1
5
1 GHz
2
3
RF IN
Z-MATCH
NETWORK
VIDEO OUT
Figure 13. Voltage Doubler Circuit.
4
6
5
FREQUENCY (GHz): 0.9-0.93
Figure 9. RF Impedance of the
HSMS-285x Series at -40 dBm.
Figure 11. Input Impedance.
915 MHz Detector Circuit
Figure 10 illustrates a simple
impedance matching network for
a 915 MHz detector.
The input match, expressed in
terms of return loss, is given in
Figure 12.
Such a circuit offers several
advantages. First the voltage
outputs of two diodes are added
in series, increasing the overall
value of voltage sensitivity for the
network (compared to a single
diode detector). Second, the RF
impedances of the two diodes are
added in parallel, making the job
of reactive matching a bit easier.
[3]
[4]
Agilent Application Note 963,
Impedance Matching Techniques for Mixers and Detectors.
Agilent Application Note 956-4,
Schottky Diode Voltage Doubler.
[5]
Agilent Application Note 965-3,
Flicker Noise in Schottky Diodes.
HP [ AGILENT(HEWLETT-PACKARD) ]
