Datasheet

LTC15 4 0
14
1540fb
For more information www.linear.com/LTC1540
 LINEAR TECHNOLOGY CORPORATION 1997
LT 0817 REV B • PRINTED IN USA
www.linear.com/LTC1540
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TYPICAL APPLICATION
PART NUMBER DESCRIPTION COMMENTS
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Figure 10. Nanopower Field Detector
RF Field Detector
Figure 10 shows the complete circuit for a field detector
which was tested at 445MHz. A transmission line is used to
match the detector diode (1N5712) to a quarter-wave whip
antenna. The 0.23λ wavelength transmission line section
transforms the 1pF (350Ω) diode junction capacitance to
a virtual short at the base of the antenna. At the same time
it converts the received antenna current to a voltage loop
at the diode, giving excellent sensitivity.
The rectified output is monitored by the LTC1540 compara
-
tor. The internal reference is used to set up a threshold of
about 18mV at the inverting input. A rising edge at the
comparator output triggers a one shot that temporarily
enables answer back and any other pulsed functions.
The total supply current is 400nA. Among other monolithic
one shots, the CD4047 draws the least amount of transient
current.
–
+
LTC1540
2V
TO 11V
2
1540 F10
10nF
0.23λ
1N5712
10nF
12M
180k
λ /4
10k
FB
7
6
5
8
3
4
1
Q
Q
CMOS
ONE SHOT
(CD4047)
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