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HSMS-2862-TR1G

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18
7
The characterization of the surface mount package is
too complex to describe here — linear equivalent circuits
can be found in AN1124.
Detector Circuits (small signal)
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]
Moreover, since external DC bias sets the
video impedance of such circuits, they display classic
square law response over a wide range of input power
levels
[2,3]
. These circuits can take a variety of forms, but
in the most simple case they appear as shown in Figure
9. This is the basic detector circuit used with the HSMS‑
286x family of diodes.
Output voltage can be virtually doubled and input
impedance (normally very high) can be halved through
the use of the voltage doubler circuit
[4]
.
In the design of such detector circuits, the starting point
is the equivalent circuit of the diode. Of interest in the
design of the video portion of the circuit is the diode’s
video impedance — the other elements of the equiv‑
alent circuit disappear at all reasonable video frequen‑
cies. In general, the lower the diode’s video impedance,
the better the design.
The situation is somewhat more complicated in the
design of the RF impedance matching net work, which
includes the pack age inductance and capacitance
(which can be tuned out), the series resistance, the
junction capacitance and the video resistance. Of the
elements of the diode’s equiv alent circuit, the parasitics
are constants and the video resistance is a function of
the current flowing through the diode.
[1]
Avago Application Note 923, Schottky Barrier Diode Video
Detectors.
[2]
Avago Application Note 986, Square Law and Linear Detection.
[3]
Avago Application Note 956‑5, Dynamic Range Extension of Schottky
Detectors.
[4]
Avago Application Note 956‑4, Schottky Diode Voltage Doubler.
[5]
Avago Application Note 963, Impedance Matching Techniques for
Mixers and Detectors.
HSMS-285A/6A fig 12
VIDEO
OUT
RF
IN
Z-MATCH
NETWORK
L
1
DC BIAS
VIDEO
OUT
Z-MATCH
NETWORK
L
1
DC BIAS
RF
IN
Figure 9. Basic Detector Circuits.
HSMS-285A/6A fig 13
1 GHz
2
3
4
5
6
0.2 0.6 1
2
5
Figure 10. RF Impedance of the Diode.
R
V
= R
j
+ R
S
0.026
R
S
= R
d
-
I
f
The sum of saturation current and bias current sets
the detection sensitivity, video resistance and input RF
impedance of the Schottky detector diode. Where bias
current is used, some tradeoff in sensitivity and square
law dynamic range is seen, as shown in Figure 5 and
described in reference
[3]
.
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.
[5]
Design work begins
with the RF impedance of the HSMS‑286x series when
bias current is set to 3 µA. See Figure 10.
8
915 MHz Detector Circuit
Figure 11 illustrates a simple impedance matching network
for a 915 MHz detector.
The HSMS‑282x family is a better choice for 915 MHz ap‑
plications—the foregoing discussion of a design using
the HSMS‑286B is offered only to illustrate a design
approach for technique.
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
RETURN LOSS (dB)
0.9
-20
FREQUENCY (GHz)
0.915
0
-10
-15
0.93
-5
100 pF
VIDEO
OUT
RF
INPUT
WIDTH = 0.017"
LENGTH = 0.436"
WIDTH = 0.078"
LENGTH = 0.165"
TRANSMISSION LINE
DIMENSIONS ARE FOR
MICROSTRIP ON
0.032" THICK FR-4.
0.030" PLATED THROUGH HOLE,
3 PLACES
0.094" THROUGH, 4 PLACES
FINISHED
BOARD
SIZE IS
1.00" X 1.00".
MATERIAL IS
1/32" FR-4
EPOXY/
FIBERGLASS,
1 OZ. COPPER
BOTH SIDES.
HSMS-2860 fig 15
Figure 11. 915 MHz Matching Network for the HSMS-286x Series at 3 µA 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 12.
Figure 12. Input Impedance.
The input match, expressed in terms of return loss, is
given in Figure 13.
Figure 13. 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.
Figure 14. 2.45 GHz Matching Network.
Figure 15. Physical Realization.
2.45 GHz Detector Circuit
At 2.45 GHz, the RF impedance is closer to the line of
constant susceptance which passes through the center
of the chart, resulting in a design which is realized
entirely in distributed elements — see Figure 14.
In order to save cost (at the expense of having a larger
circuit), an open circuit shunt stub could be substituted
for the chip capacitor. On the other hand, if space is at a
premium, the long series transmission line at the input
to the diode can be replaced with a lumped inductor. A
possible physical realization of such a network is shown
in Figure 15, a demo board is available from Avago.
CHIP CAPACITOR, 20 TO 100 pF
HSMS-2860
HSMS-285X fig 20 was 17
VIDEO OUTRF IN
Figure 16. Test Detector.
9
Two SMA connectors (E.F. Johnson 142‑0701‑631 or
equivalent), a high‑Q capacitor (ATC 100A101MCA50 or
equivalent), miscellaneous hardware and an HSMS‑286B
are added to create the test circuit shown in Figure 16.
The calculated input impedance for this network is
shown in Figure 17.
Figure 19. Input Impedance. Modified 2.45 GHz Circuit.
This does indeed result in a very good match at midband,
as shown in Figure 20.
HSMS-0005 fig 21 was 18
FREQUENCY (GHz): 2.3-2.6
HSMS-285X fig 22 was 19
RETURN LOSS (dB)
2.3
-20
FREQUENCY (GHz)
2.45
0
-10
-15
2.6
-5
HSMS-0005 fig 23 was 20
FREQUENCY (GHz): 2.3-2.6
2.45 GHz
HSMS-285X fig 24 was 21
RETURN LOSS (dB)
2.3
-20
FREQUENCY (GHz)
2.45
0
-10
-15
2.6
-5
Figure 17. Input Impedance, 3 µA Bias.
The corresponding input match is shown in Figure 18. As
was the case with the lower frequency design, bandwidth
is more than adequate for the intended RFID application.
Figure 18. Input Return Loss, 3 µA Bias.
A word of caution to the designer is in order. A glance
at Figure 17 will reveal the fact that the circuit does
not provide the optimum impedance to the diode at
2.45 GHz. The temptation will be to adjust the circuit
elements to achieve an ideal single frequency match, as
illustrated in Figure 19.
Figure 20. Input Return Loss. Modified 2.45 GHz Circuit.
However, bandwidth is narrower and the designer runs
the risk of a shift in the mid band frequency of his circuit
if there is any small deviation in circuit board or diode
character istics due to lot‑to‑lot variation or change in
temper‑ature. The matching technique illustrated in
Figure 17 is much less sensitive to changes in diode and
circuit board processing.
5.8 GHz Detector Circuit
A possible design for a 5.8 GHz detector is given in Figure
21.
20 pF
VIDEO
OUT
RF
INPUT
WIDTH = 0.016"
LENGTH = 0.037"
WIDTH = 0.045"
LENGTH = 0.073"
TRANSMISSION LINE
DIMENSIONS ARE FOR
MICROSTRIP ON
0.032" THICK FR-4.
Figure 21. 5.8 GHz Matching Network for the HSMS-286x Series at 3 µA Bias.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18

HSMS-2862-TR1G

Mfr. #:
Buy HSMS-2862-TR1G
Manufacturer:
Broadcom / Avago
Description:
RF Detector 4 VBR 0.3 pF
Lifecycle:
New from this manufacturer.
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