Digital Signal Processing Reference
In-Depth Information
1
10
6
10
6
868
×
2045
×
0.8
0.6
0.4
0.2
0
1
×
10
8
1
×
10
9
1
×
10
10
Frequency
R
j
=
250
R
j
=
2500
R
j
=
25 k
Figure 4.82
The factor
M
describes the influence of the parasitic junction capacitance
C
j
upon
the output voltage
u
chip
at different frequencies. As the junction resistance
R
j
falls, the influence
of the junction capacitance
C
j
also declines markedly. Markers at 868 MHz and 2.45 GHz
the Schottky detector and thus also the junction resistance
R
j
of the Schottky diode.
The HF equivalent circuit of a voltage doubler correspondingly consists of the parallel
connection of two Schottky diodes.
In order to now achieve the required power matching between the antenna and the
Schottky detector, the input impedance
Z
rect
of the Schottky detector must be matched
by means of a circuit for the impedance matching at the antenna impedance
Z
A
.In
HF technology, discrete components, i.e.
L
and
C
, but also line sections of differing
impedances (line transformation), can be used for this.
At ideal matching, the voltage sensitivity
γ
2
xs
(in mV/
W) of a Schottky detector
can be simply calculated (Figure 4.83; Hewlett Packard, 963, 1089):
µ
0
.
52
γ
2
=
1
(
4
.
104
)
R
j
R
L
+
2
C
j
R
s
R
j
)
·
(I
s
+
I
b
)
·
(
1
+
The theoretical maximum of
γ
2 lies at 200 mV/
µ
W (868 MHz) for a Schottky
diode of type HSM 2801, and occurs at a total diode current
I
T
=
I
s
+
I
b
of 0.65
µ
A.
The saturation current
I
s
of the selected Schottky diode is, however, as low as 2
µ
A,
which means that in theory this voltage sensitivity is completely out of reach even
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